Description of Case

Vertical upward and downward two-phase flow has applications in the chemical, refrigeration, nuclear, geothermal, and petroleum industries, whether in an adiabatic flow or in an application of the boiling phenomenon.

The two-phase flow phenomenon in vertical pipes is of prime interest because of the significant influence of the interaction of gravity, buoyancy, and inertia forces on the individual phases. It has been observed and experimentally verified that two-phase flow parameters such as flow patterns, interaction of forces, and void fraction are affected by the change in flow direction from vertical upward to vertical downward. The accurate prediction of void fraction in vertical pipes is of immense importance in determining the hydrostatic component of the two-phase pressure drop. This variation in void fraction resulting from the change in flow direction requires a reliable, accurate, and flexible correlation to predict void fraction.
Description of Problem

Many researchers have contributed to the development of the void fraction correlations in vertical orientation, with more emphasis on the vertical upward orientation. The theoretical considerations used for the development of these correlations are based on the separated flow model or the drift flux model. Most of these correlations are flow pattern–specific and are limited in their application in terms of the range of flow variables. The void fraction correlation proposed by Woldesemayat and Ghajar1 is independent of the flow direction, system pressure, and fluid thermophysical properties; however, it is subject to some inaccuracies in predicting the void fraction (α), typically in a range of 0 < α < 0.4 in vertical upward and downward flow directions. For industrial applications involving two-phase heat transfer and pressure drop in vertical pipe orientations, accurate prediction of void fraction is key in designing and sizing the flow lines. To choose the right correlation for the desired application from a pool of available correlations in the literature, it is of the utmost importance to sort out and identify the top-performing correlations applicable to a range of flow variables and pipe orientations.

Description of Solution

A total of 52 void fraction correlations were identified and tested against a comprehensive database of 1,208 void fraction data points for vertical upward orientation; 26 correlations, including some developed for upward orientation, were tested against 909 data points in vertical downward orientation. The results of the comprehensive performance analysis pointed to the Rouhani and Axelsson² and Gomez and associates³ correlations as outstanding performers in predicting void fraction for vertical upward and downward flow, respectively. Both correlations were based on the concept of drift flux model initially proposed by Zuber and Findlay.⁴ These correlations suggested expressions for distribution parameter (C_o) and drift velocity (U_gm), respectively. The general structure for drift flux model is

where the ± sign indicates the use of positive and negative drift velocity in void fraction correlations based on the drift flux model for vertical upward and downward orientations, respectively. The use of this sign is justified in description of results section, below.

For vertical upward flow, Rouhani and Axelsson² proposed an expression for the distribution parameter:

The drift velocity was expressed as

Gomez and associates³ proposed a correlation for vertical upward bubbly flow, but this was found to perform better for vertical downward flow and independently of the flow patterns. They fixed the distribution parameter at C_o = 1.15 and expressed the drift velocity in terms of void fraction, written as

In these equations, the subscripts g, l, m and s stand for gas, liquid, mixture, and superficial, respectively, and

• C_o=distribution parameter in drift flux model
• D = pipe diameter (m)
• g = acceleration due to gravity (m/s²)
• G = mixture mass flux (kg/m²s)
• U = velocity (m/s)
• U_gm = drift velocity used in drift flux model (m/s), expressed as U_gm = U_g – U_m
• x = flow quality
• α = void fraction
•  ρ = phase density (kg/m³)
• σ = gas-liquid interface surface tension (N/m)
• θ = pipe orientation (degrees)

Description of Results

Before discussing the results of the performance analysis of the correlations, it is necessary to mention the difference in the void fraction in vertical upward versus downward flow. The experimentally measured void fraction values in vertical upward and downward flow were compared for similar input superficial phase velocities, and it was found that the void fraction in vertical downward flow is consistently higher than that in vertical upward flow. Typically, in a void fraction range of 0 < α ≤ 0.3, the difference between the void fraction for the two orientations is remarkable, but this difference is reduced as the void fraction increases, and the two values approach unity. This deviation in the void fraction values for the two orientations is a result of the buoyancy force acting to reduce the actual gas velocity in downward flow, thus increasing the residence time of the gas phase in a pipe and serving to increase the actual gas velocity in the upward orientation, reducing the residence time of the gas phase in a pipe. This results in the higher void fraction in downward flow compared with upward flow.

To take into account this difference in void fraction resulting from change in flow direction, typically in the range of 0 < α ≤ 0.3, negative drift velocity is used in the drift flux model to calculate void fraction in vertical downward flow. The use of negative drift velocity can be justified by the fact that the slip ratio for this range of void fraction in vertical downward flow is less than unity, translating to the lower gas velocity than liquid velocity and hence the negative drift velocity. Although for void fraction corresponding to the slip ratios higher than unity the drift velocity should turn positive, the incorporation of negative drift velocity does not introduce significant errors in the prediction of the void fraction, since at higher values of void fraction in comparison to the high gas and liquid superficial velocities, the magnitude of drift velocity virtually becomes negligibly small. An illustration of the void fraction results using positive and negative drift velocity in the Gomez and associates³ correlation is shown in Figure 1. As depicted in Figure 1(A), for the low range of void fraction, 0 < α ≤ 0.3, the change in drift velocity significantly affects the prediction of the void fraction in upward and downward orientation. In contrast, the difference between the void fraction values decreases with increasing void fraction, as shown in Figure 1(B).

Fig. 1A

Fig. 1B

Figure 1 Performance of the Gomez and associates³ correlation using positive and negative drift velocities.

The performance analysis of the different void fraction correlations against eight different pipe diameters and six different fluid combinations showed that for vertical upward flow Rouhani and Axelsson,² Nicklin and coworkers,⁵ and Hasan⁶ provided the top-performing correlations, whereas for downward flow Gomez and associates,³ Cai and colleagues,⁷ and Rouhani and Axelsson² were judged to have provided the best void fraction correlations. The expressions and accuracy of all these correlations are reported in Ghajar and Tang⁸ and Bhagwat.⁹ The void fraction correlations for downward flow were verified against 11 pipe diameters and four different fluid combinations. The range of variables against which these correlations can be employed successfully are listed in Table 1. However, the comparison of all these top-performing correlations for the entire data set and void fraction range given by Ghajar and Tang⁸ in each orientation showed that the Rouhani and Axelsson² and Gomez and associates³ correlations yielded a consistent and better performance compared with the other correlations and hence are designated as the best correlations to predict void fraction in vertical upward and downward flow, respectively. More details about the performance analysis and the accuracy of different correlations for downward flow can be found in Bhagwat.⁹ The quantitative performance of the best correlation for each orientation is tabulated in Table 2.

Re_sl and Re_sg are the superficial liquid and gas Reynolds numbers, respectively.

A graphical representation of the performance of the Rouhani and Axelsson² and Gomez and associates³ correlations for different fluid combinations is depicted in Figures 2 and 3 for vertical upward and vertical downward orientations, respectively.

Figure 2 Performance of the Rouhani and Axelsson² correlation for the entire data set with different fluid combinations for vertical upward two-phase flow

Figure 3 Performance of the Gomez and associates³ correlation for the entire data set with different fluid combinations for vertical downward two-phase flow

Wider Applicability of Results

The void fraction correlations listed in Table 2 can be used to predict void fraction in vertical upward and downward flow, including both boiling and nonboiling industrial applications. For enhanced heat transfer, the energy industry often employs vertical heat exchangers with downward flow. The void fraction predicted by the recommended correlations here can be used in the general two-phase heat transfer correlation proposed by Ghajar and Tang (http://engineeringcases.knovelblogs.com/2010/03/24/estimations-of-heat-transfer-in-nonboiling-two-phase-flow-with-a-general-correlation). In the case of the deep oil wells and flow lines usually encountered in the transportation of oil and natural gas and having significant elevation head, hydrostatic pressure drop is dominant in comparison to the frictional pressure drop. The correlations identified above can be embedded into the simulation tools and models used in the petroleum industry to perform hydrostatic pressure drop calculations and can play a vital role in the design of suitable pumping system and flow lines to prevent a well from dying.

References

1. Woldesemayat, M. A., and Ghajar, A. J. Comparison of Void Fraction Correlations for Different Flow Patterns in Horizontal and Upward Inclined Pipes. International Journal of Multiphase Flow, vol. 33, no. 4, pp. 347–370, 2007.
2. Rouhani, S. Z., and Axelsson, E. Calculation of Void Volume Fraction in the Subcooled and Quality Boiling Regions. International Journal of Heat and Mass Transfer, vol. 13, no. 2, pp. 383–393, 1970.
3. Gomez, L. E, Shoham, O., Schmidt, Z., Chokshi, R. N., and Northug, T. Unified Mechanistic Model for Steady State Two Phase Flow: Horizontal to Vertical Upward Flow. Society of Petroleum Engineers Journal, vol. 5, pp. 339–350, 2000.
4. Zuber, N., and Findlay, J. A. General Electric Report: GEAP–4592, 1964.
5. Nicklin, D. J., Wilkes, J. O., and Davidson, J. F. Two Phase Flow in Vertical Tubes. Institute of Chemical Engineers, vol. 40, pp. 61–68, 1962.
6. Hasan, A. R. Void Fraction in Bubbly and Slug Flow in Downward Vertical and Inclined Systems. Society of Petroleum Engineers: Production and Facilities, vol. 10, no. 3, pp. 172–176, 1995.
7. Cai, J., Chen, T., and Ye, Q. Void Fraction in Bubbly and Slug Flow in Downward Air-Oil Two Phase Flow in Vertical Tubes. International Symposium on Multiphase Flow, Beijing, China, 1997.
8. Ghajar, A. J., and Tang, C. C. Void Fraction and Flow Patterns of Two Phase Flow in Upward, Downward and Horizontal Pipes. Advances in Multiphase Flow and Heat Transfer, vol. 4, Chap.  7, pp. 231–267, 2010.
9. Bhagwat, S. M. Study of Flow Patterns and Void Fraction in Vertical Downward Two Phase Flow. M.S thesis, Oklahoma State University, Stillwater, 2011.

Description of Case

Combined heating and power  (CHP) systems have many potential benefits when used to provide electricity and heat for a commercial building, including increased power reliability, reduced costs, reduced emissions, reduced primary energy consumption, and improved power quality¹.

A CHP system consists of a prime mover that generates electricity for use in a building and produces heat as a by-product. The heat energy is captured and provided to the building as space heating or hot water. The sizing of the CHP system, its component efficiencies, and whether it operates at a partial load are all factors that affect system performance^2,3. However, a CHP system often is sized to provide a constant base load of electrical output where additional electricity needed can be purchased from the grid. This alleviates the reduced efficiencies associated with partial load operation and does not require knowledge of the partial load performance of the power generation unit. The International Energy Administration has identified CHP as part of a strategy to reduce greenhouse gas emissions⁴. U.S. greenhouse gas emissions are primarily energy-related carbon dioxide emissions, and electric power production is the largest contributor to U.S. emissions. In 2009, 19% of the total energy consumed in the United States was used by the commercial sector⁵. For this reason, several model buildings representing a wide variety of building types in the commercial sector are investigated in this article to consider the reduction of emissions that result from using a CHP system in place of a conventional reference case in which all electricity is purchased from the grid. Similarly, the operational cost and primary energy consumption using a CHP system for each building are compared with those of the reference case.

The buildings analyzed were in Chicago, IL. Nine different types of commercial buildings were analyzed, allowing the identification of the best candidates for reduced emissions and energy consumption with CHP. To incorporate NOx and CH4 emissions in addition to CO2 emissions, the carbon equivalent is used to assess the overall global warming potential of the emissions associated with a particular case. The operational cost analysis determines whether monetary savings are indicated with a particular building type, and when the CHP system would cost more than the reference case, the monetary value of carbon credits necessary to make up for the additional cost is calculated.

Emissions from the Use of the CHP System

Figure 1 shows the total emissions obtained from either the reference system or the CHP system. The emissions caused by providing electricity and heat to the building using the reference system are shown on the left. These emissions result from the production and distribution of power plant electricity and the use of a boiler to provide heat. The emissions caused by providing the same amount of electricity and heat with a base-loaded CHP system are shown on the right. Since the CHP system analyzed in this case is base-loaded, electricity has to be imported from the grid and fuel has to be imported to satisfy the thermal load. Therefore, the emissions resulting from the CHP system operation include the emissions from the power generation unit (PGU) as well as the emissions from supplemental power plant electricity and boiler heat.

The emissions from the reference case (a building using conventional technologies) and the emissions obtained when the CHP system is operated can be determined as follows:

Equations (1) and (2) are general and can be used to determine the amount of CO2, NOx, and CH4 emissions by using the emission factors for CO2, NOx, and CH4, respectively. These emission conversion factors depend on the location where the facility is installed and on the fuel mix used to generate electricity in that location.

The carbon equivalent, a parameter used to compare the emissions from various greenhouse gases on the basis of their global warming potential, can be determined as follows:

*This investigation accounts only for CO2, NOx, and CH4 in the carbon equivalent calculations.

 Figure 1

Results

To show the potential emissions reduction from the use of CHP systems, benchmark buildings developed by the U.S. Department of Energy were used to simulate the CHP system’s performance.⁶ The benchmark buildings were simulated by using the weather data of Chicago, IL. Table 1 presents building and utility cost information for the evaluated buildings. The electric and gas utility rates considered in this investigation are average annual rates.⁷  The CO2 emission conversion factors for electricity and natural gas for the city of Chicago are 341.7 g/MJ and 52.1 g/MJ, respectively.⁷  The total carbon equivalent conversion factors for CO2, NOx, and CH4 for the city of Chicago are 0.2727, 80.7272, and 6.2727, respectively7. Table 2 shows the size of the PGU used to simulate each building. The PGU size that was selected was based on the minimum electricity needed by the building in an hour. The PGU was sized using this criterion to guarantee that no excess electricity is produced, since net metering or selling electricity back to the grid is not available at all locations.

Table 1. Building Information and Utility Cost for the Evaluated Buildings in Chicago, IL⁷

Table 2.  PGU Size Used to Simulate the Evaluated Buildings

Figure 2 shows a comparison of the CO2 emissions of the reference building and the building using a CHP system. In general, it can be seen that the use of a CHP system reduces the CO2 emissions for all the evaluated buildings. The building that shows the highest reduction of CO2 emissions is the small hotel (23%); the building that shows the lowest reduction of CO2 emissions is the large hotel (17.3%). The same analysis was done to determine the emissions of NOx and CH4 , and it was found that the use of a CHP system reduces the NOx and CH4 emissions for all the evaluated buildings as well.

Figure 3 shows the carbon equivalent for the reference buildings and the buildings using a CHP system as well as the reduction obtained with the use of the CHP system. For all the buildings, the use of a CHP system reduces the carbon equivalent. The maximum reduction in kilograms was obtained for the hospital (1,380,374 kg), and the minimum reduction was obtained for the full-service restaurant (46,780 kg). The maximum and minimum reductions in percentage were achieved for the small hotel (27%) and the large hotel (19.8%), respectively.

Now that it has been shown that the emissions can be reduced by using a CHP system, parameters such as operational cost can be used to determine the overall performance of the CHP system. Figure 4 shows the variation in the carbon equivalent and operational cost for all the evaluated buildings. In this figure it can be seen that four of the seven buildings show a reduction in emissions as well as operational cost when a CHP system is used. These buildings are the large hotel, supermarket, small hotel, and hospital. In contrast, the remaining buildings show an increase in the operational cost while reducing emissions. For these particular cases, it is important to mention that if carbon credits could be used, the higher operational cost could be offset and the use of CHP systems would become economically attractive. For example, the use of a CHP system at a full-service restaurant reduces the CO2 emissions by 94,467 kg/year while increasing the operational cost by $4,374/year. Therefore, a minimum carbon credit of approximately $21.6/kg of CO2 is required to offset the different in the operational cost.

Figure 2. Comparison of the CO2, emissions of the reference building with those of the CHP building

Figure 3. Comparison of the carbon equivalent of the reference building with the carbon equivalent of the CHP building and the reduction obtained with the CHP application

Figure 4. Variation of the carbon equivalent and operational cost of the CHP building with respect to the reference case

Conclusions

The use of a CHP system always reduced the emissions of CO2 as well as the carbon equivalent for all buildings analyzed for the city of Chicago. Reductions of CO2 and carbon equivalent up to 23% and 27%, respectively, are achieved for the small hotel building. However, it is important to mention that for some buildings the use of CHP systems increases the operational cost while reducing emissions. In these cases, if carbon credits are available, the buildings that showed increased operational cost with the use of a CHP system may be able to reduce costs on the basis of their reduced emissions when the monetary value of the carbon credits is sufficient. Also, if additional benefits such as power quality and power reliability could be considered in an economic analysis, the use of CHP system could be more feasible and attractive from the economic point of view.

References

1. Mago, P. J., Chamra, L. M., and Hueffed, A. A. Review on Energy, Economical, and Environmental Benefits of the Use of CHP Systems for Small Commercial Buildings for the North American Climate. International Journal of Energy Research, vol. 33, pp. 1252–1265, 2009.
2. Hueffed, A. K., and Mago, P. J. Influence of Prime Mover Size and Operational Strategy on the Performance of Combined Cooling, Heating, and Power Systems under Different Cost Structures. Journal of Power and Energy, vol. 224, no. 5, pp. 591–605, 2010.
3. Dorer, V., and Weber, A. Energy and Carbon Emission Footprint of Micro-CHP Systems in Residential Buildings of Different Energy Demand Levels. Journal of Building Performance Simulation, vol. 2, no. 1, pp. 31–46, 2009.
4. International Energy Agency. Combined Heat and Power: Evaluating the Benefits of Greater Global Investment, 2010. http://www.iea.org/Papers/2008/chp_report.pdf.
5. U.S. Energy Information Administration. Annual Energy Review. 2010. http://www.eia.doe.gov/emeu/aer/contents.html.
6. U.S. Department of Energy. Existing Commercial Reference Buildings Constructed in or after 1980. 2010. http://www1.eere.energy.gov/buildings/commercial_initiative/after_1980.html.
7. U.S. Department of Energy. EnergyPlus Energy Simulation Software. 2010.

Background and Challenges

Hydrogen produced from water splitting and clean energy sources is predicted by many to be a clean fuel that will serve as a substitute for conventional fuels because its oxidation does not emit greenhouse gases. Numerous thermochemical water splitting cycles have been proposed for clean hydrogen production. The copper-chlorine (Cu-Cl) cycle has a relatively low temperature requirement compared with other cycles and therefore is viewed as a promising method.

The cycle consists of three chemical reactions, as shown in Table 1:

Table 1.

The Cu-Cl cycle has been demonstrated in proof-of-principle tests, and a key issue is whether the cycle can be scaled up to a larger engineering and commercial scale. The challenges for scale-up must be overcome, and solutions must be obtained. This article will examine the scale-up feasibility, particularly by enlarging by 1,000 times the cycle from proof-of-principle tests (equivalently 3 g H₂ per day) to a large engineering scale (equivalently 3,000 g H₂ per day). Since the Cu-Cl cycle consists of one electrolytic and two thermal reactions, this article will focus on the scale-up of thermal reactions at the Clean Energy Research Laboratory (CERL) at the University of Ontario Institute of Technology (UOIT).

Scale-Up Methodology

For the two endothermic reactions—hydrolysis and oxygen production—the scale-up can be achieved by adopting the following strategy:

1. Construct low-temperature units to study the necessary safety improvements and flow characteristics. In these units, no chemical reactions occur but the processing rate is equivalent to 10,000 times the hydrogen production scale of proof-of-principle tests. The fixed bed equipment and experimental loop are illustrated in Figure 1.

Figure 1:  Fixed bed low-temperature reactor (10,000 times the processing scale of proof-of-principle tests)

2. Design and build reactors to study the actual chemical reactions at high temperatures. Both the hydrolysis and the hydrogen production reactors were designed and operated at 1,000 times larger scales than proof-of-principle tests. The equipment and the experimental loop for the oxygen production step are shown in Figure 2.

Figure 2:  High-temperature oxygen production reactor (1,000 times the processing scale of the proof-of-principle tests)

3. The scale-up was conducted with the goal of system integration of unit operations in terms of chemical stream composition and quantification, reaction thermodynamics and kinetics, and improvements in the energy efficiency of the whole cycle.

Results and Discussion

Figure 3 shows the friction factor results obtained from the experimental fixed bed unit at various flow conditions at 10,000 times the processing scale of the proof-of-principle tests. The results provide valuable scale-up data to improve packed bed designs and safety by limiting the adverse impact of undesirable pressure drops during operation at a larger engineering scale.

Figure 3: Measured and predicted friction factor for the packed bed at various operating conditions (1,000 times the scale of the proof-of-principle tests)

Table 2 shows the experimental results of the reverse reaction of oxygen production at various operating conditions. At an engineering hydrogen production scale, the reverse and undesirable side reactions of the oxygen production step can be minimized or avoided.

Table 2.

Figure 4 illustrates the estimated heat transfer to a hydrolysis reactor for 100 metric tons of hydrogen production (a large commercial-scale plant). When the ratio of water to cupric chloride exceeds 1.5—i.e., 3 times the stoichiometric requirement—the evaporation heat requirement of water will exceed the reaction enthalpy.  As a consequence, the hydrolysis reactor will function like a steam generator. Further research is needed to determine whether the excess steam requirement of hydrolysis is caused by thermodynamic or engineering limits. However, it is desirable for the ratio of water to cupric chloride to be below the transitional point of 1.5 so that the heat transfer rate to the hydrolysis reactor is directed primarily to the reaction enthalpy. Another option is to utilize a separate steam generator. This can control the ratio of water to cupric chloride more readily to be closer to the stoichiometric value and provide more flexibility for the selection of reactor types because other intake forms of CuCl₂, such as solid powder and slurry, can be introduced into the hydrolysis reactor. In addition, external water such as make-up water of the Cu-Cl cycle rather than water from the electrolytic cell also can be utilized. It is crucial to reduce the water requirement of the hydrolysis input stream to improve the overall thermal efficiency of the Cu-Cl cycle.

Conclusions

This article shows that the Cu-Cl cycle can be scaled up to a large engineering scale, and it has been scaled up 1,000 times from proof-of-principle tests. Experimental results were obtained from both the low-temperature and high-temperature units for the flow characteristics, safety improvements, reaction thermodynamics, kinetics, and chemical stream quantification. Future research is recommended to achieve system integration and improvement of the system’s thermal energy efficiency. Also, the economics of the Cu-Cl cycle at large industrial scales should be examined further.

Figure 4: Comparative roles of reaction enthalpy and water evaporation in the hydrolysis reactor.

Description of Case

With PDM as the foundation, a small engineering services firm has adapted lean principles to product design in an attempt to reduce waste, drive efficiencies, and deliver better value to customers.

When your business is providing engineering services, the mantra has to be all about delivering optimal value to customers. After all, you’re not only competing with other engineering services firms, you’re also up against low-cost overseas outsourcing providers and what might be the most formidable contender: a customer’s internal engineering team.

With customer value as a guiding principle, Inertia Engineering + Design (IE+D) set out from its inception in 2004 to be the small engineering services shop that leveraged big ideas to achieve its goals. Proven concepts such as concurrent engineering and lean principles serve as the foundation for IE+D’s product development philosophy. The 10-person firm also is aggressively leveraging sophisticated design platforms and collaboration tools, including product data management (PDM), in an effort to reduce waste and garner efficiencies as part of a broader strategy to differentiate itself from the rest of the pack.

The idea behind leveraging lean principles to create a different kind of design and engineering services shop was rooted in several early career experiences. Initially, as a process engineer at an injection molding plant, I was exposed to the principles of lean as they relate to the groundbreaking Toyota Production System. One of the company’s clients was Toyota, and the car giant’s operations team provided rigorous hands-on training in principles and methodologies such as time-and-motion studies, pull production systems, and visual manufacturing control systems as part of an effort to keep its vendor partners up to speed. With that training under my belt and with time spent in other engineering roles, it became clear that although there were many trained and knowledgeable experts willing to serve up engineering services, few were focused on the customer experience and even fewer were committed to delivering value.

PDM at the Core

With that in mind, IE+D’s charter was to apply lean principles to product development to create efficiencies and cut waste from the overall design process. As with any kind of transformative initiative, there was a need for some core building blocks. In this case, one of the key foundational elements is a PDM system, which serves as a central repository for all design data and project documentation. This ensures that everyone has access to the right version of the design no matter where he or she is located, greatly reducing the possibility for error and rework while facilitating greater reuse of parts and designs when and where it makes sense.

Along with version-control capabilities, the PDM system, in this case, the SolidWorks Enterprise PDM, is instrumental in facilitating standardization and automation, two other guiding principles of IE+D’s lean approach. Using the built-in workflow capabilities of SolidWorks Enterprise PDM, IE+D is able to map out key business processes and workflows to ensure that the design process follows the same path every time, no matter where team members are located and regardless of the client. Using principles gleaned from the early Toyota Production System training, the team at IE+D generated standardized work procedures for every part of its design process, using the Advanced Product Quality Planning (APQP) standard widely embraced by the automotive industry as a base, with minor modifications to address some unique requirements.

The PDM system also facilitates automation, another tactic for wringing waste out of the product development process. Drawing on SolidWorks Enterprise PDM, IE+D has created templates and workflows around a number of common tasks so that information is populated automatically into documents and reports, eliminating the need for laborious data entry and at the same time reducing the possibility of errors.

Concurrent engineering, another core pillar of the IE+D design philosophy, also is enabled by the PDM platform. Because all the data and detailed drawings exist in a central repository with the proper version controls, IE+D engineers and design partners are able to move forward on other related tasks—for instance, simulating kinematics or creating final drawings—well before parts and assemblies are fully designed. Being able to perform key development milestones simultaneously compared with using a traditional serial approach garners huge efficiencies, allowing IE+D to serve its clients more effectively by helping them get products to market in a more timely fashion.

People, Process, Tools

Although PDM definitely lies at the heart of IE+D’s strategy, other tools and methods are used to foster collaboration between team members and clients and drive waste out of the design process. To optimize communications between far-flung teams and clients, IE+D employs Zoho Projects and Google Docs online project management applications. With these tools, team members can share and edit documents, get real-time dashboard status updates on project tasks, and collaborate through online chats or Web meetings.  Additionally, these online project management and collaboration tools allow our customers to participate more actively in our projects with full transparency about the work being done.

In keeping with IE+D’s core focus on customer value, the firm has taken steps to ensure that it can wring more out of its resources. IE+D has forged relationships with low-cost design and manufacturing partners in China and outsources manufacturing work there to reduce costs when appropriate while employing a small team of engineers to oversee projects. Another new tactic gleaned from lean manufacturing practices is a two-shift operation designed to improve productivity. One engineering team works the traditional shift from 8 a.m. to 4 p.m., with the other on board from 3 p.m. to 11 p.m.

For IE+D, the biggest challenge has been finding team members who are passionate about the directive around customer value and flexible enough to make the changes needed to allow for the continuous improvements that go along with lean practices. Even with the core PDM and collaboration tools, communication among extended teams can be tricky, especially when one takes into account the remote partners and the second shift.

To facilitate communication between personnel, IE&D has instituted a 15-minute “huddle meeting,” held daily at 3:33 p.m. ET, where both day and afternoon shift personnel exchange reports on work in progress and any problems. Second shift personnel also take 30 minutes to update their counterparts and solicit feedback. There are different routines for the China-based team. Engineers there conduct a daily 15-minute huddle with the director of operations, who is fluent in Chinese, via Skype, and tasks are assigned and tracked by using Zoho Projects.

For the offshore engineering team, IE&D’s strategy is to compartmentalize projects because real-time collaboration and communication becomes more difficult when workers are not in the same office. Typically, the offshore team will focus on manufacturing and assembly drawings as well as finite element analysis studies because these are labor-intensive projects that require a lower level of collaboration.

With its course set on lean engineering services, IE+D is delivering on its promise of supplying value to customers.  Thanks to PDM and the concurrent engineering practices, IE+D was able to help a customer design an industrial vacuum trailer (Figure 1) in less than eight weeks, a 50% decrease compared with traditional methods. The same scenario occurred with a project for a child car seat. By applying lean practices and leveraging the outsourcing and two-shift operations, the IE+D team slashed development time on the car seat by 30%.

Figure 1.

The QuickSider battery-powered urban delivery vehicle (Figure 2) is another telling example of how a focus on lean practices and collaboration pays off. As project lead, IE+D coordinated the design effort for its client, Unicell Ltd., a truck body manufacturer, along with close to a dozen other engineering contractors and specialists. Thanks to processes that support streamlined collaboration, IE+D was able to help reduce the development cycle for QuickSider on the order of 30 to 40%.

Figure 2.

These are all great examples of how lean practices deliver, but IE+D sees greater opportunity in honing its approach to delivering more value to customers while still making a profit. The goal is not to make lean practices an end point but to develop an ongoing strategy for continuous improvement.

Description of Case

As an airline passenger is waiting in the terminal for his flight, he decides to plug in his tablet to charge the battery. As soon as he connects the AC/DC adapter to the tablet he hears a snap and he sees an error message on the tablet’s screen communicating that there is a power failure. Upon inspection he discovers that the connector has jarred loose and thus his battery is not charging his tablet.

These problems are not uncommon for some of the mobile products that are being sold in the market place. Seeing the individual’s frustrations from meager designs can have a dampening effect in terms of revenue and a company’s reputation. To overcome some of the quality issues in connectivity the design engineer needs to consider many characteristics of the connector. Characteristics such as electrical, mechanical, reliability, and environmental will require a detailed examination and definition. This article will explore what the considerations are on how to specify or design a connector system that is appropriate for certain applications. Then the focus will shift to devise a general guideline in which the designers can utilize to insure that a connector system can be a reliable one.

Connector Structure

Connectors have been in the electronic industry for quite a long time. With the advent of more miniaturization of electronics their complexities have grown by leaps and bounds. Connectors today have to support many high-speed electronic circuits in a system, whereby at the same time, they have to utilize as little real-estate possible.

In electronics, connectors are electro-mechanical devices that transfer and join electrical signals and power to other parts of the electronic system. Connectors can range in size from just a few millimeters in low power applications to feet in high power generation. One can surmise that the number of circuits and the power usage through the connector will determine a connector’s physical size.

Some connector systems can be static where they are only plugged in once during the life of the product to connector systems, others are extremely dynamic where they are inserted and extracted many times during its life. An example of a static connector system is one used for video inside the display of a tablet. A dynamic connector system would be one for plugging in power for charging in a smartphone. Connectors are constructed with certain materials that can withstand different environmental and manufacturing conditions to enhance the connector’s performance and reliability.

In general, a connector system is usually constructed from two pieces: a male and female, or a plug and a socket. There are different types of connector systems, the primary ones are electronic board-to-board , wire-to-board, and wire-to-wire connector systems where some of these are in Surface Mount Technology (SMT). There are others, such samples are below:

Fig 1: SMT Board-to-Board Connector

Fig 2: Wire-to-Board Connector

Fig 3.: Wire-to-Wire Connector

Depending on the connector, a plug or a socket, are structured utilizing several components; the housing and the electrical contacts are the two fundamental components. The other components that make up a connector are the support accessories such as EMI shielding and grounding, a polarization feature, a bracket, or a strain relief feature. I’ll discuss further on a strain relief feature below.

Fig 4: Connector Housing with Contacts

Connector Components

The connector housing is usually made of some specific polymer and/or metallic housing. The plastic housings are made of high temperature thermo-plastic to withstand manufacturing and other environmental conditions such as in a SMT (Surface Mount Technology) reflow process. Some examples of high temperature thermoplastics are Liquid Crystal Polymer (LCP), Polyphenylene Sulfide (PPS), or a High-Temperature Nylon, and there are others. SMT connectors should be able to withstand temperatures up to 260c for at least a minute without degradation.

As for the contacts for both the socket and plug, they are usually made of certain base metals such as phosphor bronze, copper alloys, or perhaps, tin with intermediate plating deposited such as nickel or tin. Outer surface plating or finishes for contacts consists of inert metals for a certain thickness such as of gold or palladium for low current applications. For high current or power applications usually tin or silver are utilized. These contacts are usually formed or stamped into different geometries to provide sufficient contact normal force for a long-term reliable connection along with insertion and extraction cycle life depending on the application. Additionally, the contact geometries will determine the electrical bandwidth of the connector and how fast electrical signals can operate without seeing any type of degradation.

One of the serious errors that engineers often make is when they inadvertently mix metal platings on contacts. For example, a socket may contain gold as its outer surface finish while the plug may consist of tin. When the plug and socket are mated, the different platings will react slowly due to the galvanic properties of the metals. After a period of time, along with humidity and the environment, galvanic corrosion occurs and leads to open circuit connection in the contacts, thus a connection failure. A critical connector commandment is never mix contact interface platings in a mated connector system or you will compromise the reliability of the design.

Mounting Connectors
Depending on the type of connectors, they can be mounted on a PCB utilizing Plated Thru-hole (PTH) or SMT or a mix of the two mounting technologies including a press fit type. Other types of connectors can be mounted on brackets with wires attached or even a flexible circuit . Connectors can be mounted vertically, horizontally or even sideways on the edge of the PCB.

Connectors that are mounted on PCBs usually have certain features with them to enhance mechanical reliability and/or solderability of the connector system. Such features could be locating and retention leads and/or strain relief feature.

Strain Relief

This article begins with connector snapping off a tablet, a problem which I experienced, is an example of a strain relief issue. Strain relief is a feature that is built into the connector to relieve mechanical stress on any component on the connector system. Without strain relief a catastrophic failure may occur in the connector system over time. Strain relief can come in many forms and are too long to list in this article, however, I will discuss one example below.

Take the case above, a connector snapping off or where the solder joints were weakened, eventually cracking and failing. The connector that failed was a power connector that essentially was used to charge the battery for the tablet’s mobility. The SMT connector was a two-circuit type with dc power and ground. Both contacts of the connecter were part of the leads that were soldered on the pads of the PCB. The solder joints of the connector were subjected to a high number of mechanical stresses since the number of insertions and extractions that the connector was exposed to were going to be very high.

This failure was compounded by the fact that the grounding contact of the connector was sizable such that every time the power cable was inserted into the connector it caused the ground contact to deflect quite a bit. This deflection of the ground pin caused the solder joint to be subjected to abnormal dynamic forces thus causing the joint to eventually crack then fail (See Fig. 5).

Fig. 5: Power Connector Solder Repair

To overcome some of the abnormal dynamic forces on the solder joint, a strain relief feature or features are necessary. Depending on the number of insertions and extractions perhaps a redundant PTH lead on the ground SMT contact would assist to overcome some of the abnormal dynamic forces on the solder joint. Other features such as a strain relief bracket attached to the connector that attaches to the housing of the tablet may assist in relieving the stress on the solder joints. There are other improvements a design/component engineer can explore.

How to Specify and Design-in a Robust Connector System

With so many variables that can affect a connector design, I put together a list of tasks that a designer/connector engineer should investigate before a connector system can be specified and designed into a particular product. The list of tasks are as follows:

1. Determine product specifications along with any regulatory requirements whereby this new connector would operate in.

2. Determine what configuration or type of connector to utilize such as Board-to-Board, a wire-to-board, or a wire-to-wire connector system or other connector types.

3. Determine the environment that the connector is supposed to operate, such as in a commercial, industrial or aerospace environment.

4. Determine the basic requirements of the connector system such as number of circuits, method of attachment (SMT or PTH or a panel mount), and so forth.

5. Determine if the connector will be subjected to high number of insertions and extractions such as in an external I/O or power connector, such that a strain relief feature may be specified.

6. Determine detailed electrical requirements:

• Determine bandwidth or maximum speed of the signals that the connector system will be subjected to.
•  Power ratings of the individual contacts and the entire connector system. I.E. voltage and current ratings. Utilize any derating criteria for power.
• Determine shielding requirement for EMI/RFI especially for I/O connector systems.

7. Determine detailed mechanical requirements.

• Determine connector insertion and extraction forces.

• Determine the contact normal forces to determine what surface platings to utilize.

• Determine physical size and PCB footprint of the connector (if applicable).

• Determine strain relief features for the connector or cable (if applicable).

8. Determine detailed material requirements for the connector system.

• Determine plastic housing materials. Plastic will have to meet SMT or Wave solder manufacturing process requirements (if applicable). Housing may have to meet flammability ratings if the connector is being used for power, especially AC power. UL and other agency certifications maybe required.

• Determine the contact materia, the base metal and its platings. Outer surface plating or finishes should be tin for high current applications. For low current external applications ideally inert metals are utilized such as gold or palladium unless the connector utilizes a “gas tight” contact interface then a tin-to-tin contact interface can be utilized.

9. Once a connector is specified determine its mating connector. It should meet the criteria you outlined for the original connector.

10. With the worldwide green initiatives, ensure all materials meet Restrictions of Hazardous Materials (RoHS) certifications or provisions for R.E.A.C.H.

11. Last but not least, DO NOT MIX METALS IN THE CONTACT INTERFACE AREA when mating connectors. It will go a long way in insuring long-term connector reliability.

The list above is a general one as the Design/Component Engineer needs to treat the connector system as if it is a new one for their applications. Additional tasks that could be completed if necessary, especially for new connector designs are stress simulations, electrical simulations including utilizing a network analyzer for S-parameter transfer function, thermal profiling at peak rated power, testing and verifying the connector design including cycle life.

Selecting the sustainable response and short-term remediation options are only part of developing a sustainable spill response plan. Sustainability must be part of the planning and prevention process as well.

The first step to establishing a sustainable spill response program is to create a singular plan that complies with the various regulatory requirements for emergency response to spills and releases. The U.S. EPA and OSHA have stated that a single “emergency response plan” can be used to comply with both agencies’ requirements. Even local fire departments have accepted the Federal compliant emergency response plan as being compliant with their local requirements.

By having a single response plan, you reduce resources, paperwork and the amount of time needed to keep track of multiple emergency response plans. Employees have only one set of instructions, which simplifies the training process. Updates are made to a single plan and its associated copies. When a spill occurs, there is only one set of instructions to follow.

Most plans base response effort and disposal options on the size of the spill and what is spilled. Tipping over 1 quart of motor oil in the maintenance shop or treatment trailer requires less equipment and effort to clean up than responding to a crash on the highway or a collapse of a storage tank. Recovering 1 quart of motor oil for recycling or fuel blending may not be practical or cost effective, but recovering the contents of a railcar or storage tank for reprocessing or recycling could be. In fact, a portion of the escaping crude from BP Deep Horizon well was recovered and sent to a refinery for processing.

After combining the various plans into a single master plan, conduct a review of the clean up and disposal procedures for ways to increase recovery, recycling or disposal options other than landfilling. Look at what is used to contain and clean up the spilled material and impacted soils, sediments and waterways. How are the impacted media and used response equipment managed and disposed of? Are the guidelines for waste minimization and pollution prevention followed? Is recovery and reuse of the spilled material included as a disposal option? Can in-situ treatments be used?

The spill response plan should provide a list of options the on-site coordinator can select depending on the size, the location and cause of the spill and what is spilled. Most plans provide disposal selection guidelines based on amount of material generated during the clean up step and which disposal vendors have been vetted or issued a purchase order.

Sustainable selection criteria is usually limited to suggesting that the spilled material be recovered for site use if not contaminated, sending high Btu liquids to a fuel blending facility or thermal treatment of organic impacted soils. Note: sustainable disposal options for inorganic spill cleanup wastes and use of engineered absorbents are not discussed.

Most plans specify that absorbent pads, socks and booms be used to clean up a liquid spill. If there is a large amount of liquid spilled, kitty litter type absorbents are recommended and maintained as part of the spill response kit. The use of engineered absorbents as an alternative to kitty litter is part of the spill response plan.

Today’s engineered absorbents are not your father’s absorbents. Absorbent manufactures have developed low ash or biodegradable absorbents that can be destroyed through thermally treatment or composting. Other companies have developed in-situ products to minimize excavation and off-site disposal of contaminated soils and sediments for both organic and inorganic products.

Even if the composted or incinerated residue is landfilled, the treatment process has reduced the toxicity and quantity of the waste by 80 to 90%. Other absorbent materials are part of a system that can extract the absorbed liquid from the absorbent for reprocessing or recovery, especially for low Btu or inorganic liquids. Still others will neutralize corrosive liquids so they can be sent to a wastewater treatment system.

A web search for “oil absorbent vendors” identified venders such as The New Pig Corporation, Spill911, Absorbentonline.com, Gator International and a rough estimate of 100,000 additional sites that stock biodegradable or low ash absorbent products.

A similar search using “absorbent vendors” generated an equally large listing. Gather information; compare costs, benefits and ease of use. The final selection will be up to you and your company’s purchasing process. Stock these products on-site so they will be used when needed.

After selecting the absorbent materials to be used, the spill response plan must incorporate the recommended handling and disposal procedures from the absorbent supplier. Cleanup procedures may be as simple as picking up the overturned quart container, wiping down the exterior of the container and mopping up the spill. The used absorbents are placed in a plastic bag, the bag sealed tight and placed in a DOT approved trash drum, then the labeled drum is sent to a landfill.

If the bag is leaking, it must be placed in a drum containing an absorbent to solidify the liquids. Once all liquids have been solidified, the sealed and labeled drum can be landfilled. If the contents of the drum cannot be solidified, the drum must be sent to an incinerator or other disposal facility that can handle liquids.

As the spill gets larger such as overfilling the used oil storage tank, the ability to contain the spill within a shop floor or secondary containment unit becomes more difficult. Drain covers; containment booms and skimmers are needed and should be staged near the source of a spill on-site.

The selected equipment should minimize the amount of waste to be landfilled and complement the planned waste management option. On-site mobile systems such as thermal treatment, soil washing, bio-piles or chemical treatment can treat the impacted soil and sediments so they can be returned to the spill site as clean fill. The extracted material could be recovered for reuse or recycling. In-site treatment options that enhance biodegradation of organic compounds can eliminate the need to excavate.

Table-1 lists some of the response options for cleanup of common liquid chemicals using today’s engineered absorbents and in-situ or ex-situ treatments options.

If recovery is selected, the containment system needs to isolate the spilled material without affecting it.  If disposal is selected, the containment system must be compatible with the disposal option such as biodegradable if composting, have a high Btu and low ash content for thermal treatment or solidifies the liquids for landfilling.  Disposal options are listed in Table 2 for generic liquids.

Once a spilled liquid reaches permeable surfaces such as soil, rocks or sediments or enters the sewer systems or local waterways, containment, cleanup and disposal options require even more preplanning and study.  At this point, the spill begins to impact the public and coordination with local and state agencies is required.  A website such as http://www.cameochemicals.noaa.gov/ can help identify pathways that need to be remediated without creating excess waste or injuries.

Excavation of the impacted soil for off-site disposal is the quickest solution, but not the most sustainable or cost effective solution.  Depending on the amount of soil impacted, one sustainable solution could be on-site thermal treatment in a mobile unit from Clark Environmental or other vendors.

High Btu contaminants are destroyed and the treated soil can be used as backfill material instead of obtaining clean fill from an off-site facility.  On-site thermal treatment becomes cost competitive when more than 5,000 cubic yards of soil must be remediated, especially if the nearest landfill is more than 100 miles away.

Other ex-situ processing such as soil washing, stabilization and bioremediation can be conducted in mobile units that can destroy the toxic organic or non-organic contaminants and allow the treated soil or sediments to be used as fill.  These treatment processes become cost competitive with landfilling when processing over 2,000 cubic yards of solids.

To minimize long-term remediation and monitoring efforts, spray the sides and bottom of the excavation area with a chemical or biological reagent before filling in the excavation area.  Chemical and biological reagents can destroy trace concentrations of organic contaminants remaining in the soil or sediments.  Either can be sprayed on the side and bottom of the excavation using hand held spray systems available at home and garden stores.

If excavation is not an option for large-scale soil cleanup, modification of the chemical injection process has been used to destroy or recover the spilt liquid.  URS developed a process using the Badger Injection System to push the spilled liquids towards a groundwater recovery system being used to remediate impacted groundwater.  The recovered material was sent to a recycler for processing.

If mobile treatment systems cannot be considered for political or technical reasons, the sustainable disposal option is thermal treatment or destruction for organic contaminated solids or immobilization for inorganic contaminated solids.  Corrosive liquids must be neutralized before being sent.  Landfilling should be the last option and only used for waste streams that have no other viable treatment option.

If the spill enters a sewer system or waterway, sustainable cleanups become difficult to implement.  The spill needs to be contained to minimize the extent of impact.  If possible, recover the spilled material for reprocessing, reuse or destruction.  Vacuum trucks, skimmers and oil/water separators can be used to collect the floating liquids.  If recovery is not possible, consider using the sustainable processes approved for groundwater remediation.

The sustainable groundwater treatment technologies are not common practice even when cleaning up impacted groundwater.  Regulators and elected officials are accustomed to seeing skimmers and absorbent booms, as well as, pillows and pads containing and collecting spilled liquids.  Yet the justification for using chemical reagents to remediate groundwater impacted by historical releases is the same for remediating water impacts from current spills especially for large areas such as beaches and creeks.

Containment systems are needed to limit the distance the spill travels even if the source of the spill will not be completely plugged for some time.  The goal of a recovery system attached to the containment system is to collect the spilled liquids at a rate equal to or greater than the amount being released.  While recycling the spilled liquid is the preferred option, thermal treatment for organics and filtration for inorganics within the containment systems can reduce the adverse impacts caused by the release.

If petroleum sheens appear on the surface of waterways downstream of containment booms, consider spraying weak solutions of hydrogen peroxide or biodegradation products on the surface of the sheen to destroy the dissolved plume.  These products successfully destroy the groundwater plume from historical releases and can help reduce the amount of spilled material washing up on the coast and riverbanks.

Once the source of the spill had been plugged and a short-term recovery and collection has been completed, the dissolved plume, if is still present, in the soil, groundwater and sediments must be addressed.  Sustainable techniques used for cleaning up historical releases will work to clean up the areas impacted by the spill.  The only difference is that the release is relatively fresh and the composition of the contamination will more closely match the composition of pure product.

The techniques and procedures used to implement sustainable remediation plans should be applied to the cleanup of a spill to reduce the need for a long-term remediation project.  Equipment to facilitate recovery or in-situ destruction of the split liquids must be readily available and familiar to employees.  If the spill cleanup cannot be completed within 30 days, the spill response plan can be converted into a sustainable remedial action plan that will complete the cleanup with minimal process changes, secondary impacts and regulatory delays.

Description of Case

With the advent of more powerful microprocessors and microcontrollers at steadily decreasing prices in recent decades, the use of logic devices has blossomed. The feasibility of digital-based regulators and “smart” power supplies (which can monitor and report their own performance and health) has been amply demonstrated. Such supplies have become the status quo over less intelligent power circuitry among higher -end applications.

Quite often, the controller of choice for such applications is based on Harvard architecture. The Harvard architecture is most often contrasted with Von Neumann architecture. In a nutshell, Harvard architecture devices have code and data in separate memory structures, while the Von Neumann architecture has code and data in common memory structure(s). The Von Neumann architecture is arguably the most familiar style in use at this time, being represented by most commercial computing equipment such as Windows-based PCs. Harvard architecture is more suited to controller functions where there is little need to modify the code on a frequent basis. By separating code and data, the “day-to-day” functions of the controller are protected from accidental corruption by errant software routines. In practice, the rigorous application of the Harvard design philosophy leads to certain difficulties in software development. However, a hybrid Harvard architecture, which has a restricted ability to write data in code memory, has found the widest acceptance in today’s market.

One such supply under development in recent years contained two digital controllers, one a Harvard architecture device that provided self-monitoring and interface communications, and the other a DSP style workhorse controller for the internal supervisory regulation/performance of the supply. The supply was a single printed circuit board with 3 different voltage outputs, intended for usage in close physical proximity to the CPU of a server. As working voltages of CPUs come down to reduce the power dissipation requirements of ever faster switching, the current requirements go up, yet PC board electrical conductor requirements become prohibitive for transmitting large currents from a remote supply. The adopted design approach is to provide higher voltage and lower current to a high efficiency conversion/regulation point near the CPU.

While the design of the supply was in the pre-production phase it was put into production under a pilot process to prove the design and manufacturability of the supply prior to initiating volume production in an offshore facility. The final touches were being applied to the design of the hardware, software, the process and the test equipment. At this time, spurious and somewhat infrequent changes to non-volatile data memory location 0 were occurring, changing from its default value of FFh to 00h. The non-volatile memory storage was not critical to the basic functions of the supply. The customer had slated it for some future, undefined use that might never even be implemented.

At the first occurrence, collective opinion among the design and development team was to dismiss it as a random fluke. When the same error repeated 3 or 4 times in different modules over the course of a week, the error began to receive serious consideration from most, but not all, of the development team. The lead design engineer was opposed to chasing after a perceived minor hiccup with a vestigial feature of the supply.

Intermittent failures are sometimes the most troubling of all failures. In the case of mission critical applications such as medical, nuclear, airborne or space-borne equipment, such failures must be positively proven as understood and fixed prior to proceeding with the manufacture of the item. Since intermittent problems are usually difficult to catch “red-handed”, i.e., observed at the moment of occurrence, it is often tempting to sweep the problem under the rug.

In spite of conflicting opinion on how to proceed, management granted a “grudged consent” to proceed with pinpointing the failure. Over the course of several weeks of repeated testing and data analysis, no progress was made. Operating the supply for many hours under close monitoring would not reproduce the failure.

The various contributing factors to the error were taken into consideration. During teleconferences, the software development group located on the East Coast was asked to review their code, particularly any recent revisions. The hardware design and manufacturing groups, located on the West Coast, were asked to review their functions for contributing factors. The controller vendor was also asked to suggest insights into the situation.

The software development group observed that the routine by which the controller was able to update its own code, as well as, the routine that wrote to non-volatile memory storage and the DSP code updates had been implemented shortly before the problem began to appear.

The hardware design group had its history established in analog design principles. These kinds of problems previously had been attributed to design tolerances, internal part manufacturing flaws and similar analog considerations.

Manufacturing processes were reviewed, but no contributors could be found among their domain that would cause the error to occur.

During a teleconference with the controller vendor, one of the applications engineers mentioned that a similar problem had occurred in a similar design for other customers. The cause of the error in that instance had been attributed to random execution of code during the power down of the circuit. As the voltage supplied to the controller rolled off during power down, the controller was continuing to execute many, multiple processor cycles under questionable circuit conditions. This eventually proved to be the correct direction to pursue but was loudly protested by the lead hardware designer as a ridiculous course of action and a waste of time!

Under protest and close scrutiny, software development was requested to supply a temporary code revision to the routine that performed writing to the non-volatile memory. Rather than writing a default value of 0h, a request was made to change the default value written to A5h. The revised code required several days to be received from the group located on the other side of the country. The temporary revised code could only be placed in the manufacturing process with careful supervision of the modified units, lest some of the test subjects were to escape into the customers’ hands with the unproven, unqualified code inside.

These items were done, and lo and behold, after a few days of manufacturing pre-production, 3 of the test units displayed a value of A5h in non-volatile memory location 0! The final explanation is that the CPU would occasionally erroneously vector to the code at interrupt location 0 during power down which happened to be the routine which wrote values into the non-volatile memory.

Epilogue

The work around for the immediate difficulty was to implement a software revision, which caused the first non-volatile memory location to be written at the very top of memory, which was unlikely to ever be used. The final solution was never known, as the company went out of business not long after this incident, due to an unrelated manufacturing error that resulted in a multi-million dollar product recall.

All microprocessors and microcontrollers familiar to this author have a variety of input pins that can enable or disable the CPU. The pins are usually labeled as “CPU DISABLE”, “POWER GOOD” or similar. In breadboard kits supplied by vendors or educational circuits designed for introductory computer engineering courses, these pins are usually tied to whichever voltage rail will make them irrelevant to the CPU operation.

Many integrated circuit vendors market a small, inexpensive chip commonly referred to as a supervisory circuit, or brownout protector. An IC monitors operational parameters of the controller and its circuitry. Although it is a fairly small consideration, the failure to pay attention to its importance can have potentially disastrous consequences. The failure that was observed and very nearly ignored in this case study was only “the tip of the iceberg”. It is unknown what other functions might have been potentially randomly executed by the controller during power down.

A significant factor in this story concerns the subject of expertise. The lead hardware design engineer was extremely competent at the analog factors in designing power supplies, but lacked equivalent prowess in digital design. Pride and ego got in the way of listening to others and learning from them when the subject matter was outside of his area of expertise.

Description of Case

In 2009, there were 27 million displaced people due to military conflicts alone. Cyclone Nargis in Myanmar (2009), the floods in Pakistan (2010), and the 2011 earthquake and tsunami in Japan highlight the number of devastating disasters occurring around the world in the recent past. Apart from deaths, the number of displaced communities due to these anthropogenic or natural disasters is a huge issue to tackle. Provision for safe drinking water is among the highest priorities during and after any disaster situation.

On-site water treatment is considered an important strategy to cope up with prolonged disaster recovery efforts. Water taken from surface water sources such as rivers and lakes needs some treatment before it can be safe for drinking. Fallout from disasters often limits access to various resources such as chemicals, electricity, and treated water from treatment plants. When people are displaced from their homes and are gathered at one location, such as a rehabilitation camp, unavailability of resources becomes quickly apparent. Such conditions highlight the necessity of having reliable drinking water treatment systems running on alternative energy sources such as human-generated power. With the prime objective of providing safe drinking water using human-generated power (using hand pumps rather than electric pumps) in these situations, a microfilter- (MF) based, hand-pump operated water treatment unit was developed. The low-operating, trans-membrane pressure of microfiltration provided the applicability of hand-pumps to develop a sufficient driving force for filtration.

Emergency Potable Water Supply System

A dead-end, outside-in hollow fiber MF [200 kDa MWCO (molecular weight cut-off) with a 3.4 m² surface area] made of PVDF and a rotating hand pump are the most important units of the system. Pretreatment consists of a bar screen and a cloth-bag filter with pore sizes of 45 µm. The schematic diagram of the system and the actual system are given in Figures 1 and 2, respectively. In its operation, surface water is poured through a coarse screen, then a cloth screen, and finally stored in the feed tank. From there, the feed water is pumped by the hand pump through the membrane, and the filtrate is collected and stored in the filtrate tank. The system is capable of providing 400-650 L/day of treated water, sufficient to meet the daily drinking water requirement of 50-100 people under normal operating conditions.

Figure 1: Schematic diagram of the emergency potable water supply system.

Actual emergency potable water supply system:
Figure 2A: Front

Figure 2B: Rear

System Performances

The system was operated and tested with two types of raw water typical to those found in tropical regions. These two types include water from a surface water collection pond and water from a river. The basic properties of the water sources are indicated in Table 1.

Table 1

The emergency water supply system has shown excellent performances under both types of water sources. However, removal efficiencies and maintenance requirements vary with the quality of the raw water. Table 2 summarizes the system performances and operation details of the unit.

Table 2

Treated Water Quality

Treated water quality satisfies not only emergency water supply standards but also drinking water quality standards. Some of these water quality parameters are given in Table 3. Irrespective to raw water turbidity, the unit is capable of providing low turbid water, well below the upper limits of drinking water quality standards. A major advantage of this low turbid product is low requirement of disinfectants for the post disinfection processes.

The membrane separation process (PVDF membrane having 200 kDa MWCO) provides complete rejection of bacterial and protozoa pathogens in the water. The removal of protozoa and protozoa cysts is very important because of the resistance of those under a typical chemical disinfection process with chlorine compounds.

Table 3: Water Quality Parameters of the Treated Water 

However, there are possibilities of growing pathogenic bacteria in the treated water line or in storage containers. Also, pathogenic viruses that are smaller than membrane pores may escape through the membrane. To counter this, the system includes a post disinfection using a chlorine compound, Ca(OCl)₂. The treated water quality with low turbidity provides excellent conditions for further disinfection with this chlorine compound, or other forms of Cl₂. Apart from the chemical disinfection, techniques such as solar water disinfection (SODIS), can be used.

The emergency water supply systems can be operated and maintained with simple instructions—consisting of an operation and troubleshooting manual with pictorial illustrations. Further improvements to suit the system for different field conditions are under progress at the moment, with the collaboration of industrial partners.

Some additional details of the emergency water supply system are given in the next sections.

 Operating Procedure of the System

A. Pour the Surface Water Into the Feed:

(Figure 3)

(Figure 4)

B.  Rotate hand pump (clockwise). During the rotation of the hand pump, press the gas release valve until water comes out from it:

(Figure 5)

(Figure 6)

c. Collect treated water.

(Figure 7)

Figure 8

(Above) This Picture shows the comparison between feed water and treated water.

D. Disinfect using chlorine.

Daily Draining Out and Maintenance

To remove the impurities accumulated in the membrane after one working day, draining the concentrated water can be performed by opening a valve at the bottom of the housing.

• Open the draining valve for about five minutes to drain out the concentrated water.
• Rotate the rotating hand of the hand pump to let the feed water pass through the membrane for about one minute.
• Close the draining valve.
• Continue rotating the hand pump to fill the membrane housing completely.
•  Press the air release valve on the housing until water exits from this valve.

Figure 9

Cleaning frequency of the fine screen depends on the feed water quality. Normally, it is recommended to clean whenever membrane chemical cleaning is done.

Step 1: Take out the coarse screen, and then the fine screen from the system.

Figure 10

Figure 11

Figure 12

Step2: Remove the particles inside the fine screen.

Figure 13

Figure 14

Figure 15

Step 3: Shake and rub the screen in a small feed water tank, then rinse the screen with the treated water.

####

Bibliography and References:

Abeynayaka A., Nguyen T. T., Visvanathan C. & Ariyamethee P. (2010). Chemical-free and carbon neutral membrane based emergency water supply system. 8th International Symposium on Southeast Asian Water Environment, Phuket, Thailand. October 24 to 26, 2010. pp. 59-66.

Nguyen T. T.  (2010). Development of a water treatment system for emergency situations. Master thesis, Environmental Engineering and Management, Asian Institute of technology. Thailand.

Whether it’s picking the colors and trim on a pair of high-end sneakers or choosing the right set of options for a vehicle, consumers love the idea of custom tailoring the products they order on the Web. Although most online sites offer a configure-to-order buying experience, a partnership between a computer-aided design (CAD) solution provider and a high-end surfboard company has pushed the concept a step further, delivering what may be the first consumer-oriented online engineer-to-order system.

ShapeLogic, a Siemens PLM Software and Technology partner and channel partner, and Firewire Surfboards, a five-year-old high-end surfboard manufacturer, have teamed up to create Custom Board Design (CBD), an online application that lets surfers directly modify stock Firewire surfboard designs to their preferred specifications. At the heart of Firewire’s newly released CBD system is ShapeLogic’s patent-pending Design-To-Order Live! for NX. The customization solution, which is built on Siemens PLM Software’s NX CAD platform, consists of an Internet-enabled user interface, a set of intelligent parametric models of Firewire stock surfboards, and advanced three-dimensional CAD tools that work together to deliver a customer-driven engineer-to-order system.

High-Tech Materials and More

Firewire is highly regarded in surfing circles for its use of unique high-tech materials such as expanded polystyrene (EPS) foam, aerospace composites, epoxy resins, carbon rod suspensions, and bamboo decks, which serve to create boards that are lighter and more responsive than traditional mass-produced polyurethane foam offerings. Although hard-core enthusiasts are drawn in by the performance of the Firewire lineup, this segment of the market is looking to tweak board dimensions to suit their individual needs—an approach Firewire could not accommodate efficiently and cost-effectively because of the strain it put on that company’s limited engineering staff.

Most high-end custom surfboard companies create a preshape of up to 85 percent of the design and then rely on a master craftsman to shape each board to its exact specifications. The problem with this approach is that although it supports one-off custom design, the process is not repeatable, and so the customers have no guarantee that they will be able to re-create that board if they want a replacement. Firewire had honed a development process that machines boards at a much higher level than other manufacturers (closer to 97 percent to 98 percent of the design); this was a huge step forward in removing a skilled craftsman from the equation and ensuring repeatability of the design. However, the process still did not lend itself to a custom buying experience because it required hours of effort on the part of Firewire’s engineering team to fine-tune CAD models individually for each one-off design to maintain the precision required by the firm’s CNC (computer numerically controlled) machine-driven manufacturing operation.

Firewire needed to bridge the gap and find a means of automating the behind-the-scenes CAD work while creating a seamless way to translate the modified CAD models of each board directly for CNC production. It also needed to create a Web user interface that was easy to navigate, gave users the ability to input custom parameters without degrading the complex interdependencies of the surfboard CAD model, and provided a good visual sense of what the custom board would look like before an order was placed.

The Three Prongs of CBD

ShapeLogic, the key technology provider for this revolutionary system, took the first step by creating a library of highly flexible parametric models of all the Firewire base surfboard designs. This was no small task in light of the highly complex shapes and contours of a surfboard, which are not easy to replicate with traditional 3D CAD programs. Drawing on prior expertise using CAD tools to model sets of golf clubs, ShapeLogic leveraged an array of sophisticated shaping tools and modeling commands supported by NX to mimic and morph the long flowing curves that traditionally were hand carved by a master craftsman.

The second step was to leverage the NX Open application programming interface (API) with Active Server Page (ASP) scripting to create the dynamic interactive CBD application. The NX Open APIs serve as the critical link between the browser interface and the 3D parametric surfboard models, allowing the custom dimensions that customers input—changes in width or length, for example—to be fed directly into the CBD system, where they drive changes to a live CAD model. To ensure that the customers do not make modifications that compromise the integrity of the design or impede the stability of the board, ShapeLogic embedded design rules in the base 3D CAD models that limit what users can modify. Specifically, users are able to customize parameters involving length, main width, nose and tail widths, and thickness; the CBD system has been designed to control the varying offset of the original curves that the shaper so painstakingly created as it applies those new dimensions to the template and the deck foil.

The third prong of the CBD engineer-to-order system is the visualization piece, which gives online customers an opportunity to view a custom board design and ensure that it is right before committing to a purchase. The system automatically generates a 3D PDF of the custom board by using Anark Core Server, including the 3D solid model of the base design and all the exact dimensions. The custom board can be compared with the stock board to visualize the exact changes by using selective translucency. Users also can download a lightweight version of a custom 3D board model in Siemens PLM Software’s JT Open data file format, which they can share or use to get feedback on further modifications.

Once customers order the board online, CBD generates a precise CAD solid model of the custom board, and that model is inputted into the CNC machines in Firewire’s factory, where the board then is produced. ShapeLogic and Firewire are working on fully automating the handoff to the CNC machines so that engineers do not have to input the CAD models manually and reprogram to kick-start production.

Firewire is riding a wave with the CBD system, and the ability to change live 3D geometry on the Web could benefit other industry sectors by providing an effective way to offer custom design capabilities for putters, bike frames, baseball bats, and even surgical implants. With all the focus today on the customer experience, the ability to offer price-competitive custom designs may be just what a company needs to get a leg up.

Fluid flow is a transport process of unbalanced forces or stresses governed by the three primary conservation laws of momentum, energy, and mass.  Conservation laws form a fundamental basis of the solution for all engineering flow problems, with the following relation applicable to the analysis of the system or control volume, where X represents the conserved quantity:

Equation 1-1 is referred to commonly as the continuity equation.  Expressed symbolically:

Fluid flow within a system is either steady-state or transient.  Under steady-state conditions, flow properties remain constant with respect to time; consequently, the accumulation term in Equation 1-2 equals zero.  Transient conditions indicate that one or more flow properties change with time, as observed during refinery plant start-up operations or in the simpler case of fluid draining from a vessel by gravity.

The primary method of fluid transfer is a piping or conduit network of circular cross-sectional area.  Flow through a pipe is assumed to occupy the entire cross section at any specific point and can be characterized as laminar, transitional, or turbulent.  Under laminar flow conditions, all fluid elements compose a smooth, unidirectional stream.  Turbulent flow involves bulk fluid movement mixed with an irregular motion of eddies except for a laminar film at the pipe or conduit wall called a boundary layer.  The laminar and turbulent flow regimes are separated by a transition region where fluid behavior does not have a distinct pattern.

The nature of fluid flow can be determined through calculation of a dimensionless parameter known as the Reynolds number, N_Re (Osborne Reynolds, 1842–1912).  N_Re is defined as a ratio of inertial to viscous forces:

**Where ρ is fluid density (lb_m/ft³); ν is fluid velocity through the pipe or conduit (ft/s); D is the pipe inner diameter (ft); and μ is fluid viscosity (lb_m/ft•s), converted from cP.

In laminar flow viscous forces dominate, whereas in turbulent flow inertial forces prevail.  In an engineering context, flow is considered laminar if N_Re is less than 2000 and turbulent if it is greater than 4000.  The range 2000 ≤ N_Re ≤ 4000 corresponds to transitional values.

Fluid transport phenomena are largely empirical.  Only a handful of problems, such as those involving laminar flow, have exact mathematical solutions; this points out the importance of laminar flow to the process industry as well as to designers of process equipment and instrumentation.  As an example involving compressible flow, mass flow controllers (MFCs) typically utilize flow-bypass thermal technology for gases, in which an appropriately sized restriction element within the device induces laminar flow so that an accurate measurement can be obtained and reproduced.  The majority of fluid flow problems are challenging as they depend on approximations derived from correction factors determined from extensive experimentation.

The following sections present topics in modeling incompressible fluid flow through piping, orifices, valves, and pumps.  Relevant problems that involve both steady-state and transient flow conditions are discussed, with an emphasis on refinery plant applications.  Crane Technical Paper No. 410 (see Bibliography) serves as a primary resource for relations, empirical pipe flow data, and engineering data.

2. The DARCY-WEISHBACH EQUATION,  Bernoulli Theorem, and Flow Coefficient C_v.

The Darcy-Weisbach equation (Henry Darcy, 1803–1858, and Julius Ludwig Weisbach, 1806–1871) is the general relation for pressure drop and is valid for laminar or turbulent flow barring cavitation.  The Crane paper presents several variations of the relation in terms of relevant process variables, demonstrating its versatility as a flow modeling tool.  The Darcy-Weisbach equation will be derived here with respect to pressure drop ΔP and volumetric flow rate q, with the development of each expression facilitated through the use of a mechanical energy balance commonly known as the Bernoulli theorem (Daniel Bernoulli, 1700–1782).  The resulting derivations will be applied to valve characterization, including the flow coefficient C_v.  Fluid flow is assumed to be at steady state.

The Bernoulli theorem accounts for two points in adiabatic flow when friction forces dominate energy transfer:

**where P is pressure (psi or lb_f/in²), with the applicable conversion factor; g_c is a conversion factor (32.174  lb_m •ft/lb_f •s²); g is the gravitational constant (32.174 ft/s²); Z is elevation (ft); h_L represents frictional loss in the line and valves/fittings (ft)

Each term in Equation 2-1 is referred to as a head, with units of ft.  The sum of the pressure head, velocity head, and elevation head for either point (1) or point (2) defines the corresponding total head.  Friction loss head h_L designates the sum of all frictional contributions in the line between points (1) and (2) and typically is represented by individual source terms.  The conversion factor g_c provides unit detail relating lb_f to lb_m; note that some technical resources, including Crane, omit g_c, resulting in minor dimensional inconsistencies for English units.

For incompressible flow through a horizontal pipe, Equation 2-1 reduces to:

The Darcy-Weisbach equation in its simplest form presents friction loss head as the product of velocity head and a resistance coefficient:

**where f is the Darcy friction factor, and L is pipe length (ft).

To determine f, the Reynolds number N_Re from Equation 1-3 must be calculated to ascertain the nature of flow.  For laminar flow, f is independent of piping or conduits and can be calculated from the quotient (64/N_Re).  For turbulent flow, f is a function of N_Re and the relative roughness ε of the pipe walls as a ratio to inner diameter (ε/D) and becomes relatively constant with higher values of N_Re.  Moody plots (Lewis Ferry Moody, 1880–1953) provided in Crane, pp. A-23 and A-24, relate f to N_Re and ε/D for any commercial pipe, and Crane p. A-25 relates f to N_Re and D specifically for clean steel pipe.  Pipe length L equals the sum of straight pipe length and equivalent length of bends, fittings, and valves, where the equivalent length of a bend, fitting, or valve is the length of straight pipe that produces the same frictional loss.  Representative equivalent length data, specified as L/D, are provided in Crane, p. A-30.

The Darcy-Weisbach equation can be written in terms of pressure drop using Equations 2-3 and 2-2:

where K represents

Given sufficient turbulence, K is constant and pressure drop increases with the square of velocity.  From the continuity relation in Equation 1-2, velocity relates to volumetric flow rate at a specific cross-sectional area:

where q is volumetric flow rate (ft³/s), and d is the pipe inner diameter (in) with the applicable conversion factor.

Substituting Equation 2-5 into Equation 2-4 yields the Darcy-Weisbach equation in terms of flow rate:

Flow rate therefore varies with the square of the pipe inner diameter, along with the square root of the pressure drop, provided K is constant.

A convenient means of expressing valve and pipe fitting capacity and flow characteristics, particularly with control valves, is through the use of a parameter called the flow coefficient C_v.  By definition, C_v is the volumetric flow rate of water, in gal/min or gpm, at 60°F that effects a pressure drop of 1 psid across the valve or fitting.  Note that C_v has units of gpm/√ psid and can be calculated with the following relation using any liquid with a viscosity close to that of water:

where Q is volumetric flow rate (gpm), and s.g. is specific gravity

Combining Equations 2-6 and 2-7 with appropriate unit conversions (shown) and applicable substitution of conditions relates C_v to the pipe inner diameter:

The resulting equation shows that the C_v of a pipe fitting or valve changes with the square of the pipe inner diameter, assuming constant K.

Commercially, valves are provided with pressure drop and C_v test data for each available diameter.  Model flow curves, including corresponding flow coefficients, are shown in Figure 2‑1.

Figure 2‑1

Pressure drop and flow coefficient data, commercial valve

The values for C_v indicate flow curve readings at a pressure drop of 1 psid.  Each curve presents a parabolic relationship between flow rate and pressure drop, which follows from Equation 2-6.  Flow rate and C_v likewise exhibit a nonlinear relationship with valve diameter given a fixed pressure drop; both were shown as second-order in Equations 2-6 and 2-8, respectively.

3. NOZZLES and ORIFICES

Nozzles and orifices are piping ancillaries that are used primarily as metering devices in pipelines and perforated plates.  The volumetric flow rate through a nozzle or orifice, neglecting velocity of approach, presents a comparative model to the Darcy-Weisbach equation in Equation 2-6:

where C is the flow coefficient (dimensionless; not to be confused with C_v for valves), and A is the cross-sectional area of the nozzle or orifice (ft²).

Rewriting Equation 3-1 in a slightly condensed, more convenient form:

where d₀ is the nozzle or orifice diameter (in) with the applicable conversion factor.

Converting Equation 3-2 to units of gpm:

Calculation of the flow rate q or Q requires an iterative sequence as the flow coefficient C is found by trial and error.  The differential pressure and fluid properties are known.

1. Calculate β, the diameter ratio of the orifice to the inner pipe d₀/d.
2. Estimate C based on β and the perceived nature of flow by using the applicable plot in Crane, p. A-19.
3. Determine q from Equation 3-2, velocity v based on d from Equation 2-5 or Crane, p. B-14 (if applicable), and the Reynolds number N_Re from Equation 1-3.
4. Find C based on N_Re and β by using the selected plot and compare with step 2.
5. If C from step 4 is not within 5% of the initial guess, repeat the sequence with an appropriately considered new guess.

A square-edged orifice can also be sized to place in a pipe.  The differential head has to be specified; the flow rate and fluid properties are known.

1. Determine v from Equation 2-5 or Crane, p. B-14 (if applicable), and N_Re from Equation 1-3.
2. Assume a ratio for β (0.5 is suitable) and calculate d₀.
3. Find C based on β and N_Re by using the applicable plot in Crane, p. A-19.
4. Calculate d₀ from Equation 3-2 or 3-3 and compare with step 2.
5. If d₀ from step 4 is less than the initial guess by at least 5%, repeat the sequence with a smaller guess for β.

The following example illustrates the calculation of an orifice.

Figure 3-1

Strategy: In both considerations, the new orifice diameter can be solved from Equation 3-3 as a ratio of new process variables to the original.  A constant flow coefficient C thus is eliminated, simplifying the problem substantially.  For an unknown C, the five-step approach to size a square-edged orifice will be useful.  Original and new variables will be designated with the respective subscripts orig and new.

Given:

• Piping: 6” Schedule 40 welded steel
  • d = 6.065 in (Crane, p. B-16)
  • d_{0, orig} = 3 in

• Flow rates through pipe:
  • Q_orig = 250 gpm
  • Q_new = 2Q_orig = 500 gpm

• Applicable properties of water:
  • ρ_60°F = 62.37 lb_m/ft³, μ_60°F = 1.129 cP

Calculations:

New orifice characteristics assuming constant flow coefficient C

• Express volumetric flow rate Q in terms of original and new flow rates by using Equation 3-3.

• Combine the expressions and solve for d_{0, new}:

• Quantify new orifice size:

New orifice characteristics with unknown flow coefficient C (five-step approach)

1)  Determine velocity v_new and Reynolds number N_{Re, new}.

-  Look up v_new in Crane, p. B-14 (500 gpm, 6” Schedule 40).

-   Calculate N_{Re, new} by using Equation 1-3.

2) Provide initial guess for β_new.

-  Calculate ratio using d_{0, new} obtained assuming constant C:

3) Estimate flow coefficient C_new:

• Read from provided chart based on results from steps 1 and 2.

4)  Calculate d_{0, new} and compare with step 2.

• Rearrange Equation 3-3 accordingly and combine both process representations.

• Determine flow coefficient C_orig:

-  Calculate Reynolds number N_{Re, orig}:

-  Calculate diameter ratio β_orig:

-  Read from chart.

5)  Calculate the percent error e between new orifice diameters.

• Use d_{0, new} from step 2 as the base or “actual” value.

• Since e ≥ 5%, repeat from step 2 with a lesser guess.

Comparison of d_{0, new} between the two approaches

• Perform similar error calculation.

-Depending on process sensitivity, assuming constant C may be acceptable.

4. DRAINING CYLINDRICAL TANK

The rate of depth change as liquid drains from a small circular aperture in an open cylinder-shaped tank is derived in the following illustration.  The effects of friction will be ignored here and subsequently explored by using separate transient models.

Figure 4.1

Frictionless Model:  The Torricelli Equation

From the continuity equation with respect to mass m, Equation 1-2 reduces to:

Assuming incompressible flow, Equation 4-1 becomes:

The volume of liquid in the tank at any time t can be expressed as:

Differentiating both sides of Equation 4-3 with respect to t:

Equating Equations 4-2 and 4-4:

where β represents the diameter ratio D₀/D.

To solve Equation 4-5, the exit velocity must be expressed in terms of an arbitrary, fixed height of liquid in the tank, which can be considered separately as a steady-state condition and modeled by using the Bernoulli theorem.  Considering point (1) located at the surface of the liquid in the tank and point (0) at the exit, Equation 2-1 reduces to

The velocities at points (1) and (0) are related by continuity:

Substituting Equation 4-7 into Equation 4-6 and rearranging yields the Torricelli equation (Evangelista Torricelli, 1608–1647):

The term (1 – β⁴) typically makes a minimal contribution and can be ignored but will be included in the remainder of the derivation for completeness.  Substituting Equation 4-8 into Equation 4-5 yields the following simple differential equation:

The solution presents the time theoretical minimum required for the tank to drain:

Friction Model:  Square-Edged Orifice

The tank now is modeled as a vertical pipe containing a square-edged orifice.  Volumetric flow rate as presented in Equation 3-1 represents draining liquid.  Combining Equation 3-1 with Equation 4-2 yields

Equating Equation 4-11 with Equation 4-4 and performing applicable substitutions:

Unlike the frictionless model, the pressure drop must be considered at the orifice, which reduces to a function of liquid height under static conditions.  Considering points (1) and (0) as before:

This relation can be used to calculate the pressure exerted by any vertical column of liquid or the pressure drop corresponding to a friction loss head.  Substituting Equation 4-13 into Equation 4-12 and separating variables yields the following differential equation:

Since the flow coefficient C is found by trial and error, a property-dependent representation is beyond the scope of the model.  The solution therefore assumes that C remains constant over the drainage period, which is reasonable for varying levels of turbulence ( N_Re ≥ 10000 ) on the diameter ratio.  For instance, if β is assumed to have a maximum value of 0.3, C is constant for Reynolds numbers greater than about 20000.  As a technical reminder, velocity is based on tank diameter D in determining N_Re.

Integrating Equation 4-14 presents the time-depth relation for the friction model:

The solution for the frictionless model in Equation 4-10 is therefore proportional to Equation 4-15 by a factor of C, neglecting the term (1 – β⁴) and considering sufficiently turbulent flow.  For example, if C is determined as 0.6, then the time theoretical minimum for drainage will be approximately 60% of the actual.

5. PUMPS

A pump is a physical contrivance that is used to impel fluids from one location to another through piping and conduits.  The two primary categories of pumps are as follows:

• Positive displacement. Operates by filling a cavity and then displacing a specific volume of liquid. A positive displacement pump is used to deliver a constant volume of liquid for each cycle against varying discharge pressure or head and typically is used in process applications in which a constant flow rate is required under varying pressures.  Examples include reciprocating, rotary, and peristaltic.

• Kinetic or Dynamic. Produces flow by increasing the velocity of the liquid with a rotating vane impeller.  The velocity energy imparted to the liquid subsequently is converted to pressure energy by the volute casing.  Unlike positive displacement pumps, flow rate decreases with increasing downstream resistance.  Kinetic pumps are used widely in the chemical industry for numerous applications, both process and utility.  The most common example is centrifugal.

The operational performance of a pump depends on system load characteristics such as pipe friction and fitting losses.  Writing the Darcy-Weisbach equation relating volumetric flow rate to pressure in Equation 2-6 with substitution of friction loss head in Equation 2-2 yields the following parabolic correlation between flow rate and friction loss for the system, given a constant pipe inner diameter and sufficient turbulence:

where ref typically refers to design conditions.

Characteristic information about a pump is contained in a pump curve, in which pump head is plotted versus flow rate for a set of impeller sizes or rotational speeds.  Efficiency contours, net positive suction head required (NPSH_R), and horsepower curves normally are included.  Pump performance typically is evaluated by using water at 60°F; the motor horsepower must be corrected for density if the pump operates with other fluids and/or at other temperatures.  NPSH_R refers to the cavitation characteristics of the pump, which are not discussed for the purposes of this section.

The performance curves for positive displacement and kinetic pumps differ significantly.  The curves for positive displacement pumps resemble a vertical line with a slightly negative tilt, indicating that capacity is nearly independent of downstream resistance for a particular pump speed, blocked-in conditions notwithstanding.  Kinetic pump curves exhibit an exponential decay of pump head with increasing flow and will serve as the focus of the following discussion.

Figure 5‑1 presents a set of pump head capacity curves for a commercial centrifugal pump, along with a simple exemplary system curve representing Equation 5-1.

Figure 5.1

Commercial centrifugal pump and superimposed system friction curves

The ordinate axis represents static pump head in a zero-flow condition.  The system curve begins at the origin, since in this example the pump has only frictional resistance to overcome.  Given a system flow requirement of 50 to 60 gpm, the four provided impeller sizes are options.

Consider the pump curve pertaining to an impeller diameter of 5.906 inches and a 1-horsepower (HP) motor.  The point of intersection with the system curve, the condition of operation, indicates that the pump will flow approximately 54 gpm against 30 ft of frictional head generated within the system.  A centrifugal pump is specified such that the condition of operation lies within the range of its optimal efficiency, approximately 75 to 110% of the best efficiency point (BEP).  Efficiency contours are not provided in Figure 5‑1, but maximum efficiency for a pump is generally in excess of 50%; some pumps may operate at efficiencies in the range of 80 to 90%.

In sizing a pump, the pump head between two relevant system points is needed at the specified flow rate.  The Bernoulli theorem applies to the pump system in an extended form:

The term h_pump accounts for pump head taking point (1) in the suction line and point (2) in the discharge line.  The applicable system curve plots h_pump versus flow rate, the zero-flow intercept equaling the difference in total head between points (1) and (2).  A suitable condition of operation is sought between the required h_pump and a pump curve of the selected model, provided that the fluid is water or of an approximate density considering available power.  If h_pump falls between curves due to the flow rate specification, then either the system or the pump must be modified.  The discharge can be throttled to shift the system curve upward until it intersects the next larger impeller curve, although a more efficient solution is to adjust the pump speed by using a variable-frequency drive.

6. THROTTLING PUMP FLOW

The following example is reprinted with permission from Professional Publications, Inc., Chemical Engineering Practice Exam Set, 2^nd ed., by Randall N. Robinson, P.E., copyright © 1988 by Professional Publications, Inc.  The problem highlights several aspects of flow dynamics covered in the previous sections.

Figure 6.1

Problem: Calculate flow coefficient C_vfor a control valve under given throttled flow rates of water.  For the purposes of this example, C_v will be evaluated only at 50 gpm throttling.

Strategy: The flow coefficient C_v can be calculated from Equation 2-7:

The pressure drop across the control valve ΔP_CV can be calculated from Equation 4-13:

The friction loss head across the control valve h_CV can be determined from Equation 5-2, the extended Bernoulli theorem, with friction loss head h_L expressed as the sum of h_CV and pipe friction loss head h_pipe.  The system will be modeled considering point (1) at the tank level and point (2) at the scrubber inlet:

Pipe friction loss head h_pipe can be found from Equation 2-3 using point (2):

Given:

• Source tank: P₁ = P_atm, v₁ = 0 (negligible), Z₁ = 0 ft
• Scrubber: P₂ = P_atm (operates at atmospheric pressure), Z₂ = 60 ft
• Piping: 2” Schedule 40 welded steel

• d = 2.067 in (Crane, page B-16)
• L = 220 ft

• Pump head h_pump = 135 ft @ 50 gpm (from provided pump curve)
• Volumetric flow rate Q = 50 gpm (throttle target)
• Applicable properties of water:

Calculations:

Pipe friction loss head h_pipe

Determine velocity v₂:

• Look up v₂ in Crane, p. B-14 (50 gpm, 2” Schedule 40).

• Determine the Darcy friction factor f.
  • Calculate the Reynolds number N_Re from Equation 1-3 to assess nature of flow.

• Since flow is turbulent through clean steel piping, use Crane, p. A-25
• f = 0.022
• Solve Equation 6-4.

Friction loss head across control valve h_CV

• Reduce Equation 6-3 and solve.

Pressure drop across control valve ΔP_CV

• Solve Equation 6-2.

Flow coefficient C_v

• Solve Equation 6-1.

Bibliography

1.    Crane Company Engineering Division. Flow of Fluids through Valves, Fittings, and Pipe. Chicago: Crane Co. Technical Paper 410, 1965, pp. 1-1–1-7, 2-8–2-9, 2-14, 3-2, 3-4, 3-14.

2.    Darby, R.  Chemical Engineering Fluid Mechanics, New York: Marcel Dekker, 2001, pp. 1–2, 105–120, 239–247.

3.    Parker, David B.  Positive Displacement Pumps—Performance and Application.Proceedings of 11th International Pump Users Symposium, 1994, College Station, TX.
4.    Perry, R. and Green, D.  Perry’s Chemical Engineers’ Handbook, 7th ed. New York: McGraw-Hill, 1997, pp. 6-21–6-22, 10-20–10-27.
5.    Robinson, R. R.  Chemical Engineering Practice Exam Set, Second Edition.  Belmont, CA: Professional Publications, Inc., 1988, pp. 1, 34
6.    U.S. Department of Energy.  DOE Fundamentals Handbook, Thermodynamics, Heat Transfer, and Fluid Flow, Vol. 3. Washington, DC: Department of Energy, 1992, pp. 25–27.
7.    http://www.EngineeringToolBox.com.  The Engineering ToolBox.

1. http://www.engineeringtoolbox.com/water-density-specific-weight-d_595.html
2. http://www.engineeringtoolbox.com/absolute-dynamic-viscosity-water-d_575.html

Acknowledgements

The author would like to thank Karl Amo, M.S., P.E., for his expertise in technical editing, and R. A. Stearns for his helpful advice on writing style.