Control Valve Flow Coefficient (Cv) Sizing for HW/CHW Systems 3

[mepe15806]

ASHRAE Systems and Equipment Handbook indicates that control valves should be sized for a pressure drop of 25 to 50 percent of the pump head in order to obtain good control. I have seen many projects where all the valves are sized for a 5 psig pressure drop. I this "rule of thumb" method for sizing control valves valid? Where does the 5 psig come from?

 

[makeup]

At the end of the day CV's are there to get theoretical calulations to a working norm. Why not go back to the value supplier and as them???

 

[NCMog]

About 25% of the total system head is a good rule of thumb for a control valve. I've seen the 5 psig before and believe that it stems from the typical low system head seen in HHW and CHW system, and a lot of people don't know how to size control valves. Its not hard, but in HVAC I've seen a fair number of people use rules of thumb versus doing the calc.

 

[CARF]

Control Valve Handbook

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

free download - 295 pages in PDF format - Emerson Process Management - The Control Valve Handbook has been a primary reference for more than 30 years.
This third edition is a complete revision and update that includes vital information on
control valve performance and the latest technologies.

Good luck !

 

[friartuck]

Control valves should be sized on what we call 'Authority'

Control valve authority is the simple equation of control valve pressure drop/controlled system pressure drop (Including the control valve)

i.e. if you are controlling a heater battery using a diverting valve and you want a valve authority of 0.5 (Typically 0.3 to 0.5 are acceptable for good control)

and if we have a heater battery pressure drop of 20kPA,

then 0.5 = VPd/20+Vpd

in this case Vpd is also 20 since 20/20+20=0.5

If the value is lower than 0.3 then the control is poor

higher values are simply wasteful of energy

(If I knew how to send you a sketch I would)

Regards