Selecting the right valve for the process control requires that the engineer should be able to:
Valve sizing is a procedure with a set of specified process conditions that ensures the correct valve capacity.
The first step in finding the size of a valve is to calculate the flow coefficient (Cv) which is important for the system.
The process of “control valve sizing” is a procedure where the process dynamics of the system are matched to the performance characteristics of the valve. This produces a control valve of an appropriate size and type that will best meet the needs of managing flow within the processing system.
Every attempt should be made to carefully and accurately to size and select a control valve for the required application.
An undersized control valve can’t pass the required flow through it. But, actually, in several instances, a control valve is too large for the application.
An oversized valve is very sensitive to process operating conditions. Even the smallest changes in valve position will cause significant changes in inflow. This makes it difficult or even impossible for the valve to the exact tune of the required flow. A highly oversized valve will operate in an almost closed state, which is unstable and undesirable.
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Selecting a properly sized control valve is required to reach the highest degree of process control for the liquid, gas, or multi-phase fluid”.
For best process control applications:
The actuator is sized based on the thrust that is required to overcome the unbalanced forces in the valve body and seating force. Also based on stiffness necessary for stability.
Actuator speeds can be increased by enlarging the airflow ports and by installing booster relays. On/off valves, the addition of a quick dump exhaust valve will fast increase the venting rate.
It is the responsibility of the process control engineer to specify the valve failure position. It is a general practice to fail the energy supply valve (steam, hot oil, and so on) closed. Energy removing valves (cool or chiller water ) open.
The flowsheet abbreviations can be
FC – fail closed
FL – fail in the last position
FI – fail indeterminate (not exactly known)
FO – fail-open
Actuator without springs always requires a positioner.
The positioner will increase the actuator thrust by increasing the pressure or volume.
Changes in packing friction due to dirt, corrosion, lack of lubrication, the variation of dynamic forces of the process can be overcome with the addition of a positioner.
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The spring-and-diaphragm actuator is a low cost, provide high thrust at low air supply pressure.
Linear piston actuators do provide longer strokes and can operate at higher air pressures
Rotary piston actuators do operate at higher air pressures and can provide higher torque, suitable for throttling large ball or butterfly valves.
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