Answer / vipul d. soni

Maximum shut off DP is vital to size the actuator of the
valve. If the DP is less, the actuator size will be small.
As the DP increases, the actuator size also increases. If
your valve actuator is not sized properly, when there is
maximum DP across the valve (e.g., valve is fully closed),
you may not be able to open the valve. This is equally
important for regulators, control valves or shutdown valves.

Nomally at the initial design stage people are not sure
what could be the maximum DP across the valve. One safe
escape in that situation to consider the pipe line design
pressure as the maximum DP across the valve. Because at any
time the pressure in the pipe line will not be more than
the design pressure of the pipe line.

Calculate the maximum pressure drop that can be allowed to occur across a control valve, so that we can prevent cavitation / flashing from occurring. Use the following formula: Δ P allow = Km (P1 – PvapourA) where, Δ P allow - the maximum allowable differential pressure for sizing purposes, or terminal pressure drop; Km- the valve recovery coefficient from the manufacturer’s catalogue; P1 - absolute inlet fluid pressure; PvapourA - absolute fluid vapour pressure at the inlet temperature; Use Atmospheric pressure = 101.3 kPa, P1 = 1200 kPa(g), PvapourA = 4 kPa, KM = 0.72

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