Fisher Regulator Sizing

[skuntz]

I am sizing a Fisher regulator for air and I have calculated the Cv required . The product data sheet(s) however give me "wide open flow" and C1. I understand the definition of C1 but but how does this relate to my calculation? Is my criteria that I nedd to be below the wide open flow coefficient? How can I get the available Cv the valve?

 

[Latexman]

C₁ = C_g/C_v

Did they give C_g? They should have, C₁ and C_g are both needed in Fisher's universal gas sizing equation.

Good luck,
Latexman

 

[skuntz]

They give Cg but only for wide open flow. I don't think that is what I should be looking at when doing calcs for normal pressure regulation. What I am not understanding is whether to use the universal gas sizing equation or the standardized method that uses Cv only. I am filling out a data sheet & I need to report the max operating Cv of the regulator which is not (correct me if I am wrong here) the wide open flow Cv.

 

[Latexman]

Is it linear trim? If so, that makes it easy.

You have [C_v = C_g/C₁]_{wide open}, right? You also calculated [C_v]_required, right? Seems to me:

% open = [C_v]_required / [C_v]_{wide open}

Is the CV ~ 50% open (ideal) or at least within the 20-80% or 25-75% open range?

Good luck,
Latexman

 

[skuntz]

Latexman-

That's a good point, I see what you mean. So if the wide open Cv is just that - 100% valve opened, it is the same thing as the term "rated Cv" for any other control valve.

 

[steamdog]

Regulators are different than control valves when it comes to sizing. All regulators must have some 'droop' or offset flow, which is an offset to your setpoint. Most regulators are sized for a 10% droop, some for 20% and it depends on what your system can handle.
The wide open Cv or Cg is used for safety relief valve sizing only, not for sizing the valve itself.
All of the Fisher data sheets have charts for capacity and these are what I use to size them, then use the wide open Cv to size the relief (full disclosure: I work for the Fisher local business partner - rep). I would call your rep and have them size it, it is their job.

FYI: Cg is the gas flow coefficient, Cv is the universal flow coefficient, and C1 is the ratio of Cg/Cv

 

[rmw]

I wouldn't count on a self contained regulator being a linear trim. It might well be, but if so it would surprise me.

rmw

 

[Latexman]

Yes, unconventional sizing for a regulator, but it should result in a good operation. And in the real world, when the delivery pressure is a little low due to droop, that's usually easy to adjust out. Just open it up a little. Done it many times. Just be sure it's not under- or over-sized.

Good luck,
Latexman