from the instrumentation department...
allowable pressure drop across control valve
 Posted by tushar on 30 September, 2009 - 3:16 am
Dear all,

What is a maximum value of pressure drop should be considered across control valve? I read somewhere that pressure drop should not be greater than 50% of inlet pressure.

 Posted by Ronald Deepak on 18 October, 2009 - 1:08 am
Hi Tushar,

This debate on what might be an allowable pressure drop across a control valve is a very generic one. To broadly classify, we need to know if the flowing fluid is a compressible fluid (gas or vapor) or a incompressible fluid (liquid).

1. For Gas/Vapors :
For compressible fluids, when pressure drop exceeds a certain value (defined by the valve trim) and the fluid property, the flow produces a lot of noise and vibration. This pressure drop is called "Choked flow" pressure drop. Usually it is observed that when pressure drop across a valve is increased, there is a corresponding increase in the flow through the valve, as defined by the flow sizing equation (ISA 75.01.01). However, when the pressure drop exceeds the choked-flow pressure drop, any additional pressure drop will not increase the flow as per the sizing equation. Only a small increment in flow will be observed (which occurs due to the enlargement of the vena-contracta). The additional pressure drop above the choked-flow pressure drop goes towards producing Aero-dynamic noise & subsequently causes vibration. Having said this, if the noise generated is within acceptable levels, the pressure drop can still be taken across the valve. In cases where the pressure drop is too high (as in the case of Flare/vent valves), a diffuser is added downstream of the control valve. An ideally designed diffuser will take the pressure drop that is in excess of the choked flow pressure drop. A rough check to see whether the pressure drop is above or below the choking value can be done by using the below relation :

x = Fk * Xt

(x = pressure drop/inlet pressure)
Fk = Specific heats ratio factor of the fluid

To summarise : choked flow pressure drop will be : Fk*Xt*Inlet pressure

2. For liquid services, the choked flow pressure drop is dictated by the pressure drop at which cavitation begins to occur. As we all know, whenever the fluid pressure falls below the vapor pressure, the liquid starts to form bubbles. When the fluid pressure recovers once again above the vapor pressure, the bubbles already form collapse and this is termed as cavitation. Cavitation can cause serious damage to the valve internals and lead to lack of proper control. The allowable pressure drop in this case is determined by :

a. Inlet pressure
b. Pressure drop
c. Vapor pressure of the fluid
d. Valve recovery characteristics

It is to be noted that cavitation is a phenomenon that is caused by both the process AND the valve recovery characteristics. For Processes having high possibility of cavitation (e.g. pump min. flow recirculation), valves with suitable recovery characteristics (low -recovery) or anti-cavitation trim which prevent the occurrence of damaging cavitation can be used.

Ronald Deepak
jronalddeepak [at] gmail.com

 Posted by Sara Joshua on 17 August, 2012 - 11:51 pm
Dear Ronald,

Thank you for your informative post
I have question regarding fluids with Choking due to high flashing conditions. Do you recommend using downstream diffusers, e.g. for a water application with P1 = 73 kg/cm2 (g), P2 = 2.43 kg/cm2 (g), T1 = 287 degC, Q = 380 t/hr.

I understand that the flow requirement can be met by selecting an appropriate Cv value. But how can I ensure that my downstream pressure will be achieved. Would appreciate if you could share your experience on the above.

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