8 replies to this topic

[eastorca]

Hi everyone

I'm too confused.
My senior said that: three most important thing to sizing a PCV,they are Cv min, Cv normal, and Cv max.
Cv min: maximum flowrate at min valve dP
Cv max: minimum flowrate at max valve dP

My question: Which is more important between Flowrate and dP. How to decide the case to calculate Cv min, Cv max.

[breizh]

HI ,
2 papers to support your query.
Hope this helps
Breizh

[Zauberberg]

Speaking in practical terms, what you need from any control valve is:

- To ensure maximum expected flow @ minimum DP available across the valve: this corresponds to the upper range of valve Cv (high flow at smaller pressure drop requires larger Cv);

- To operate within controllable range when handling lower flows (lower range of Cv), so as to avoid oversizing the valve (common error).

A typical example is pumping lean amine to Absorber: at higher flows, you have less available pump head (moved right on the pump curve) and therefore less DP available to be used for control across the valve (lower pressure upstream of the valve). At lower amine flows, you have higher pressure upstream of the valve and you still want the valve to operate with more than 15-20% opening. Whatever is said for flow control valves, applies for PCV's as well - correlations are the same.

[eastorca]

Thank you very much.

I must confess that it's difficult to both define the system and decide on the maximum dP (min dP) case for sizing a PCV.

Cv = Q (G/dP)^0.5

In General, which parameter has the most effect on caculating Cv (between Q and dP)?

[djack77494]

In General, which parameter has the most effect on caculating Cv (between Q and dP)?

eastorca, I don't think your question makes much sense. You have a system that has three variables: Valve Cv, Flowrate, and pressure drop dP. Fix any two and the third is known. You also know the equation relating them. So, when you ask which parameter has the most effect, just plug in values of variables that you are interested in and see the effect. Pick "base values" for flow and dP, and calculate a Cv. Now double the flow, leave the dP alone, and calculate a Cv. Observe (record) the result. Go back to the equation, put in the original flow and double the dP to calculate a new Cv. Now you have: 1) the original Cv, flow, and dP, 2) the Cv with doubled flow but the original dP, and 3) the Cv with the original flow and doubled dP. Isn't that about what you want? It clearly shows the impact on Cv of doubling either of the two independent variables.

Edited by djack77494, 23 September 2010 - 03:33 PM.

[sheiko]

HI ,
2 papers to support your query.
Hope this helps
Breizh

Bonjour Breizh,
Apparement les 2 articles n'ont pas été téléchargés.
Merci

Edited by sheiko, 08 November 2010 - 10:17 AM.

[breizh]

Sheiko ,
J'ai fait du menage et la pluspart des attachements ont ete elimines !

Breizh

[kkala]

Gladly surprized to see these control valve sizing handbooks (attached to post by breizh), since I have used these of Fisher and Masoneilan (printed on paper). Some valve suppliers have issued free software for their control valve sizing (I remember a floppy advertized), any info to find it would be gladly welcomed!

[Art Montemayor]

Kkala:

It's been some years since I last obtained a free copy of the Fisher Sizing Software, but I believe it is still obtainable from Fisher (now Emerson) - or more quickly, from their local representatives. You should be able to pick up the phone in Europe and obtain a free, complete package of sizing software - complete with the instruction booklet - from your local representative. This applies to both Fisher and Masoneilan. I don't know about Samson, but I suspect that they follow suit as well.

If you can't get a positive response from your local rep, then go to the websites and contact the main headquarters for each manufacturer and I am sure you will get a positive response. Any practicing professional engineer involved with process control should be able to own a complete sizing package of the competing manufacturer's products. The sizing package is the first indication for an engineer to know if the manufacturer has a product that "fits" a proposed application. The manufacturers know this and also know that this is their strongest selling tool.

Good luck.