choked flow and positive displacment pump 1

[ARenko]

We are testing a positive displacement pump with water, applying pressure with a control valve. Pressure drop is very high - P1=1500 psig, P2=10 psig. By calculation of choked flow conditions, the max allowable pressure drop is around 1100 psi. Max flow is calcluated around 250 gpm. Actual flow from the pump is 350 gpm. We can hear the cavitation. FL value of valve is .56.

I'm trying to get my head around what is happening. Obviously the flow is not limited at 250 gpm. Do the Cv calculations become inappropirate at conditions exceeding max deltaP? The choked flow calcs are based on the calculated Cv position - is it possible the actual position is different than calculated (unfortunately we don't have valve position data)?

 

[zdas04]

You seem to be mixxing apples and ducks. Mostly anything you can pump and/or measure in gpm is not subject to choked flow.

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.

 

[TD2K]

I'm not surprised you can hear the cavitation. My spreadsheet predicts you should start having some incipient cavitation at about 500 psi dP (I guessed the vapor pressure at 0.25 psia). At about 1400 psi dP, the valve throughput is limited because of the vapor formation within the valve.

What type of valve trim is it, linear, =%? Have you got the valve Cv curve from open to closed? I would see how much % you are talking about for the 250 gpm flow you are calculating the valve should pass versus the 350 gpm the pump should be putting out. Can you do a drawdown test on the suction tank to see what the pump is moving as a check against the two different numbers?

 

[ARenko]

Zdas04, can you elaborate? Maybe I am not understanding what choked liquid flow is - you are implying that it's non-existent or happens at some conditions difficult to actually achieve. I may be confused by the control valve examples in Crane's Tech Paper 410, which check for choked flow by determining max flow and max delta-P, comparing them to desired flow and actual dP, in an example with water as the fluid.

TD2K, we are measuring flow with a coriolis meter. The valve is plug and seat type (see attached curve). I'm not suprised at cavitation either. I guess I'm really trying to figure out what the max dP and max flow in Crane's really means. I said "we are testing," but actually it was a long time ago. In looking at valves for another application and doing the calculations I'm thinking back to that testing (which was for the pump, not the valve so it didn't concern me at the time) and wondering what exactly was going on. My numbers above were based on wrong fluid SG... Using Fp=1 I get 475 psi for max dP and 198 gpm for max flow at the valve position of the calculated Cv (9.1).

 

[zdas04]

One of the least endearing features of Crane #410 is its lack of an index. Just flipping through it I found an excellent discussion of choked flow in a compressible fluid, but I sure can't seem to find any reference to choked flow in an incompressible fluid in my 2 minute scan. Cavitation, erosion, and noise are all common in liquid flows, but choking (i.e., velocity limited to speed of sound after a pressure drop) doesn't happen in liquids. If you've found something in Crane that would lead you to a different conclusion please post the version of Crane you are using and the page (or section) number.

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.

 

[hydtools]

I believe chocked flow is only defined in compressible flow. For a liquid, as long as the pump has capacity to deliver flow against pressure and the pump driver has the capacity to drive the pump at constant speed, the flow will be relatively constant as valve restriction causes increasing pressure. Pressure will rise, flow will remain constant, until something fails or opens.

Ted

 

[ARenko]

zdas, it's in chapter 7 under the control valve examples. I don't have it handy now to give the exact example number, but first there's an example problem for incompressible fluid (H2O) where they size a valve, then in the next example they "check for choked flow" in the previous example.

 

[zdas04]

Interesting. My copy (1998 reprint of the 1991 version) only has 4 chapters and two appendices and I don't see a mention of a control valve in the table of contents at all.

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.

 

[ARenko]

I have the 2009 version. Chapter 7 examples 7-27 and 7-28 are the examples I refer to. Chapter 3 has a section on cavitation, choked flow, and flashing - it refers to incompressible fluids and doesn't even mention compressible fluids directly. It basically says gas bubbles in the vena contracta takes up more space than the liquid so that the flow begins to restrict and deviate from the predicted Cv curve until the point when all liquid in the vena contracta is turned to vapor. At that point a further decrease in downstream pressure will not increase flow rate and the flow is considered fully choked.

I don't see any specific discussion on choked flow of compressible fluids, but it's basically defined in the discussion on how an incompressible fluid becomes choked.

In the sizing and selection section for incompressible fluids after the initial Cv is calculated they say to check for the possibility of choked flow using equations for Qmax and delta-Pmax. It says these are the max flow and max differential pressure at which choking occurs at the valve position of the calculated Cv.

What I'm getting from my calcs is that if I put the valve in the position of the calculated Cv and turn the pump on, then I will quickly pop the relief valve. Of course that's not how we did the testing of the pump - we started with full open valve and slowly closed until we achieved the pump test pressure. I guess we achieved the pressure at a larger valve opening than calculated and that at that position our flow rate was somewhere between the incipient cavitation point and choked flow conditions. In this region the flow rate deviates from the Cv curve. That's my current theory anyway.

 

[bingbingwang]

GOOD STORY.
you did not say the temperature of the water, i guess 20C Deg?
choked flow for liquid seems just a range between incipient bubbling and fully vapourization. for the gas choke flow is pretty clear defined as "sonic velocity in that fluid".
you are lucky you guys did not test that pump with closed valve, otherwise, the pump will be damaged in a few second! I think your valve is choked or not , depends on the vapour pressure (temperature ) of downstream line. still 350gpm flow through the valve, it means not fully vaporized inside of valve. mostly, close to severe cavitation condition.
choked flow noise sounds like jet plane but less vibration, cavitation noise is not so sharp but more vibration on the valve body and downstream line.

Get help and return with helps!!.