Flow through flow control valve - how to read it? 1

[acr2003]

Hello everybody,
I have a flow control valve which has a Cv = 0.1 and controls the flow of staurated steam. Because this is not a flowmeter, I am trying to infer the flow passing through this valve according to the pressure drop across it. As I said, my fluid is 300F saturated steam and I am using the following equation:
flow (gallons/min) = (7.9) x (Cv) x (sqr.rt (pressure drop / density of fluid))
I am not sure if this would be a correct equation for this case, as I heard this equation is mainly used for liquids. Can I used it anyway? If not, what would be the correct one for saturated steam? Could anyone please advise me?
Thanks,
- acr -

 

[jsummerfield]

Control valve plugs are available with several characteristic relationships. These include linear, quick opening, equal percentage, parabolic, modified parabolic, etc. These characteristics have an associated curve relating the percentage of stem opening with percentage of rated valve coefficient. However, many manufacturers do not publish the precise data.

You need the stem position to rated coefficient data at various percentages or the curve fit data.

John

 

[btrueblood]

Hmm, not sure of the units in your equation. The correct equation should be:

Q = Cv [(P1 - P2)/(sg)][sup]1/2[/sup]

Where Q = gallons per minute, P1 and P2 are in psi (pounds per square inch) and sg is the fluid specific gravity (ratio of fluid density to that of water at 60 deg. F). The 7.9 factor may be the right correction to apply to convert your density units to specific gravity, or it may not be... but it doesn't look familiar to me.

As jsummerfield points out, you should also know whether the Cv data you have is correct for your stem position, and whether the Reynold's number of your flow is roughly close to the Reynold's number that the Cv was measured at, and even then understand that the Cv may be different for your valve due to uncertainty in measuring stem position, manufacturing tolerances, etc. etc.

 

[gmorin]

Please note all of the warnings above. If you're looking for any kind of accuracy, get a flowmeter.

I've used the following correlation with reasonable results. It's better than the water correlations, anyway. I'm not sure of the ultimate source - I'm getting it out of the back of a valve catalog (Edwards-Vogt)

Dry sat steam (non-choked):
W=2.97 Cv * [(P2-P1)*P1]^0.5
P1, P2 in PSIA - P1 is upstream, P2 downstream
W in lb/hr

Note that P1<2*P2. Otherwise you have choked flow and that's another can of worms, but a simpler formula:

W=1.82 Cv*P1

Another option is to use the water correlations to convert the valve restriction into equivalent pipe length and do a pressure drop vs flow chart from there, but I'm not sure that'd be more accurate.

Good Luck

Greg

 

[acr2003]

Thanks for all the replys guys, they were all very useful.
Greg, you hit straight where I needed. I do not have a flowmeter right now, unfortunately, so I do really have to count on the formulas at this time. In any case, your explanation was exactly what I was looking for. One thing: my inlet pressure (upstream) is actually 75psi and the outlet pressure coming from the valve is around 5psi, which according to the formula gives me a choked flow, right? I am not sure how bad that could be to my line, but I should then be using the second formula you provided me.
Thanks a lot for the help.
- acr -

 

[gmorin]

75 psi going to 5 psi is definitely choked flow. Remember to use absolute units (unless you are and the 5 psi is a vacuum system)

This simplifies the flow "measurement" - you don't need to be concerned about the downstream pressure.

Is this an on-off application or a modulating control valve? If it's a modulating valve, the pressure differential would generally dictate multi-stage trim, which makes things more complex and decreases whatever little accuracy you have.

Final warning - use this as an estimate only.

Good luck

Greg

 

[davefitz]

If you have choked flow, then you need to use a different calculation. Choked flow is linearly proortional to upstream pressure and independent of downstream pressure, if you substitute sv= rT/P in the formula.

From the ISA handbook of control valves, for choked flow:

W = 0.667*63.4*Cv*SQRT{Xt*Pi*Fk/sv,i}
where Fk = 0.7/R, R= ratio of spec heat = Cp/Cv
Xt= valve body oblique shock wave coefficient
Cv= valve Cv for current % open
W= lb/hr
Pi= inlet press psia
sv,i= inlet spec vol, ft3/lb

In any case, it will not be very accurate to use the valve plug position to indicate flow- it will be a very approximate indicator.