need help understanding zone valve Cv

greenmountain

I'm pretty confused by Cv values and how to size the valves.
If I have a zv with a Cv of say 7.0, and a "normal flow range" of 6-10gpm, does that mean that the valve would act as a flow limiter if the pump was producing 16gpm?

GroundUp

Cv is a number relating to pressure drop at a given flow rate. A Cv of 7.0 would imply a 1.0 PSI pressure drop at 7.0 GPM. AT 16 GPM, that same 7.0 Cv valve would create 2.29 PSI of pressure resistance, or 5.28ft of head.

EDIT: I now realize I have been wrong about this my whole life. The equation is not linear as I'd been taught. The square root of the pressure drop in PSI multiplied by the Cv value equals GPM. So in this case we'd want to do 16/7= 2.286 and then square that quotient so 2.286 x 2.286=5.22 PSI. Never too old to stop learning I guess.

As with anything, pipe included, more flow will create more resistance to flow. The Cv rating is needed to appropriately size the circulators, piping, etc to make a proper system.

offdutytech

Another consideration in choosing valves is also if it needs to be a linear valve or equal percentage valve. 
Here is a very quick overview of that.

https://www.buildingcontrolsgroup.com/news-events/valves-the-difference-between-equal-percentage-linear-flow/

Another great tool is Siemens Valve Selection Tool or Belimo also have apps / web based selection tools that help you to understand how to size valves based on CV and characteristics. 

Rich_49

https://www.womackmachine.com/engineering-toolbox/data-sheets/how-to-use-flow-coefficients-cv-for-hydraulic-fluids/


https://www.youtube.com/watch?v=Q99htzgeVL0

hot_rod

Here is a spread sheet we built to run the calculation , box 3 allows Cv and gpm inputs to show pressure drop in psi and ft. head

JohnGellatly

Hey Bob how do we get that spreadsheet ...

hot_rod

See if this comes across.

Should be self explanatory. If you know any two values you can solve for the third.

The 3'rd box is most often used. Enter the Cv of the valve, and the flow rate you intend to run, hit enter. See pressure drop in psi and feet of head.

mattmia2

Valves with different Cv can be used to roughly balance the system then the balancing valves in each zone can fine tune it.

hot_rod

Yep, it is always a best practice to size a valve by the flow rate, not pipe size. the Cv number indicates the flow at 1 psi drop.

With zone valves, there is a wide selection of Cv available, here are the 5 sizes Caleffi offers.

Notice also the shut off pressure change with Cv size. So a building like a hotel, dorm, etc with multiple small load air coils. Those typically get a 1 or 2.5 Cv valve as they generally have a single, large circulator developing higher delta P than a common residential system. So a 1 Cv valve get you 75 ∆P