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Sizing Control Valves-Outlet Pressure
6

Sizing Control Valves-Outlet Pressure

Sizing Control Valves-Outlet Pressure

(OP)
Hello everyone,
When looking at sizing control valves, what outlet pressure should be used in the calculation?  So lets say I have a valve that has an inlet nitrogen gas pressure of 120 psig.  The outlet pressure used in the correlation should it be the expected pressure drop through the valve or the pressure far downstream of the valve?  So lets say the source about 25 feet downstream of the valve and at atmospheric pressure normally.  Should I used delta P of 120 psig?

Thanks.

RE: Sizing Control Valves-Outlet Pressure

First,

If you really need a downstream pressure control valve, you need it because you will have to hold a downstream pressure to some value.  If you don't know the value, why do you need a control valve?

Typically a downstream pressure control valve application will be to hold pressure below the allowable pressure of another pipeline, below some allowable pressure of a piece of equipmet, or above the NPSHR pressure equivalent of a pump, or at a minimum pressure needed to produce a reaction for some process at a refinery, so usually all those pressures are well known.

The set point (pressure setting) of a pressure control valve should be adjusted to the point where the control valve's pressure sensor is located.  For example, if a valve must be set to control the pressure of a vessel 1 mile downstream of the valve to <= 500 psig, you should set the valve's set point to 500 psig + the pressure loss in the 1 mile of pipe between the valve and the vessel, PROVIDED THAT that set point is not higher than the allowable operating pressure of the vessel, since if the vessel outlet is closed by mistake or for some emergency reason and the inlet is left open, it would be possible for the valve's set pressure to eventually reach or exceed the vessel pressure, as flow from the valve to the vessel finally comes to a stop.

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Sizing Control Valves-Outlet Pressure

(OP)
Can't the valve just be for flow control also (say water needs 10 GPM but too much would waste money and too little would hurt a scrubber).  What would you size the outlet pressure for there (still MAWP of the scrubber?)

Thanks.

RE: Sizing Control Valves-Outlet Pressure

2
Control valves are usually referred to by their control function.  If they control pressure, they are pressure control valves "PCV"; if they control flow, they are flow control valves "FCV"; if they control temperature they are temperature control valves, etc.  Note that each type of valve is positioned based on a particular type of control signal, from a pressure signal, flow signal or temperature signal, respectively.

Since you asked "what outlet pressure should be used", I thought it was reasonable to think that was your intended contro function and you were sizing a "downstream pressure control valve".

Yes, of course you could size a "flow control valve" for a particular flowrate where the positioner is being driven by a flowrate signal from a flowmeter of some kind.  In that case, you know the flow you want to hold, so you size the valve using that flowrate and using the Valve Cv at say about 75-80% open.  That will give you a specific pressure drop at that flowrate.  Then you check your piping pressures to find the minimum possible pressure at the flow control valve's inlet, subtract the pressure drop and you know the outlet pressure, assuming its not below 0 psia.  If its below 0 psia, or its below some other minimum pressure you think you should be holding for some reason, product vapor pressure, or other reasons as per previous post, you resize the valve a little larger.  If you valve becomes too large, you look at ways to increase the inlet pressure.

It is also sometimes possible to cross a particular desired control function, say flow rate control, with a different type of transducer signal, say pressure signal, but it is a more complex relationship and imprecise method for which you must be able to establish a very good relationship between flow and pressure that will hold for all pressures in your piping system.  Your piping system must show relative stability between the cross-controlled variables.  For example, when you increase a solar hot water flow to raise temperature of your bath water, you should ALWAYS get more hot water.  If the water is cool before the sun comes up in the morning, your valve control function will not have the desired effect and you might flood the bathroom.

Say you want to have 10 gpm flowing from the end of a pipe open to atmosphere.  You know atmospheric pressure is usually 14.696 psia with relatively small variations and your hydraulic analysis shows that by holding 30 psia at the inlet to the last pipe, you will discharge 10 gpm.  There you can try to size a control valve to hold 30 psia, when it has a 15 psi pressure drop when flowing 10 gpm.  Then you can add 14.696 + pressure drop of last pipe, say 1.5 psi = 16.196 psia (1.5 psig) and get the set pressure at the downstream side of the control valve, 1.5 psig.  Then add the 15 psi to 16.196 psia to get the inlet pressure required at the valve = 31.196 psia (16.5 psig).  Everything will be just fine, until your valve inlet pressure starts to change.  If the inlet pressure goes up, more flow will start to move across the valve, which will increase the pressure drop across the valve.  The valve sees the discharge pressure also goes up and starts closing to reduce it.  If the valve closes the appropriate amount, it is possible that your flowrate is maintained at 10 gpm, but don't count on it closing an appropriate amount, because it doesn't really care about your 10 gpm.  It cares about the 1.5 psig setting, so the flowrate will be whatever it must be to get the 1.5 psig outlet pressure.

So, after all is said and done, maybe it works, maybe it doesn't, but what is true is that you should have a very stable system to try to use cross control.  It also requires a very good match of pressure drop and flow change.  Its not easy to do.  The system must be relatively stable.  Then, you realize that, if your system is stable enough to do this, then you might not really need a control valve, as you could probably control flow just as well with a manual plug valve that you came around once a day or week to make some fine adjustments.  That's a bit of an exaduration, but makes my point that a manual valve might do just fine if the system is stable enough to allow crossing control.  You just have to check and make sure it will work, because if it doesn't, things can get bad quickly.

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Sizing Control Valves-Outlet Pressure

2
Everything BigInch said was good.  Print it off and save it in your valve sizing binder.  

For your SPECIFIC case, you have N2 at 120 psig (135 psia).
Once your discharge is lower than about half the inlet pressure :67.5 psia, 52.5 psig,  you will get choked flow.(and yeah, I'm using 15 psi for atmosphere just because this morning I don't feel the decimals are all that important for ther sake of this exercise). Choked flow is widely misunderstood because it is poorly labeled, but all it really means is that your downstream ressure no longer affects the flowrate.  So for s FIXED orifice, if you start out with 120 in and 119 out, you will get a flowrate.  If you drop the outlet pressure to 118, 117.....70, 69 the flow will increase every time.  But at some magical point, the velocity in the orifice hits sonic and a shockwave forms across the orifice.  All the downstream N2 molecules can no longer communicate with the upstream N2 molecules to let them know that "there's plenty of extra room downstream of the orifice-c'mon down".  Here's where it gets confusing for many: If you increase the size of the orifice (open the valve)  YOU STILL GET MORE FLOW. The real significant thing about choked flow to a valve engineer is that you have to recognize it mathematically and pop over to a different equation.  FLow is no longer a function of DP but a function of P1-Pcrit which can be a lot less than actual DP.  That's why they call it "choked": There can be a lot less flow happening than if you use the full DP in the calculation.  

So in your specific application, you more than likely have critical drop/choked flow and it doesn't much matter what you use for a downstream pressure. Any value less than about 52 psig should give the same result if your math is right.  THEN you need to look at pipe velocity and noise, because if you are flowing sonic downstram of the pipe you will make an awful scream and you will get calls from the AirForce complaining about the noise.  

I ran this for a 1" valve (you didn't specify) at 100F ( you didn't specify that either) in my Flowserve sizing program.  I get 877.513 scfm flowrate calculated for every value less than 40 psi down, using a Valtek Mk1. Small variations will occur with other valve types because Fl is used in the calculation for critical drop and that varies among valve styles.  

RE: Sizing Control Valves-Outlet Pressure

When calculating the capacity coefficient, use the far downstream pressure for the pressure drop.  The lowest actual pressure just after the vena contracta is difficult to predict.  Your objective is to control the flow or downstream pressure at least five pipe diameters after the valve.

RE: Sizing Control Valves-Outlet Pressure

(OP)
When you have choked flow at the valve, are you at sonic velocity in the pipe after the valve?

Thanks.

RE: Sizing Control Valves-Outlet Pressure

Just inside and in the imediate vicinity of the valve.  The flow streamlines expand just afterwards and slows down.

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Sizing Control Valves-Outlet Pressure

(OP)
So another thing is that we do need to keep pressure downstream of the valve below 4" WC to help protect the baghouse we have downstream.  So our N2 supply pressure is around 120 psig.  I believe we need to drop the pressure in stages (PCV which drops from 120 to 60, the the FCV which drops from 60 to 4" WC while controling).  Going from 120 to 4" probably will cause control problems.

Thanks

RE: Sizing Control Valves-Outlet Pressure

That's not much pressure range allowed for control swings.  You'll need some fine tuning possibilities there.  I think two regulators would not be out of the question; the last one a finely tunable needle valve type.

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Sizing Control Valves-Outlet Pressure

(OP)
Could you please give me an example on valve sizing?  I'm looking to size a valve to provide 75 SCFM nitrogen maximum, 50 SCFM normally, and 25 SCFM minimum.  Upstream pressure is 120 psig and upstream temperature is 70F.  Downstream pressure can not be any more than 4" WC (usually atmospheric).

Thanks.

RE: Sizing Control Valves-Outlet Pressure

Just find a CV as near as possible to these characteristics,

a Cv at 60-70% open that will give you a flow of 50 SCFM,
a Cv at around 80% open (max % open, more or less) that gives a flow of 75 SCFM and a Cv at around 30% (min % open)open that gives a flow of 25, while using a 120 psi pressure drop across the valve.  If the pressure CANNOT exceed 4" WC and that figure is CRITICAL to prevent bags from being destroyed or something, I'd put a pressure relief valve set at 4" WC and able to flow 75 SCFM somewhere between the regulator and the bag plant.

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Sizing Control Valves-Outlet Pressure

I ran the numbers through the Flowserve sizing program:

70F N2 120 psig P1, 4" WC(G)P2,
at 75 SCFM: Cv required is 0.962
at 50 scfm, the Cv required is 0.642.  
At 25 scfm, Cv required is 0.321

This puts you pretty squarely in the realm of the Kammer 30,000, Masoneilan 28000, Samson 240, and Baumann Little Scotty type globe valves.  A 1" Worcester CPT control Ball valve with a 15-degree seat looks good, too.  73%, 57%, and 42% of stroke respectively.  

Velocity in a 1" pipe is Mach 0.289, which is about as high as you really want to go.  


RE: Sizing Control Valves-Outlet Pressure

Thanks. I wish we could post pictures in this forum.  It would have been neat to post a screen shot from the sizing program.  

RE: Sizing Control Valves-Outlet Pressure

or use the image tag:

[img http://....]

I2I

RE: Sizing Control Valves-Outlet Pressure

Typical picture an engineer would post.......

Back to the topic:
>>>When you have choked flow at the valve, are you at sonic velocity in the pipe after the valve?<<<

Maybe.  Depends upon the pipe size, downstream pressure, and flowrate.  If you are sonic it means you need to re-size your pipe, so the velocity does not exceed Mach 0.7.  Above that, you're gonna make a lot of noise even with noise attenuation valve trim.  Sonic flow is just a cascade of standing shock waves, and it really screeches.  

RE: Sizing Control Valves-Outlet Pressure

(OP)
OK so what is good practice for my case if I want to drop the pressure in steps.  Have a PCV drop the pressure from 100psig to 8 psig, then have the FCV drop the pressure from 8 psig to 4" WC (8" is the MAWP) or the opposite?  The FCV will be the one that is variable (actuator with a PID loop to control flow on demand) while the PCV will me a manual valve set and let go.

Thanks.

RE: Sizing Control Valves-Outlet Pressure

If the source is clean nitrogen, then the process conditions you described can be well met by a valve which will frictionally choke the flow, as opposed to acoustically choke the flow. An example of a valve that will frictionally choke the flow is the CCI Self-drag valve. Yuo would only need one valve in this case, not two, and there would be no noise issues, as might happen if you used a conventional valve that had acoustically choked flow.

RE: Sizing Control Valves-Outlet Pressure

This is not an application for a CCI Drag valve.  Those are for 3 or 4 leagues more severe than this.  

I wouldn't even bother staging this one.  The pressure is not high, the flow is small, and your limitation on control will be the sensitivity of the pressure transducer.

RE: Sizing Control Valves-Outlet Pressure

(OP)
So you are saying that the control valve can take the 120 pressure drop at 0-100% of total opening?  We wouldn't need to drop the pressure before the valve?  

Thanks.

RE: Sizing Control Valves-Outlet Pressure

Sorry, I got a phone call in the middle of that last post and had to bail before I got to explain.  

Staged pressure reduction stations go back to when this was all done with regulators.  Even before I was in the business.  Regulators have, among their other characteristics, a property called Inverse Sympathetic Ratio. It's caused by the forces exerted on the plug by the process medium, compared to the forces exerted on the diaphragm by the sensed pressure.  Nothing "Automatic" inside a regulator, just proportional force balance.  3% is not an unusual number for ISR. So if the upstream pressure increases by 10psi, the downstream pressure DECREASES by 0.3 psi. You'd be shut down.  Or, of your N2 system goes a little soft by 10 psi, the outlet pressure of the regulator would INCREASE by 0.3 psi.  That would be a HUGE swing for you in this application since your setpoint is only 0.1445 psi (4" WC). If you staged (2) 3% regulators, the outlet pressure swing would drop to 0.03^2 or less than a tenth of a percent.  You wouldn't even SEE the output change due to swings in upstream system pressure unless you were watching verrry closely. So why don't you just use regulators? Regulators have other fun characteristics such as droop and high seat leakage but I won't go into all that here.

But you are not looking at regulators. You are looking at a control valve.  With a control valve, the motive force comes from an independent compressed air system, with a PID controller telling the valve where to go. It looks at your downstream pressure, compares it to setpoint, and makes any corrective action necessary to keep it to zero error within the limits of sensitivity of the pressure transducer reading it.  You only need a Cv of about 1, so the orifice is about 1/4".  Not much force is required on an unbalanced plug  to overcome 120 psi DP across a 1/4" hole.  Your thumb will do that.  

You can very easily take that kind of drop in one stage through one valve and control accurately. I listed a number of valves in an earlier post that would do just that.    

RE: Sizing Control Valves-Outlet Pressure

(OP)
The valve will not be controlled by pressure.  The purpose of this control valve will be to add nitrogen to a pipe if the LEL gets too high in the header.  The header has a baghouse that can only take 8 " WC.  So I will have relief in the header at 6" WC.  So as LEL gets ligher, the valve will open and vice versa.  So my question will I be able to keep the outlet pressure of the valve at 4"WC with 100 psig inlet pressure (at 0-100% open) to the valve providing 200 SCFM at 50% open?  Or would I need a control valve with a regulator downstream to make sure I have only 4" WC at full open to full close valve positionings?

Thanks.  

RE: Sizing Control Valves-Outlet Pressure

We can re-scale the valve so you get 200 scfm N2 at 50%.  So you need a Cv of about 8. I don't know why you want to limit the valve to 50% but I trust you have your reasons. It appears if you want to control on the other points we evaluated earlier that you are compromising resolution.

200SCFM will flow if the Cv is 2.56. So just give the valve a signal that will not cause it to open to a Cv of greater than 2.56,(look at the specific curve for the valve you are considering)  and you won't exceed 200 scfm.  

Note that if you direct the flow directly at the bags it will probably do the same thing as the pulsers and knock the dust cake loose, so you might want to arrange a sparger ring inside the chamber to distribute the N2 uniformly.   

An you still don't need two stages.  

RE: Sizing Control Valves-Outlet Pressure

(OP)
The reason I want the valve to get 200 SCFM at 50% is for a safety margin in case the LEL gets even higher in the system.  

So I'm new at control valves so if I size a control valve at 50% stroke and 120 PSIG pressure drop for a Cv of 2.56, the Cv will increase as the valve opeens to 75%.  Now will the delta P stay at 120 PSIG or will the outlet pressure of the valve increase?  So does a higher Cv just mean a higher flow or can the pressure increase also (getting above 4" WC and opening the relief valve)?

Thanks for the learning.

RE: Sizing Control Valves-Outlet Pressure

As you open the valve from 50% to 75% the flow through the valve will increase.

Will the downstream pressure increase?  Depends on the hydraulics in the downstream system.  If the capacity of the system is 'large' compared to the additional flow, the rise in pressure will be minor (there will be a rise in pressure since more flow needs a higher dP but the effect might be minor).  

Now, in your case, if the downstream pressure is 4" WC and that's the pressure needed to flow through the ducting/piping and bag filters to the atmosphere and you increase (let's say for discussion purposes) the flow by 50% by going from 50% open to 75% open, you need about 2x dP.  In other words, the 4" WC will increase to 8" WC or it would if you didn't have a relief device set at 6" WC.

RE: Sizing Control Valves-Outlet Pressure

(OP)
So how would I size a control valve to make sure I never exceeded 4" WC but still was able to get at minimum 200 SCFM nitrogen into the system (not staging the pressure drop)?  Would I have to size the control valve to have a maximum of 200 SCFM at around 80-90 % open?

Thanks.

RE: Sizing Control Valves-Outlet Pressure

1. Staged pressure drop is unrelated to, well, much of anything if you are not going to monitor and control the downstream pressure.

2. Exceeding 4" WC is, as TD2K stated, dependent on whether your baghouse can accept an extra 200scfm. I can't imagine that there would be a problem because 200 scfm into a baghouse is a tiny fraction of the what I estimate to be the normal flow, the bags are open to atmosphere, and if nothing else the N2 will flow upstream.  If you completely clog the bags and you have a High LEL, lifting the vent in the baghouse is not your primary problem.  

If 200 scfm is truly your maximum flow, then buy a valve that will be wide open at slightly above 200 scfm.  You had other operating points, and this will give you the best resolution for your"normal"operation.  Nitrogen ain't free, and controling it accurately will pay off in N2 savings as well as your primary purpose, safety.  

RE: Sizing Control Valves-Outlet Pressure

How do you prevent the downstream pressure from exceeding 4" WC?

That's the job of the pressure transmitter controlling the control valve.  As the downstream pressure (assuming that is what you are controlling) varies, the controller readjusts the opening on the control valve to return the pressure to its set-point.  The valve does not directly control the downstream pressure.  

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