Electricity , Power generation ,Thermal power plant , Solar Energy , Generators , Green Energy

Doubly Fed Induction Generator in Wind Energy Conversion System (WECS)

2014-02-04T18:00:00.000+02:00

Doubly Fed Induction Generator in Wind

Energy Conversion System (WECS)

-The "Danish Concept", as shown in fig.1, is the first technology design for low-power wind turbine for power energy conversion. This conventional fixed speed generator (FSG) design uses squirrel cage induction generator directly connected to the electricity grid via a step up transformer. The Danish Concept turbine scheme delivers its rated output power at constant wind speed with a fixed ratio gearbox. At wind gusts (high wind speed), a great variation to the electrical output power is observed, that affects the system stability from grid view. Also it affects the wind turbine performance due to the high mechanical stresses.

Fig.1 - The Danish Concept scheme

-Adjustable speed generators (ASG) design offer more advantages over the Danish Concept turbine such as improving power quality, improving efficiency, reducing the mechanical stresses at high wind speed, and dynamic compensation for power and torque. Two different designs of Adjustable speed generators are commonly used for wind energy conversion system;

(i) Direct-in-line system, as shown in fig.2, it uses synchronous generator that its stator directly connected into the grid through a full capacity power converter. With the use of power converter, the generator can operate within full speed range. The only disadvantage of this scheme is the high cost of the full rated power converter whose rating is 1 p.u. to system power as it is connected directly in series with the generator.

Fig.2 - Direct in line system scheme

(ii) Doubly Fed Induction Generator (DFIG), as shown in fig.3, it uses wound rotor induction generator that its rotor is connected through a power converter and its stator is directly connected into the grid. With the use of power converter in this scheme, it allows a bidirectional power flow in the rotor circuit and increases the wind speed ranges of the generator.

Fig.3 – Doubly fed Induction Generator scheme

-Among various alternatives to construct grid-connected wind turbines, the ones based on doubly fed induction generators (DFIGs) are dominating the energy market due to their outstanding merits ;

(i) Reduced mechanical stress,

(ii) Improved Power Quality,

(iii) Improved system efficiency,

(iiii) reduced acoustic noise,

(v) Reduced cost of inverter set as it is connected in rotor circuit.

How Wind Turbines Work!!!

2011-09-27T11:43:00.002+02:00

Wind ENERGY

How Wind Turbines Work!!!

-Wind is a form of solar energy. Winds are caused by the uneven heating of the atmosphere by the sun, the irregularities of the earth's surface, and rotation of the earth. Wind flow patterns are modified by the earth's terrain, bodies of water, and vegetation. Humans use this wind flow, or motion energy, for many purposes: sailing, flying a kite, and even generating electricity.

-The terms wind energy or wind power describes the process by which the wind is used to generate mechanical power or electricity. Wind turbines convert the kinetic energy in the wind into mechanical power. This mechanical power can be used for specific tasks (such as grinding grain or pumping water) or a generator can convert this mechanical power into electricity.

-So how do wind turbines make electricity? Simply stated, a wind turbine works the opposite of a fan. Instead of using electricity to make wind, like a fan, wind turbines use wind to make electricity. The wind turns the blades, which spin a shaft, which connects to a generator and makes electricity. Take a look inside a wind turbine to see the various parts in the following figure.

-This aerial view of a wind power plant shows how a group of wind turbines can make electricity for the utility grid. The electricity is sent through transmission and distribution lines to homes, businesses, schools, and so on.

Types of Wind Turbines

-Modern wind turbines fall into two basic groups: the horizontal-axis variety and the vertical-axis design, like the eggbeater-style Darrieus model, named after its French inventor.

-Horizontal-axis wind turbines typically either have two or three blades. These three-bladed wind turbines are operated "upwind," with the blades facing into the wind.

Sizes of Wind Turbines

-Utility-scale turbines range in size from 100 kilowatts to as large as several megawatts. Larger turbines are grouped together into wind farms, which provide bulk power to the electrical grid.

-Single small turbines, below 100 kilowatts, are used for homes, telecommunications dishes, or water pumping. Small turbines are sometimes used in connection with diesel generators, batteries, and photovoltaic systems. These systems are called hybrid wind systems and are typically used in remote, off-grid locations, where a connection to the utility grid is not available.

Inside the Wind Turbine

Anemometer:

Measures the wind speed and transmits wind speed data to the controller.

Blades:

Most turbines have either two or three blades. Wind blowing over the blades causes the blades to "lift" and rotate.

Brake:

A disc brake, which can be applied mechanically, electrically, or hydraulically to stop the rotor in emergencies.

Controller:

The controller starts up the machine at wind speeds of about 8 to 16 miles per hour (mph) and shuts off the machine at about 55 mph. Turbines do not operate at wind speeds above about 55 mph because they might be damaged by the high winds.

Gear box:

Gears connect the low-speed shaft to the high-speed shaft and increase the rotational speeds from about 30 to 60 rotations per minute (rpm) to about 1000 to 1800 rpm, the rotational speed required by most generators to produce electricity. The gear box is a costly (and heavy) part of the wind turbine and engineers are exploring "direct-drive" generators that operate at lower rotational speeds and don't need gear boxes.

Generator:

Usually an off-the-shelf induction generator that produces 60-cycle AC electricity.

High-speed shaft:

Drives the generator.

Low-speed shaft:

The rotor turns the low-speed shaft at about 30 to 60 rotations per minute.

Nacelle:

The nacelle sits atop the tower and contains the gear box, low- and high-speed shafts, generator, controller, and brake. Some nacelles are large enough for a helicopter to land on.

Pitch:

Blades are turned, or pitched, out of the wind to control the rotor speed and keep the rotor from turning in winds that are too high or too low to produce electricity.

Rotor:

The blades and the hub together are called the rotor.

Tower:

Towers are made from tubular steel (shown here), concrete, or steel lattice. Because wind speed increases with height, taller towers enable turbines to capture more energy and generate more electricity.

Wind direction:

This is an "upwind" turbine, so-called because it operates facing into the wind. Other turbines are designed to run "downwind," facing away from the wind.

Wind vane:

Measures wind direction and communicates with the yaw drive to orient the turbine properly with respect to the wind.

Yaw drive:

Upwind turbines face into the wind; the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. Downwind turbines don't require a yaw drive, the wind blows the rotor downwind.

Yaw motor:

Powers the yaw drive.

This Info are from http://www1.eere.energy.gov

Diesel Generator sizing - How to Determine What Size You Need??

2011-09-22T11:28:00.000+02:00

Diesel Generator sizing - How to Determine What Size You Need??

-Getting a generator that can handle all your power generation needs is one of the most critical aspects of the purchasing decision. Whether you are interested in prime or standby power, if your new generator can't meet your specific requirements then it simply won't be doing anyone any good because it can put undue stress on the unit and even damage some of the devices connected to it. Unfortunately, determining exactly what size of generator to get is often very difficult and involves a number of factors and considerations.

-Making a choice amongst single phase, three phase, kW, KVA, welder, standby or motor starting generators can be mind-boggling. To prevent such confusion, this article was developed to help you get a better idea of how the sizing process works and some key things to keep in mind.

Generator Size Variations

-With the latest advancements in the field of electrical engineering, generators are now available in a wide range of sizes. Generators with power supply capacities of 5kW to 50kW are readily available in the personal and home use markets, while industrial generators are anywhere from 50kW to over 3 Megawatts. Handy and portable gen sets are available for homes, RV's and small offices, but larger businesses, data centers, buildings, plants, and industrial applications need to use the much larger sized industrial generators to meet their higher power requirements.

Generator Sizing - How Much Power

-Many people believe smaller generators can be used for standby electric power because they are not running all the time. This is not only a myth but can actually be very detrimental. Unfortunately, generator under sizing is one of the most common mistakes committed by buyers. Not only does it involve the risks of damaging your new asset (the generator), but it can also damage other assets connected to it, create hazardous situations, and even limit overall productivity of the unit and/or the business relying on it. If nothing else, the key thing to remember here is that more is always better than less.

Know Your Requirements

Going to a dealer and buying the best or cheapest generator available without any other consideration is clearly not the best approach. It is always better to delve deep into your power generation requirements before making a choice. You can do this in the following ways:

- Make a list of the items that need to be powered by the generator

- Make a note of the starting and running wattage of the respective items

- Calculate the total power requirements in KVA or KW

Advantages of choosing the right size generator

Now that you have an idea on how to choose the appropriate size of generator to suit your needs, here's just a few of the benefits obtained by going through that process:

- No unexpected system failures
- No shutdowns due to capacity overload
- Increased longevity of the generator
- Guaranteed performance
- Smoother hassle-free maintenance
- Increased system life span
- Assured personal safety
- Much smaller chance of asset damage

Ingress Protection Codes and Classification IP CODE

2011-09-20T01:38:00.000+02:00

Ingress Protection Codes and Classification IP CODE

- The IPCODE (IP code) is used to stipulate the environmental protection applicable for specific equipment enclosures.
-When we purchase any electrical equipment, mechanical devices, household instruments, or enclosures, it is important to identify what degree of IP rating or IP Code (Ingress Protection) the equipment offers. Ingress of solid or liquid particles into the equipment can be dangerous to humans as well as machinery.
-The intention of this rating is to provide consumers and end users clearer details than advertising terminology such as "rainproof" or "waterproof."
-The IP code is used to stipulate the environmental protection used for specific equipment enclosure.
**Normally an IP code has two numbers:

Protection from solid particles
Protection from liquids

As an example, with the rating "IP 23," the first digit "2" describes the level of protection from solid objects and second digit "3" describes the level of protection from liquids.

2 = Protected against solid objects up to 12mm, e.g. fingers.
3 = Protected against direct sprays of water up to 60° from the vertical.

IP Standards

An engineer working within industry should understand the types of enclosures provided against hazardous parts and the ingress of solid foreign particles for all equipment located in the plant. Also during design of machines, an engineer should take care of IP codes of equipment, considering its application and the surrounding condition of placement.
The applicable European standards for ingress protection are:

BS EN 60529 Specification of Degrees of Protection Provided by Enclosures
IEC 529 Specification of Degrees of Protection Provided by Enclosures

Both of these provide a numerical code to classify the degree of protection offered. Please click on the following link to view the table.

This information source from ‘’ ContractorsUnlimited.co.uk ‘’.

OPERATION JOB OR TECHNICAL PLANNING ??

2011-09-19T18:43:00.000+02:00

HELLO EVERYONE

I want to share with you an important question , and I hope you have a reply for me ?
What would you choose if there are two offers for work and why ?

The first Offer , is to work as an operational Engineer at Electricity Power Generation Utility and you have to make the proper control and optimization for the whole unit.

The seconde Offer , is to work at Technical Planning Engineer for a company in telecommunication Sector , and you have to make proper tenders and recommendation for purchasing and order of Electrical Equipments like Diesel Units , Batteries , Medium Voltage Panels , UPS , INVERTERS , Rectifirers.

I want your replies so soon , please , and thanks for your interest.

Khaled Hamza

BOILER DRUM ROLES

2011-07-18T20:45:00.000+02:00

BOILER DRUM ROLES

- Steam drums are used on recirculating boilers that operate at subcritical pressures. The primary purpose of the steam drum is to separate the saturated steam from the steam-water mixture that leaves the heat transfer surfaces and enters the drum. The steam-free water is recirculated within the boiler with the incoming feedwater for further steam generation. The saturated steam is removed from the drum through a series of outlet nozzles, where the steam is used as is or flows to a superheater for further heating. (By definition, saturated steam is pure steam that is at the temperature that corresponds to the boiling temperature at a particular pressure. For example, saturated steam at a pressure of 500 psia has a temperature of 467°F.)

The steam drum is also used for the following:

1- To mix the saturated water that remains after steam separation with the incoming feedwater.

2- To mix the chemicals that are put into the drum for the purpose of Corrosion control and water treatment.

3- To purify the steam by removing contaminants and residual moisture.

4- To provide the source for a blowdown system where a portion of the water is rejected as a means of controlling the boiler water chemistry and reducing the solids content.

5- To provide a storage of water to accommodate any rapid changes in the boiler load.

- The most important function of the steam drum, however, remains as the separation of steam and water. Separation by natural gravity can be accomplished with a large steam-water surface inside the drum. This is not the economical choice in today’s design because it results in larger steam drums, and therefore the use of mechanical separation devices is the primary choice for separation of steam and water.

- Efficient steam-water separation is of major importance because it produces high-quality steam that is free of moisture.

This leads to the following key factors in efficient boiler operation:

1. It prevents the carry-over of water droplets into the superheater, where thermal damage could result.

2. It minimizes the carry-under of steam with the water that leaves the drum, where this residual steam would reduce the circulation effectiveness of the boiler.

3. It prevents the carry-over of solids. Solids are dissolved in the water droplets that may be entrained in the steam if not separated properly. By proper separation, this prevents the formation of deposits in the superheater and ultimately on the turbine blades.

**(carry-over is the passing of water and impurities to the steam outlet)

**The term critical pressure is the pressure at which there is no difference between the liquid and vapor states of water; i.e., the density is identical. This occurs at 3206 psia.

This information source from ‘’ Steam Plant Operation 8th Ed. - Everett B. Woodruff ‘’ .

AUTOMATIC VOLTAGE REGULATOR (AVR)

2011-07-11T17:49:00.002+02:00

-I will talk today with you about an important equipment used in power system utilities, it is the Automatic Voltage Regulator (AVR). From its name it is a regulator which regulates the output voltage at a nominal constant voltage level.

Role of AVR

AVR (Automatic voltage regulator) has following roles.

1- To regulate generator terminal voltage.

Mainly generator under no-load condition, AVR regulates the generator voltage to voltage setter (90R).

*AVR detects terminal voltage and compare with voltage setter (90R).

*AVR regulates field current via the Exciter.

*Generator terminal voltage is regulated by field current.

V_t< 90R _ Field current will be increase

V_t > 90R _ Field current will be decrease

2-To adjust MVars (Reactive power).

When the generator connected to power grid, AVR adjust reactive power by regulate generator voltage.

MVar (Reactive power: Q) is regulated by generator terminal voltage. Therefore AVR can regulate MVars.

Vt is increased _ MVars will be increase

Vt is decreased _ MVars will be decrease

Hence;

To increase MVars _ 90R raise

To decrease MVars _ 90R lower

3-To improve the power system stability.

There are two stability

-Transient stability …… Improved by AVR

-Dynamic stability ……. Improved by PSS (power system stabilizer)

* Improve the Transient stability

Transient stability is improved by high initial response characteristic. In the fault condition, Field voltage is increased to keep the generator voltage constantly. If the excitation response is slow, it will not able to keep voltage and the generator cannot keep synchronizing.

* Improve the Dynamic state stability

Dynamic stability is improved by Power System Stabilizer (PSS). PSS is provided in order to improve the power system dynamic stability. PSS will control the excitation to reduce the power swing rapidly.

4-To suppress the over-voltage on load rejection.

When the load rejection, field current and field voltage should be reduced rapidly to keep terminal voltage constantly and prevent overvoltage.

This information source is MELCO CO. for CEPC generator

Gas Recirculation Fan ( Structure & Control )

2011-07-03T00:30:00.000+02:00

Gas Recirculation Fan

- Gas Recirculation Fan (GRF) draw gas from a point between the economizer outlet and the air heater inlet and discharge it into the bottom of the furnace outlet.

- Recirculation gas introduced in the vicinity of the initial burning zone of the furnace is used for steam temperature control, while re circulated gas introduced near the furnace outlet is used for control of gas temperature.

-The following picture will show the full specification of the GRF ( motor and impeller ).

-Due to large GRF duty , Bearings are equipped with water cooling flexible line , the bearing at the motor side is fixed to the shaft, while at the counter to the motor side is set free for the shaft expansion caused by the heat of high temperature gas in the casing during the continuous operation.

-Because the fan treats the high temperature gas, it is adopted with a turning device, If the fan stopping remain the high temperature gas in the casing, the impeller has the unbalance by heat effect. So the purpose of the no unbalance of the heat effect drive the turning device regular time at about 50 rpm until the economizer heat decreased to a safety limit about 100 centigrade.

Gas Recirculation control

-RH outlet steam temperature is normally controlled by regulating the gas recirculation flow. Increased flue gas flow over Reheater of the convection heating surfaces increases the heat absorption and RH temperature is increased. The amount of gas recirculation flow is controlled by positioning the inlet dampers on the two Gas Recirculation Fans (GRF).

(1)During start -up (approximately < 25% boiler load), GRF inlet damper position is control led by the function generator from Boiler Master. Because RH temperature feedback control is difficult during the start-up due to the slow response.

(2) Dynamic feed forward signal (BIR) is added to GRF inlet damper to improve RH steam temperature control during load change.

(3)High and low limit from Boiler Master for GRF inlet damper are provided. High limit is to avoid the unstable combustion and the over current of GRF. And low limit is to protect the water wall and avoid the high NOx.

Keeping Control of Drum Levels

2011-07-02T20:47:00.000+02:00

Keeping Control of Drum Levels

“THIS ARTICLE GIVES US DIFFERENT TECHNIQUES ABOUT BOILER CONTROL ESPECIALLY DRUM LEVEL CONTROL,”

-Power plants are designed to operate for decades, provided they undergo regular repair, upgrade and improvement. Much of the time, those maintenance actions are minor. But plant managers expect a few big-ticket expenditures.

-Public Service Electric & Gas Co. (PSEG), for instance, spent $1.3 billion and more than 7 million hours of labor during the 2008 to 2010 period upgrading emissions controls at its Hudson and Mercer, N.J. coal-fired plants. By installing scrubbers, selective catalytic reduction, baghouses and activated carbon injection, PSEG reduced NOX emissions by more than 95 percent, SO2 by more than 94 percent, particulates by more than 99 percent and mercury by more than 90 percent. On a smaller scale, PSEG’s 753 MW Mercer Generating Station experienced problems controlling the water levels in its four Foster Wheeler drum boilers. In that case, it was simply a matter of replacing the original actuators on the feedwater valves with new electraulic actuators.

-“We would have excursions in drum levels and had to wait until it settled down before the operator was comfortable with moving on line,” said PSEG Controls Engineer, Mark Maute. “It’s not a problem anymore. There is no slop in the controls and it runs right where it is supposed to.”

Hitting the Set Point

-Boiler efficiency and overall plant performance depend on being able to accurately control the water levels in the drum. Several problems affect that ability. To begin with, there is the issue of shrink and swell. As steam demand increases, the drum pressure drops initially, causing bubbles to form below the surface of the water and producing a rise in the drum level (swell). When demand decreases, pressure in the drum increases and water levels drop (shrink). This is usually addressed by using a cascade/feed-forward control strategy that takes readings from steam flow, feedwater flow, drum level and drum pressure transmitters and adjusts the feedwater accordingly.

-The feedwater control strategy must also integrate with the combustion control strategy. The firing rate set point demand is calculated as a function of the steam flow and the main steam header pressure. Accurate control of the feedwater loop is necessary to maintain stable combustion flow. If the plant requires steam pressure and flow to remain fairly constant but the feedwater loop is unstable due to poor controllability, the combustion controls will have to continually adjust in attempting to follow unstable changes in the feedwater flow and keep the steam demand at the set point.

-To accurately control the volume of water entering the drum, the feedwater regulator valve needs to accommodate a wide range of operating conditions. During start-up and low-fire conditions, the valve sees high inlet pressures but low flow, so the valve requires anti-cavitation trim to address the full pressure drop across the valve. As the boiler load increases, the valve passes more flow, the inlet pressure drops and the outlet pressure to the drum rises. As a result, the valve must have a large capacity with minimal pressure drop. Some plants use two valves in parallel, one designed for start-up conditions and the other for mid- to full load. The other approach is to use a single valve with a characterized disk stack designed to accommodate varying flow conditions.

-Whichever approach is taken, the feedwater regulator valve must have an actuator that can smoothly and accurately adjust feedwater flow. The valve doesn’t need to be extremely fast, because the digital control system will tune the loop for optimum operation. But the actuator should respond to the command without added delay and execute the command without overshooting or undershooting.

-Pneumatic actuators cannot achieve the highest level of control performance because air is compressible. When the command is given to increase the air pressure and move the valve, there is a lag while the air pressure increases to the point where it is high enough to overcome the static friction of the actuator and start it moving. Pneumatic actuators also tend to overshoot the set point. This dynamic is known as hysteresis, and is a common occurrence in pneumatic control valves. “Smart” pneumatic positioners slow the actuator as it nears the set point to reduce the amount of overshoot, but the adverse affect is that they also increase the “deadtime” in the process loop and ultimately inhibit the controls engineer in optimum loop-tuning. It is common for pneumatic actuators to add seconds of deadtime into the process loop, making them more difficult to tune for high levels of process performance.

-One alternative for improved control is for a plant to use hydraulic actuators. Hydraulic fluids are incompressible by nature, have immediate response and move to position in a stable and repeatable manner without overshooting and without hysteresis. The detriments of electro-hydraulic control systems (EHC’s) are in that they are more expensive than pneumatic systems and require a network of components: an external gravity fed reservoir, constantly running motors/pumps, expensive filtering systems and typically handling of fire retardant oil. Because of these components, EHCs have given maintenance department’s headaches for years. Although the performance of hydraulic systems is undeniably more robust and more accurate, the trade-offs associated with maintaining these systems offset their inherent benefit.

Gaining Control

-To address its feedwater problems, PSEG decided to use an “electraulic” (electro-hydraulic) actuator from Rexa. The Rexa actuators are self-contained units that combine hydraulic, electronic and mechanical technologies. Rexa actuators provide performance commensurate with typical hydraulic systems (0.05 percent resolution and 70mSec deadtime), but eliminate the maintenance-intensive requirements associated with EHCs.

-Electraulic actuators have two main components, a power module and a control enclosure and operate by moving hydraulic fluid from one side of a double-acting cylinder to the other.

-Inside the power module are a motor, gear pump, flow match valve, 60cc thermal expansion/make-up oil reservoir, heater and thermostat. Upon receipt of a control signal, the pump delivers oil at a nominal 2,000 psi to one side or the other of a hydraulic cylinder, causing motion in the desired direction. The hydraulic cylinders come in either linear or a rack-and-pinion rotary design. A position sensor is mounted within or adjacent to the cylinders and provides position feedback to the control electronics.

-The control sub-assembly contains the central processing unit (CPU), power supply and motor drivers. The CPU typically contains a microprocessor, an analog-to-digital converter, a position transmitter, limit switches and warning and alarm systems. The power supply takes the incoming AC power and coverts it to the voltages required by the control components. The motor driver receives commands from the CPU and sends control signals to the motor. Outside of the enclosure (or inside when conditions require) is a two-line display giving actuator status and a five-button keypad to set up and calibrate the actuator.

-When used to control a feedwater regulator valve, the CPU receives a control signal from the DCS and converts it to a target position for the actuator. It then compares current position of the actuator as reported by the feedback assembly with the desired new position. If the difference is outside the preprogrammed range the CPU will send a signal to start the motor, which then drives the reversible hydraulic pump to pressurize one side of the cylinder or the other, moving the piston in the desired direction. Once it reaches the new position, the pump shuts down and check valves close. This locks the hydraulic fluid in the cylinder and maintains the actuator position, without having to keep the motor running, until a new signal is received from the DCS.

‘By Joe Zwers , freelance writer’

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http://science-hamza.blogspot.com/

Air Ejectors (2 of 3)

2011-03-27T18:59:00.000+02:00

Air Ejectors (2 of 3)

-An air ejector or steam ejector is a device which uses the motion of moving fluid (Motive Fluid) to transport another fluid (Suction fluid). It is has a wide range of application in steam ejector in boiler condenser, fresh water generator and in priming the centrifugal pump.

-It works on the principle of convergent /divergent nozzle as it provides the venturi effect at the point of diffusion as the tube gets narrows at the throat the velocity of the fluid increases and because of the venturi affect it pressure decreases, vacuum will occur in the diffuser throat where the suction line will be provided.

-The operating medium of an air ejector can be either high-pressure gas or liquid. This is passed through a nozzle and the pressure energy is converted into velocity energy. The high-velocity fluid aspirates the air and the non-condensable gases and the mixture is projected into a diffuser which reconverts the velocity energy into pressure energy.

-Steam is suitable operating medium and is used in the steam-jet air ejector. The steam consumption is controlled by the compression ratio of the air and this factor influences the decision to adopt either single or multiple stage ejectors for a particular condition.

-To meet the requirements when raising vacuum a starting ejector is provided. This is high capacity, high steam consumption ejector, of single stage and without an after condenser.

-A main air ejector with standby unit is usually provided for normal operation. The heat in the operating steam is partially recovered in the condensate which flows through the inter and after coolers.

-If an installation is to operate with a direct cooling system, as is provided by the sea or a river, vacuum as high as 29.2 in.Hg would be expected during the winter months.

-A cooling tower installation will operate at a slightly poorer vacuum than a direct cooled station and this vacuum can usually be handled by a two-stage ejector.

CONDENSER – AIR EXTRACTION EQUIPMENT ( 1 Of 3 )

2011-03-26T14:14:00.000+02:00

CONDENSER – AIR EXTRACTION EQUIPMENT

- Turbines are designed for a particular operating conditions like steam inlet pressure, steam inlet temperature and turbine exhaust pressure/ exhaust vacuum, which affects the performance of the turbines in a significant way. Variations in these parameters affect the steam consumption in the turbines and also the turbine efficiency. Theoretical turbine efficiency is calculated as work done by the turbine to the heat supplied to generate the steam.

- Higher exhaust pressure/ lower vacuum, increases the steam consumption in the turbine, keeping all other operating parameters constant. Exhaust pressure lower than the specified will reduce the steam consumption and improves the turbine efficiency. Similarly exhaust vacuum lower than the specified, will lower the turbine efficiency and reduces the steam consumption.

- The air extraction equipment must be capable of meeting two conditions; one met during normal operation, the other when raising vacuum on the turbo-generator unit.

- When raising vacuum the air extraction equipment must deal rapidly with a large quantity of air and sufficient capacity must be installed to reduce pressure quickly in the condenser to a level which allows the turbine to be started. The last row blades of a 500 MW turbine will overheat if they run at speed and at low load in a poor vacuum. Thus, a vacuum of 20 in.Hg must be obtained before steam is admitted to the turbine and a vacuum of 26 in.Hg for full speed. It is important that the time taken to bring a turbo-generator on load shall not be increased by sufficient air extraction capacity.

- Under normal operating conditions the quantity of air to be exhausted is lower. It consists of air leakage into the condenser via flanges and glands and also of incondensable gases that are present in the steam exhausting from the turbine. Air and incondensable must be removed from the condenser as their presence in any quantity impairs the heat transfer capability of the condenser and hence its performance. Conversely, excessive air extraction capacity should not be run, as this involves unnecessary running costs and also results in the extraction of an unwanted quantity of water vapor from the condenser steam space.

“We will talk about the equipment for the extraction in the 2 upcoming articles”

Plant Safety: Learn from the Mistakes of Others

2011-03-25T13:42:00.000+02:00

Plant Safety: Learn from the Mistakes of Others

“THIS ARTICLE GIVE US LESSONS FROM AN ACCIDENT HAPPENED AT U.S. WITH HEAT EXCHANGERS EXPLOSION,”

-On January 27, the U.S. Chemical Safety Board (CSB) released a case study examining the causes of a heat exchanger rupture and ammonia release at the Goodyear Tire and Rubber Plant in Houston in 2008. Use these lessons learned to check your employee tracking system and to determine whether your plant has installed valves that violate the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code.

-The CSB event summary begins by noting that the accident occurred on June 11, 2008, when an overpressure in a heat exchanger led to a violent rupture of the exchanger that hurled debris, which struck and killed a Goodyear employee walking through the area. The heat exchanger contained pressurized anhydrous ammonia, a colorless, toxic chemical used as a coolant in the production of synthetic rubber; five workers were exposed to ammonia released by the rupture.

-On the day prior to the accident, maintenance work required closing several valves on the heat exchanger. CSB investigators found that workers closed a valve that isolated the exchanger from a relief valve, to replace a burst rupture disk located below the relief valve.

-The next day, at about 7:30 a.m., an operator closed another valve—this one blocking a second, automatic pressure control valve—to begin cleaning the process line with steam. The operator was unaware that the isolation valve was also closed, thus leaving no means of relieving excess pressure in the exchanger. As a result, pressure continued to increase until the heat exchanger exploded violently.

-The report further notes that maintenance work activity was not properly communicated between maintenance and operations personnel, resulting in a subsequent shift not being notified of the isolation of the pressure relief line.

Plant Design Mistakes

-The CSB’s final report outlines several lessons learned, including the need to adhere to the existing ASME Boiler and Pressure Vessel Code.

-CSB Investigations Supervisor Robert Hall said, “We found the accident likely would not have happened had operators followed the ASME code. It’s crucial that workers continuously monitor an isolated pressure relief system throughout the course of a repair and reopen blocked valves immediately after the work is completed.”

-The CSB’s report notes that the ASME code states that “Overpressure protections shall be continually provided... whenever there is a possibility that the vessel can be over-pressurized by a pressure source.”

‘This article for, Dr. Robert Peltier , PE “

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http://science-hamza.blogspot.com/

BOILER (MASTER FUEL TRIP)

2011-03-23T23:05:00.000+02:00

Steam Generator ( boiler ) is very important equipment at thermal power units , Boiler is the steam supply for the unit to rotate the turbine's rotor , hence produce electricity at Generator's stator.

Boiler usually equipped with solar and Main gas firing equipments , and you can imagine that huge boiler disappear in seconds unless operator take his precaution from operation hazards., hence Boiler always equipped with a protection relays to trip the boiler out of service if hazards conditions has happened , these relays are enclosed at huge protection system is called ( Master Fuel Trip ).

From its name ( Master Fuel Trip ) , it produces trip signal for fuel systems from operation , then trip the boiler out of service away from danger. this relay needs many conditions to operate , these conditions will be found at the following figure.

MASTER FUEL TRIP , CONDITIONS :-

1-REHEATER PROTECTION
2-DELAYED LIGHTING
3-LOSS OF ALL FUEL INPUT
4-DRUM LEVEL HI HI
5-DRUM LEVEL LOW LOW
6-ALL FD FAN STOP
7-FURNACE PRESSURE HI HI
8-CCS WATCHDOG ABNORMAL
9-COMBUSTION AIR FLOW LOW LOW
10-CONDENSER PROTECTION
11-PARTIAL LOSS OF FLAME
12-BOILER EMERGENCY TRIP
13-LOSS OF ALL FLAME
14-HP/LP BYPASS VALVE ABNORMAL CLOSE
15-ALL FUEL TRIP DELAY TRIP

LOAD SWING TEST

2011-03-22T23:36:00.000+02:00

YESTERDAY , We made a test at our power station ( 2*350 MW - Gas/Oil Fired ) , this test is LOAD SWING TEST.

This test is implies the load of the unit at 350MW , for a while . and collect our data from the unit especially the boiler and Turbine system. , Then Decrease the sharply to the lowest load at 250 MW .

Hold time at this minimum load and collect data , and identify the performance of BOILER MASTER and GOVERNOR behaviors, and judge on the performance for these equipment.

Repeat this swing two and three times about this range , and collect the data for the system.

What Happened with our unit ??

It is noted that when decreasing the load , the water heaters ( Medium Pressure Heaters ) are going out of service because of hi level , this is very dangerous for the behavior of steam unit.

because when water heaters , the cascaded drains causes to trip all the heaters from service , and suddenly the load may increase sharply , especially the high pressure heaters.

Pressure Measurement

2011-03-21T16:49:00.000+02:00

Pressure Measurement

-A major concern in process control applications is the measurement of fluid pressure. The term fluid means a substance that can flow; hence, the term applies to both liquids and gases. Both will occupy the container in which they are placed.

-The pressure is equal to the force applied to the walls divided by the area that is perpendicular to the force. For a liquid at rest, the pressure exerted by the fluid at any point is perpendicular to the boundary of the liquid. Pressure is defined as a force applied to, or distributed over, a surface area as:-

P=F/A

Gauge and Absolute Pressure

-Absolute pressure is the pressure measured above total vacuum or zero absolute, where zero absolute represents a total lack of pressure.

-Gauge pressure is the pressure measured above atmospheric or barometric pressure. It represents the positive difference between measured pressure and existing atmospheric pressure.

Pressure Gauges

-Pressure gauges are used for local indication and are the most common type of pressure-measurement instrument used in process industries. Pressure gauges consist of a dial or indicator and a pressure element. A pressure element converts pressure into a mechanical motion.

-Most mechanical pressure elements rely on the pressure that acts on a surface area inside the element to produce a force that causes a mechanical deflection. The common elements used are Bourdon tubes diaphragms and bellows elements.

-The following figure shows one of the most common and least expensive pressure gauges used in the process industries. This pressure gauge uses a “C” type Bourdon tube. In this device, a section of tubing that is closed at one end is partially flattened and coiled. When a pressure is applied to the open end, the tube uncoils. This movement provides a displacement that is proportional to the applied pressure. The tube is mechanically linked to a pointer on a pressure dial to give a calibrated reading.

- A Diaphragm is another device that is commonly used to convert pressure into a physical movement. A diaphragm is a flexible membrane. When two are fastened together they form a container called a capsule. In pressure- measuring instruments, the diaphragms are normally metallic. Pressure applied inside the diaphragm capsule causes it to expand and produce motion along its axis.

- A diaphragm acts like a spring and will extend or contract until a force is developed that balances the pressure difference force. The amount of movement depends on how much spring there is in the type of metal used.

- To amplify the motion that a diaphragm capsule produces, several capsules are connected end to end, as shown in the following figure. You can use diaphragm- type pressure gauges to measure gauge, absolute, or differential pressure.

- The Bellows pressure element is very similar to a diaphragm-type gauge in that it converts a pressure into a physical displacement. The difference is that typically the movement in a bellows is much more of a straight-line expansion. A typical bellows-type pressure gauge is manufactured by forming many accordion-like pleats into a cylindrical tube

-You can use the motion that the pressure input signal produces to position a pointer, recorder pen, or the wiper of a potentiometer.

BOILER STEAM PURITY

2011-03-20T19:42:00.000+02:00

BOILER STEAM PURITY

- It is important that the steam leaving the boiler drum should be free from impurities which could be deposited in the superheater or the turbine.

- Such impurities may arise from two distinct causes, one being the property which steam at high temperature and pressure has of dissolving significant amounts of certain substances, notably SILICA & CAUSTIC SODA.

- the other cause is more familiar, namely priming or carry-over, where there is incomplete separation of the two phases in the drum and droplets of boiler water are entrained in the steam leaving the drum.

- Impurities caused by steam solubility can only be controlled by careful regulation of the boiler water analysis, particularly with regard to alkalinity and silica, and it is accepted that if the silica in steam doesn’t exceed 0.02ppm , it will cause no trouble by deposition in the superheater or the turbine.

- The concentration of silica in the boiler water which is in equilibrium with 0.02ppm in the saturated steam varies with the operating pressure as shown in the following figure.

Feed Forward Control Loops

2011-02-09T00:33:00.000+02:00

Feed Forward Control Loops

-A feedback control loop is reactive in nature and represents a response to the effect of a load change or upset. A feedforward control loop, on the other hand, responds directly to load changes and thus provides improved control.

-In feedforward control, a sensor is used to detect process load changes or disturbances as they enter the system. A block diagram of a typical feed- forward control loop is shown in the following Figure. Sensors measure the values of the load variables, and a computer calculates the correct control signal for the existing load conditions and process set point.

-Feedforward control poses some significant problems. Its configuration assumes that the disturbances are known in advance, that they will have sensors associated with them, and that no important undetected disturbances will occur.

-Therefore, feedforward control is more complicated and more expensive, and it requires the operator to have a better understanding of the process than does a standard feedback control loop. So, feedforward control is generally reserved for well-understood and critical applications.

Ratio Control Loops

2011-02-09T00:32:00.000+02:00

Ratio Control Loops

- Based on hardware, ratio control is quite often confused with cascade control. The basic operation of ratio control, however, is quite different.

- Ratio control is often associated with process operations in which two or more streams must be mixed together continuously to maintain a steady composition in the resulting mixture. A practical way to do this is to use a conventional flow controller on one stream and to control the other stream with a ratio controller that maintains flow in some preset ratio or fraction to the primary stream flow. A preset ratio regulates the flow of the controlled variable. For example, if the ratio is 10 to 1, then for every gallon per minute of the uncontrolled variable that is flowing, ten gallons per minute of the controlled variable are allowed to flow.

- Use the signal from the uncontrolled flow transmitter, or wild flow, as the ratio input of the ratio controller. Multiply the value by an adjustable factor or ratio setting to determine the set point of the flow controller. The process variable to the controller is the flow of the controlled stream. The output from the ratio controller adjusts the control valve.

- You can use a ratio control loop with any combination of suitably related process variables, and the control action selected is normally proportional plus integral. The response to process upsets of a control loop with ratio control is the same as the response found in a single-feedback loop.

Cascade Control Loops

2011-02-06T15:58:00.000+02:00

Cascade Control Loops

-The general concept of cascade control is to place one feedback loop inside another. In effect, one takes the process being controlled and finds some intermediate variable within the process to use as the set point for the main loop.

-Cascade control exhibits its real value when a very slow process is being controlled. In such circumstances, errors can exist for a very long time, and when disturbances enter the process there may be a significant wait before any corrective action is initiated. Also, once corrective action is taken one may have to wait a long time for results. Cascade control allows the operator to find intermediate controlled variables and to take corrective action on disturbances more promptly. In general, cascade control offers significant advantages and is one of the most underutilized feedback control techniques.

-An important question you may confront when implementing cascade control is how to select the most advantageous secondary controlled variable. Quite often, the designer has a large number of choices. The overall strategy or goal should be to get as much of the process lag into the outer loop as possible while, at the same time, getting as many of the disturbances as possible to enter the inner loop.

-The next two figures illustrate two different cascade control arrangements for a furnace that is used to increase the temperature of a fluid that is passing through it. In both cases, the primary controlled variable is the same, but in each case a different intermediate controlled variable has been selected. The question then is which type of cascade control is best.

-To determine the best cascade control arrangement, you must identify the most likely disturbances to the system. It is helpful to make a list of these in order of increasing importance. Once this has been done, the designer can review the various cascade control options available and determine which one best meets the overall strategy outlined earlier: to make the inner loop as fast as possible while at the same time receiving the bulk of the important disturbances.

-If both controllers of a cascade control system are three-mode controllers, there are a total of six tuning adjustments. It is doubtful that such a system could ever be tuned effectively. Therefore, you should select with care the modes to be included in both the primary and secondary controllers of a cascade arrangement.

-For the secondary (inner or slave) controller, it is standard practice to include the proportional mode. There is little need to include the reset mode to eliminate offset because the set point for the inner controller will be reset continuously by the outer or master controller. For the outer loop, the controller should contain the proportional mode. If the loop is sufficiently important to merit cascade control then you should probably include reset to eliminate offset in the outer loop. You should undertake rate or derivative control in either loop only if it has a very large amount of lag.

-The tuning of cascade controllers is the same as the tuning of all feedback controllers, but the loop must be tuned from the inside out. The master controller should be put on manual (i.e., the loop broken), and then the inner loop can be tuned. Once the inner loop is properly tuned, the outer loop can be tuned. This allows the outer loop to “see” the tuned inner loop functioning as part of the total process or as the “all else” that is being controlled by the master controller. If you follow this general inside-first principle when tuning cascade controllers, you should encounter no special problems.

Sorry Message

2011-02-05T23:20:00.000+02:00

Hello everyone , I am so sorry for my absence from the blog and new posts. that is because the current accidents in Egypt.

So forgive my absence , and I will be more clever the next days , thank you.

DEAERATOR ( PART 2 )

2011-01-24T16:33:00.002+02:00

DEAERATOR ( PART 2 )

OPERATION PROCEDURES

** Pre-start checks and operations

(1) Vent valve shall be wide open.

(At low load start, it may be necessary to throttle valve slightly to increase the operating pressure, but vent valve not to be fully close in any case.)

(2) Steam, condensate or water and feed water suction valve shall be closed.

(3) All block valve of instrument is to be open.

(4) All control, alarm and switch is on proper function.

(5) Temperature pressure gauge and instrument shall be calibrated.

--------------------------------------------------------

** Initial state (STARTUP)

Before water and steam flow into the equipment, check below contents.

(1) Open the vent completely. In case of very low pressure operation vent throttle valve is slightly closed to maintain the pressure of tank But don't close vent completely at any case.

(2) Check the safety valve.

(3) Check to see that all local instruments are operation and indicating correctly.

(4) Check the degree accuracy of temperature gage, pressure gage and all measurement instruments.

--------------------------------------------------------

**Normal Start-up

(1) Open slowly the condensate or water supply valve.

(2) Fill-up water to storage tank up to 60-70% level.

(3) Stop the condensate or water supply.

(4) Open slowly the sparger steam supply valve to heat the water within storage tank.

(5) After water temperature is increased to proper temperature, close the sparger steam supply valve and stop and valve operation until vapor fume disappear from vent pipe.

(6) Above step 1- 5 may be omitted if feed water can be recirculated through minimum flow line.

(7) Open slowly the condensate or water supply valve to 15-30% load flow rate.

(8) Open step by step the main steam supply valve until operating pressure will be rate.

(Caution: Do not fill the deaerator with steam first and then supply the condensate or water. This operation will cause the damage of internals and the partial collapse of shell.)

(9) Adjust the steam supply valve to Auto mode.

(10) Open minimum flow valve of feed water pump discharge.

(11) Start feed water pump.

(12) Recirculate feed water through minimum flow line until feed water temperature is increased to near operating temperature.

(Step 10 & 12 can be omitted in case that water is heated by steam of step 1-5)

(13) During recirculation operation, drain valve shall be open to protect the increase of water level and the suspension of eater supply.

(14) Adjust the water supply valve to Auto mode when water level is near normal level.

(15) Open slowly the discharge valve of feed water pump and supply the feed water to boiler or its related equipment.

(16) Load of deaerator will be increased as the discharge valve of feed water pump is more open.

(17) After normal operation is maintained with proper load than 30minutes, vent valve can be closed to one or two turns.

(Normally it indicates sufficient venting if steam fume rises approximately one meter above the end of vent pipe.)

(18) Now deaerator is ready for continuous operation.

--------------------------------------------------------

**Normal Shutdown

(1) Close slowly the boiler feed water pump discharge valve.

(2) Close the condensate or water supply valve.

(3) Isolate the main steam supply valve.

(4) Open the atmospheric vent valve fully to expel any remaining steam in tank.

(5) If it should be necessary to drain the tank. Start drain after wide opening of vent valve.

(Draining without sufficient venting will cause the undue stress on shell by vacuum.)

--------------------------------------------------------

**Emergency Shutdown

(1) Isolate the main steam supply valve.

(2) Close the condensate or water supply valve.

(3) Close the boiler feed water pump discharge valve.

(4) Open the atmospheric vent valve fully to expel any remaining steam in tank.

(5) If it should be necessary to drain the tank.

Start drain after wide opening of vent valve.

(Draining without sufficient venting will cause the undue stress on shell by vacuum.)

“This article for, Dearator & Storage tank O&M manuals , SPC. LTD “

DEAERATOR ( PART 1 )

2011-01-23T20:52:00.001+02:00

DEAERATOR ( PART 1 )

FUNCTION

-The function of the deaerating heater is to remove dissolved non-condensable gases and to heat boiler feed water. A deaerating heater consists of a pressure vessel in which water and steam are mixed in a controlled manner. When this occurs, water temperature rises, and all non-condensable dissolved gases are liberated and removed and the effluent water may be considered corrosion free from an oxygen or carbon dioxide standpoint. Free air or other non-condensable gases should be vented prior permitting the fluid to enter the deaerator.

-A deaerating heater is the watch dog of boiler plant as it protects the feed pumps, piping, boiler and any other piece of equipment that is in the boiler feed and return cycle from the effects of corrosive gases, i.e. oxygen and carbon dioxide, to a level where they are no longer a corrosion factor.

Principles of Deaeration

-There is physical law which states that the solubility of any gas in a liquid is directly proportional to the partial pressure of the gas above the liquid surface. Another law states, the solubility of a gas in a liquid decreases with an increase in temperature of the liquid. Experience has shown that more rapid and more complete removal of non-condensable gases from a liquid is obtained when the liquid is vigorously boiled or scrubbed by condensable or carrier gas bubbles.

-Therefore, essentially the deaerating heater must first heat the feed water to as high a temperature as possible, i.e. to the temperature corresponding to the steam pressure, It must vigorously boil and scrub the heater water with fresh steam, which can carry to the liquid surface any traces of oxygen and carbon dioxide.

-The partial pressure of the oxygen and carbon dioxide in the steam atmosphere must be maintained as low as possible, particularly at the point where the deaerated water separates from the steam. Non-condensable gases must be continually withdrawn from the heater at the rate at which they are being liberated.

Deaerator Heater

-A deaerating heater utilizes steam by spraying the incoming water into an atmosphere of steam in the preheated section (first stage). It then mixes this water with fresh incoming steam in the deaerator section (second stage).

-In the first stage the water is heated to within 1.1C of steam saturation temperature and virtually all of the oxygen and free carbon dioxide are removed. This is accomplished by spraying the water through self-adjusting spray valves which are designed to produce a uniform spray film under all conditions of load and consequently a constant temperature and uniform gas removal is obtained at this point.

- From the first stage the preheated water, containing minute traces of dissolved gases, flows into the second stage. This section consists of either a distributor or several assemblies of trays,

Here the water is in intimate contact with an excess of fresh gas-free steam. The steam passes into this stage and it is mixed with the preheated water. Deaeration is accomplished at all rates of flow if conditions are maintained in accordance whit design criteria. Very little steam is condensed here as the incoming water has a high temperature caused by the preheating. The steam then rises to the first stage and carries the small traces of residual gases. In the first stage most of the steam is condensed and remaining gases pass to the vent where the non condensable gases flow to the atmosphere, A very small amount of steam is also discharged to the atmosphere which assures that the deaerating heater is adequately vented at all times.

- The water which leaves the second stage falls to the storage tanks where it is stored for use. At this time the water is completely deaerated and is heated to the steam saturation temperature corresponding to the pressure within the vessel.

VENT

-Efficient removal of the non-condensable gases from the deaerating heater requires that the vent valve be opened sufficiently to allow complete discharge of the gases passed to the vent condenser outlet pipe. The maximum concentration of the non-condensable gases such as oxygen, carbon dioxide passing out the vent depends on the degree of condensation produced by the steam and gas mixture passing through and around the spray created by the special spray valve.

“This article for, Dearator & Storage tank O&M manuals , SPC. LTD “

On/Off Control

2011-01-22T20:02:00.000+02:00

On/Off Control

- Suppose a process operator has the task of holding the temperature, T, near the desired temperature, Td, while making sure the tank doesn't overflow or the level get too low. The question is how the operator would cope with this task over a period of time. He or she would manually adjust the hot water inlet valve (HV-1) to maintain the temperature and occasionally adjust the outlet valve (HV-2) to maintain the correct level in the tank.

- The operator would face several problems, however. Both indicators would have to be within the operator's view, and the manual valves would have to be close to the operator and easy to adjust.

- To make the operator's work easier, suppose we installed electrically operated solenoid valves in place of the manual valves, as shown in the next figure. We can also install two hand switches (HS-1 and HS-2) so the solenoid valves can be operated from a common location. The valves can assume two states, either fully open (on) or fully closed (off). This type of control is called two-position or on/off control.

- Assume for the moment that the level is holding steady and that the main concern is controlling temperature. The operator has been told to keep the temperature of the fluid in the tank at 100°F. He compares the reading of the temperature indicator with the selected set point of 100°F. The operator closes the hot water valve when the temperature of the fluid in the tank rises above the set point. Because of process dead time and lags the temperature will continue to rise before reversing and moving toward the set point. When the temperature falls below 100°F, the operator opens the hot water valve. Again, dead time and lags in the process create a delay before the temperature begins to rise. As it crosses the set point, the operator again shuts off the hot water, and the cycle repeats.

- This cycling is normal for a control system that uses on/off control. This limitation exists because it’s impossible for the operator to control the process exactly with only two options.

This on/off type of control can be expressed mathematically as follows:

e = PV – SP

In the on/off control mode, the valve is open valve when the error (e) is positive (+), and the valve is closed when e is negative (–).

General Requirements of a Control System

2011-01-16T23:39:00.000+02:00

General Requirements of a Control System

The primary requirement of a control system is that it be reasonably stable. In other words, its speed of response must be fairly fast, and this response must show reasonable damping. A control system must also be able to reduce the system error to zero or to a value near zero.

SYSTEM ERROR

The system error is the difference between the value of the controlled variable set point and the value of the process variable maintained by the system. The system error is expressed in equation form by the following:

e(t) = PV(t) – SP(t)

where:-

e(t) = system error as a function of time (t)

PV(t) = the process variable as a function of time

SP(t) = is the set point as a function of time

SYSTEM RESPONSE

-The main purpose of a control loop is to maintain some dynamic process variable (pressure, flow, temperature, level, etc.) at a prescribed operating point or set point. System response is the ability of a control loop to recover from a disturbance that causes a change in the controlled process variable.

-There are two general types of good response: underdamped (cyclic response) and damped. (Figure 1) shows an underdamped or cyclic response of a system in which the process variable oscillates around the set point after a process disturbance. The wavy response line shown in the figure represents an acceptable response if the process disturbance or change in set point was large, but it would not be an acceptable response if the change from the set point was small.

- (Figure 2) shows a damped response where the control system is able to bring the process variable back to the operating point with no oscillations.

Control Loop Design Criteria

- Many criteria are employed to evaluate the process control’s loop response to an input change. The most common of these include settling time, maximum error, offset error, and error area.

- When there is a process disturbance or a change in set point, the settling time is defined as the time the process control loop needs to bring the process variable back to within an allowable error. The maximum error is simply the maximum allowable deviation of the dynamic variable. Most control loops have certain inherent linear and nonlinear qualities that prevent the system from returning the process variable to the set point after a system change. This condition is generally called “offset error” . The error area is defined as the area between the response curve and the set point line as shown by the shaded area in (Figure 3).

- These four evaluation criteria are general measures of control loop behavior that are used to determine the adequacy of the loop’s ability to perform some desired function.

REFERENCES , ( THOMAS HUGHES , MEASUREMENT & CONTROL BASICS )

Elements of a Process Control System

2011-01-15T16:16:00.001+02:00

Elements of a Process Control System

INTRODUCTION

-The term automatic process control came into wide use when people learned to adapt automatic regulatory procedures to manufacture products or process material more efficiently. Such procedures are called automatic because no human (manual) intervention is required to regulate them.

-All process systems consist of three main factors or terms: the manipulated variables, disturbances, and the controlled variables. Typical manipulated variables are valve position, motor speed, damper position, or blade pitch. The controlled variables are those conditions, such as temperature, level, position, pressure, pH, density, moisture content, weight, and speed, that must be maintained at some desired value. For each controlled variable there is an associated manipulated variable. The control system must adjust the manipulated variables so the desired value or “set point” of the controlled variable is maintained despite any disturbances.

Elements of a Process Control System

-The following figure illustrates the essential elements of a process control system. In the system shown, a level transmitter (LT), a level controller (LC), and a control valve (LV) are used to control the liquid level in a process tank. The purpose of this control system is to maintain the liquid level at some prescribed height (H) above the bottom of the tank. It is assumed that the rate of flow into the tank is random. The level transmitter is a device that measures the fluid level in the tank and converts it into a useful measurement signal, which is sent to a level controller. The level controller evaluates the measurement, compares it with a desired set point (SP), and produces a series of corrective actions that are sent to the control valve. The valve controls the flow of fluid in the outlet pipe to maintain a level in the tank.

-Thus, a process control system consists of four essential elements: process, measurement, evaluation, and control. A block diagram of these elements is shown in the following figure. The diagram also shows the disturbances that enter or affect the process. If there were no upsets to a process, there would be no need for the control system. The figure also shows the input and output of the process and the set point used for control.

PROCESS

In general, a process consists of an assembly of equipment and material that is related to some manufacturing operation or sequence. In the example presented, the process whose liquid level is placed under control includes such components as a tank, the liquid in the tank, and the flow of liquid into and out of the tank, and the inlet and outlet piping. Any given process can involve many dynamic variables, and it may be desirable to control all of them. In most cases, however, controlling only one variable will be sufficient to control the process to within acceptable limits. One occasionally encounters a multivariable process in which many variables, some interrelated, require regulation.

MEASUREMENT

- To control a dynamic variable in a process, you must have information about the entity or variable itself. This information is obtained by measuring the variable.

- Measurement refers to the conversion of the process variable into an analog or digital signal that can be used by the control system. The device that performs the initial measurement is called a sensor or instrument. Typical measurements are pressure, level, temperature, flow, position, and speed. The result of any measurement is the conversion of a dynamic variable into some proportional information that is required by the other elements in the process control loop or sequence.

EVALUATION

- In the evaluation step of the process control sequence, the measurement value is examined, compared with the desired value or set point, and the amount of corrective action needed to maintain proper control is determined. A device called a controller performs this evaluation. The controller can be a pneumatic, electronic, or mechanical device mounted in a control panel or on the process equipment. It can also be part of a computer control system, in which case the control function is performed by software.

CONTROL

The control element in a control loop is the device that exerts a direct influence on the process or manufacturing sequence. This final control element accepts an input from the controller and transforms it into some proportional operation that is performed on the process. In most cases, this final control element will be a control valve that adjusts the flow of fluid in a process. Devices such as electrical motors, pumps, and dampers are also used as control elements.

------------------------------------------------------------------------------

Process and Instrumentation Drawings

-In standard P&IDs, the process flow lines, such as process fluid and steam, are indicated with heavier solid lines than the lines that are used to represent the instrument. The instrument signal lines use special markings to indicate whether the signal is pneumatic, electric, hydraulic, and so on. two types of instrument signals are used: double cross-hatched lines denote the pneumatic signals to the steam control valve and the process outlet flow control valve, and a dashed line is used for the electrical control lines between various instruments. In process control applications, pneumatic signals are almost always 3 to 15 psig (i.e., pounds per square inch, gauge pressure), and the electric signals are normally 4 to 20 mA (milliamperes) DC (direct current).

- A balloon symbol with an enclosed letter and number code is used to represent the instrumentation associated with the process control loop. This letter and number combination is called an instrument identification or instrument tag number.

- The first letter of the tag number is normally chosen so that it indicates the measured variable of the control loop. In the sample P&ID shown, T is the first letter in the tag number that is used for the instruments in the temperature control loop. The succeeding letters are used to represent readout or passive function or an output function, or the letter can be used as a modifier. For example, the balloon in Figure marked TE represents a temperature element and that marked TIC is a temperature-indicating controller. The line across the center of the TIC balloon symbol indicates that the controller is mounted on the front of a main control panel. No line indicates a field-mounted instrument, and two lines means that the instrument is mounted in a local or field-mounted panel. Dashed lines indicate that the instrument is mounted inside the panel.

- Normally, sequences of three- or four-digit numbers are used to identify each loop. In our process example, we used loop numbers 100 and 101. Smaller processes use three-digit loop numbers; larger processes or complex manufacturing plants may require four or more digits to identify all the control loops.

- Special marks or graphics are used to represent process equipment and instruments. For example, in our P&ID example in Figure two parallel lines represent the orifice plate that is used to detect the discharge flow from the process heater.