Flow Control - Pressure Regulation 1

[Newgeer]

Hi All,

I am working on a farm-scale ethanol plant project and have a question regarding flow control. Our reflux apparatus consists of a centrifugal pump and a flow control valve. The pump generates 60psia upstream of the valve. The downstream pressure is 15psia. Not wanting a 45psi pressure drop across the valve, I sized the valve for a 5psi pressure drop. Now how do I get a d/p of 5psi? Should I put a pressure regulator before the control valve to drop the upstream pressure to 20psia? Or should I put a backpressure regulator after the control valve to raise the downstream pressure to 55psia? Assuming I am even on the right track, I would rather use a backpressure regulator downstream of the control valve because I happen to have one on hand that would work.

Thanks in advance,

Newgeer

 

[JLSeagull]

I would expect your reflux apparatus to include piping and fittings, changes in elevation etc. in addition to the centrifugal pump and control valve. What was the basis of selecting a pump that produces 60psia head upstream of the valve? Perhaps it was a pump that was available and you can waste 45psi pressure drop across the valve. Consider the piping hydraulics.

You should have no problem selecting a control valve to handle the full pressure drop. However the capacity calculation for the control valve should consider the minimum pressure differential. Thus you still need to know the other pressure factors.

 

[pleckner]

You don't want only 5 psid across the valve, it won't control very well if at all. Stick with a minimum of 10 to 15 psid.

But the real answer is to evaluate system hydraulics as suggested by JLSeagull. This seems odd to have a pump producing so much head.

Please provide more details about your system and the pump. There are many ways to improve on your system and more details will help us guide you.

 

[Newgeer]

I apologize for not being more specific. The plumbing consists of one elbow and about four feet of 1/2" pipe, with zero elevation change. The column runs at atmospheric pressure. So the pressure downstream of the control valve is 15psia or very close to it. The flow through the valve is only 1.1USgpm normal, and 1.6USgpm maximum. I didn't want 45 psi pressure drop across the valve because I couldn't find a valve small enough. I also don't have a lot of choice when it comes to pumps this size. Another reason I went with a 5psi pressure drop is that if we increase the production rate of our plant, the same valve can be used by raising the pressure differential.

What do you mean by "minimum pressure differential"? Should the pressure drop be more than 5psi? Do you see any other areas of concern, from the information I have given you?

 

[Newgeer]

You folks were right about the pump. I had the flow curve right in front of me, and I read 60ft head as 60psi. So that's about 41psia?

 

[MortenA]

a valve is a valve is a valve. The valve itself does not "know" whether you want to control upstrream pressure, downstream pressur or flow.

You will use an instrument to measure variable that you want to control and then you adjust the valve opening accordingly.

No matter what the system will be limited defined by your boundries.

Best regards

Morten

 

[BigInch]

Seems like your valve should be set up as a pressure control valve, not a flow control valve.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[dcasto]

Investigate a VFD on the pump.

 

[BigInch]

There doesn't appear to be any indication of a variable flow that might justify a VFD.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Newgeer]

Well...I am trying to control the flow using a control valve. A positive-displacement pump with a VFD would offer good flow control. However I don't think a VFD will give the same result with a centrifugal pump as the pump is designed to produce a constant head.

It appears I tackled the problem bass-ackwards. I should have selected a valve based on the available pressure drop. Instead I am trying to alter the pressure drop to work with a valve I already have.

 

[MortenA]

Newgeer: You are on track now.

The dP is defined by your system, and your required flow rate. If you are conserned that your valve may be damaged by the dP then you must look for another valve. On occation several valves in series may be required in order to "distribute" the dP - but thats another matter.

Best regards

Morten

 

[JimCasey]

Sorry to be jumping in so late.

If your pump makes 60 feet of head (26.01 psi, if the fluid is water) and the pressure downstream of the valve is atmospheric, then the differential across the valve is 60 feet. What is, is. If you try to run the valve open enough to only make 5 psid, then the only way to do that is to have the pump sooooo far out on its curve that your're probably into pump runout and you will be flowing a lot more than your design flow.

If your valve is oversized and won't control at such a low flow corresponding to the pump curve at 60 feet, you might stick an orifice in the line downstream of the valve
to take some of the drop.
I don't think it will make that much difference.
The valve alone will need a Cv of about 0.48.
If the valve is taking only a 5 psi drop, the valve will need a Cv of 0.71. The orifice will need a Cv of 0.653, which is pretty close to 10 x 1/16" diameter holes.

Those calcs assume you really meant that the pump discharge pressure is 60 feet absolute, but is am suspicious and believe the pump pressure is 60 feet GAGE ( 40.7 psia) but in that case if the valve was the only restriction in the system the Cv of the valve would need to be about 0.376. SO with all the manipulating of the pressure drops the Cv still hasn't changed bt a factor of 2.

If you are using a 1" valve with a reasonably typical wide open Cv of 14 and also a reasonably typical equal percentage characteristic, the valve would be running about 8-10% of stroke. Not optimum, but in this case it would not be damagine either---just poor control resolution.

The actual cure is to open up the pump and chuck the impeller into a lathe and to reduce the diameter of the impeller so that it will only MAKE about 8-9 feet of head at 1.6 gpm. Pump literature will show pump curves for different diameters.

 

[BigInch]

Not too bad advice up to the last paragraph, but I don't think you should be talking about modifying a pump impeller to get an 86% reduction in head!

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Newgeer]

The pump literature lists 5 impellers, the smallest of which generates 28 feet of head. The control valve is a Fisher easy-E with a 1/4" port. The Cv is 0.075, 0.175, 0.641, 1.52 at 10%, 30%, 70% and 100%, respectively. Assuming negligible pressure loss due to piping, will those Cv figures give decent control characteristics if I use the smallest impeller? Or should the impeller be cut down? BTW, the liquid being handled is ethanol with a SG of about 0.80.

 

[dcasto]

A VFD for this small application is $200.

 

[BigInch]

My last comment goes for VFDs too.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[JimCasey]

I re-ran your numbers through my sizing program.
I still believe that your pump output is 60' GAGE, but correcting for SG of alcohol, and assuming 1/4" trim, I get a Cv required of 0.30 at 1.1 gpm, and 0.41 at 1.6 gpm. This should put you just about mid-stroke on the valve.

 

[Newgeer]

The flow curve shows 0 flow at 0 head. So the head must be gage. It would appear that if the valve is mid-stroke at 1.1 gpm, then flow control shouldn't be a problem. Thank you JimCasey, and all who responded.

Newgeer