Mechanical Engineer questions

How to find out length of holding tube?

2015-09-13T01:18:00.004-07:00

Pasteurization requires that the milk is in custody for a specified time. The appropriate length for the holding tube can be calculated by the flow rate (Q) of milk and the diameter (D) of the holding tube using the formula:

Length of holding tube, L = Q x holding time/A x η

A = Area = pi/4 x sqr D

Where an efficiency factor, η, of 0.85 is used in the model.

How to find out Power input into the motor compressor?

2015-09-13T01:13:00.003-07:00

Power input into the motor compressor is obtained by the following formula:

Pcomp =M x (h2 - h1)

Pcomp = Power input into the motor compressor

M = The mass flow rate of the refrigerant

h1 = Enthalpy at inlet to the process,

h2 = Enthalpy at outlet to the process,

How to find out work done by compressor?

2015-09-13T01:12:00.000-07:00

The work done to compress the refrigerant vapour is obtained by applying to the compression process:

Wcomp= h2 - h1

Wcomp = Work done during the process

h1 = Enthalpy at inlet to the process,

h2 = Enthalpy at outlet to the process,

This is the work done by compressor per kg of the refrigerant.

What is emergency procedure for Ammonia?

2015-09-13T01:08:00.001-07:00

Emergency procedure is providing to workers with detailed directions in case of an emergency. The written emergency procedures must consist of specific details pertaining to the following:

Location of the system fire valve and fire hydrant system

Emergency personal protective equipment and its location

Evacuation procedure and a check system to ensure that all personnel are evacuated

Notifying fire department and other emergency response units

Emergency lighting

Notifying workers of the emergency location

Controlling materials that may become dangerous during the emergency

Search and rescue

Notifying adjacent plant of the emergency

Why Ammonia is used as a refrigerant (R717)?

2015-09-13T01:06:00.002-07:00

Pure ammonia comes in two forms: gas and liquid.

Ammonia is a naturally occurring substance that has been successfully used as a refrigerant in industrial refrigeration plants for over 130 years. Liquid ammonia is a clear fluid that evaporates quickly at room temperature. Liquid ammonia also has a high compression ratio. The ratio of liquid to gas is 1 to 800, which means that 1 litre of liquid ammonia

expands to form 800 litres of gas. It is a colorless gas that liquefies under pressure and has a pungent odor. It is also much lighter than air. If ammonia gas escapes from a refrigeration system or a storage container, it tends to collect in high areas

or ceilings. Ammonia has negligible environmental impact with zero ozone depleting potential (ODP= 0), no direct global warming potential (GWP = 0) and low operational emissions due to high efficiencies.

Ammonia carries a B2 safety classification, meaning that it has a medium flammability and high toxicity risk. A major ammonia spill is potentially disastrous because liquid ammonia evaporates quickly when exposed to air and creates an explosive fire hazard at high concentrations. The ignition energy required to ignite Ammonia is 50 times higher than that of natural gas and will not burn without a supporting flame. Due to the high affinity of ammonia for atmospheric humidity it is otherwise rated as ‘hardly flammable’. Ammonia can cause chemical burns on all body surfaces. Ammonia vapour reacts with moisture in the air to form moist ammonia, which attacks copper, zinc, tin, cadmium, and most of their alloys. Ammonia is toxic to the skin and mucus membranes, but has a characteristic, sharp smell, which gives a warning below concentrations of 300 ppm. In amounts of 300 to 400 ppm, prolonged exposure will become unpleasant, and in amounts over 700ppm it can cause burns and serious damage to eyes. In amounts of 5,000 ppm or above, exposure can be lethal to humans within five minutes. Ammonia gas is very irritating to the eyes, nose, and respiratory system, which makes it easy to detect low concentrations in the air.

COP of major compressor types

2015-09-13T01:04:00.002-07:00

The average COP of the major compressor available in the market is given below:

Compressor Type	Capacity	COP-Avg. rating
Open Reciprocating type	5 kw to 225 kw	2.9
Screw type	80 kw to 2000 kw	3.25
Hermetic Scroll type	3 kw to 50 kw	3.0
Hermetic reciprocating type	1 kw to 50 kw	2.0

How to find out cooling capacity of bulk milk cooler?

2015-09-13T01:02:00.000-07:00

Cooling capacity of bulk-milk cooler find out by the following formula:

Q=V x Rho x Cp x (T1 – T2)/t

Where;

Q - Cooling capacity of bulk-milk cooler, kcal/h

V - milk volume inside bulk-milk cooler, cum

Rho - milk density, kg/cum

Cp - milk specific heat, kcal/kg.C

T1 - milk temperature at any loading time, in degree C

T2 - milk temperature at the time which gives the temperature of safe storage, in degree C and

t - cooling hours, h.

Refrigeration load calculation for cold store

2015-09-09T09:54:00.000-07:00

Example-
One cold store dimensions is -10 m x 10 m × 5 m = 500 Sq mtr, External store surface area is 385.75 Sq mtr, Storage temperature is minus 20 Degree C, maximum ambient temperature is 35 Degree C. Insulation thickness is 250 mm. Consider two men are working inside the cold room. Calculate total calculated refrigeration load.

Calculation:
(1) Insulation heat leak through walls, floor and roof
Conductivity of polystyrene 0.033 kcal/h m C
Surface area of store = 385.75 Sq mtr
Temperature difference between ambient and store 35 Deg C and -20 Deg C = 55 Deg C
Thickness of polystyrene = 250 mm
Heat leak in Kcal/hr = 385.75 x 55 x 0.033/ 0.250 = 2800 kcal/hr

(2) Product load
5.5 kcal/kg for fresh fish load at an average temperature of -20 Deg C
Fish loaded per day 25 000 kg
Product load = 25000 x 5.5 / 24 = 5729 kcal/hr

(3) Fan load
2 x 200 W = 400 kcal/hr

(4) Air changes
Average of 2.5 air changes in 24 h
Store volume = 500 Cum
Heat gain (35 C and 60% R.H. air) 40 kcal/Cum
Air change heat gain = 500 x 2.5 x 40/ 24 = 2083 kcal/h

(5) Lights during working day
Light load during working day- 1000 W = 860 kcal/h

(6) Men working
1 man working at -20C gives off 370 kcal/hr
2 men working is equivalent = 740 kcal/hr

(7) Defrost heat
1 defrost of 8000 W for 1 h (recovered over 6 h) = 1142 kcal/hr

Total calculated refrigeration load

= sum of Items (1+2+3+4+5+6+7)

= (2800+5729+400+2083+860+740+1142)=13754 kcal/hr

What is water boiler? Hindi meaning of water boiler.

2015-09-05T20:47:00.000-07:00

A container in which water is boiled or a sealed vessel where water is converted to steam. Hindi meaning Bhatti.

Water boiler is a closed vessel or arrangement of enclosed tubes in which water is heated and circulated, either as hot water or as steam, for heating or to supply steam to drive an engine or turbine.

Industrial Boilers must be constructed in accordance with the ASME Boiler and Pressure Vessel Code:

Section I is for the high-pressure boilers. Additionally, ASME B31.1 is the required code for power piping installation for high-pressure boilers.

Section IV is for low-pressure steam and hot water heating, and hot water supply boilers.

Note:-ASME means American Society of Mechanical Engineers.

What is Anaerobic Treatment?

2015-03-20T09:30:00.005-07:00

Process Principle:

Microbial reactions take place in the absence of molecular/ free oxygen, Reactions products are carbon dioxide, methane and excess biomass.

Applications:

Net Sludge Yield: Relatively low
Waste water with medium to high organic impurities (COD > 1000 ppm) and easily biodegradable waste water e.g. food and beverage waste water rich in starch/sugar/alcohol
Post Treatment : Invariably followed by aerobic treatment

Example Technologies:

Continuously stirred tank reactor/digester, Up flow Anaerobic sludge Blanket, Ultra High Rate Fluidized Bed reactors etc.

What is Aerobic treatment?

2015-03-20T09:27:00.001-07:00

Process Principle: Microbial reactions take place in the presence of molecular/ free oxygen, Reactions products are carbon dioxide, water and excess biomass.

Applications:

Net Sludge Yield: Relatively high
Waste water with low to medium organic impurities (COD < 1000 ppm) and for waste water that are difficult to biodegrade e.g. industry waste water, municipal sewage, refinery waste water etc.
Post Treatment : Typically direct discharge or filtration

Example Technologies: Activated Sludge e.g. Extended Aeration, MBR, Fixed Film Processes e.g. Trickling Filter/Bio tower

What do you mean by Post Welding Heat Treatment?

2015-03-11T18:23:00.003-07:00

As per ASME code, for pressure vessels and piping, if the thickness of the parts being welded exceeds a specified value, PWHT cannot be avoided. Charpy test properties of the material and the minimum service temperature required should determine this. In general, US & UK codes limit the thickness of C / C-Mn Steels to 32 mm, beyond which PWHT cannot be avoided whether it be pressure vessel, piping or storage tank.

As per ASME code VIII Div.1 UCS-79 as it stated that the required material will need heat treatment when the resulting fiber elongation is more than 5% from the as rolled condition.

When comes to the low alloy steels, ASME B31.3 has set a limit thickness of 19 mm, above which PWHT cannot be avoided.

Refer ASME Section VIII - Div. 1, UCS-56 for Post Weld Heat Treatment requirements detailed. Also refer UCS-68 where it is mentioned that PWHT cannot be avoided when minimum design metal temperature is colder than -55 degree F.

What is difference between tube and a pipe?

2015-03-11T18:22:00.002-07:00

Tube is specified by its outside diameter. Tubing is normally "cold drawn" seamless with the O.D. the same as the size designation (i.e.: 1" (25mm) inch tubing has an O. D. of 1 inch (25mm)). Tube is OD x wall unless DOM (Drawn over Mandrel) which is technically OD & ID, DOM in SSID (Special smooth ID) is ID and Wall.

Pipe is specified by its nominal inside diameter and Pipe has a wider variance in wall thickness. Pipe is to the pipe charts as per SB's comments NPS and schedule. Pipe can be made off the "pipe charts" but the reality is the more generous tolerances vs tube and uses the NPS for the first designation. It gets a few wrinkles in welded vs seamless for tolerances. 1-1/2" SCH40 Pipe, the 1-1/2" is the inside dimension but the actual inside dimension might be 1-5/8" instead.

Pipe is made seamless or rolled and welded (sometimes referred to as ERW pipe) . The pipe sizes up to 12' has a nominal size which is close but not the same as the actual O. D. Pipe in sizes 14" and larger have an O. D. the same as the nominal size. Both the seamless pipe and the ERW pipe comes in different wall thicknesses depending on the pressure/temp criteria. Pipe normally comes in lengths called "single random" (20'+/-) or "double random" (40'+/-). ERW pipe is also furnished in very long lengths, spooled on large coils for pipeline needs. Pipe 2" (50mm) and smaller is normally joined by threading or socket weld. Pipe 3" (80mm) and larger is normally joined using butt weld pipe fittings.

In addition tube is usually flexible or bendable, whereas pipe is usually rigid.

Can it is possible to weld Cast Steel Material?

2015-03-11T18:21:00.001-07:00

Yes, it is true and yes this material could be welded. It can be welding as per following standard.

ASTM 216/216M -"Specification for steel castings, carbon, Suitable for fusion welding for High temperature service"

Bad reputation of castings are due to,

1. Most of the casting standards are lenient in composition, heat treatment and inspection.

2. The control exercised by the foundries, due to lack of knowledge or economics, in both composition and heat treatments. Due to this the quality factor on general castings (derating factor) is specified in the ASME/VIII which with the proper NDE and testing could be again made 100%.

However since these additional NDE requirements are not mandatory (supplementary requirements) would be enforced only if specified in specific purchase orders. These (Radiography, LPI, MPI etc) ensures the structural rigidity.

It is possible for welding but definitely not recommended for strength concerns. Cast Iron/steel are brittle usually goes with high carbon content. Localized heat in the weld will result to residual stress on the spot and surrounding material, thus consequent to breaking and material fatigue when subject to external forces or pressure.

Why diesel engines have heavier flywheel than petrol engines?

2015-03-11T09:39:00.001-07:00

Diesel engine has higher compression ratio, hence it is subjected to very high cylinder pressures which may cause higher vibrations. In order to counter the vibrations the diesel engines have heavier flywheel than petrol engines.

Diesel engine is bigger causing it absorbing vibration, a lot of compression going on in this cycle. The principle of a flywheel is to reduce variations in the rotational speed of the shaft to which they are attached. As such they dampen an impulsive drive or load.

All internal combustion engines have speed variations and offer an impulsive drive due to their cyclic processes of induction, compression, combustion and exhaust. The extent of the speed variations is dependent on the number of cylinders and the engine configuration. A single cylinder engine exhibits the most significant difference. The momentum (stored energy) of the flywheel reduces these variations by accepting and discharging energy as required.

Diesel engines due to their higher compression ratios are more subject to crankshaft speed variation and hence require heavier (higher moment of inertia) flywheels.

What is Difference between Nozzle & Diffuser?

2015-03-11T09:37:00.001-07:00

Nozzle: Reduction in pressure and increase in velocity of fluid

Diffuser: Reduction in velocity and Increase in pressure of fluid

A nozzle accelerates the flow of a gas or fluid with a resulting decrease in the pressure while a diffuser does pretty much the opposite; it slows down the flow of the gas or fluid and has a resulting increase in the pressure. The mass flow, for a system that has both nozzle and diffuser, remains the same but there is a decrease (nozzle) or increase (diffuser) in the cross sectional area of the boundary of the system.

In Incomprehensible Flow.

In Nozzle Static Pressure is decrease and Velocity is increased.

In Diffuser Static Pressure is increased and velocity is decreased.

Nozzle in compressible supersonic Nozzle

Nozzle has generally three sections.

1. Convergent..2. Divergent. 3. Throat.

In subsonic flow Mach No<1 : Static Pressure is decreased velocity increase

In throat Mach No=1 Flow is Sonic.

In divergent section Velocity is further increased to supersonic Mach No>1 unlike in incomprehensible flow.

Nozzle speed up or increase the flow of substance (gas or liquid); while diffuser reduces the flow of material.

What is the full form of DTSI? How does this technology work?

2015-03-11T09:35:00.003-07:00

The full form of DTSI is "Digital Twin Spark Ignition."

Two sparks, offset by a time interval, is used to ignite the fuel air mixture. The initial spark burns most of the mixture while a second spark is used to burn the rest. This increases fuel economy as less unburnt fuel escapes through the exhaust port. It also provides increased power.

A knock occurs when the fuel is ignited while the piston is undergoing the compression stroke not when it is going back. When the second spark fires, the amount of charge in the cylinder is considerably less and therefore the pressure spike created by it does not produce any jerking effect.

DTSI system has been the driving technology in the new generation motor cycles to be more fuel economical & to meet the emission standards. We are all aware of Internal Combustion engine with single spark-plug system. DTSI engines use two spark plug one at the top of the combustion chamber & the other somewhere in-between the TDC & BDC of the piston. The piston position is sensed digitally -the top spark plug is activated & as the piston moves down with the power stroke the second spark plug is activated.

With the first spark the combustion mixture burns & produces the required force but the flame propagation dies of early without the whole air+fuel mixture being burnt completely. Activating the second spark plug digitally helps in burning the complete A/F mixture resulting in fuel economy & better emission control.

What is difference between design and drafting?

2015-03-11T09:33:00.006-07:00

Design – It is conceptualization of an idea based on theory and actual practice or experience.

Drafting – It is putting the conceptualized idea in paper which would be the basis or reference in its implementation.

Design - It is the complete process of calculations and drawing.

Drafting - It means drawing. It is the part of design.

Design - It is knowledge base concept

Drafting – It is art.

Design - It is a raw idea conceptualized through gathering of facts, ideas and research; a product of an initial study.

Drafting – It is when putting such design into its detailed drawing/plan, complete with dimensions ready for implementation.

A designer is someone with subject matter knowledge and is experienced enough to make decisions influencing a design.

A draftsman delineates a design without influencing that design.

Drafting is the visual representation of a Design.

Difference can be explained by just a simple example..

A rivet is required to join two plates for a particular load. so, now engineer will decide the diameter and material for the rivet, this is design....but a draftsman will draw this rivet for manufacturing.

Design is more of a process to create or evolve an idea to resolve a problem which involve - engineering principles or fundamentals, innovation, aesthetics, ergonomics, market research and many other things that may come depending on the domain.

It is a technique to present your ideas - it be just 2D Drafting or 3D drafting or simple hand sketching or a digital sketching. All these are just drafting techniques.

Design includes sizing of an equipment, identifying the important components which form a part of the same, the material of construction (MOC) or the mechanical properties like tensile strength, hardness etc, wall thicknesses, need for any protective coating, type of auxiliary equipment needed, drive equipment ratings like motor kW, Gear Box ratings, size of shafting, pulleys, belts, bearings, protective equipment & instrumentation etc. The Designer may utilize necessary Programs for reducing the drudgery of manual calculations or avoid any mistakes which are likely while using Excel sheets.

After the calculations are done, components & aux equipment are decided & sizes are fixed by the Designer, the draftsman draws these details with CAD (earlier it was done manually), so that the Designer's ideas are visually translated on the drawing The drawing generally shows Part Nos., Bill of Material, quantities, weights, MOC, welding / bolting / flange details etc so that any Engineer can interpret how the equipment is to be manufactured. The drawing also shows the mounting details of the equipment & the drive for the knowledge of the Civil Engineer drive details for the Electrical Engineer & Instrumentation mountings for the knowledge of C & I Engineer.

How to calculate the ID FAN & FD fan Capacity?

2015-03-08T09:58:00.003-07:00

For the FD, calculate your boiler´s total air requirements. The total air flow is the sum of theoretical air plus excess air. Once you have the air in mass units ( kg/s , lb/h ) convert it to volume units ( multiply by the specific volume of air at a standard temperature, for example 20 deg C and correct for the ambient temperature and altitude of your locality ).

For the fan discharge pressure, you will need to add all of the pressure drops of your boiler system ( air pressure drop in air preheater and, in the air ductwork, in the burner wind box and register. Flue gas pressure drops in the radiant superheater panels, in the reheater, in the convective superheater panels, in the boiler bank, in the economizer banks, in the gas side of your air preheater and, in the scrubbers, cyclone dust collectors and bag or electrostatic collectors)

If your boiler system is of the balanced draught type, the FD handles air pressure drops and the ID fan flue gas pressure drops and the flue gas flow converted to units of volume flow. Remember to correct the flue gas pressure drops for altitude.

If you have a package boiler or a field erected oil or gas fired unit, the FD fan handles ALL of the pressure drops.

Pressure drops have to be corrected for altitude and the FD fan vendor can select the fan for a design air temperature greater than the ambient temperature provided for the location of your boiler.

Margins are added on the total volumetric air, flue gas volumetric flows and on pressure drops, since wear and time reduce fan capacity. For industrial oil and gas fired boilers 15% on flow and 30% on pressure drops are common.

How to calculate the vessel component thickness at hydro test pressure?

2015-03-08T09:56:00.004-07:00

The ASME Code Section VIII, Division 1, does not have mandatory rules for the calculations of vessel component thickness for hydrostatic test condition. The requirement for such calculations comes from the customer in its vessel specification. As the test is for short duration at ambient temperature, the customer specification often requires that no components of the vessel to be subjected to above the 90% of the material yield strength during the test.

The pressure due to liquid head is always added to the design internal pressure and the total pressure is used to calculate the thickness in the UG-27 (shell) and UG-32 (heads). The Code allowable stress value at the design temperature is used in the formula. The operating liquid may not be always full of the vessel. For example, consider a tall distillation tower, where liquid level of sp. gr. greater than water is at the maximum just below the lowest down comer, which could be only one-sixth or less of the tower height.

In the hydro test, the vessel is filled with water (sp. gr. 1.0) at room temperature. Using the allowable stress at the design temperature would be too conservative and design will be uneconomical. The test condition is very short and the allowable stress used in this case is from 0.8Sy to 0.9Sy or as per the client specification. The ASME Code Section VIII, Div. 1, does not have a mandatory allowable stress value in this case.

A typical calculation procedure is to design the vessel for its specified design conditions (UG-22). After establishing component final thicknesses and the hydrostatic test pressure (UG-99), compute the stress level in each component for the test pressure plus the applicable hydrostatic head using the Code respective formula with un-corroded (new) thickness. Compare the computed stress value with the 90% of the yield strength (or as specified by the customer). Increase the thickness of a component if necessary. If you are using a computer program like COMPRESS or PV Elite, option is provided for such calculations.

What is spanner size for Heavy Hex Stud and Nuts?

2015-03-08T09:54:00.001-07:00

There are a lot of tables, but you can use formula of nominal stud diameter x 1.5 + 1/8 inch in most cases, for example a 1-1/2" stud uses a 2-3/8" wrench size.

What is different between class-(#) & rating?

2015-03-08T09:52:00.003-07:00

ASME B16.5 or ASME B16.47 contain definitions of class, pressure rating,, and pressure-temperature rating.

"Pressure Rating Designation

Class, followed by a dimensionless number, is the designation for pressure-temperature ratings as follows:

Class 150, Class 300, Class 600, Class 900 etc..

"Pressure-Temperature Ratings

Pressure-temperature ratings are the maximum allowable working gage pressure...at the temperature shown in Tables...for the applicable material and class designation..."

Rating is the qualifying parameter.

Class is a designation or name for rating.

The pressure classes were originally designated as “pound”, that is, Class 150 = 150 pound flange. The relationship between pressure class and rating for flanges is a complicated one and is defined in ASME B16.5. For most carbon steels, the rating of a Class 150 flange is 285 psi at 100 F. Similarly, the rating of the Class 300 flange is 740 psi. For threaded and socket welding fittings, the relationship between pressure class and rating is found in ASME B16.11.

it is important to recognize that a Class 150 (150-lb) flange is not rated for 150 psig at the same temperature that a Class 300 (300-lb) flange is rated for 300 psig, etc. That is why it is necessary to look at the Pressure-Temperature tables. Do not make an assumption about what the flange class will be without checking the tables. Note that the ratings also change when the flange material changes.

For example, if you want a standard carbon steel A-105 flange for a design pressure of 100 psig and a design temperature of 400 deg F, you will need to use a Class 300 flange. Many of us might assume that a Class 150 flange was acceptable if we were not familiar with the rating tables.

How we can size safety valve for water surge tank?

2015-03-08T09:01:00.000-07:00

The flow rate Q, can be calculated using the tank volume,

Q=dV/dt

Where,

V- tank volume,

B- the bulk modulus of water,

dP/dt -the known pressure surge rate,

Q=dV/dt=(dP/dt)(V/B)

dV= (V/B)(dP)

Since water is virtually incompressible, only a small flow of water needs to be released at a rate on par with the pressure surge rate to prevent rupturing the tank.

What is the method for measure carbon percentage in steel?

2015-03-08T08:59:00.004-07:00

Laboratory chemical wet analysis test.

XRF portable machine.

X-ray Gun.

The most popular standard for this type of analysis in metallic materials is ASTM-E415. This is considered the most accurate method.

ASTM E-415- Standard Test Method for Analysis of Carbon and Low-Alloy Steel by Spark Atomic Emission Spectrometry.

What is condition monitoring?

2015-03-08T01:41:00.003-08:00

Condition Monitoring, is all about proactive/predictive maintenance. How we use gathered data such as maintenance history, visual inspection, oil sample history, etc, and generate backlog activity to prevent further damage

Condition monitoring is the process of determining the condition of machinery while in operation. The key to a successful condition monitoring program includes:

Knowing what to listen for
How to interpret it
When to put this knowledge to use

successfully using a program enables the repair of problem components prior to failure. Condition monitoring not only helps plant personnel reduce the possibility of catastrophic failure, but also allows them to order parts in advance, schedule manpower, and plan other repairs during the downtime.

On-line monitoring is practiced by the installed supervisory instrumentation of the major plant equipment. This includes instrumentation such as vibration, pressure, differential pressure, temperature, flow, tank level, analyzers of critical process parameters. Trending the monitored parameters helps to determine the status of the equipment and schedule the maintenance work accordingly.

Off-line monitoring covers the measurements of wear, clearances& fits on various internal mating parts of the major equipment. These measurements are recorded during the scheduled inspection plans and compared with the allowable values specified by OEMs. This will help to decide when replacement is due and when to schedule the next maintenance prior to catastrophic failure occurs.