Sizing a Valve with Cv 1

[RontotheB]

Hello,

I was wondering how to best size a solenoid valve. I know that one specification that can be used for this purpose is Cv (flow coefficient). How would I use this parameter?

I have 3750 psig on one side and 0 psig on the other (so the differential is 3750 psi). I'm using nitrogen gas at about room temperature, at a flowrate of about 1 cubic ft/s. The smallest flow orifice is 0.0653" in diameter, and the largest is 0.25" in diameter. How do I determine if a particular Cv is too low or too high for me? Would this help me choose my valve better?

 

[BigInch]

Got just the thing for you,

The Fisher Control Valve Handbook (Free online edition)

http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf

http://virtualpipeline.spaces.msn.com

 

[davefitz]

I suggest you use the valve sizing rules outlined in the "ISA handbook for control valves".

In that approach , each valve that handles a compressible gas has a Cv flow rating and an Xt factor for choked compressible flow effects. The case you described is choked compressible flow.

As I recall ( to be confirmed)

W (lb/hr) = 63.4*Cv*Y*SQRT ( X*Pi/sv,i) ; Pi in psia, sv in ft3/lbm

Y*=1-0.33*X/(Xt*Fk), but Y= Max (Y*, 0.67) { choked flow occurs when Y=0.67}

X= DP/Pi, Fk= R/1.4 where R = ratio of spec heat

Xt = valve oblique shock wave index , approx = 0.85 for globe valve

perhaps in your case Xt=0.85, Fk=1.0, X=1.0, Y*=0.61, Y=0.67
you can solve for Cv knowing W, Y, X, Pi, sv,i.

 

[BigInch]

Dave,

See Fisher Control Valve Handbook page 119

Sizing Valves for Compressible Fluids

Following is a six-step procedure for
the sizing of control valves for compressible
flow using the ISA standardized
procedure.

And... you don't have to shell out for it.

http://virtualpipeline.spaces.msn.com

 

[GSTeng]

BigInch hit the nail onthe head. I have used this book to size control valves and it is very good. The other option would be to go here

http://www.emersonprocess.com/fisher/products/firstvuesizing/firstvuesizing.html

and download the firstvue program from fisher. you can use it to tell you the Cv that you need and then compare it to the values of the solonoids you have. It uses ISA standard and it also free. It is not that user friendly but if you do a lot of valve sizing it is good.

OR you could call the local fisher representative and him or her size the valve, they use this same program.