Use of Cv for Valves in Gas Application

[bthompson]

I have found specifications for valves (internal) where the Cv value of the valve is specified and the valve is almost exclusively used for gas application (dry Nitrogen). I haven't worked on this stuff for a while, so I reviewed the derivation of the classic Cv equation. This derivation assumes the fluid is incompressible. Check out fisherregulators.com - one of their "technical monographs" goes over this in excellent detail. For a new application, I know what my minimum flow requirements are and I know what my pressure range is. Why not simply stay away from Cv and specify flow and pressure? Is it appropriate to use Cv for gas application (obviously I think not)? If not, is there an equivalent term used for gas application?

 

[TD2K]

You still need some term to express how much gas the valve will flow for a given pressure and temperature AND pressure drop.

While Cv was originally developed for liquids, it's also used by a lot of valve vendors for gas services with appropriate correction factors to take account of any compressibility effects.

Fisher uses Cg for gas services which is Cv/C1. The reason Fisher developed the use of Cg is that they state that they found valves with similar Cvs (eg, they would flow the same amount of liquid for the same dP) 'might' not flow the same amount of gas for the same dP. Thus, they developed Cg and publish these for their valves and use them for gas sizing. They also have Cs factors for use with steam, similar reasons.

 

[bthompson]

Thank you TD2K. This makes sense. I do have an understanding of the pressure drop range and the max operating pressure requirement. And, of course I'm using good old ROOM temp. All that stuff, along with a flow requirement, I think specifies what I need in terms of orifice size (this is a 2-way valve app) Can you point me to a good technical discussion of the development of Cg? I don't recall seeing on on Fisher's web site.

 

[davefitz]

A more consistent calc method may be to use the ISA equations for compressible flow thru a valve. Each valve is characterized by a incompressible coefficient ( Cv) plus a compressible flow coeficient ( Xt) , which is a measure of the oblique shock waves formed during choked flow.

refer to ISA Handbook of control valves by J W Hutchinson

x= DP/Pi
Fk = (K) / 1.4 ( K= gas ratio of heat capacities)
Xt= valve body compress coef
Y=1-0.33( X/Fk/Xt)
w ( lb/hr) = 63.3CvY SQRT{xPi/sv}, sv =inlet spec vol ft3/lb,
but subject to the limit that the max value of X is :
X <= {KxXt}, which occurs at the choke point.

 

[TD2K]

bthompson, I think you are going to have to request the derivation/development of Cg from Fisher or your Fisher rep. I 'think' what I have seen of it (and gotten from talking to Fisher reps) is in some older reference books of theirs but it wasn't to the depth that I think you are looking for.

There's no real detail to the background of Fisher's Cg equation in their catalogue that I have here. I'll see if I have anything at home in my other reference material but I wouldn't hold my breath.

 

[bthompson]

Thanks again TD2K. And thanks to davefitz. I am very pleased with this forum. I will check into the ISA Handbook of control valves. Does anyone else out there know of a derivation of &quot;Cg&quot;, as described by TD2K and as evidently is used by Fisher??

TD2K: I'm sure I do not have a Fisher rep per se. You seem to be fairly familiar with them. Do you think they'd oblige me considering we do no business with them (that I know of)?

 

[RobV]

bthompson

Like TD2K said the standard Cv equation for incompressible flow was modified primarily to correct the units, i.e. SG to more specific gas law for density, USgpm to scfh, etc. The 'new' developed formula gave incorrect reading at dp/p ratios larger than something like 0.02 due to compressililty.
Also the 'choked flow' range predicted was incorrect when reviewing different valve styles.
In the end they discovered that the new gas formula didn't account for the difference in valve type, i.e. high recovery and low recovery valves. The basic liquid formula does by limiting the DP used in the equation to that of choke flow which is calcualted using the valve recovery coefficient Km (FL²).
To account for this diff. in high and low recovery the C1 (recovery)factor was introduced together with the Cg factor.

I've got a copy of the Fisher handbook which discribes this section a little more in detail. I can get you a scanned copy of this section if you leave yr e-mail.

Next to the ISA formula described above which I believe Fisher have now also adopted you can also check the formula used by Mokveld being:

US units
Cv =(Q*SQRT(G*T))/(833*Fl*P1*(Y-0.148Y^3))
in which Y=1.63/Fl*SQRT(DP/P1) with a maximum of 1.5

Q=flow scfh
G=specific gravity (air=1)
T=temp in rankine
P1 inlet pressure psia
Dp=pressure drop psi

In the end, using Cv for gas and liquids is a logical step as we can now check different valve type and sizing/selection from Vendors.

 

[TD2K]

Fisher is pretty good IMO. Do a search on the web for Fisher, you'll find their website and it should have a link to the rep for your area. The reps have access to a lot of background information and I've found them quite helpful in the past. If you are reasonable with your requests for their time, I think they wouldn't have a problem answering your questions.

There's a small book called 'Control valve handbook' Fisher publishes, don't know if it is still in print, my copies are pretty old. In section 3, they go into sizing of liquid valves using Q = Cv (dP/SG)^0.5. In the next section on gas, there is the comment 'A sizing procedure for gases can b4 established based on adaptiations of the basic liquid sizing equation. by introducing conversion factors to change flow units from gpm to cfh and to relate SG to meaningful terms of pressure, a equation can be derivived for the flow of air at 60F. .....'. They then do into the details of their correction factor for gas compressibility that they use in their gas sizing equations.

While a lot of this same information I recognize is in their catalogue 10, I'm not sure all of it is, there 'seems' to be some background information here that isn't in catalogue 10.

 

[bthompson]

Thanks again for all the responses TD2K and for the new offer from RobV. I'm at bthompson@helixtechnology.com I really appreciate your help. If you guys ever have any questions about Cryogenics (not freezing people ) or high vacuum pumping (molecular flow only) or industrial diaphragm or bourdon tube based pressure gauges (previous life), I'll do my best to return the favor. Thanks again for everyone's help. RobV, I look forward to your email.

 

[TD2K]

Shucks, and I have this head sitting here that I've been trying to figure out what to do with, for a second there I thought I had my answer ;-)