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Flow Control Valve, Delta P calculation questionHelpful Member! 

ELH (Nuclear) (OP)
14 Jan 03 15:48
I'm trying to find the actual pressure drop across a flow control valve.  I was able to find the standard pressure drop using Crane 410 and the "rated Cv" of the valve from the vendor drawing.

My question is in regards to the valves %open.  This valve is used as a throttle-type flow control valve, and I was wondering how the %open would affect the pressure drop across this valve.

I am assuming that the "rated Cv" is for the valve in full-open condition.  How would I integrate the normal operating position of 60% to calculate a more accurate delta P across this valve?

Thanks,
Liz
Helpful Member!  TD2K (Chemical)
15 Jan 03 0:11
You will be able to get a Cv versus %opening for a control valve from the manufacturer.  With that, you simply read off the Cv for the %open and then calculate the dP from your equation.

If it's not a control valve, say a small hand globe valve, then you'll need to talk to the manufacturer to see if this have this information.  If it's a metering type valve, they typically do.  If it's more of a generic globe valve, they might be able to give you an idea if it's a linear trim in which case the %opening is directly proportional to the Cv (eg. 50% open = 50% of the full open Cv).

If the trim is more of an equal percentage or quick opening, you can use generalized charts (see a control valve sizing manual) to estimate the Cv of the valve BUT it's a LOT less accurate as those curves are only approximations of the Cv characteristic, not a guarantee.
Lakeshore (Mechanical)
3 Feb 03 12:24
Keep in mind, Cv is calculated between 5.0 and 15.0 psid, with the valve full open.  What you need is the valve's flow curve at a specific delta P.  The manuf. should have the curve (with hysteresis) at a specific differential pressure.  Honestly, the best thing to do is test the valve yourself at your conditions if you need accuracy.  Get a scale, a bucket, your valve, a water supply, a stop watch... and let the good times roll!

Good luck!

Best regards,

Lakeshore

ELH (Nuclear) (OP)
3 Feb 03 12:56
Thanks for the advice... as for testing it myself, I don't work in an environment where that's entirely possible being a nuclear plant and all.  But thanks again!
Tremolo (Nuclear)
28 Feb 03 18:29
ELH,

I don't know if I'm too late to answer your question, but here it is anyways.

We have done a lot of modeling of gate and globe valves for nuclear plants.  We have had good success with this method.

If all you have is the Cv for a full open globe valve, try the following which is based on a curve fit of manufacturer's data.

Cv(t) = Cv0*(1-t/tau)^1.5

where,

Cv(t) is the flow coefficient as a function of time
Cv0 is the full open Cv value
t is the time and it goes from zero to tau
tau is the total stroke time

If the stroke is linear over time, then (1-t/tau) is simply the fraction of the full stroke.  at t=0, the valve is full open; at t=tau, the valve is full closed.

As an example, if you are half way through your stroke (t=0.5*tau), then Cv(t) = Cv0*0.5^1.5 = 0.35*Cv0.

Note: the exponent 1.5 applies only to globe valves.

I can provide you with comparisons of this correlation against manufacturer data.  If interested, let me know.
hacksaw (Mechanical)
28 Feb 03 22:15


you need a certain amount of system pressure drop across the valve in order for it to modulate the flow. the valve is just a variable restriction (the manufacturer has characterized it in water testing).

You can visualize the details of the sizing requirements most easily when you plot your system pressure curve versus flow for several valve openings.


4carats (Chemical)
15 Jul 03 17:41
The Installed Characteristic (i.e. after installation of the valve in the process) can be quite different from the Inherent Characteristic (i.e. flow versus lift as you obtain from valve manufacturer's catalog).  A valve with linear characteristic can behave like a valve with quick-opening; a valve with equal percent charactertic can behave like linear. It also depends the on the fraction of the dynamic pressure taken by the valve, its over-sized factor and its rangeability. Some chemical engineering texts cover this subject.
AirTrainer (Industrial)
16 Jul 03 16:25
I've read your question about five times so far, and I'm focused on your use of the word 'actual'.  Real world flow characteristics for any device are best evaluated in place.

I generally don't rely upon a manufacturer's info unless I have prior experience with them, and know their published values to be the result of good science, as opposed to good marketing.

Does your situation allow for the installation of pressure sensing devices before and after the flow control?

Best regards....
Scotsinst (Petroleum)
24 Jul 03 13:36
I am not sure what your question is.  There is no such thing as a "standard" pressure drop.  The pressure drop across the valve is simply the upstream pressure less the downstream pressure.
1.  Do you know the flowrate and fluid properties?
2.  Does the manufacturer list the valve characteristic (linear, =% etc) as well as full-open Cv?

You need to know the flowrate, liquid SG (or gas properties), valve percent opening and valve characteristic to calculate the pressure drop.
mtmurthy (Industrial)
3 Aug 03 23:43
The general formula Q=CV* sqrt(dp/G ),
from which you can calculate dp across
the valve by using the cv value from
the manufacturers Cv table.(control valve manufacturer list cv for % openings 10 to 100% open ) But manufacturer's
table are only inherent characteristic. If
you want dp with installed conditions,
you can workout by calculating pipeline losses
by installing the valve in a system. But once
again if you look for MORE ACCURATE value, then
you need to install & measure it.

newengineer1 (Mechanical)
20 May 04 7:36
Bypass TCV calculation!

DESCRIPTION OF THE PROBLEM:
Reactor water storage tank temperature is maintained between 29?C and 35?C. In winter spring and autumn time water circulates trough tube side of the heat exchanger and returns to the storage tank. Steam passes trough the shell side of the heat exchanger and heating the refueling water. The steam flow trough the heat exchanger is controlled by the Fisher TCV2886.

RWST heater steam inlet pneumatic control valve TCV2886 (fail closed) modulates to maintain discharge temperature at a pre-set temperature of 150?F (66?C). Valve operation is automatic in response to a signal from a filled bulb temperature detector.

TCV2886 is selected by the design steam flow of 5000 lb/hr, which is based on maintaining the refueling water storage tank as I already mentioned between 29?C and 35?C at outside conditions -17?C and wind 7 m/s.

This drastic condition are not very often in or region. So in great deal the TCV2886 is half closed or even more. In these days the valve opening is about 5 to 10%. But the real problem accurse when the TCV2886 starts to oscillates (open/close). With the valve closening the steam downstream of the valve condenses and stays in line from TCV2886 to heat exchanger. With reopening of the valve steam push the condense into the heat exchanger what makes water hammers.

So we like to dispatch of water hammers with making bypass line 1" with new TCV control valve, which will mentained the the temeprature in tank in spring and autumn period.

What literaure for TCV calculation do you recommend?
Scotsinst (Petroleum)
20 May 04 14:31
Change to condensate control and run with the tubes partially flooded.
Use water outlet temperature cascading to Flow controller in condensate line.

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