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Location: LA, CA
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Control Valve Sizing

08/21/2009 5:21 PM

Hi,

I am trying to size some control valves but I do not have enough information like the GPM or Pressure drop to correctly size it. I was wondering if it was possible to size the valve by looking at the number rows of fins located at the CHW Coil Has anyone ever had to do that before? if so how would you accomplaish that?

The line is 1"

Valve is a 3 way

Control needs to be proportional

Any help would really be appreciated

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#1

Re: Control Valve Sizing

08/21/2009 7:17 PM

Oh my...

In your case, I'd find a way to measure it as a last option. Check out any pump capacity in the circuit to try to find at least the approximate flow and static pressure.

If still not possible, a valve with internal passages in the same size your line would be something to consider, if the valve is a straight,-flow valve type. But try to figure out the first advice, because an increase in valve size means an increase in square power in price... as you may already know.

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#2

Re: Control Valve Sizing

08/22/2009 1:02 PM

If you need this for estimation purpose,the thumb rule the control valve size will never be more than line size. Hope this is fine for you.

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#3

Re: Control Valve Sizing

08/23/2009 9:46 AM

exosquad-

Without knowing the GPM of your process fluid it is just a "guess" on sizing the correct valve. Without spending any additional money, can you measure the coil size or find a coil and/or air handler make/model/serial number and contact the manufacturer for specifications? Also, depending on the control accuracy, a larger or smaller valve than required will produce variable results for your process. A valve that is too small will not provide sufficient capacity. A valve that is too large will produce temperature swings at normal loads and produce noisy operation at low load conditions.

If you are in the facilities or control industry, you may want to purchase an ultrasonic flow meter. You then can measure the flow rate at the location of where you will install the valve. You must measure this process at full load with no restrictions such as an old valve, etc., therefore giving you the GPM of the process.

Typical ultrasonic flow meter:

http://www.omega.com/Green/pdf/FD613_614.pdf

Once the GPM is known, accurate valve sizing is simple:

Find the Cv of the required valve:

Cv=(GPM) / SqRt ((delta P) / (specific gravity))

Most controls contractors use 5 psig for the delta P

Specific Gravity of water is 1

So your calculation is highly dependent on knowing the GPM

Once the Cv is known, you can go to any valve manufacturer's catalog and select the appropriate valve. NEVER undersize the valve. A slightly oversized valve is acceptable, unless you are controlling steam. Then you would use a 1/3 - 2/3 set up. But that discussion is for another day! Good Luck!

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#4

Re: Control Valve Sizing

08/24/2009 9:30 PM

If you know the required thermal performance of the CHW coil, and the temperature difference in the CHW supply and return lines, you might be able to estimate the GPM flow rate.

With a 8 or 10 degree F temperature differential, you will have around 3 to 2.5 GPM per Ton of cooling.

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