The control valve is a device meant to control the flow rate or flow direction of the fluid in the process.
It is referred to as the final control element. This control valve will adjust the flow passage as directed by a signal controller to modify or control the flow rate.
The basic and most common terminologies for understanding the control valve applications are discussed below.
But in some cases these terminologies might be confusing but refer to the definite components, devices, and processes.
An actuator is a pneumatic, hydraulic, or electrically powered device meant to supply force and motion for the closing and opening of the valve.
The bench set of the control valve is defined as the maximum pressure required for stroking the valve actuator to the rated valve travel from a fully open 100% position to a fully closed 0% position without any valve forces.
The closure member in the control valve is the movable part that is positioned in the flow path to modify the rate of flow of fluid flowing through that control valve.
Flow Coefficient, Cv:
The flow coefficient is defined as the relative measure of the valve efficiency. The Cv value is a constant in relation to the control valve geometry. The flow coefficient is used to predict the flow rate through the valve for particular valve travel.
The valve trim is the internal part of the valve to modulate and control the flow of the fluid. The valve trim is available in various designs.
Some of them include:
- Anti-cavitation trim: It is a combination of control valve trim by its geometry to reduce the tendency of the controlled liquid to cavitate.
- Anti-Noise trim: It is a combination of control valve trims by its geometry to reduce the noise generated by fluid flowing through the valve.
- Balanced trim: The balanced trim in the control valve is designed to minimize or reduce the total static and dynamic fluid flow forces acting on that trim.
- Reduced trim: The reduced trim in the control valve has a smaller flow area than the actual flow area of that valve.
- Soft-seated trim: The soft-seated trim in the valve is made with an elastomeric, plastic or other readily deformable material. Used for closure component or seat ring to provide tight shutoff with minimal actuator forces.
The characterized trim in the control valve provides predefined flow characteristics. This characterized trim is combined with the characterized disc for the high close-off capabilities of the valve to ensure a true equal percentage ﬂow characteristics to achieve superior flow control.
Erosion Resistant Trim:
This erosion-resistant trim is a valve trim that has been designed with special geometry and surface materials to resist the corrosive effects on the controlled fluid flow.
Valve Failure Modes:
The failure mode in the valve is the position in which the valve closure member of the valve is moved when the source of actuating energy fails.
The following are the basic types of valve failure modes
- Fail-Close: It is a condition where the closure member of the valve is moved to the closed position when the source of actuating energy fails.
- Fail-Open: It is a condition where the closure member of the valve is moved to the open position when the source of actuating energy fails.
- Fail-in Place: It is a condition where the closure member of the valve is positioned at the present state (either open or closed position) when the source of actuating energy fails.
The fail-safe is the property of a particular control valve and its actuator causes the closure member of the valve to fully open, fully close, or remain in an existing state when the source of actuating energy fails.
The fail-safe action sometimes demands the use of auxiliary controls that are connected to the actuator. Here the control valve design must comprise the fail-safe action for the plant and personal protection in cases of emergencies or disasters.
It is the region in the flow stream near the orifice where the diameter of the stream is less, and the velocity of fluid flow is at its maximum. Even the static pressure and the cross-sectional area of the fluid is at their maximum.
Throttling is defined as the action of a control valve to control the fluid by adjusting the position of the closure member. The variable pressure drop across the control valve occurs due to throttling action.
Valve travel is the movement of the closure member from the fully closed position to the fully open position.
The valve gain is the relationship between valve travel and the flow rate through the valve. It is described by means of a curve on a graph expressed by an installed or inherent characteristic of the valve.
A valve is said to have dual seating when it uses a flexible material or composition material such as TFE, Kel-F, or Buna-N, etc. for primary sealing and secondary sealing (metal to metal seat).
The primary sealing provides a tight shutoff and the secondary sealing provides backup to the primary sealing with a Class IV shut-off.
- The flow characteristic is defined as the relationship between valve capacity and valve travel.
- It is graphically represented in the curve.
- These include inherent and installed characteristics.
- The inherent characteristic is obtained by testing the valve with some fluid and a constant pressure drop across that valve.
- The pressure drop across the valve varies with valve travel when installed with pumps, pipes, and fittings in the system
- When an actual flow of the valve in the system is plotted against the valve opening, this curve obtained is known as the installed flow characteristic.
- To produce a particular curve the valves are characterized by shaping the plugs, orifices, or cages.
The term reverse acting has several senses that depend upon the device used. The reverse acting type actuator is the device where the actuator stem may revoke when diaphragm pressure is increased. And the reverse acting valve is one that has a seat orientation and plug push down to open.
The output signal is inverse proportional to the set point in the case of the reverse-acting positioner.
It is considered as the flow of fluid in the direction opposite to the standard direction. The pressure drop is higher during reverse flow. Here the rotary valves are viewed as bi-directional control valves.
The stem guide is a bushing that is fitted closely to the valve stem and aligned with the seat. Stem guiding in the valve is the most essential to minimize the packing leakage.
It is the pressure supplied at the inlet port of pneumatic instruments like actuators, controllers, positioners, or transducers. The commonly used supply pressure for the instruments is 3 to 15 psig.
The term turn down ratio in the valve for the particular system describes the relationship between the minimum flow rate and maximum flow rate conditions. For example: If the minimum flow rate is 10 G.P.M and the maximum flow rate 100 G.P.M the turndown would be 10.0
The seat leakage in the control valve is the quantity of fluid passing through it when the valve is in the fully closed position with the specified temperature and differential pressure.
Seat Leakage Class:
The seat leakage class categorizes the leakage tolerance for various valve trims. And this was established by ANSI B16.104.
Liquid Pressure Recovery Factor:
The pressure recovery factor in the case of non-vaporizing liquid service is defined as the ratio to the flow coefficient (Cv) based on the pressure drop at the vena contracta to the usual valve flow coefficient (Cv) based on the overall pressure drop across the valve.