Interview Q&A


Hi Chemical Engineers,

Here we have provided few possible Interview questions which are commonly asked in Interviews for any chemical engineer positions. Please read them and appear well for the Interview. Also kindly share if you come across any Interview questions, technical contents  etc so that we can share them to whole chemical engineer’s community.

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Question & Answers
1) When to use double pipe heat exchanger and shell and tube heat exchanger?

Double pipe heat exchangers are used when the heat transfer area is small say up to . If we connect them in series to increase the heat transfer area it will require much space as well the pressure drop will be higher due to more fittings. Also we can’t increase no of passes for either side fluids. The double pipe HE can’t be used for dirty fluids due to choking and cleaning is tougher. The advantage is it is simple to construct and easy to operate. But in shell and tube HE, we can pack a large heat transfer area within a small volume. As the numbers of tubes are more in a shell and tube HE, we can expect a higher turbulence which will result in higher heat transfer rates. Dirty fluids also can be handled owing to easy cleaning.



2) What is Biot No?

Biot no is a dimensionless no defined as Bi= hL/kb (No unit).
h= convective heat transfer co efficient unit is W/
L= characteristic length (Volume of the body/ surface area of the body and unit is metre)
kb= thermal conductivity of the body(W/mK)
It is the ratio between convective heat transfer away from body to that of conductive heat transfer within the body. If it is above one, means convection is more and less than one means conduction is more.
3) What should be the pressure to be taken for pressure test for pipe and reactor before placed in service?

For pipes the test pressure should be double the proposed application pressure and for reactors test pressure should be 1.5 times the reaction pressure. It means if we wish to subject a pipe to 10 bar we have to do pressure testing at 20 bars whereas for the reactor it would be 15 bars.
4) Which liquid should be on shell side of a shell and tube HE and why?

Even though there is no rule of thumb the following points are taken into consideration normally. 1) The corrosive fluid shall pass through the tube side as the replacement of tubes is easier and cheaper than shell side 2) The toxic, hazardous fluid shall pass through tube side. Because in case of any leakage it won’t get exposed to atmosphere.
5) How we can find leakage in any tube in shell and tube HE?

We have to remove the bonnets of left and right side of the HE. We have to force a liquid thro’ the shell side with high pressure. It will penetrate the leaked tube and come out via the same tube. Thus we can find the leakage.
6) What is the difference between vapor and gas?

A vapor is formed by heating any liquid and it can be condensed at atmospheric conditions either by reducing temperature or by increasing pressure. But a gas has already above the critical temperature and can’t be condensed by application of above methods. First it has to be brought below critical temperature. Then only it can be condensed.


Source:Click Here
7) What is the difference between evaporation and boiling?

Evaporation happens at any temperature whereas boiling occurs only at a single temperature for a single component like water. For example the water at sea surface evaporates every day. It may happen either at or , whereas water boils only at when the vapor pressure becomes equal to atmospheric pressure.
8) Why the hot liquid in heat exchanger, reactor jacket is passed from bottom to top?

If we pass the liquid from top to bottom, it will flow fast by gravity itself. So it will have less contact time with the heat exchanger/reactor surface which will result in poor heat transfer. That’s why the hot fluid should be passed from bottom to top to maximize the contact time. The same is applicable for cold fluid also in reactors.

9) Why steam enters top side of jacket in reactor?

If we pass the liquid from top to bottom, it will flow fast by gravity itself. So it will have less contact time with the heat exchanger/reactor surface which will result in poor heat transfer. That’s why the hot fluid should be passed from bottom to top to maximize the contact time. The same is applicable for cold fluid also in reactors.

10) When we need pumps in parallel and pumps in series?

If we need a higher discharge flow (Q) we have to go for pumps in parallel arrangement. If we need a higher head (H) we need to go for pumps in series.

For more:

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11) Please give a comparison between orifice meter and venturi meter?

1) The orifice plate can easily be changed to accommodate widely different flow rates, whereas the throat diameter of a venturi is fixed, so that its range of flow rates is circumscribed by the practical limits of Dp.

2) The orifice meter has a large permanent loss of pressure because of the presence of eddies on the downstream side of the orifice-plate; the shape of the venturi meter prevents the formation of these eddies and greatly reduces the permanent loss.

3) The orifice is cheap and easy to install. The venturi meter is expensive, as it must be carefully proportioned and fabricated. A home made orifice is often entirely satisfactory, whereas a venturimeter is practically always purchased from an instrument dealer.

4) On the other hand, the head lost in the orifice for the same conditions as in the venturi is many times greater. The power lost is proportionally greater, and, when an orifice is inserted in a line carrying fluid continuously over long periods of time, the cost of the power may be out of all proportion to the saving in first cost. Orifices are therefore best used for testing purposes or other cases where the power lost is not a factor, as in steam lines.

5) However, in spite of considerations of power loss, orifices are widely used, partly because of their greater flexibility, because installing a new orifice plate with a different opening is a simpler matter. The venturi meter can not be so altered. Venturi meters are used only for permanent installations.

6)It should be noted that for a given pipe diameter and a given diameter of orifice opening or venturi throat, the reading of the venturi meter for a given velocity is to the reading of the orifice as (0.61/0.98)2, or 1:2.58.i.e. orifice meter will show higher manometer reading for a given velocity
than venturi meter).


Courtesy:1)Click Here

12) When to use gear pumps?

When we need to handle high viscous liquids we need gear pumps as they deliver a higher discharge pressure (even up to 200bars) than centrifugal pumps. They are used to pump paints, resins, adhesives, pitch. diesel, crude oil etc. They are positive displacement pumps.


13) Why centrifugal pumps are widely used in process industries?

Centrifugal pumps ate widely used in process industries. Because
1) They are simple to construct and operate
2) Installation and maintenance are easy
3) Casing can be made of up variety of materials
4) Noise free operation
5) No need of safety valve at discharge side in case of single stage centrifugal pumps
6) Can handle variety of liquids. The impeller MOC can be altered accordingly.

14) Give brief classification of pumps.

Please refer below chart to have a basic idea on centrifugal pump

pumps classification
15) What is surging in compressors and how it can be prevented? .

In centrifugal and axial compressors when the suction volume falls below a certain limit i.e. when the compressor is not getting enough volume of gas to compress, the gas from discharge side will flow back to the compressor( which would reduce the speed of compressor )and join together with the suction side to increase the volume input. Thereafter the speed of compressor picks and it come to normal operation. But meanwhile such reverse in flow will cross huge pressure gradient across the compressor and vibration thus may damage the impeller. This momentary reverse flow lapses for very short time

To prevent surging we need to install an anti surging system. The anti surging system shall have a mechanism to measure the suction flow and discharge flow time to time along with a control valve. When the suction volume falls below a certain set value a part of discharge gas will be directed towards the suction to maintain the minimum suction volume. As normally compression leads to increase in temperature the gas recycle sis taken after the “after cooler” to ensure the discharge temperature is within control.


16) Differentiate compressors, fans and blowers? .

Fans, blowers and compressors are differentiated based on the method used to move the air and specific ratio (specific ratio= discharge pressure/suction pressure). As per the American Society of Mechanical Engineers (ASME) the compression ratio for fans is up to 1.11, blowers- 1.11 to 1.2 and for compressors above 1.2
17) What is the difference between HAZOP and HAZAN? .

18) Why we use LMTD to calculate overall heat transfer co efficient in shell and tube than arithmetic average?.

In a heat exchanger the heat being lost by the hot fluid as well as the heat gained by the cold fluid is not linear. So we need to use logarithmic average than arithmetic average to calculate correct value of overall heat transfer co efficient.


19) What should be the packing size in packed columns?.

The size of packing should be approximately 1/8th of the internal diameter of packed column for optimum pressure drop

20) When to use absorption factor method to calculate no of plates?

If the operating data line and equilibrium line in the Mccabe thiele method runs parallel the no of theoretical plates would be infinite. So it would be impossible to find the no of plates as the both won’t touch at any point. So we need absorption factor method wherein no of theoretical plates can be found by Fenske equation.
21) How to convert ppm in to percentage?

We have to divide by 104 to for the ppm to get converted to percentage. To convert percentage to ppm we have to multiply by 104.
How to calculate?
ppm(parts per million) means out of million i.e. out of 106
Percentage means out of 100 ie out of 102
Let us assume we have to convert X ppm into percentage
106 parts of substance is having x parts of a component
102 parts of the substance would have (102 * X) / 106 = X/104

23) What is critical insulation thickness for pipes?

We know that by adding more insulation to a wall always decreases heat transfer. The thicker the insulation, the lower the heat transfer rate. This is expected, since the heat transfer area A is constant, and adding insulation always increases the thermal resistance of the wall without affecting the convection resistance.
Adding insulation to a cylindrical piece or a spherical shell, however, is a different matter. The additional insulation increases the conduction resistance of the insulation layer but decreases the convection resistance of the surface because of the increase in the outer surface area for convection. The heat transfer from the pipe may increase or decrease, depending on which effect dominates.

Consider a cylindrical pipe of outer radius r1 whose outer surface temperature T1 is maintained constant. The pipe is now insulated with a material whose thermal conductivity is k and outer radius is r2 .Heat is lost from the pipe to the surrounding medium at temperature T∞ with a convection heat transfer coefficient h. The rate of heat transfer from the insulated pipe to the surrounding air can be expressed as

The value of r2 at which heat transfer rate reaches maximum is determined from the requirement that (zero slope). Performing the differentiation and solving for r2 yields the critical radius of insulation for a cylindrical body to be

rcr,cylinder = k/h

Note that the critical radius of insulation depends on the thermal conductivity of the insulation k and the external convection heat transfer coefficient h.The rate of heat transfer from the cylinder increases with the addition of insulation for r2< rcr, reaches a maximum when r2= rcr, and starts to decrease for r2> rcr. Thus, insulating the pipe may actually increase the rate of heat transfer from the pipe instead of decreasing it when r2< rcr.

Courtesy: Click Here

24) What is priming in centrifugal pumps?

Priming is a technique used to drive away the air entrapped in the suction line of a centrifugal pump. If the air present in the suction line are not removed the pump won’t be able to suck the liquid from the reservoir as air is lighter medium whereas liquid is heavier medium. So the whole suction line and the part of casing is filled with water and the air is removed via the air vent. Also if air is allowed to enter to the impeller they will damage the impeller by flashing (cavitation).

Normally cavitation won’t occur in pump which is continuously in service as there is a little chance for the air to get into the suction line.

Another answer from “mcnallyinstitute” is as follows

Unlike a positive displacement pump that can pump a liquid to any head as long as the pump body is strong enough, and there is enough horsepower available, the centrifugal pump can only pump a liquid to its rated head. You’ll recall that this head was determined by, and limited to the diameter of the impeller and the impeller speed (rpm.)

Since the weight of water is approximately 8000 times that of air (50 miles vs. 34 feet or 80 Km. vs. 10 meters) the centrifugal pump can produce only 1/8000 of its rated liquid pressure. In other words, for every one foot water has to be raised to prime the pump, the centrifugal pump must produce a discharge head of approximately 8000 feet (each meter requires a head of 8000 meters) and that is impossible with conventional impeller diameters and speeds.

Below given the steps involved in priming of a centrifugal pump whose sump is below the pump level.


1.) Discharge isolation valve 2.) Shutoff valve 3.) from outside supply. 4.) Foot valve 5. Check valve

Steps involved

I. Close the discharge isolation valve.
II. Open the air vent valves in the casing.
III. Open the valve in the outside supply line until only liquid escapes from the vent valves.
IV. Close the vent valves.
V. Close the outside supply line.

Source: 1) Click Here
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Follow up question: Is priming needed if the reservoir is located above the pump level?
No. because already the suction line would be filled with liquid and there will be no trapped air.
25) Why PD pumps need safety valve at discharge side whereas a centrifugal pump doesn’t need it?

Because the positive displacement pumps create a higher discharge pressure. (Even up to 200bars) In case by mistake the discharge valve is closed the higher delivery side pressure will damage the discharge line and the pump itself. To avoid we need a safety valve which may recycle a portion of liquid from delivery side to the reservoir.
26) Why steam ejectors are located above 10.33 meters only/ What is Barometric leg ?

When steam ejectors are in operation a higher vacuum is created inside the system. If the system comes in direct contact with atmosphere the atmospheric air would enter the system and break the vacuum. So we use water as a seal. But water also will be sucked by the vacuum to a height of 10.33 meters which is known as Barometric leg. If the steam ejector is kept below 10.33 meters the barometric leg would be unstable and water will enter the system. (10.33 Meters of water = 760 mm of Hg)


N.B: The barometric line should be straight to ensure a perfect vacuum.
Source: Click Here
27) What is the size of nozzle in steam ejectors?

One to three millimeters diameters.


In the above diagram Part 2 is the nozzle.
28) Why Steam ejectors are used in series?

If the system under which vacuum needed is large i.e. if we have to suck large volume of vapors from the system a single ejector can’t handle the entire load. So we need more ejectors in series.


Schematic Diagram of two stage condenser


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Sub question: Why an inter condenser is needed between two stages of ejectors?

In inter condenser the vapor from a particular stage is cooled and thus the load to the next stage is reduced. Only the incondensable (the part of vapor that can’t be condensed) move to the next stage which would leave the system along with next stage condensed vapor.
29) What is MSDS?

A MSDS (material safety data sheet) is a sheet which gives all information such as physical, chemical properties of the chemical being handled, TLV, antidote, safety precautions, fire extinguisher type to be used etc. Everyone who handles a chemical substance should be well aware of the MSDS of that particular product.
30) What is the role of casing in centrifugal pumps?

The casing is designed in such manner its cross section area increases from the side of entry of liquid i.e. the liquid enters in a small cross section and moves along higher cross section. Due to that the part of its kinetic energy is converted into pressure energy.



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31) What is the difference between PFD and PID?

The PFD (process flow diagram) gives us picture about stages of the process. But the PID (process instrumentation diagram) shows us the location of valves, pumps, compressors, utilities etc in the process.



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32) What is the material of construction(MOC) of HE?

The material of construction of heat exchanger depends upon the properties of the liquid / vapor being handled, pressure and temperature conditions etc. Normally used materials are carbon steel, stainless steel, nickel, nickel alloys or other special alloys.
33) Why earthing /grounding done in reactors. Flammable liquid storage vessels on transfer times?

Earthing is done in reactors (always) and solvent containers at the time of solvent transfer to ground the static electricity that may be generated.


Grounding of solvent vessel on solvent transfer

Source: Click Here

Source:Click Here
34) Why Stainless Steel(SS) is not corrosive?

The SS (stainless steel) contains some percentage of chromium. The chromium reacts with oxygen (which is present in air) to form Cr2O which prevents further oxygen to react with Iron (Fe) of SS to form rust.

Source:Click Here
35) When vacuum distillation is needed?

In case if the substance being distilled may degrade before it reaches its boiling point we have to boil it a lower temperature than its boiling temperature. That’s why we need vacuum distillation.
36) What is the use of plate efficiency in distillation column?

Plate efficiency in plate column is used to convert the theoretical number of plates into actual number plates. As any plate can’t perform ideal we have to multiply its efficiency with theoretical no of plates to get actual plates required.
37) What would be impact if we increase and decrease the size of packing in packed column?

Increase in size of packing will give lower mass transfer rate and lower pressure drop. Decrease in packing size will lead to higher pressure drop and high mass transfer rates.
38) Why we use tangential entry in cyclone separators?

If the dust laden gas (gas with dust particles) enters straightly inside the cyclone separator it won’t acquire swirl motion. Only tangential entry sets up such swirl motion (centrifugal force) due to which separation of solid- gas is achieved.



Source: 1)Click Here
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Follow up question

Why the the gas outlet duct of cyclone separator is extended below the inlet pipe level?
The level of outlet duct is always extended below the entry level to avoid short circuiting. Otherwise the incoming dust lade gas willgo thro exit without any separation. Please look at the image one. The outlet pipe level is set below entry level of incoming gas.


Source:Click Here
39) When to use gate valve and globe, ball?

Please look at the below pictures(From left to right)
1)Ball Valve- On off with quick operation 2) Globe valve– Throttling purpose 3) Gate valve– On off purpose


Follow up question:

Please differentiate globe valve & gate valve

•Flow valves, globe valve and gate valve have similar external housing but operational functions of both differ little. The main difference between globe and gate valve is in the form of the inner elements and the form of the flow inside the both valves . Globe valve has very good throttling ability while gate valve is not used for throttling.Gate valve is best suited for isolation and globe valve is best suited for regulating or throttling the flow.

•Globe valve cannot seal tight as compared to gate valve. Gate valve has tight shut-off ability in pipe lines. Because of this tight shut-off ability of gate valve, it is used upstream of globe valve. You might have seen this gate and globe valve combination at the discharge pipe of pumps.

•Globe valve requires greater torque to operate as compared to gate valve. This greater power requirement for globe valve is also considered when designing automatic and motor operated actuators for valves.

• Gate valve is not designed to regulate flow, and if done so then we observe noise, vibration and seat / disk damage in gate valves. While globe valve can be sometimes used for stop start of flow in case of unavailability of gate valve. Gate valve is generally used for isolation.

• Gate valve can be used in both directions as designs are available for both directions of flow. Same gate valve can have flow in both directions. While globe valve has flow direction usually indicated on it and is used in compliance with that.

•Valve Opening and closing geometry of disk and seat is also a difference between globe and gate valve. In a gate valve, the closure element is a plate or disk which travels parallel to the plane of the seat. The gate in a dam is a well known example. In a globe valve, the closure element is usually called a "poppet", and it travels perpendicular to the plane of the seat

•The globe valve can be repaired in place while nearly all gate valves need to be removed from the piping system to have leakage problems repaired.

•A globe valve can be opened against a high differential pressure, while a gate valve will bind and cannot be opened. If you try to open against a high DP, the seat will score and

•A globe valve has near linear characteristics of Cv vs. % open, while a gate valve has a severe parabolic characteristic and thus cannot be used for throttling

Source:Click Here
40) When extended surface HE is required?

When the temperature difference between two fluids is less to achieve higher heat transfer rate we may have to increase the external surface area. That’s why we need extended surface heat exchangers.

Below given few images of extended surface tubes


Source: Click here
41) How liquid nitrogen is saved in tanks?

Nitrogen is available in liquid form after compression. If it is exposed to atmosphere directly or comes in contact with heat it would flash (evaporate). So we need to protect the nitrogen storage from the outside temperature. The boiling point of nitrogen is -195.80C. To achieve that it is stored in an inside vessel with an outer vessel surrounding it. The annular space between the two vessels is evacuated to ensure the outside temperature won’t enter inside as no medium is there. This method is similar to what we see in a thermos flask


Source: Click Here
42) In which type of condensation (Film type, drop wise) heat transfer would be higher?

In drop wise condensation the drops formed will fall and the new steam that enters will find a new surface to contact, and to transfer heat. But in film type condensation a film will be formed on the tube wall which will prevent the fresh steam to contact the tube surface. So naturally drop wise heat transfer will be higher.

Please look at the below image. The left side shows the dropwise condensation and the right side shows film type condensation.

Also You can watch this video to get a clear idea

Click Here
43) What is the use of baffles in heat exchangers?

Baffles installed in a heat exchanger acts as an obstruction in the flow path of shell side liquid. It reduces the effective cross section area through which the liquid travels. Due to this the velocity and turbulence of liquid is increased thus resulting in high heat transfer.

Theoretical Support
Please refer the Donohue equation to calculate individual heat transfer co efficient


Please look at the first component of right side. Which is Reynolds no in which Ge denotes mass flow rate which is velocity(m/s)* density(kg/m3 ).This shows if velocity is increased h0 also will increase.

Please look below to have few more images of baffles


44) Why baffles are used in reactors?

The purpose of baffles in reactors is as same as that of heat exchangers. They increase turbulence thus causing higher rate of heat transfer, mixing thus ultimately reaction rate.


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45) Both the Nusselt number and the Biot number have the same form. What are the differences between them in terms of the variables employed and their physical significance?

Both the Biot number and the Nusselt number are of the form (hL/k). However, for the Biot number, the thermal conductivity k used is that for the solid; for calculating Nusselt number the k value as that of the fluid. The Biot number is a measure of the ratio of the temperature drop in the solid material and the temperature drop between the solid and the fluid. The Nusselt number is a dimensionless version of the temperature gradient at the surface between the fluid and the solid, and it thus provides a measure of the convection occurring from the surface.
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46) What are the color codes for cylinders of Oxygen, Carbon di oxide, and Chlorine?

Please look below chart

Source:Click Here

47) Please provide the color coding of pipe lines in process industries.

48) What is fouling and scaling?

Fouling is any resistance offered to heat transfer such as deposits of algae etc on the surface of heat exchanger tubes. Scaling is form of fouling.

Source:Click Here
49) What is dry bulb temperature, wet bulb temperature?

Dry bulb is the temperature of ambient air measured by thermometer. It is called “Dry bulb temperature” because the temperature shown by thermometer is not affected by the moisture content of the air.
The Wet Bulb temperature is the temperature of adiabatic saturation. This is the temperature indicated by a moistened thermometer bulb exposed to the air flow.

Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin. The adiabatic evaporation of water from the thermometer and the cooling effect is indicated by a “wet bulb temperature” lower than the “dry bulb temperature” in the air.

Courtesy:Click Here

50) What is dew point and why it is important in instrument air?

The Dew Point is the temperature at which water vapor starts to condense out of the air (the temperature at which air becomes completely saturated). Above this temperature the moisture will stay in the air.

Electronic instruments uses instrument air received from instrument compressors must be free of any moisture. Because even a very small amount of moisture may condense and give enormous deviation between actual value and displayed value which would result in many problems. So the instrument air should be free of moisture. This is ensured by keeping the dew point below -400C or below.
51) Are carbon steel storage tanks appropriate for NaOH solutions?

NaoH( Caustic Soda) is basic in nature and MOC to handle has to be chosen with utter caution. Otherwise corrosion will lead to severe damage to the container. Carbon steel tanks with no stress relieving can be used for NaOH concentration below 20% by weight, up to about 160o F and a 30-50% solution can be used, up to about 120 o F. However if we provide stress relieving bends and welds the service area can be extended further say .concentration below 20% by weight, can be used up to about 220 o F and a. 20-30% concentration , up to about 210 o F. Beyond this area we may need nickel alloys


52) Why Spherical vessels are preferred over other shapes for high pressure storage purposes?

Sphere has a strong structural strength compared to other structures. In spherical shaped vessel stress is evenly distributed internally and externally. So there are o weak points. Also it has smaller surface area per volume ratio. Due to this the amount of heat conducted through its wall to inside will be less compared to other structures. However manufacturing cost is high for spherical storage compared to cylindrical or rectangular storage.
Also Sphere storage results in the lowest land space for pressurized storage and offers lower costs for foundations, coatings, accessories, and piping than other options.
53) Which is the ideal shape to store LPG and why?

LPG storage is done under high pressure around 200 psi. So we need to design the vessel somewhat higher pressure bearing capacity to be on safe side.
1) To make it economical a curved surface is necessary to minimize the wall thickness. A bullet or a cylindrical tank with hemispherical or elliptical heads is the most economical. Because if the bottoms of a cylindrical tank have a spherical or elliptical shape, the welding line is subjected to pulling stress only.
2) Also if we fill a tank with flat bottoms, the inside pressure tears the welded area (the joined area between the cylindrical body and the flat end) and the tank may burst.
However apart from cylindrical vessels, spherical shape storage tanks also being used for LPG storage
54) What is the difference between light and heavy crude oil?

Light crude oil is the one which is having lower density and heavy crude has high density. So light crude oil is easy to get refined and the cost is higher than heavy for the same reason. The density is measured by API gravity which has value between 10-70. The API gravity has an inverse relation with the density of crude oil. It means if API gravity is high the crude is lighter.

Source: Mr. Brian Esteban Dowd(Sales & Marketing at Focus Economics in Linkedin)
55) What is sweet crude oil and sour crude oil?

Sweetness of crude oil is term related to the sulfur content. If the sulfur content is more than 0.5% the crude is called as “Sweet crude” and anything below this value is “sour crude oil” The light and sweet variety is preferred to its heavy and sour counterpart because of the amount of processing necessary to refine the oil to remove impurities to make fuels such as gasoline and diesel.

Source: Mr. Brian Esteban Dowd(Sales & Marketing at Focus Economics in Linkedin)

56) Please differentiate between absolute pressure, gauge pressure and differential pressure?

Gauge pressure: Gauge pressure is the pressure that is indicated by a pressure gauge like bourdon gauge. It takes the atmospheric pressure as its reference. Otherwise any pressure that is above the atmosphere in which the gauge is put is measured as gauge pressure.

Absolute pressure: Absolute pressure is measured with reference to a perfect vacuum.
Absolute pressure= Gauge pressure+ Atmospheric pressure
Gauge pressure=Absolute pressure- Atmospheric pressure


Differential pressure: Differential pressure is the difference in pressure between two points those are having different pressure. Either a simple manometer or DP transmitters based meters can be used to measure such differential pressure.


Please refer the above image. DP is the differential pressure that exists between high pressure and low pressure side.
bourdon gauge


57)Please explain function of bourdon tube used in bourdon gauge

The bourdon tube inside the bourdon gauge has C shape with oval cross section .It has two ends one is stationery and the next end is free to move. Bourdon tube has an oval cross section so that it gives maximum deflection in vertical direction. When an external pressure is applied the oval shape tends to become circular causing movement of tube and in turn needle (pointer) connected to it through a sector pinion. The movement of needle (pointer) will be opposite to movement of tube. The movement of the free end of the Bourdon tube is proportional to the difference between the external atmospheric pressure and internal fluid pressure.

The material of construction of Bourdon tube should have elastic properties so that its movement can be manipulated to measure pressure. Normally used materials are Phosphorous bronze, Berrilium copper and stainless steel. But if it is exposed to a corrosive liquid then alloy K Monel is used.
Courtesy: Instrumentation reference book edited by Walt Boyes

bourdon gauge

58) What are types of orifice plates and theirs application?

1)Concentric plate
In this type of plate the center of orifice is concentric with center of pipeline. This model is used for ideal liquid, steam and gases.

Square edge, concentric orifice plate

2)Eccentric plate
This type of plates are off center to the pipe. They are used to handle liquid containing slurry, oil and for wet steam.
Eccentric Orifice Plate, Square edged


Segmental Orifice Plate

The segmental orifice place has the hole in the form segment of a circle. This is used for colloidal and slurry flow measurement. This avoids build up of material in front of plate. .
The upstream side of the bore is shaped like a flow nozzle while the downstream side acts as a sharp edge orifice plate. They are recommended for measurement of viscous fluids which have pipe Reynolds numbers below 10,000. An increase in the viscosity of a fluid flowing through a sharp edge orifice will increase the diameter of the vena contracta (point of lowest static pressure downstream the orifice bore), which results in a decrease in differential pressure. However, an increase in the viscosity of a fluid flowing through a flow nozzle increases the friction drop in the flow through the nozzle, which results in an increase in the differential pressure. The quadrant radius orifice plate combines these two effects to produce a constant coefficient

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59) What is known as upstream and downstream in fluid flow?

“Upstream “in fluid dynamics means the side which comes before any mechanical device installed in the flow path. It can be a flange, valve or orifice plate. When we move along the direction of fluid flow the inlet side is called as ‘”upstream” and the outlet side is called as “downstream”. The difference in pressure between these two sides is used to calculate the volumetric flow.
60) What is schedule no & Nominal pipe size of pipes?

Schedule no and nominal pipe size or nominal bore are sizes used to express pipe thickness. When a pipe is put in high pressure, temperature or corrosive application its thickness play a vital role. So apart from material of construction we need to go for an appropriate thickness of pipe to prevent/ minimize erosion.

Nominal pipe size (or) Nominal bore

It is a North American set of standard sizes for pipes used for high or low pressures and temperatures. "Nominal" refers to pipe in non-specific terms and identifies the diameter of the hole with a non-dimensional number. For example – "2-inch nominal steel pipe" consists of many varieties of steel pipe with the only criterion being a 2-inch diameter hole through the center. That means 2 inch internal diameter.

Schedule No;

Schedule no is somewhat opposite to NPS. Schedule no is a measure of wall thickness of any pipe provided the OD(Outer diameter) of the pipe is fixed.

The higher the schedule, the thicker is the pipe wall and thus the weight of pipe for a given length. For example:

2-inch nominal size steel pipe: schedule 40 has a wall thickness of 0.154 inches and schedule 80 has a wall thickness of 0.218 inches.

4-inch nominal size steel pipe: schedule 40 has a wall thickness of 0.237 inches and schedule 80 has a wall thickness of 0.318 inches



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