Are you searching for Control valves jobs? Then we will help you out by providing you with Control Valves Interview Question and Answers on our wisdomjobs site page. To be precise about the Control values is a valve used to control fluid flow by varying the size of the flow passage as directed by a signal from a controller. This enables the direct control of flow rate and the consequential control of process quantities such as pressure, temperature, and liquid level. If you are good at all concepts on control Valves then there are various leading companies that offer you job positions like Control and Valve Installers and Repairers, Service Engineers- Control Valves, Senior Manager-inside Sales- Control Valve and along with these there are many other roles too.
Question 1. Why Do Different Control Valves Have Different Characteristics?
Question 2. Definition Of Linear And Equal Percentage Characteristic?
Linear – For equal stem movements the change of flow resulting from the movement is constant throughout the stroke.
Equal Percentage – For equal stem movements the change of flow resulting from the movement is directly proportional to the flow rate immediately before the change took place.
Besides the loop gain and installed characteristic considerations, equal percentage valve trim will generally give better rangeability and better control at low flow rates. Linear trim will give better control at flow rates over 50% of the valve capacity.
Question 3. What Is The Trim In A Control Valve?
The trim consists of the parts of the valve that affect the flow through the valve. In a standard globe valve the trim would just be the plug and seat. In a special valve the trim would consist of the plug, seat and retainer (or disk stack).
Question 4. Why Is Reduced Trim Required In Control Valves?
Question 5. What Is Meant By Critical Pressure And Critical Temperature?
Critical temperature is that above which a fluid cannot be liquefied by pressure alone. Critical pressure is the equilibrium or vapour pressure of a fluid at its critical temperature.
Question 6. Are Safety Valves, Regulators And Isolating Valves All Examples Of Control Valves?
Normally the term control valve is used to describe a valve that controls flow with an externally adjustable variable restriction. Safety valves and isolating valves should not be referred to as control valves without a qualifier such as safety control valve or on/off control valve. Regulators should be referred to as self-regulating control valves to avoid confusion.
Question 7. Is Flow Through A Control Valve – Turbulent Or Laminar?
Question 8. What Is Cavitation?
Cavitation is a condition that occurs in liquid flow where the internal pressure of the liquid, at some point falls below the vapour pressure and vapour bubbles form and at some other point downstream rises above the vapour pressure again. As this pressure recovers so the bubbles collapse, and Cavitation takes place
It is possible to predict where cavitation will occur by looking at the pressure conditions and the valve recovery factor. However, it is important to recognise that the damage that occurs is dependent on the energy being dissipated and is thus flow dependent.Cavitation sounds like stones passing through the valve.
Question 9. What Effect Does The Positioner Cam Have On A Valve Characteristic?
The feedback cam in the positioner controls the relationship between the control signal and valve position. With a linear cam at 50% signal the valve will be 50% open.
It is possible to alter the apparent characteristic of a valve by changing the shape of the cam e.g. for a ball valve that has an inherent equal percent character it is possible to make it appear linear so that the flow rate through the valve at 50% signal is half of the maximum flow – the valve will however only be 25% open to achieve this result.
From the control point of view there are advantages in doing this, but changing the valve characteristic and keeping the linear cam in the positioner is a better technical solution if it is possible.
Question 10. What Is Flashing?
Flashing is a condition that occurs with liquid flow where the pressure falls below the vapour pressure and remains below it. There are then two phases flowing (i.e. liquid and vapour) downstream.
Severe damage can occur inside a valve due to erosion caused by the impact of liquid droplets travelling at high speeds.
Question 11. What Is Choked Flow?
Question 12. How Can Cavitation Damage Be Contained?
Three methods exist for treating cavitation in control valves – the first is to ensure that the plug and seat are made of a material that can resist the damage (e.g. stellite hard facing). The second is to control where the bubbles collapse and keep this away from vulnerable components (see Cav Control trim). The third is to control the pressure drop and velocities to ensure that the liquid pressure does not fall below the vapour pressure – thus eliminating cavitation altogether.
Question 13. How Can Flashing Damage Be Contained?
Flashing cannot be eliminated in the valve – if the downstream pressure is less than the vapour pressure then flashing will occur.
To minimise the damage:-
Question 14. Definition Of Linear And Equal Percent Characteristics?
Equal Percent characteristics.
The change of flow resulting from a fixed increment of valve travel is directly proportional to the flow immediately before the change took place.
The change in flow resulting from a fixed increment of valve travel is constant throughout the whole stroke.
Question 15. How Is The Characteristic Determined In A Globe Valve?
There are several ways of altering the characteristic in a globe valve depending on the particular design.
Question 16. Is The Velocity Of A Fluid In A Control Valve Critical?
The velocity is one of the more important considerations in sizing a control valve. For long life on liquid applications the velocity at the exit of the valve body should be less than 10 m/s. This compares with generally accepted line velocities of about 3 m/s, which explains why control valves often are smaller than the line size.
On gases and vapours the velocity at the exit of the valve body should be less than 0.33 Mach (1/3rd of sonic) for noise control valves and less than 0,5 Mach where noise is not a consideration.
Question 17. What Is The Difference Between A Liquid, A Vapour And A Gas?
These are all different states or phases in which a fluid can exist. H20 exists as a solid (ice), liquid (water), vapour (saturated steam), and a gas (superheated steam) – it depends on the temperature and pressure which phase is current. Practically the most significant difference between liquids and vapours/gases is the compressibility. Liquids are for most practical purposes incompressible where as the density of gas and vapours varies with pressure.
Question 18. What Is A Desuperheater And How Does It Differ From An Attemporator?
Question 19. What Is The Difference Between Installed And Inherent Characteristics?
The inherent characteristic is a plot of the flow rate through a valve (or Cv) against percentage opening with a constant pressure drop across the valve.
This is the result of a workshop test where the upstream and downstream pressure are held constant and the only variables are the flow rate and opening of the valve.
The installed characteristic is the plot of flow against opening using actual pressure drops experienced in practice. Due to the fact that in most applications the pressure drop increases as the flow rate drops, the installed characteristic will normally change from =% towards linear, and from linear towards quick opening.
Question 20. Why Are Control Valves Sometimes Very Noisy?
Noise is created by an object vibrating. Valve components will tend to vibrate whenever they are subjected to high velocity turbulent flow. Standard control valves will therefore tend to be noisy on high pressure drop applications particularly where flow rates are high, since the low pressure experienced downstream of the seat ring (at the vena contracta) is accompanied by very high velocities reaching as high as the speed of sound. Special low noise valves are designed to drop pressure gradually so that velocities are controlled at low levels.
Question 21. Can Two Control Valves Be Used In Series In High Pressure Drop Applications?
Dropping the pressure across two valves rather than one is theoretically better. However, in practice, the two valves will not usually control well together unless the process can operate with a very low proportional band with slow response times.
A better, and usually less expensive approach is to use a valve that is designed with multiple pressure drop restrictions inside the trim.
Question 22. Can Two Control Valves Be Used In Parallel To Handle High Turndown Applications?
Two valves in parallel working on split range signals can give very high turndown capability. The situation that should be avoided if possible is that the larger valve operates in the “cracked open” position – one way to avoid this is to program the PLC or DCS to shut the small valve and use only the larger unit once the capacity of the small valve is exceeded.
An alternative to two valves in parallel is to select a valve with a high rangeability such as a vee-ported ball valve.
Question 23. What Is The Difference Between Rangeability And Turndown?
Generally the term rangeability is used to describe the capability of a control valve (i.e. the ratio of the maximum Cv of the valve to the minimum Cv at which it can control) whereas the term turndown is generally used to describe the requirement of an application (i.e. ratio of Cv at maximum conditions to Cv at minimum condition).
Note that the rangeability of a valve must be greater than the ratio of the Cv of the valve when fully open to the calculated Cv for the minimum conditions of the application.
Question 24. What Process Date Is Required To Size A Control Valve?
Question 25. What Is Incipient Cavitation?
Incipient means “starting” – “incipient cavitation” begins when the pressure first dips below the vapour pressure and continues until the flow becomes choked at which point “full cavitation” is said to take place.
Question 26. What Is The Difference Between A Diffuser Plate And A Choke?
A diffuser is a plate with a large number of small holes in it that is installed in the downstream pipework. On gas and vapour applications it creates a back pressure between the valve and plate, and this enables a smaller value to be selected than would otherwise be possible, due to the lower velocity at maximum flow. The overall noise level produced will be lower as the overall number of pressure drop stages are increased.
A choke is a restriction orifice and is a plate with one central hole. It is used with liquid flows and is also installed in the downstream pipe work to create backpressure. The purpose is to reduce the pressure drop across the valve at the maximum flow rate either to eliminate cavitation or to reduce the intensity of the damage to the valve.
Question 27. What Is A Field Reversible Actuator?
The actuators for many control valves are either spring-to-open or spring-to-close. The Mitech control valve actuator has all the parts necessary to reverse the action – this will normally take place in a workshop on site.
Question 28. Will Separable Flanged Valves Seal In A Pipeline?
The sealing face is part of the valve body and so the separable flanges are only there to hold the body in the line – they are not required to seal.
Question 29. What Is Vapour Pressure?
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?
The specific gravity of gas is the ratio of the density of the gas to the density of air both measured at standard conditions of 101,3kPa and 15°C .
Question 31. What Is Meant By Cryogenic?
Cryogenic valves operate at temperatures below minus 100°C.
These valves have extended bonnets to remove the stuffing box and actuator away from the source of cold and are made of materials such as stainless steel Monel or bronze that do not become too brittle at these temperatures.
Question 32. What Materials Can Be Used For Oxygen Service?
Question 33. Why Do Oxygen Valves Require De-greasing?
In the presence of most oils and greases oxygen will burn or explode. Even the oil deposited on a component by an uncovered hand is sufficient to cause a problem, which is why plastic gloves should be used when building degreased valves.
Question 34. Why Do Some Control Valve Actuators Have A Small Internal Fail Action Spring And Some Are External And Much Larger?
A piston actuator piped up double acting and operating with full supply pressure of about 500 kPa is very stiff and can normally operate satisfactorily with the flow direction either under the plug or over. This enables the flow direction to be chosen to assist with the fail action, which means that only a small bias spring is necessary inside the actuator to start initial movement in the right direction in the event of air failure. In the case of diaphragm actuated valves, the stiffness is much lower and so the flow direction must always be under the plug, resulting in the need of a heavy spring to give fail closed action. This cannot be fitted inside the actuator.
Question 35. Why Is Live Loading Sometimes Offered On Valves?
Live loading reduces the need for routine maintenance in the plant.
Live loading is recommended on applications where a leak along the valve shaft would be likely to cause damage to the shaft and packing. High-pressure water and steam applications are examples of where live loading is advantageous.
Question 36. Why Is Energy Dissipation An Important Factor In Control Valve Selection?
All Control valves cause pressure drop in the fluid as it passes through the valve. Since pressure is a form of Potential Energy, this means that a certain amount of energy is converted from potential energy into some other form. The higher the Pressure Drop and the greater the flow rate then more energy will be dissipated. Depending on the type of valve and the trim design this energy can cause significant damage to valve components due to cavitations and high velocities, or can be environmentally unfriendly because of high noise levels produced. Through the careful choice of valve type and correct trim design it is possible to minimize the adverse effects of high levels of energy dissipation.
Control Valves Related Interview Questions
|AutoCAD Interview Questions||Digital Electronics Interview Questions|
|Switching Interview Questions||Solid Works Interview Questions|
|Control Systems-Electrical Engineering Interview Questions||Electrical Power System Interview Questions|
|Industrial Instrumentation Interview Questions||Switchgear Interview Questions|
|Electrical Drives Interview Questions||Flow Meter Interview Questions|
Control Valves Related Practice Tests
|AutoCAD Practice Tests||Digital Electronics Practice Tests|
|Switching Practice Tests||Solid Works Practice Tests|
|Control Systems-Electrical Engineering Practice Tests|
All rights reserved © 2020 Wisdom IT Services India Pvt. Ltd
Wisdomjobs.com is one of the best job search sites in India.