Valve Authority for Chiller Valve in Variable Primary Variable Secondary System 1

[IanVG]

I think I've got a good grasp on what valve authority is for individual circuits and how to apply it for designing & selecting AHU/FCU coil valves, but I cannot seem to find good guidance specifically for valve authority for variable primary and variable secondary systems. I got a full hydraulic model built out for a plant that delivers chilled water to six buildings. I am replacing the plant's chiller, and have several scenarios modeled out for various seasons and cooling demands. My question is how do I calculate the valve authority for the chiller inlet or outlet chilled water valve? I've got a total pressure drop across the system of 15.3 psi differential (psid), so using a valve authority of 50% (where I assume anywhere between 25% and 75% is acceptable), I would select a valve with a 100% open pressure drop of 7.65 psid. This means I am effectively increasing the pump head by 50%. Does good valve selection and valve authority apply differently for variable primary and variable secondary systems? I can upload sketches if necessary to clarify.

 

[HVAC-Novice]

You mean modulating valves? Use PICV if they are available in the size required. they mainly limit the maximum design flow. But as a side effect they they have a very linear response (flow vs. actuator position). Just select it for the flowrate needed and you are done.

 

[IanVG]

Hey HVAC-Novice, thanks for the response. Yes, I mean a modulating control valve. The ePICV's that I have gotten spec'd from Belimo for a 6" pipe are around 5x the cost of a pressure dependent control valve (PDCV). I do agree in that PICV's in general are the way to go in terms of design, but I know our shops have had problems with the manual PICV's (mPICV) as the spring cartridges would get clogged up with material from the piping system. Hence, the reason I am being told to run with pressure dependent control valves.

 

[HVAC-Novice]

We usually add strainers upstream of PICV. Without those, yes I see that being a problem.

Are both valves you compare from Belimo? I suspect in 6" size they don't offer the old mechanical (without flowmeter) PICV.

Yes, the ePICV cost more. But they also let you monitor the flow and you don't need a balancer. And you don't run into the danger of too much flow if the actuator mal functions and that AHU steals 3 times the design flow from the system. You also get the linear response (i.e. 25% signal will give you 25% of heat output). So the control loops will be more stable. For the non-e valves they use a laser-cut ball to get the exact linear response. For the e-PICV they can program it to get the linear response.

I can't find an old Carrier document that explained valve authority. But some rules of thumb (based on absolutely nothing!) either say to select the valve with a pressuredrop of what the coil loop is, or what half the system pressure drop is. I'm sure if you talk to Belimo or someone they can give guidance. There are only so many Cv values the valves are available in.

I'm not sure if this is available in 6", but if you can get a ball valves, that will work best long term.