Control Valves in parallel

[Iyer9]

Friends,

Please clarify my doubt:

I have 4 fully identical control valves (A,B,A1,B1).

A gas at constant pressure is fed on the upstream.

First 2 valves(A,B) are on the main line, one is acting as monitor and other as throttling control valve.
Second set of valves(A1,B1) are connected as bypass to the first set of valves.

Say for example:

If inlet pressure is 200 psig & drop across each valve is 50 psig.

Down stream pressure will be(considering bypass line is isolated) 100 psig, which is (200 - (50+50)).

If both main line and bypass are open, what will be effective drop across all the four valve and what would be the downstream pressure.

should we follow rule similar to electrical while resistors are in parallel,

Main line drop : 50 + 50 and bypass line drop : 50 + 50
(1/100 + 1/100)

Thanks

 

[TD2K]

If I understand the system, A and B valves are in series, each taking a 50 psi pressure drop. A1 and B1 are a parallel pair of valves not in service.

If both pair of valves are put into service, the downstream pressure will depend on the capacity of the downstream system and the Cv of the valves (operating and wide open depending on what cases you want to consider). There's no way to answer it with the information you've provided.

Let's say these valves were between a pair of steam headers, the upstream operating at 200 psig and the downstream at 50 psig. If you opened the second pair of valves there might be no change in the downstream pressure depending on the capacity of the low pressure system relative to the flow through these valves.

If instead the low pressure side of these valves flowed through a long pipe system to say atmospheric pressure (100 psig to 0 psig). If I opened second set of valves so I had two flow paths with identical flow characteristics (eg, set by the valves' Cv) you'd initially have twice as much flow through the two sets of valves. To get that flow through the downstream piping, the pressure downstream of the valves would have to increase and would settle out at 160 psig.

Since you said the valves were throttling, if you said that the valves went fully opened then it's impossible to estimate the downstream pressure. If the valves' Cv is relatively very large, the pressure drop through them could be minor leading to a pressure downstream of them close to the upstream pressure: 200 psig.

 

[TD2K]

Actually, for a gas you would have to consider the compressible flow behavior rather than incompressible flow which I used in the above example but the same conclusion holds, there's no way to simply state what the change in the downstream pressure will be when the second pair of valves are put into service.

 

[zdas04]

Could you provide a drawing? If I'm picturing your system right, then A and B are in series and A1 B1 are in series, and the two pairs of series valves are in parallel, but I'm not sure of that. What I am sure of is control valve operation will either try to maintain upstream pressure, downstream pressure, or flow (usually programmed as dP). I've never heard of using a control valve to provide a constant dP and can't imagine the process that would require it, but that is probably just a gap in my experience and imagination.

My experience with control valves in either parallel or i series is that the interaction is far from trivial and you often have to rethink your control criteria to get actual field results to match your desired results.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[BigInch]

I don't understand the control function. You say one valve is monitoring and the other is throttling.
Monitoring what? Upstream Pressure, Downstream Pressure, Differential Pressure, or Flow? Is it throttling to maintain a constant flow, or constant upstream pressure, or constant downstream pressure?

If the control function is put on standby, opening the other set of valves could have one of several effects.
You can't look at a system as some independently functioning valves without knowing how extra flow will affect the upstream or downstream piping. You need to know how the upstream system capacity relates to the downstream system capacity with any given change in flow.

If there is excess capacity downstream to carry away the extra flow, change may be small. But...

If the Downstream System Packs
If the extra flow packs up the downstream system, pressure throughout the system will tend to increase.

1.) If the valves are set to control downstream pressure, they will begin to close and reduce flow to keep the downstream pressures from building, which will tend to raise upstream pressure.

2.) If the valves are set to control upstream pressure, they will begin to open so as to keep the tendency of the system pressure to increase from increasing that upstream pressure.

3.) If they are set to hold constant flow, they will begin to open more. They would have to now that the downstream pressure is going higher. That would pack the downstream piping even more and possibly eventually reaching the capacity of the downstream system, at which point the downstream and upstream pressure will begin to increase. Eventually the valve would open all the way and system pressure would begin increasing everywhere.

Upstream System Evacuates
If the upstream system can supply the extra flow without reducing pressure, change may be small. If the increasing flow tends to evacuate the upstream piping and reduce pressure upstream of the valves, the upstream pressure, and the pressue in the whold system will tend to fall.

1.) If the valves are set to hold downstream pressure, they will begin opening to try to hold downstream pressure. If they can't when full open, the upstream pressure will fall as well.
2.) If the valves are set to hold upstream pressure, they will begin closing to try to hold upstream pressure. If they can't, then they will continue closing until they are fully closed and pressure is held.
3.) If the valves are set to hold flow, they will try to open, either reaching downstream system capacity and holding upstream pressure, or continuing to open and reducing upstream pressure in the process. Possibly, as they continue to open, reducing upstream pressure to the point where that matches downstream pressure.

Valves are part of the piping system. You can't tell what the valves will do, unless you know how the upstream capacity relates to the downstream capacity. It's like trying to figure out where your pump will operate by ignoring the system curve and only looking at the pump curve. That only works on the test bench.



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