Control Valve Sizing

[MFJewell]

Just looking for a quick peer check on this. I am looking to control flow to 3500 GPM (this is for a new installation of a air separation vessel on chilled water system). I did some quick calcs and it appears I may need to use a 6 inch control valve since 12" and 8" seem too large. Does this seem logical?

 

[MFJewell]

Also checked the 6" and it comes out at 3197 GPM. Looks like it'll be 8".

 

[MFJewell]

No comments?

 

[quark]

What is the actual pressure drop when you select 6". If it a proportionate control valve then 8" is bigger and you have a narrower window of control.

 

[quark]

That is about 33 psi if your 6" valve Cv is 604.

 

[MFJewell]

The pressure drop across the valve should be fairly constant. This valve will be installed between two pressurized headers. I am putting about 2-3% of total system flow through it, so it shouldn't impact the header pressures (the headers are 60"). I also checked my numbers against two flowseal valves and confirmed that the 6" was too small. With the 8" I should end up at 62% open at the design point. Checking for choked flow on the 8" showed that I should be able to flow up to 5900 gpm and up 61 dP.

I also checked a similar configuration using real data from a side stream filter on the same system (I have actual data for dP, flow rate, and % open on the control valve). When I used the same method to calculate what the flow rate should on that system, I got 1948 GPM versus 1952 measured at the flowmeter (0.2% difference).

 

[quark]

62% is OK. Need not bother about choked flow in your case.

 

[ThurstonC]

Here is a free control valve sizing program in Excel on

http://www.industrydocs.website

The logic is accessible, and it includes documentation.

 

[MFJewell]

Here is a free control valve sizing program in Excel on

http://www.industrydocs.website

The logic is accessible, and it includes documentation.

Thanks for the link, but I have since written my own valve sizing program in excel, which is what I'll use going forward.

 

[MFJewell]

Just a quick update, put the system in service today. The calculator that I made in excel showed that for 3000 GPM, I needed to be 41.6 degrees open (46% open) for the dP at the time (45 PSI). The actual data was almost dead on. We ended up setting flow right around 3100 gpm and the valve was just under 50% open.