6 replies to this topic

[Takechi]

Dear all

 

I am a new member for this forum.

Could you please make me more understand.

 

1. I would like to know whether downstream pressure of control valve depending on %operation valve?

2. As I saw in datasheet in as-buit document of my plant, they alway design control with constant pressure drop. Thus, I would like to know that what different between design control valve for constant pressure drop and variable pressure?

 

 

Anupong

 

[fallah]

Anupong,

1) The approximate upstream/downstream Pressure of the CVs are specified at FEED stage based on pressure profile of the relevant plant and will be finalized at detail design stage using hydraulic calculation.
2) Pressure drop across CV could be different at min/normal/max flow rate which is mainly attributed to the different pressure drops at different flow rates.

[Takechi]

Dear all

 

I use wrong word.

 

1. I would like to know whether downstream pressure of control valve depending on %opening valve?

 

Anupong

[latexman]

1. Yes.

[breizh]

http://www.hassmfg.com/vscform.pl/

 

Let you consider this resource to clear your doubt ,if any.

Breizh

[katmar]

To get a clear picture of what is happening you have to first look at the entire system from the source of pressure to the discharge point, and then look at the section from the valve to the discharge point.

 

To keep things simple let us assume that the source of pressure is constant.  As you change the valve position the pressure drop across the valve will change (as described by the calculator referenced by Breizh).  Since the source pressure is constant, if the pressure drop across the valve changes then the pressure drop across the rest of the line must change to compensate.  For example, if you open the valve the pressure drop across it will reduce, but the flow rate will increase and therefore the pressure drop through the rest of the line will increase so that the overall pressure drop remains constant.

 

The section from the valve to the discharge point will not have changed physically, but the flow rate will have increased so the pressure drop also increases.  So opening the valve raises the downstream pressure and closing the valve lowers the downstream pressure.  The simplistic way to look at it is to say that with the valve opening more it is allowing more of the pressure to pass through to the downstream side, but this is not correct.  The real reason is the change in flow rate as described above and the resulting pressure drop across each section.

[villivord]

Yes, the downstream pressure changes as the valve opening increases/reduces as katmar explains but it is the system which determines the pressure drop across the control valve. A control valve is installed to generate a certain pressure drop which is required by the system. To determine this pressure drop in control valve sizing, one should determine the pressure drop from the source (for example a centrifugal pump) to the inlet of the control valve and the pressure drop from the outlet of the control valve to the discharge point. By doing this, you know exactly the pressure at the inlet and outlet of the control valve and it is this difference in pressure that the control valve has to generate. 

Edited by Borpe_, 28 January 2017 - 03:49 PM.