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Top 100+ Control Valves Interview Questions And Answers

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Top 100+ Control Valves Interview Questions And Answers

Question 1. Why Do Different Control Valves Have Different Characteristics?

Answer :

Some valves have an inherent characteristic that can not be modified, which include full port ball valves and butterfly valves. For other valve types, which includes globe, the characteristic can be modified to in shape the utility.
Ideally the inherent valve characteristic have to be chosen to provide an hooked up characteristic as near as viable to linear (see inherent vs mounted feature). This enables the loop to stay tuned at all conditions with the equal calibration settings.
Question 2. Definition Of Linear And Equal Percentage Characteristic?

Answer :

Linear – For equal stem actions the alternate of go with the flow resulting from the motion is regular in the course of the stroke.

Equal Percentage – For same stem actions the trade of drift resulting from the motion is at once proportional to the glide price without delay earlier than the trade took place.

Besides the loop advantage and set up feature issues, identical percentage valve trim will commonly give better rangeability and better manage at low float fees. Linear trim will supply higher manipulate at drift rates over 50% of the valve capability. 

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Question three. What Is The Trim In A Control Valve?

Answer :

The trim consists of the components of the valve that affect the drift thru the valve. In a popular globe valve the trim might just be the plug and seat. In a unique valve the trim would consist of the plug, seat and retainer (or disk stack).

Question 4. Why Is Reduced Trim Required In Control Valves?

Answer :

Control valves are sized consistent with the application necessities and must satisfy both Cv and pace standards.
Reduced trim is used where it's miles important for the valve to have a Cv capacity smaller than the maximum possible in that size of valve.
The most not unusual reason for decreased trim is that the glide charge is low for the scale of valve required – especially wherein 25mm valves have been particular because the smallest length for use. Some plant life stipulate that no manage valve ought to be less than two sizes smaller than the road size, other that the valve should no longer be much less than half of the line length.
The 2nd motive is that on excessive stress drop gas or vapour applications the valve invariably is sized on the opening port velocity limits and the Cv required is tons less than the whole bore Cv.
Question five. What Is Meant By Critical Pressure And Critical Temperature?

Answer :

Critical temperature is that above which a fluid cannot be liquefied by way of strain by myself. Critical pressure is the equilibrium or vapour strain of a fluid at its vital temperature.

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Question 6. Are Safety Valves, Regulators And Isolating Valves All Examples Of Control Valves?

Answer :

Normally the time period manage valve is used to explain a valve that controls flow with an externally adjustable variable restrict. Safety valves and isolating valves need to not be called manipulate valves with out a qualifier which include protection manage valve or on/off manipulate valve. Regulators must be referred to as self-regulating manipulate valves to keep away from confusion.

Question 7. Is Flow Through A Control Valve – Turbulent Or Laminar?

Answer :

Flow thru manage valves is almost usually turbulent.
Laminar float takes vicinity with liquids operating at low Reynolds numbers. This happens with drinks which are viscous, working at low velocities. Laminar go with the flow in gases and vapours very seldom will be skilled in method vegetation.
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Question 8. What Is Cavitation?

Answer :

Cavitation is a condition that takes place in liquid waft in which the inner stress of the liquid, sooner or later falls below the vapour strain and vapour bubbles form and at some other point downstream rises above the vapour strain again. As this stress recovers so the bubbles fall apart, and Cavitation takes region

It is possible to are expecting where cavitation will arise via searching on the stress situations and the valve recuperation aspect. However, it is important to understand that the damage that occurs is depending on the strength being dissipated and is therefore flow structured.Cavitation sounds like stones passing thru the valve.

Question 9. What Effect Does The Positioner Cam Have On A Valve Characteristic?

Answer :

The remarks cam in the positioner controls the connection among the control signal and valve function. With a linear cam at 50% signal the valve will be 50% open.

It is viable to modify the apparent characteristic of a valve by way of changing the shape of the cam e.G. For a ball valve that has an inherent same percent man or woman it is possible to make it appear linear in order that the float rate through the valve at 50% sign is half of the most go with the flow – the valve will however simplest be 25% open to reap this result.

From the manage point of view there are benefits in doing this, however changing the valve feature and retaining the linear cam in the positioner is a higher technical answer if it's miles possible.

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Question 10. What Is Flashing?

Answer :

Flashing is a condition that takes place with liquid waft in which the pressure falls underneath the vapour stress and stays underneath it. There are then  levels flowing (i.E. Liquid and vapour) downstream.

Severe damage can arise interior a valve due to erosion because of the impact of liquid droplets traveling at high speeds.

Question 11. What Is Choked Flow?

Answer :

Choked float (otherwise referred to as crucial flow) takes region in a valve whilst an growth in strain drop across the valve now not has any effect on the flow rate via the valve. It takes place while the speed of the gasoline or vapour reaches sonic (Mach 1) on the vena contracta.
Choked drift isn't always necessarily a hassle in valves but does need to be taken into account in the Cv calculations. For liquids, choked drift shows the onset of complete cavitation, which generally requires special steps to be taken to reduce damage.
With easy gases there's no hassle with choked glide. Use the choked strain drop in any equation to calculate Cv or flow quotes. High noise tiers may be generated.
Solid particles in fuel flow will reason erosion because of the high velocities worried. With liquids complete cavitation will occur whilst the drift is choked.
High healing valves, consisting of ball and butterfly, turns into choked at lower pressure drops than low recuperation valves such as globe which provide a more confined glide course while fully open.
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Question 12. How Can Cavitation Damage Be Contained?

Answer :

Three strategies exist for treating cavitation on top of things valves – the primary is to ensure that the plug and seat are product of a material which can withstand the damage (e.G. Stellite hard going through). The 2nd is to govern wherein the bubbles crumble and hold this away from susceptible components (see Cav Control trim). The third is to manipulate the pressure drop and velocities to ensure that the liquid pressure does not fall below the vapour pressure – consequently casting off cavitation altogether.

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Question 13. How Can Flashing Damage Be Contained?

Answer :

Flashing can not be eliminated within the valve – if the downstream stress is much less than the vapour pressure then flashing will arise.

To minimise the damage:-

Hard face trim (the use of difficult facing materials including Stellite, or Tungsten Carbide)
Use more erosion resistant frame cloth
Increase length of valve, for this reason decreasing the velocity
Use attitude valve – glide over plug
Question 14. Definition Of Linear And Equal Percent Characteristics?

Answer :

Equal Percent traits.

The trade of waft attributable to a set increment of valve travel is immediately proportional to the float without delay before the alternate passed off.

Linear traits.

The change in flow on account of a fixed increment of valve tour is regular during the entire stroke.

General rules.

Use Equal Percent if in doubt.
Use Linear for level manage.
Use Equal Percent for strain manipulate.
Use Linear when the pressure drop throughout the valve is a big percentage of the total strain drop.
Question 15. How Is The Characteristic Determined In A Globe Valve?

Answer :

There are numerous methods of altering the feature in a globe valve relying at the particular design.

The maximum not unusual is to apply the profile on the the front of the plug head. In this situation the seat ring and retainer are not modified. If the plug is cage guided the feature of the valve is typically determined by way of the retainer or disk stack with the plug having a flat face. As the plug moves up, it uncovers extra float paths.
A collection of small holes at the lowest of the retainer with large holes at the pinnacle will give a bi-linear characteristic, which can be designed to offer outcomes much like identical percentage.
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Question sixteen. Is The Velocity Of A Fluid In A Control Valve Critical?

Answer :

The pace is one of the greater important considerations in sizing a control valve. For long lifestyles on liquid programs the speed at the go out of the valve body ought to be less than 10 m/s. This compares with commonly conventional line velocities of about 3 m/s, which explains why manipulate valves often are smaller than the line size.

On gases and vapours the speed on the exit of the valve body have to be much less than zero.33 Mach (1/3rd of sonic) for noise control valves and much less than 0,five Mach where noise is not a attention.

Question 17. What Is The Difference Between A Liquid, A Vapour And A Gas?

Answer :

These are all distinct states or stages wherein a fluid can exist. H20 exists as a strong (ice), liquid (water), vapour (saturated steam), and a gas (superheated steam) – it relies upon on the temperature and stress which phase is contemporary. Practically the most good sized difference among drinks and vapours/gases is the compressibility. Liquids are for maximum realistic functions incompressible where because the density of gas and vapours varies with pressure.

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Question 18. What Is A Desuperheater And How Does It Differ From An Attemporator?

Answer :

A desuperheater is a tool that is used to manipulate the addition of water to superheated steam to lessen the temperature to within 10°C of saturation.
An attemporator additionally adds water to steam to manipulate its temperature but the set point temperature is higher and the downstream steam is still superheated.
Generally desuperheaters are used in process plants wherein the steam is used for heating. Attemporators are used greater in energy stations for interstage temperature manage.
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Question 19. What Is The Difference Between Installed And Inherent Characteristics?

Answer :

The inherent characteristic is a plot of the drift rate through a valve (or Cv) towards percent starting with a steady strain drop across the valve.

This is the end result of a workshop test where the upstream and downstream pressure are held steady and the only variables are the glide charge and starting of the valve.

The mounted function is the plot of waft towards beginning using actual pressure drops experienced in exercise. Due to the reality that during most packages the pressure drop will increase because the float rate drops, the established feature will generally exchange from =% in the direction of linear, and from linear towards quick establishing.

Question 20. Why Are Control Valves Sometimes Very Noisy?

Answer :

Noise is created by means of an item vibrating. Valve additives will generally tend to vibrate on every occasion they're subjected to excessive velocity turbulent go with the flow. Standard manipulate valves will consequently have a tendency to be noisy on excessive stress drop programs specifically where waft charges are high, because the low strain experienced downstream of the seat ring (on the vena contracta) is observed by using very excessive velocities achieving as high as the velocity of sound. Special low noise valves are designed to drop pressure steadily in order that velocities are controlled at low tiers.

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Question 21. Can Two Control Valves Be Used In Series In High Pressure Drop Applications?

Answer :

Dropping the strain across two valves in preference to one is theoretically better. However, in exercise, the two valves will now not typically control nicely collectively until the technique can operate with a totally low proportional band with sluggish response times.

A higher, and generally much less costly approach is to apply a valve that is designed with more than one strain drop restrictions within the trim.

Question 22. Can Two Control Valves Be Used In Parallel To Handle High Turndown Applications?

Answer :

Two valves in parallel operating on split variety alerts can deliver very excessive turndown capability. The situation that have to be avoided if possible is that the larger valve operates in the “cracked open” position – one manner to avoid that is to application the PLC or DCS to close the small valve and use best the larger unit once the ability of the small valve is exceeded.

An opportunity to two valves in parallel is to select a valve with a high rangeability including a vee-ported ball valve.

Question 23. What Is The Difference Between Rangeability And Turndown?

Answer :

Generally the term rangeability is used to explain the functionality of a manipulate valve (i.E. The ratio of the maximum Cv of the valve to the minimal Cv at which it may manipulate) while the time period turndown is usually used to explain the requirement of an utility (i.E. Ratio of Cv at maximum situations to Cv at minimum situation).

Note that the rangeability of a valve need to be extra than the ratio of the Cv of the valve when fully open to the calculated Cv for the minimum conditions of the software.

Turndown applies to the software and is the ratio of the calculated Cv at most situations to the calculated Cv at minimal
Rangeability applies to the valve and is the ratio of the Cv of the valve absolutely open to the minimal Cv at which it may manage
The rangeability of the chosen valve should exceed the turndown necessities of the application.
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Question 24. What Process Date Is Required To Size A Control Valve?

Answer :

Medium – What is passing through the valve? – if it's far a unique liquid give specific gravity (at flowing temperate), critical strain, vapour strain and viscosity.
Pressures – What is the most pressure that the valve needs to be rated for? What are the upstream and downstream pressures for every of the most, regular and minimal drift costs.
Flow prices – Maximum, regular and minimal. The most is used to select the valve size, the minimum to test the turndown requirement and the ordinary to look where the valve will control.
Temperature – Maximum temperature for design plus temperatures at most, ordinary and minimal waft situations.
Please see the relevant enquiry sheets for extra information that can assist in the sizing and choice of the manage valve required.
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Question 25. What Is Incipient Cavitation?

Answer :

Incipient means “beginning” – “incipient cavitation” starts whilst the pressure first dips underneath the vapour strain and keeps till the glide becomes choked at which factor “full cavitation” is said to take vicinity.

Question 26. What Is The Difference Between A Diffuser Plate And A Choke?

Answer :

A diffuser is a plate with a huge wide variety of small holes in it this is hooked up in the downstream pipework. On gas and vapour packages it creates a returned pressure among the valve and plate, and this permits a smaller cost to be decided on than might otherwise be viable, due to the decrease velocity at most go with the flow. The universal noise degree produced can be decrease as the general range of pressure drop tiers are increased.

A choke is a restrict orifice and is a plate with one vital hole. It is used with liquid flows and is likewise hooked up within the downstream pipe paintings to create backpressure. The purpose is to lessen the strain drop across the valve on the most waft charge both to get rid of cavitation or to reduce the intensity of the damage to the valve.

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Question 27. What Is A Field Reversible Actuator?

Answer :

The actuators for lots manipulate valves are both spring-to-open or spring-to-near. The Mitech manage valve actuator has all of the components essential to opposite the action – this may normally take area in a workshop on site.

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Question 28. Will Separable Flanged Valves Seal In A Pipeline?

Answer :

The sealing face is part of the valve body and so the separable flanges are simplest there to preserve the body in the line – they're not required to seal.

Question 29. What Is Vapour Pressure?

Answer :

The terms vapour strain applies to a liquid, and is the herbal equilibrium stress that exists inside a closed vessel containing the liquid.
Vapour strain varies with temperature.
The vapour stress of water at ambient temperature of approximately 25°C is inside the order of 4 kPa(a). This method that water will boil at 25°C if the outside pressure is decreased to an absolute stress of 4 kPa. At one hundred°C the vapour stress of water is one zero one kPa(a), because of this that water will boil at 100° C at sea degree where the atmospheric stress is ready a hundred and one kPa(a).
Question 30. Specific Gravity Is The Ratio Of The Density Of A Liquid To The Density Of Water – What Is The Specific Gravity Of Gas?

Answer :

The specific gravity of gasoline is the ratio of the density of the fuel to the density of air each measured at preferred situations of one zero one,3kPa and 15°C .

Question 31. What Is Meant By Cryogenic?

Answer :

Cryogenic valves perform at temperatures below minus a hundred°C.

These valves have extended bonnets to remove the stuffing container and actuator away from the supply of cold and are made from materials which includes stainless steel Monel or bronze that don't become too brittle at those temperatures.

Question 32. What Materials Can Be Used For Oxygen Service?

Answer :

Monel, bronze and austenitic stainless steel (e.G. 316) are the satisfactory substances for oxygen provider in order of preference. The better the rate the higher the fabric to be used.
Velocities need to no longer exceed 40 m/s inside the valve body with Monel and bronze and ought to be much less than 20 m/s with stainless-steel.
Question 33. Why Do Oxygen Valves Require De-greasing?

Answer :

In the presence of maximum oils and greases oxygen will burn or explode. Even the oil deposited on a issue by means of an uncovered hand is sufficient to cause a problem, that is why plastic gloves have to be used whilst building degreased valves.

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Question 34. Why Do Some Control Valve Actuators Have A Small Internal Fail Action Spring And Some Are External And Much Larger?

Answer :

A piston actuator piped up double performing and running with full deliver strain of approximately 500 kPa is very stiff and may generally perform satisfactorily with the flow route both under the plug or over. This allows the float course to be selected to help with the fail action, this means that that best a small bias spring is vital in the actuator to begin initial movement within the proper path within the occasion of air failure. In the case of diaphragm actuated valves, the stiffness is a good deal lower and so the drift direction must always be below the plug, ensuing within the want of a heavy spring to give fail closed motion. This can not be outfitted in the actuator.

Question 35. Why Is Live Loading Sometimes Offered On Valves?

Answer :

Live loading reduces the want for ordinary maintenance within the plant.

Live loading is suggested on programs in which a leak alongside the valve shaft would be likely to reason harm to the shaft and packing. High-pressure water and steam packages are examples of wherein stay loading is tremendous.

Question 36. Why Is Energy Dissipation An Important Factor In Control Valve Selection?

Answer :

All Control valves cause stress drop inside the fluid as it passes via the valve. Since strain is a shape of Potential Energy, which means that a certain amount of strength is transformed from capacity strength into some different shape. The better the Pressure Drop and the greater the float fee then greater power can be dissipated. Depending on the kind of valve and the trim design this power can cause giant damage to valve additives due to cavitations and high velocities, or can be environmentally unfriendly because of high noise stages produced. Through the cautious desire of valve type and correct trim layout it's far possible to limit the unfavourable consequences of high ranges of electricity dissipation.

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