Conditions to be met by the instrumentation specialists for control valves and on/off valves
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This post was last edited by SACREDSOUL on 2019-7-2 at 13:26. For control valves, it is necessary to know the pressure difference when the valve is closed, as well as the pressures before and after the valve. For on-off valves, it is necessary to know the pressure difference when they are closed; it doesn’t matter if there is no pressure before or after the valve. Also: The pressures before and after the valve refer to the conditions during operation, right? ； The pressure difference when the valve is closed – does it refer to the pressure difference at the moment of closure, or the pressure difference at the final stable state after closure?
I actually just want to ask about shutting down the pressure difference; could someone downstairs explain it?
For requirements and typical forms, refer to the Design Specifications for Automatic Control Engineering of Chemical Processing Units. The pressure behind the valve is the pressure in the downstream system. The manufacturer uses the pressure difference to calculate the torque force.
Pressure difference at full valve closure. Only then can the appropriate torque for the actuator be determined
For on-off valves, it is generally sufficient to know the pressure difference when they are closed. The pressures before and after the valve are typically calculated assuming a pressure difference of 0 when it is open. Since the valve has only two states, the driving force during operation can be calculated based on the pressure difference when it is closed. The pressure difference when the valve is closed; for emergency shut-off valves, it is calculated based on the pressure difference at the moment of closure. This is because the inertia of the fluid in the pipeline affects the closing speed (this applies only to liquids; gases do not have this issue). The marked emergency shut-off valve also does not need to consider this issue.) ； In the case of a normal cut-off, it is calculated based on the pressure difference at the final stable stage. In most cases, the design pressure of the pipeline can simply be used as the pressure difference for selecting the on/off valve. Simple, brutal, and foolproof.
Thank you, that’s very detailed. Could you talk more about the situation with control valves?