Pressure drop 1

[deccansher]

Hi,
Wanted to know if say for a flow control valve, which has a fixed upstream and downstream pressure, does the pressure drop across the control valve change with change in flow?

 

[MortenA]

Your valve cant both control upstream and downstream pressure. Its either or.

Best regards

Morten

 

[quark]

In other words, there is absolutely no change in flowrate when the upstream and downstream pressures are fixed.

 

[dcasto]

We may want to quiz this question more. The upstream pressure may be fixed by another system (Valve, gas supply system with multiple control devices), hence fixed upstream, then the valve he is describing may be the one controlling the downstream pressure. Or the opposite where another system (Valve, speed controlled pump) is controlling downstream pressure and this new valve the upstream?

As for change in flowrate with a, lets used a different word, controlled up and down stream pressures, then yes the valve will be changing positions as the flowrate changes given the assumptions in the first paragraph.

deccansher, any more information on how you can get both up/down pressured fixed?

 

[CJKruger]

deccansher,

No, the pressure drop stays the same and the valve opening change with flow rate.

 

[rzrbk]

If upstream and downstream pressure are fixed then flow will change as the CV of the valve changes with change in position.

There are several places in a plant this might happen, but the most common would be a drain from a large vessel, or even at your home water faucet. (ignore small line losses as they would not be a significant cause of the flow change)

 

[MortenA]

My first may have been a too tight interpretation of the question? I read that if the Pup and P down is fixed (the dP is thus fixed)then pressure drop of course wont change?

But the flow may change and the P up and P down may remain the same because the valve opening changes.

Best regards
Morten

 

[reena1957]

Assume the source pressure and Sink pressures remain same due to other constraints and controls as rzrbk suggests. If there is a significant drop between the source and the CV as well as the CV and the Sink, then as the flow varies, the u/s and d/s pressures of control valve has to vary irrespective of opening of CV as for a given flow, the CV cannot influence the u/s pressure drop from source to CV NOR the d/s pressure from CV to Sink. Since these two are fixed for a given flow, the CV ensures by proper opening, the correct pressure drops occur in both these sections for the desired flow OR if the opening is fixed, the flow gets fixed so that the pressure drops are appropriate for that flow.In other words there is only one degree of freedom, when there is a CV and none if there is no CV(in case source and sink pressures are fixed).