Pressure drop across a control valve
Pressure drop across a control valve
(OP)
Hi,
For a flow control valve, having a fixed upstream pressure and downstream pressure, how does the pressure drop across the control valve vary with change in flow...or does the velocity change and pressure drop is constant?
For a flow control valve, having a fixed upstream pressure and downstream pressure, how does the pressure drop across the control valve vary with change in flow...or does the velocity change and pressure drop is constant?





RE: Pressure drop across a control valve
David
RE: Pressure drop across a control valve
And in this case, the total pressure drop constists of line pressure drop and control valve dP.
For example, assume that you have total pressure drop of 5 bar and control valve dP=2bar.
Available pressure drop for piping is 5-2 = 3bar
Now, you throttle your control valve so that control valve pressure drop becomes 4 bar.
You have only 1 bar pressure drop available for the piping therefore your flow through piping will be reduced.
Generally the pressure drop across piping is proportional to square of volumetric flow rate.
The resultant flow will be
Qthrottle = Q1*sqrt(1/3)
where Q1 = original flow rate
and Qthr = throttled flow rate
If Q1 was 100m3/hr then throttled flow rate will be 57.7 m3/hr.
Regards,
ifreeman
p.s. [ "Realistic Control-Valve Pressure Drops", Chemical Engineering 1987 Sep., J.R. Connel ] <-- This will be a good starting point.
RE: Pressure drop across a control valve
You ask about the pressure drop across the valve only, so I don't see how that addresses the question you asked, so I'll try...
You ask, "For a flow control valve, having a fixed upstream pressure and downstream pressure, how does the pressure drop across the control valve vary with change in flow...or does the velocity change and pressure drop is constant?"
Pressure drop across a valve is described by the valve's Cv Flow coefficient. Cv has units of gpm/(psi)^0.5, where gpm is flowrate in gallons/minute and psi is the differential pressure across the valve in psi.
So a Cv of 100 would mean (Pi-Po)^2 * 100 = Flow_gpm
where Pi = valve inlet pressure
Po = valve outlet pressure
This yields, (Pi-Po)^2 = Flow_gpm/100
So, discussing only the pressure drop at the valve and disregarding what might happen with total pressure drops in the upstream or downstream piping system, you can see that if Pi and Po remain fixed, and the Cv of 100 remains constant, there is NO change in flow across the valve, nor consequently, velocity either.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Pressure drop across a control valve
My comments to the other post are, that I agree totally only with Quark, for the following reasons,
MortenA says,
My response there is,
Actually its neither, your valve (position and corresponding Cv) controls the differential pressure across the valve alone, nothing else.
Quark says,
My response is, Correct I agree! I'll go back and put a star there.
dcasto says,
He sees that the new valve might not have any effect on upstream and downstream pressures if some other equipment is controlling those, and seems to imply that the control valve will change position to match whatever flowrate is going through it, since the flowrate through the new control valve is actually being controlled by the upstream and downstream equipment setpoints.
However my response there is,
It could work something like that, but why is the control valve needed, if upstream and downstream devices are controlling pressure and flow through that pipe segment now?
CJKruger says,
My response is,
If the pressure drop stays the same, any change in valve position would probably change the pressure drop, if it was not stagnated, hence the valve position change would change the Cv and the flow would not stay the same.
I leave the floor open for return fire.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Pressure drop across a control valve
The situation deccansher describes occurs when you have a high pressure and low pressure separator with a control valve in the liquid line between them. The separator pressures are controlled by other control systems.
Thus, the dP across the liquid control valve remains almost constant, and the valve opening changes to change the flow rate.
The only change in valve dP is due to the change in friction loss in the piping to/from the separators. This effect is normally small.
RE: Pressure drop across a control valve
Consider n pipes of different diameters Dn, carrying a turbulent flow of an incompressible fluid with a constant ΔPf. The velocity on each pipe would be proportional to Dn0.5, that is to say the flow rates would be proportional to Dn2.5.
It seems to me that, as with any other pipe flow restriction, when keeping the ΔPf constant by external means, the flow rate through the valve would be directly affected by its % opening (stem position) in some, probably non-linear, relation.
RE: Pressure drop across a control valve
Yes I agree with your answer and that your scenario is possible. I just wanted to emphisize those cases where its difficult to maintain constant pressure in both vessels. When I said "probably", I said that to discount the case where the upstream and downstream pressures can actually be held constant at whatever flow is going across the valve. Sometimes constant pressure is the desired operating method, however at large flows across the valve, the upstream vessel pressure tends to be reduced and the downstream vessel pressure tends to increase and the practicality of keeping them both constant is likely to decrease. If it is really possible to hold upstream and downstream constant... at any flowrate, I should/could have kept my big mouth shut.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Pressure drop across a control valve
You are right if you are talking about "dynamic behavior"
of piping system with relatively small system volume.
But, as you know, most of our work is done on "steady
state" basis. And the question given by deccansher was not
about maintaining the constant pressure at upstream/downstream. So I can say this is a steady state problem.
To make it more clear, I would like to suggest more defined problem here.
Imagine that there is a water tap which supply water to grass.
The source(water supply) pressure will be 'almost' constant
as the water system is very big and the destination
pressure(atmospheric pressure) is also constant.
Now I think we can concentrate on the control valve
drop vs. flow rate(therefore velocity etc.).
Regards,
ifreeeman
RE: Pressure drop across a control valve
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Pressure drop across a control valve
Thanks for the replies. Biginch and ifreeman in particular thanks a lot.
Let me make things little simpler......as i understood in school that we can have two types of control valves....pressure control valves and flow control valves.
My question is with regard to the 2nd option of a flow control valve.....
To elucidate with an example.....take a storage tank that has a valve at its outlet which discharges to ambient pressure. Now assuming negligible pressure drop for the line and the overhead tank always maintains the same liquid head.....the dP across the valve is fixed???
By changing the control valve opening....i control the flow! Does the dP ever change???
Hope this will help to resolve....or am i missing something?
Thanks again,
deccansher
RE: Pressure drop across a control valve
The control valve has a pressure drop associated with it as does the piping. The control valve is nothing more than a variable orifice. If I do not change the size of the orifice and piping, keeping everything the same, flow will not change, it can't. There is no motivatio to do so. As soon as I change the orifice size, I've changed the differential pressure and this will affect flow. The overall system pressure will not change (point A and point B) but the inbetween pressure drop(combination pipe and control valve) have changed. This is what happens at home in your sink. Opening the valve reduces the pressure drop of the control valve but this must be compensated by an increase in piping pressure drop. This is accomplished by a flow increase, and vise-versa.
RE: Pressure drop across a control valve
Opening the valve will tend to reduce the system hydraulic gradient, however if the system pressures upstream and downstream somehow compensate for the reduced hydraulic gradient and maintain their same potential at a new flowrate, the new flowrate will be achieved at steady state. Likewise the reverse is also true.
Taking a "free body" diagram of the valve alone, disregarding what equipment is upstream or downstream, a flow change TENDS to change dP and a dP change TENDS to change flow. If the change, whatever it was, is not compensated for by a set point at some other piece of equipment, the change propagates through the entire system until it finds a new equilibrium point.
In your constant liquid level tank question, the flow will increase when you open the valve and pressure drop will remain the same. The REASON for that is that YOU are specifying a boundry condition, "The system somehow can compensate for the reduced hydraulic gradient caused by the reduced dP when the valve opens. YOU say, "THE LIQUID LEVEL REMAINS CONSTANT. A "natural" system boundary condition (one requiring no intervention) at the tank would be one where the water level would normally decrease, if you didn't keep on refilling it "somehow". The other boundry condition at the valve discharge is that, pressure remains constant at atmospheric pressure, which is another "natural" boundry condition that requires no intervention to maintain.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Pressure drop across a control valve
A pilot-plant chap could tell us that a constant level upstream water tank can easily be achieved by providing a convenient overflow to the tank which is being continuously fed with an adequate excess.
Which brings us back to the ubiquitous Darcy-Weisbach equation and its applications to pipe fittings.
RE: Pressure drop across a control valve
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Pressure drop across a control valve
Of course there is an infinite combinations of Pup, Pdown, Cv and flow rates that will produce the same Pup and Pdown but the dP remains the same. But strictly thats reading something into what deccansher writes that isn't there?
RE: Pressure drop across a control valve
My conclusion (and also the suggestion from both the members) is that the upstream and downstream pressures are to be considered in the immediate vicinity of the valve. The gross upstream and downstream pressures give us wrong direction and thus violate natural laws. The pressure drop is very significant even for 1 m length pipe when acted upon 1m static head in a pure gravity flow.
The pressure, just upstream the valve, is lower incase of a open valve than a partially closed valve due to higher flowrate. But the extra pressure drop required during the reduced flow is contributed by the valve. Even the downstream pressure is held constant, say by not providing any pipe section downstream the valve, the pressure drop varies for every valve position and thus flowrate.
I did the same thought experiment of various orifice sizes as pleckner did and that solves the issue. Still, I am not sure about a case where a sandwich type buttervalve is directly bolted to a tank without any downstream piping. Now if we operate the valve at various positions, the flowrate should change yet maintaining constant upstream and downstream pressures, if we top up the tank at a rate equal to that of outflow. We shouldn't bother whether this is practically possible or not. We should have the theoretical answer.
There are two quack ideas as I never observed the actual case. Either the flow shouldn't change or the entire tank should offer resistance to the flow. That is the pressure at the bottom of tank may vary at various flowrates
I am interested to continue the discussion, in any case.
RE: Pressure drop across a control valve
I think this latest discussion is all due to the fact that, to find total pressure drop, either the inlet or the outlet pressure must be known and its sometimes difficult to separate the theoretical pressure drop of an individual element (or sum of several) from that practical reality. P1 - Cv dP^2 = P2
Now let P1 and P2 float. Note that it can be Cv for a valve, k for a pipe, maybe L*(r1^2-r2^2)*Q for a reducer, or whatever for any individual flow element, but the sum of all of them only ever equals dP for the system of elements, until you specify either P1 or P2. Even if flow is known, either P1 or P2 must also be known to solve the hydraulic system equations.
Even with a butterfly or sandwich check valve bolted to the tank, theoretically there is still an infitesimal length of pipe both upstream and downstream of the valve's "orifice".
OK, let's take it another step farther and consider just a hole in the side of a tank.
Now, theoretically and practically, inside the tank itself, as a fluid particle goes from rest away from the tank outlet to some infitesimal velocity, there is the transformation from static to dynamic head, and there is also the exit flow coefficient, Cd, to deal with that drops the hydraulic gradient as a fluid particle drifts from an area under the influence of only the liquid surface static pressure, accelerates and moves into the tank outlet and out into free air or into the inlet just before arriving at a valve's orifice. That loss can be significant, depending on shape and form of the outlet at the tank wall, a nozzle protrusion, [] or O x-section, curved or square, sharp or blunt edge, etc.
But, as soon as you say, "constant level"... you've fixed the upstream pressure and can then calculate downstream pressure, or v/v.
Was that it?
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Pressure drop across a control valve
RE: Pressure drop across a control valve
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Pressure drop across a control valve
Yes, it is as simple as that and somehow I got distracted by the constant pressure difference of gravity flow (I mean your 18th Dec post). Pressure drop by the valve is the significant factor when the valve is directly bolted to the tank.
Reena,
Like I mentioned earlier, at the full rated flow of an opening (straight conversion of static head to velocity head), the pressure drop in a 1m piping is quite significant and is about 65% for a 1" line.
RE: Pressure drop across a control valve
Thanks for the responses. I am glad the question was not as simple as i thought it would be....anywyas.....continuing the discussion further...i had two more thoughts-
1. For sizing a valve...the Cv is determined for a constant pressure drop across a valve?
2. for a good orifice, doesnt that pressure finally recover itself to about 100% of the original (theroetically possible?)
So where is the pressure drop now across the orifice? So why would you have a varying pressure drop across the valve?
RE: Pressure drop across a control valve
If there is a pressure drop with a straight pipe section, why shouldn't it occur with a flow restriction ?
RE: Pressure drop across a control valve
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Pressure drop across a control valve
Secondly, there is permanent pressure drop due to restriction characteristic of the orifice. This can't be regained.
RE: Pressure drop across a control valve