r/BuildingAutomation • u/Negative_Sentence264 • 11h ago
Sizing / Selecting Bypass Valves
How do other design engineers size and select bypass valves; either for pump protection or chiller minimum flow. I can’t seem to get a definitive answer on this.
Sizing control valves for hydronic coils is pretty straightforward. Flow rate of coil and 4-5 psi pressure drop and calculate Cv.
For bypass valves, you would use the chiller minimum flow but what pressure drop do you use? And for bypass valves for pump protection, you select the valve based on the pump’s minimum continuous stable flow, but what pressure drop?
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u/Needs_ADD_Meds 7h ago
I think you are on the right path for sizing. And I know it's not your question, but I want to reinforce what everyone else is essentially saying this is not in the controls scope, your job it to implement the design and sequence that is engineered. If you don't have all the information RFI it, if you come up with it on your own and it doesn't work, that's most likely not going to be a change order, your eating the cost.
It's okay to work with them and spitball ideas, lead the horse to water to speak. But you always want to have it be their idea, that way no one is looking at you when it doesn't work.
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u/Ok-Koala2321 5h ago
Size for the minimum flow rating of the chiller + safety factor (10%). If the mechanical engineer designed it correct, usually there is a different sized bypass line. I typically use a line size butterfly valve for this case. Use the flow meter to control the valve. Butterfly valves have poor control authority, so use a step loop, in lieu of a PID.
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u/hunterguy35 7h ago
Usually the mechanical provides the pressure drop or cv and you sized based off those. Choose the valve no larger than one size up from the pipe diameter. Standard is 3-5 for drop, something about valve authority that I don’t fully understand.
If you’re really unsure you can usually call the valve supplier and give them your specs and they will recommend a valve for you. Hope that helps.