Understanding control valve bench set
C ontrol valves are frequently reused, disassembled for maintenance, or 'stroke tested' before installation. A key factor to ensuring that control valves perform as expected, is understanding how operating conditions, changes in packing material, spring compression, and other influences contribute to overall control valve performance.
During control valve and actuator sizing, several factors must be considered to ensure the actuator can move and maintain the valve plug in place while operating at process conditions.
Factors influencing control valve performance under operating conditions include:
Diaphragm loading pressure , which is frequently 3-15 psi, but may be significantly higher, especially when a positioner is being used.
Diaphragm effective area is defined by the size of the actuator. Most manufacturers offer a variety of actuator sizes.
Valve plug stroke and travel is determined by sizing the valve body to meet flow requirements.
Secondary forces acting on the valve plug in order of importance include static unbalance, dynamic unbalance, packing friction, stem size, and diaphragm hysteresis.
Spring characteristics are the most active of all influences and should be determined after all other factors have been considered.
A control valve and actuator assembly is tested on a bench, not under actual operating conditions. To meet leakage criteria, the actuator must be assembled to provide seating force beyond that required to stroke the valves fully open (or closed). Bench set consist of considering all the factors and forces, selecting a spring with the correct spring rate, and setting an initial compression on the spring. The result is a control-valve assembly that responds differently when tested on the bench, than when it is installed in the process where all active forces are in effect.
Before reusing a control valve in a different application, mixing and matching valves and actuators, adding or removing a positioner, changing components that influence hysteresis, or adjusting spring compression, it is best to discuss the planned changes with the original valve manufacturer.
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