12 replies to this topic

[juuichi]

Hi fellow forumers,

 

I have got a question which is puzzling to me with regards to control valve.

 

For a control valve I know that when the opening increase, flowrate also increase in actual.

 

However,

When valve opening increases, the pressure drop across it should reduce. And when the pressure drop across it is reduced then it should lead to a decrease in flowrate since the pressure drop across the valve drives the flow. This is counter-intuitive to what I said earlier which should give rise to an increase in flowrate.

 

What is wrong with my understanding here?

 

[fallah]

juuichi,

 

For a fixed opening such as a restriction orifice you are right, but in a control valve with variable orifice, in fact you would reduce the flow by opening reduction (more restriction) that in turn will lead to higher pressure drop across the valve...

[mansari]

Cv of any valve increases as does the opening of the valve. Therefore the valve would be capable of delivering more flow at higher opening. Pressure drop is goverrned by the system where contribution of the valve orifice is not significant. Normally the pressure drop of the system is fixed therefore for a fixed dP, increase in opnening of the valve would simply result in more flow. Variable system pressure drop is different matter and considered during the sizing exercise so that the valve must perform to meet the system demand

[shan]

A valve is a simply pressure drop element. The flow rates at downstream valve are always equal to flow rates at upstream valve.
If the flow rates are identical, the pressure drop will be larger when the valve opening is smaller. If the valve openings are identical, the pressure drop will be larger when the flow rate is higher.

[juuichi]

Agreed with what were said above. Clear explanations. So pretty much what drives the flow is the overall pressure drop of the system. 

 

However with mansuri's explanation, for a fixed dP system increase in the control valve opening will result in more flow. In that case since the dP across the whole system is fixed, I don't see how increasing or decreasing the control valve can affect the flowrate according to theory, since the flow should be the same with the same dP across the system, irregardless of the opening of the control valve.

 

It would definitely help if any of you guys can send me any illustration of pressure profiles of different flows across systems as it would be very clear to me. Or any documents/readings that can explain this well. Thank you guys.

[Zunair]

Dear @juuichi,

@Fallah has given a very good explaination let me elaborate it a little.

 

In case of fixed opening (for an orifice) the governing factor is the system's pressure,hence the pressure drop across the orifice derives the flow.This situation becomes idential in a control valve when it is fully opened and your understanding is correct.

 

Now lets suppose the control valve is fully closed.There will be no flow.During operation of the control valve from 0% to 100% the governing factor for the flow is partially shared by the corosponding opening of the flow area i.e Control Valve.Since the opening is incerasing the flow will eventually increase untill the system pressures completely become the governing factor for the flow. 

 

Hope this would be helpful for you.

[kumarpradeep2007]

juuichi,

 

If I understood you corectly - You are confused over Pressure drop across control valve (Resistance to flow)  and Pressure difference between source and destination system ( Driving force).

 

Let me put in simple way. If your upstream to control valve source at say 10.0 Kg/cm2(a) and down stream destination at 5.0 Kg/cm2(a). Suppose your control valve is in closed position. At this position upstream of control valve will see 10.0 Kg/cm2(a) and downstream 5.0 Kg/cm2(a). For now I am assuming no other hyrdraulic losses. In this scenario Pressure drop across CV and diff. P will be 10.0-5.0 = 5.0 Kg/cm2(a).

Now as as you start opening CV down stream pressure will be slightly higher than 5.0Kg/cm2(a) say 5.5 Kg/cm2(a). In this scenario your delta P across valve will be 10.0-5.5=4.5 Kg/cm2(a) but diff pressure b/w source and destination will be same 10.0-5.0 =5.0 Kg/cm2(a).

( As we assume that source and destination is at fixed P).

 

So you can see as as you increase valve opening your delta P across CV will reduce i.e. flow resistance is decreasing which will lead to increse of flow.

 

flow = driving force (Pressure difference b/w source and destination) / resistance (Delta P across valve)

 

I hope this will help you.

 

Regards,

Pradeep Kumar

[Shivshankar]

Hi

 

Now you can apply above all inputs to imagine how control valve works. Find below link and imagine for fixed orifice.

 

∆P = Q^2

 

Differential pressure is proportional to the square of the flow rate. If you double the flow the differential is pressure is four times and so on.

 

Differential pressure as a function of the flow

 

Flow                 DP

100%              100

 50%                 25

 20%                   4

 10%                   1

 

Edited by Shivshankar, 25 April 2013 - 02:44 PM.

[Dacs]

Heads up for a wall of text

 

You have to look at the whole picture to understand where control valve (or any pressure letdown elements) come into play in hydraulics.

 

Normally, you'd have a source (with fixed pressure P1) and a destination (fixed pressure P2) connected by a pipe with a control valve on it (or an ordinary valve for that matter).

 

If the valve is closed, then no flow goes through with it and the pressure upstream and downstream of the valve would be P1 and P2, respectively.

When you start opening up a valve, there would be a path where a flow from a higher pressure (P1) to a lower pressure (P2) can occur, and in fact, flow must occur. The amount of flow will depend on these:

1. Pressure loss upstream of valve

2. Pressure loss downstream of valve

3. Pressure loss across the valve itself.

 

And the summation of the pressure losses (sumDelF) above must match the pressure differential (P2-P1, assuming no static head exists for the benefit of discussion).

 

I think it's safe to say that for the vast majority of hydraulics, pressure loss at #3 is controlling, but it doesn't mean that losses from #1 and #2 can be ignored.

 

As you open the valve further, less constriction occurs on the valve, so more flow has to be allowed. Still, sumDelF=P2-P1 has to be satisfied at all times, so the reduction of pressure drop in #3 has to be accounted for. And it will be accounted for by an increase in pressure drop for both #1 and #2 (because pressure drop is proportional to increase in flow).

 

So to answer your question:

For a control valve I know that when the opening increase, flowrate also increase in actual.

This is true, provided that the pressure at inlet and outlet of valve remains the same.

 

When valve opening increases, the pressure drop across it should reduce. And when the pressure drop across it is reduced then it should lead to a decrease in flowrate since the pressure drop across the valve drives the flow. This is counter-intuitive to what I said earlier which should give rise to an increase in flowrate.

You have to realize that what you only did when you open a valve is to have a larger area for a flow to go through.  The flow across the valve still depends on how much upstream and downstream pressure you have on your valve.

 

Since you have a bigger area for flow to pass through, it follows that you'd have more flow on your valve (less resistance to flow). While it's tempting to assume that the pressures upstream and downstream would be P1 and P2 respectively, we also have to account for the pressure losses on your piping, which in turn depends on flow.

 

So there should be a point where all balances out, when for a given flow F, the pressure losses for both upstream and downstream piping will result in an upstream (P1-delPupstream) and downstream pressure (P2+delPdownstream) that will in turn, results in the same flow F for a given valve opening (constriction).

 

As pointed by the other members here, valve opening represents resistance to flow. Just think of a valve opening as a length of pipe (with a given diameter). When it's opened 100%, the length is at a minimum. As you close the valve, this length increases and it goes into infinity as you close it fully. It might not be a good analogy but I hope you get the drift.

 

However with mansuri's explanation, for a fixed dP system increase in the control valve opening will result in more flow. In that case since the dP across the whole system is fixed, I don't see how increasing or decreasing the control valve can affect the flowrate according to theory, since the flow should be the same with the same dP across the system, irregardless of the opening of the control valve.

While your system dP (P2-P1) is fixed, the resistance to flow is not, at least for the valve. Closing the valve is akin to increasing the flow resistance so by closing the valve, you'd have less flow (and it goes vice versa).

[Lai.CY]

Hello Juuichi,

 

 

Your confusion comes into the picture because you are not comparing the scenarios with the same upstream pressure.

 

You can push 100m3/hr of flow through a 2" pipe, provide you don't mind the massive pressure loss which will result a bigger pump (if any), higher piping design pressure (for hammer or pressure build up), vibration (high velocity), etc and so on.

 

But, if we're talking about the same upstream pressure, then your pressure drop is limited. Your flow will definitely increase with lower pressure drop (less restriction).

 

Imagine you have 10 cars, in a 10 lanes tunnel, reducing it to just 5 lanes... Of course, the cars will slow down at the restriction entrance and at the outlet, where there are no restriction, the cars can speed up (because you only have 5 cars with 10 lanes downstream!)...

Now, imagine again, with 10 cars on a 10 lanes tunnel, going into 3 lanes...

 

Sorry if my example sounds stupid...

[sam03]

Hello...........Everyone

Can any one help me to calculate the pressure drop across an On/off valve at different opening position..............suppose 12" on/off valve having a response time of 5s and passing a 440Kg/s of water at inlet pressure of 20bar............then how the pressure across the valve will vary. one more thing how %tag opening of valve with time can be found......

 

 

 

Thanks in advance

Edited by sam03, 31 October 2013 - 02:44 AM.

[MOMO]

Is the a rational explanation why DP across a given valve at a given flow rate and opening will be different than when this same valve is installed in a system with Pump, Elbow and restriction?

I'm trying to comprehend the notion of "Installed Valve Characteristic" vs "inherent valve characteristic". To me those advocating for such a concept are comparing apple to peanuts....

Thanks for your contributions

[dehn0045]

MOMO - Remember that if a valve opening and flow rate are fixed (assuming also that fluid properties are also fixed, which you didn't say but I am assuming is implied) then the pressure drop across the valve will also be fixed.  Therefore if you add a pressure drop  at the outlet of the valve, then the inlet pressure will need to be increased by this amount to keep everything constant.
 
The "installed" and "inherent" valve characteristic are looking at the same valve, but with different assumptions.  Inherent characteristic assumes that the upstream and downstream pressure drops are negligible, and you have an unlimited supply of flowing fluid.  Sometimes you can get close to this ideal, but it is never truly the case.  Also, "characteristic" is looking at the flow across the valve at the entire range of valve opening.  When the pressure drop of the rest of the system (excluding the valve) becomes large relative to the pressure drop across the valve, then the "installed" characteristic diverges from the "inherent" characteristic.  Consider this example:
 
A pump is supplying fluid to a control valve that is discharging to atmosphere.  As the valve opens, the flow across the valve increases.  However, as the flow through the pump increases, the pump discharge pressure drops.  This means that the supply pressure to the inlet of the valve and the pressure drop across the valve decrease as the flow is increased.  This means that the flow across the "installed" valve shows a different curve than a valve in a hypothetical system that does not have upstream/downstream pressure changes.  Below, red would be the hypothetical "inherent" and blue the hypothetical "installed" characteristics.