Control Valve interview question and answers
1. What is the valve cone?The valve cone may be a movable component inside the valve that bears on the valve seat. By means that of the cone stroke the valve is opened, throttled or closed.
2. What is the valve seat?The valve seat is that the apron for an outlined flow through opening within the valve through that the medium to be regulated needs to flow.
3. How to define seat leak class for control valves?A control valve’s ability to shut off must do with several factors. Seat material, actuator thrust, pressure drop, and also the kind plug (balanced or unbalanced) will all play a section in however well a specific control valve shuts off. Seat leak Classifications There is literally six completely different seat leak classifications as outlined by IEC 60534-4 std or ANSI/FCI seventy.2. They normally range from class IV to class Vl class IV is additionally referred to as metal to metal. it’s the type of leak rate you’ll expect from a valve with a metal plug and metal seat. Class Vl is understood as a soft seat classification. Soft Seat Valves area unit those wherever either the plug or seat or each area unit made of some reasonably composition material like Teflon or similar.
4. What is the difference between flashing and cavitation?Cavitation may be a condition that happens in liquid flow wherever the inner pressure of the liquid, at some Location falls below the vapour pressure and vapour bubbles formation takes place. After this time the pressure rises higher than the vapour pressure once more and through this pressure recover stage (recall additionally the conception of FL) the bubbles collapse, and cavitation takes place. Flashing may be a condition that happens with liquid flow wherever the pressure falls below the vapour pressure (same as cavitation) and remains below it. There are a unit then 2 phases flowing (i.e. liquid and vapour) downstream and no collapsing of bubbles takes place.
5. Why a control valve on refrigerant Service can not be mounted with actuator in horizontal position?The working rule of the extended bonnet (named “cold box”) employed in refrigerant service (below -100°C) is predicated on the principle that portion of LNG that evaporates and stagnates within the higher portion of the bonnet contains a operate of thermal insulation between the process fluid and also the gland packing. In this manner ice formation on valve stem is avoided. To grant this profit the stem/actuator axis should be in upright position. so valves put in on a vertical pipe (with the actuator in horizontal position) cannot work properly.
6. What is the distinction between “Rangeability” and “Turn-down” in an exceedingly steam DE-superheating system ?
The term “rangeability” commonly refers to a mechanical conception applied to one throttling device. F.i. for a control valve the “rangeability” is magnitude relation between the max flow (or Cv) and also the minimum governable flow (or Cv).
When 2 of such devices area unit associated, in our case on water and steam line, the minimum and most governable flow of the system as an entire can be tormented by different factors.
Typically in steam acquisition systems some prescriptions in terms of steam velocity area unit needed for a decent performance (not under..) and thus the operating range of the system is also under the “rangeability” of every individual device.
Therefore the magnitude relation between max. and minimum governable flow of the complete system is what we tend to decision “turn down”.
7. How will cavitation damage be contained?
Several ways exist for treating cavitation in control valves. The primary is to pick out valves with high values of fl (recovery factor) eliminating cavitation.
The second is to make sure that the plug and seat area unit product of a material which will resist the damage (e.g. stellite hard facing).
The third is to control wherever the bubbles collapse and keep this aloof from vulnerable elements (for instance on the brink of internal surface of valve body or pipe).
8. What is the pressure setting range?
The pressure setting range indicates the range at intervals that a valve may be set or used.
Example: The indication pressure vary 4-6 bar means that for a pressure reducing valve the outlet pressure may be set between four and six bar for an overflow valve (back pressure regulator) – the I/L pressure may be set between four and six bar for a hurt and emission valve the valve may be used for this system pressure range
9. What is the reduction ratio?
The reduction ratio indicates the maximum. ratio of I/L and outlet pressure which might be reduced among one step by a pressure reducing valve.
Example one : Reduction ratio 20:1 (the regulation data sheet of the valve indicates 20)
Example two : The face value for the outlet pressure 1.2 bar is that the highest permissible pv = 20 x one.2 bar = 24 bar
10. What is a control line?
The control line takes the I/L and outlet pressure to be regulated from the pipeline and transfers it to the control unit.
Not each pressure control valve needs a control line. The control line may also be accustomed set pressures at locations remote from the valve.
11. What is the distinction between a start-up and continuous bleeding valve?
The start-up bleeding valves bleed plants with low internal pressure throughout start-up or filling. The float acts directly onto the cone. Such valves have an oversized seat diameter to make sure quick bleeding at a pressure below 0.1 bar.
During operation they’re unbroken closed by the inner pressure of the vessel. just in case of a suddenly occurring vacuum the valves open and compensate the pressure. so damages caused by a vacuum are going to be avoided.
The continuous bleeding valves area unit accustomed evacuate the air accumulating throughout operation. they’re supplied with a lever transmission so they additionally operate at terribly low or terribly high pressures. If air intake is to be avoided the outlet is to be supplied with a non-return valve. In such a case they’re going to act as bleeding valves while not venting operate.
12. What is Nm³ (cubic metre standard), Sm³ (American kiloliter standard) and Bm³ (cubic metre throughout operation)?
Nm³ : Volume of a medium (liquid or gas) at the standard state one bar (abs) 20°U Sm³: -> engl: the american variant of Nm³
Bm³ : Volume of the medium (liquid or gas) within the operative mode. i.e. at operative pressure and operative temperature.
Attention: Bm³ and Nm³ could usually differ from one another owing to the compressibility of the medium. In such a case the differentiation between Bm³ and Nm³ is of specific importance.
13. What is the distinction between a pressure reducing valve and a pressure maintaining valve?
Both valves keep the chosen set pressure constant. The pressure reducing valve will thus on the downstream facet while the pressure maintaining valve will thus on the upstream facet of the valve (UV=backpressure regulator).
Which parameters area unit necessary for the choice of the nominal diameter except the Kv(s) values)?
The medium, flow rate, viscosity and also the pressure range to be regulated.
14. What will control unit mean?
The pressure to be regulated is transferred to the pressure control valve mechanism via the control unit.
The pressure functioning on the control surface closes (pressure reducing valve) or opens (backpressure regulator) the valve against the spring force.
Three construction varieties are distinguished:
2. piston with O-ring and different seals
15. Why is it better to increase the pipeline in some cases?
In several cases the valve is sufficiently dimensioned for an exact application with regards to the Kv(s) value, however it would be too little in respect of nominal diameter and also the resulting flow velocities.
Too high flow velocities lead to noise, pressure rise and wear and tear.
On alittle scale the flow velocity may be reduced to a permissible value because of the extended pipeline. The pipeline extension is to be disbursed by the client.
16. What is the distinction between safety valve and back pressure regulator?
The safety valve is to securely discharge the medium at a planned nominal value throughout an exact pressure relief process.
The overflow valve (backpressure regulator) limits the pressure upstream of the valve to the set (but alterable) face value.
The backpressure regulator solely discharges the amount of the medium required to achieve the face value.
17. Why may be a control line needed?
Some valve designs need control lines to securely transmit the desired set pressure to the control unit.
Control lines will enhance the control properties and area unit accustomed notice special applications (e.g. regulation of differential pressure, of quantities, or vacuums).
18. What is the control deviation & its types should it be?
In the range from ten % to seventy % of the possible rate of flow the pressure variation and also the set value (psoll) should not vary over + five %. The control deviation indicates the extent of the variation.
19. Why will a leakage line be necessary?
In case of poisonous or dangerous media the valve should be supplied with a closed spring cap (with adjusting screw seal) with leak line connection.
During the on-site assembly a leakage line needs to be arranged that drains the medium safely just in case the control unit becomes defective.
20. What is the differential pressure, Δp?
The differential pressure defines the pressure distinction between I/L pressure and outlet pressure. If pressure ranges area unit given, the littlest pressure distinction has relevancy with regards to valve design..
21. What is a released valve?
On a released valve the forces working on the cone area unit compensated on an equally sized release surface because of the I/L and outlet pressure. so the valve features significantly higher control properties.
22. What will exhausting facing mean?
It is suggested that the cone for pressure control valves for liquids and with a pressure drop of abt. > twenty five bar be designed with a tough facing layer. This may prevent early wear and tear because of cavitation.
23. Why is that the flow velocity thus important?
In pipelines bound flow velocities area unit allowable depending on the medium. additionally to the KV(s) value the flow velocity may be a decisive value for valve orienting .
24. What is an outlet pressure pH (p2)?
The outlet pressure indicates the pressure within the system downstream of the valve.
25. What is a pressure drop?
The pressure drop indicates the active pressure distinction between the effective I/L and outlet pressure.
25. What is a pressure drop?
The pressure drop indicates the active pressure distinction between the effective I/L and outlet pressure.
26. What will kV value mean?
The kV price may be a reference value for the rate of flow of valves:
27. What will Kvs value mean?
For engineering a necessary kV value is calculated on basis of the given operative conditions. normally AN allowance of half-hour is additional. The kV value of the valve ought to reach this Kvs value.
What is an I/L pressure pv (p1)?
The I/L pressure indicates the pressure within the system upstream of the valve.
28. What is a pressure reducing valve?
Pressure reducing valve reduces a higher inlet pressure to a constant downstream pressure regardless of demand and supply pressure fluctuations. It enables delivery of water at safe pressures and adequate levels in various applications such as main feed, fire and irrigation systems.
29 . What is a pressure relief valve?
Pressure relief valve relieves excess pressure while maintaining a minimum upstream pressure. It prevents downstream demand from sacrificing supply of an upstream zone. It is typically installed in protection of in-line distribution piping at booster pump stations.
30. What is the rate of a flow control valve?
Rate of flow control valve maintains a maximum flow rate setting downstream regardless of pressure changes. It is typically installed within distribution systems and process control applications.
31. 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.
32. 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 the two valves in parallel is to select a valve with a high rangeability such as a V-port ball valve.
34. Is flow through a Control Valve – Turbulent or Laminar?
Flow through control valves is almost always turbulent. Laminar flow takes place with liquids operating at low Reynolds numbers. This occurs with liquids that are viscous; working at low velocities. Laminar flow in gases and vapors very seldom will be experienced in process plants.
35. The control valve size is essentially always equal to the line size. Is this correct?
- The control valve does not need to be of the same size as the pipe. Don’t assume a 4” valve will handle the flow in a 4” line. The flow path is the critical factor and shape of the flow passage in the valve can be a problem. Look at the pressure through the valve and if the valve creates a pressure drop, then a slightly larger valve may be needed. Always calculate the flow coefficient. Even for a given size of valve, there is no industry-wide standard pressure for valve sizes; no two manufacturers design a 4″ valve the same way, and different designs have different pressure considerations. Contrary to conventional wisdom, it is better to make an error in under sizing a control valve than to oversize it.
36. Is the velocity of a fluid in a control valve critical?
The velocity is one of the most 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 vapors 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.
37. What is a “modulating pressure independent” control valve?
A modulating pressure independent control valve does not allow a change in flow rate when the pressure differential across the valve changes. Flow only changes with a change in valve position.
38. What pressure and temperature requirements should be considered in selection of control valves?
The selection of a control valve must be in accordance with established material properties. ANSI Standard B16.34 lists the allowable temperature and pressure ombinations for a variety of body materials. Standard pressure classes include ANSI Classes 150, 300, 600, 900, 1500, 2500 and 4500. For a given body material, each class denotes the maximum working pressure for a given operating temperature. Due to loss of material strength, the allowable working pressure decreases as the operating temperature rises. Lower temperature limits are also specified due to loss of ductility in some materials. Although a pressure class is usually indicated for an application, proper attention to selection of the pressure class is crucial to the safe operation of the control valve.
39. What’s the difference between balanced and unbalanced valves?
Outlet balancing is required for proper valve functioning in those applications where the outlet pressure is more than 25% of the inlet pressure. When the outlet pressure is relatively high compared to the inlet pressure, the normally closed poppet will sometimes be pushed up off the internal orifice by the back pressure on the valve. Outlet balancing prevents the backpressure from acting on the poppet.
40. How do the characteristics of the actuator influence the control valve operation?
The valve and actuator are engineered to work together. Most manufacturers provide an anti-hunting algorithm in the valve that prevents the valve from modulating until it exceeds the tolerance of the valve. Use spring-loaded diaphragm type actuators where practical. Spring-less operators and cylinder operators are acceptable only when spring-loaded diaphragm type actuators cannot provide the desired performance. Select the actuator so that the valve or damper that it controls will fail safe. Fail safe is defined as lock in position or take a position (either open or closed) that will result in the least upset.
41. What type of end connections specified for control valves?
Most control valve specifications request raised face flange connections suitable for bolting to the process piping. In power piping and nuclear applications, butt weld ends suitable for welding to the process piping are typical. For limited applications, screwed end connections (NPT) and socket-weld ends are specified for globe valves in sizes 2-inch and smaller. Some high pressure applications may require the use of RTJ (ring-type joint) flanges. Flangeless, wafer-style connections are specified in applications where rotary valves are suitable for the process conditions. These connections are usually raised face.
42. What are the common types of flow characteristics for control valves?
There are three basic types of control valve characteristics.
These are quick opening, linear, and equal percentage. A quick opening valve would flow approximately 70% of its full flow at a valve opening of approximately 30%; a linear valve would flow approximately 50% of its full flow at a valve opening of approximately 50%; and the equal percentage valve (logarithmic) would flow 30% at approximately 70% of its full opening. So the question is – where would you use one style of valve over another?
- A quick-opening characteristic is recommended for shut off service only. Not much use for modulating process control except in ON-OFF application.
- If the piping and downstream equipment cause significant resistance to the system, equal percentage will provide better control. This is a condition, when the valve pressure drop at the maximum flow rate is less than about one third of the overall system pressure drop (valve + line).
- If most of the pressure drop is taken through the valve and the upstream pressure is constant, a linear characteristic will provide better control. Also use linear characteristic when a PLC is used to control the valves in a system. Avoid linear flow characteristics at low flow conditions. Control valves with equal percentage flow characteristics are generally specified for most applications. Both types offer the best throttling control. In case of doubt, use equal percentage.
43. What is booster how its work in control valve ?
A pneumatic relay that is used to reduce the time lag in pneumatic circuits by reproducing pneumatic signals with high-volume and or high pressure output. These units may act as volume boosters or as amplifiers. A 1:2 booster will take a 3 to 15 psig input signal and output a 6 to 30 psig signal. It has also been shown that a booster may improve the performance of a control valve by replacing a positioner. It can provide the same stroking speed and can isolate the controller from the large capacitive load of the actuator.
44. What is Bubble tight ?
A commonly used term to describe the ability of a control valve or regulator to shut off completely against any pressure on any fluid. Unfortunately it is completely unrealistic. Control valves are tested to ANSI B16.104 and FCI70-2-1976 which is the American National Standard for Control Valve Seat Leakage. This standard uses 6 different classifications to describe the valves seat leakage capabilities. The most stringent of the these is Class VI which allows a number of bubbles per minute leakage, depending on the port size of the valve. The correct response to the question “Will that valve go “Bubble Tight”? is to say this valve is tested to meet Class VI shutoff requirements.
45. What is Bubble tight ?
A valve with a circular body and a rotary motion disk closure member which is pivotally supported by its stem. Butterfly valves come in various styles including eccestric and high-performance valves. Butterfly valves are high recovery valves and thus tend to induce cavitation in liquid services at much lower pressure drops and fluid temperature than the globe style valves, many people limit the travel of the valve at 60 degrees of rotation on throttling services. This can also help keep the valve out of cavitation problems.
46. What is Cavitation ?
In its simplest terms cavitation is the two stage process of vaporisation is simply the boiling of a liquid which is also known as flashing. In a control valve this vaporisation takes place because the pressure of the liquid is lowered. Instead of the more common occurrence where the temperature is raised. As fluid passes through a valve just downstream of the orifice area, there is an increase in velocity or kind of energy that is accompanied by a substantial decrease in pressure or potential energy. This occurs in an area called the vena contracta. If the pressure in this area falls below that of the vapour pressure of the following fluid, vaporisation (boiling) occurs. Vapour bubbles then continue downstream where the velocity of the fluid begins to slow and the pressure in the fluid recovers. The vapour bubbles then collapse or implode. Cavitation can cause a Choked Flow condition to occur and can cause mechanical damage to valves and piping.
47. What is control valve?
A power operated device used to modify the fluid flow rate in a process control system. It usually consists of a body or valve and an actuator, which responds to a signal from the controlling system and changes the position of a flow controlling element in the valve.
48. What is Cv ?
The valve flow coefficient is the number of US. gallons per minute of 60 degree F water that will flow through a valve at a specified opening with a pressure drop of 1 psi across the valve.
49. what is Equal Percentage characteristics of the valve.
A term used to describe a type of valve flow characteristic where for equal increments of valve plug travel the change in flow rate with respect to travel may be expressed as a constant percent of the flow rate at the time of the change. The change in flow rate observed with respect to travel will be relatively small when the valve plug is near its seat and relatively high when the valve plug is nearly wide open.
50. Why Extension Bonnet used in control valve.?
A bonnet with a packing box that is extended above the body to bonnet connection so as to maintain the temperature of the packing above (cryogenic service) or below (high temp service) the temperature of the process fluid. The length of the extension depends on the amount of temperature differential that exists between the process fluid and the packing design temperature.
51. what is Flow Characteristic of the valve ?
The relationship between valve capacity and valve travel. It is usually expressed graphically in the form of a curve. Control valves have two types of characteristics inherent and installed. The inherent characteristic is derived from testing the valve with water as the fluid and a constant pressure drop across the valve. When valves are installed into a system with pumps, pipes and fittings, the pressure dropped across the valve will vary with the travel. When the actual flow in a system is plotted against valve opening, the curve is known as the installed flow characteristic. Valves can be characterised by shaping the plugs, orifices, or cages to produce a particular curve. Valves are characterised in order to try to alter the valve gain. Valve gain is the flow change divided by the control signal change. This is done in an effort to compensate for nonlinearities in the control loop.
52. what is Hard Facing ?
A property of metals that is discussed frequently when speaking of various component parts used in valve construction particularly valve trim. There are two hardness scales which are commonly used, Rockwell & Brinell.
53. what is Rangeability of the valve ?
The range over which a control valve can control. It is the ratio of the maximum to minimum controllable flow coefficients. This is also called turndown although technically it is not the same thing. There are two types of rangeablility – Inherent and installed inherent rangeablility is a property of the valve alone and may be defined as the range of flow coefficients between which the gain of the valve does not deviate from a specified gain by some stated tolerance limit. Installed rangeablility is the range within which the deviation from a desired installed characteristic does not exceed some state tolerance limit.
54. what is Turndown ?
A term used to describe the ration between the minimum and maximum flow conditions seen in a particular system. Example: If the minimum flow were 10 G.P.M and the maximum flow were 100 G.P.M the turndown would be 10.1. This term is sometimes incorrectly applied to valves. See rangeability.
55. What is Vena Contracta ?
The location where the cross sectional area of the flow stream is at its minimum size, where fluid velocity is at its highest level and where fluid pressure is at its lowest level. The vena contracta normally occurs just downstream of the actual physical restriction in a control valve
56. What is Venturi Valve ?
A reduced bore valve. A valve having a bore smaller in diameter than the inlet or outlet. For example, an 8″x 6″ x 8″ ball valve has 8″ inlet and outlet connections while the ball and seats are 6″. The flow through a venture valve will be reduced because of the smaller port. Venturi valves can often be economically substituted for plug valves.
57. What is Tensile test ?
A destructive test performed on a specially machined specimen taken from material in its delivered condition to determine mechanical properties, such as tensile strength, yield strength, and percent elongation.
58. What is Solenoid Valve ?
A small electrically operated valve used in the control piping of powered by hydraulic or pneumatic cylinder operators.
59. what is SEAT of the valve ?
That part of a valve against which the closure element (gate, ball) effects a tight shut-off. In many ball valves and gate valves, it is a floating member containing a soft seating element (usually an o-ring).
60. What is Limit Switch ?
An electrical device providing a signal to a remote observation station indicating when the valve is in the fully open or fully closed position. Usually a component of a valve operator.
61. What is Flashing ?
Occurs, for liquids only, when the pressure in the valve body falls below the liquid vapour pressure and when the bubbles thus formed remain as vapour in the fluid, owing to the fact that the downstream pressure of the control valve is at or below the liquid vapour pressur.
62. How to Calibrate valve positioner of the valve ?
If calibrating the valve positioner separately, connect an input test pressure regulator or hand pump, and monitor the input pressure applied with a pressure standard. If there is no supply air, connect the required supply air to the positioner. Apply the pressure for the desired test points and record valve position.
For example, assume our valve positioner is 3-15 psig input = 0-100% valve position. In this case, apply 3.0, 6.0, 9.0, 12.0, and 15.0 psig. The expected valve positions should be 0, 25, 50, 75, and 100%, respectively.
The valve position indicator on the stem usually marks off in 5% or 10% increments. Therefore, a best estimate of the valve position may be all you can obtain. In other cases, a valve position detector provides a remote indication to a DCS. In such cases, ensure both indicators are working properly.
As to control valve calibration, the process is similar to positioner calibration in that one applies a pressure signal to the actuator and then tallies the resulting valve position. This step can take place with the positioner calibration, if applicable, and it can happen in conjunction with I/P calibration.