What is a valve Cv value?

Simply stated, the Cv value of a valve is the number of U.S. gallons per minute (GPM), at 60˚F, that will flow through the valve, at a pressure drop of exactly 1 PSI. (Pounds per square inch)

How is this value determined?
Actually, it is very simple.
The flow is physically measured in GPM, with a pressure of 1 PSI applied to the inlet of the valve and 0 PSI at the outlet, with no back pressure.

Therefore, a 2” full port valve would have a different Cv value than a
2” reduced port valve, obviously because the actual area or flow path is different.

What is back pressure?
Back pressure is the pressure (PSI) present at the outlet or downstream side of the valve.

Where does back pressure come from?
Many sources, other valves downstream, fittings, reduced pipe size, etc…
Each and every restriction causes a certain degree of back pressure.

Do all types of valves have Cv values?
The answer is NO they do not.
Typically, proportional or modulating valves, also spring loaded check valves, would not have a Cv,

Why?
Simply because more than 1 PSI would be required to even overcome the spring pressure and operate the valves, or in the case of proportional valves, the opening of the valve is controlled by a separate device.

Cv values apply to full open/ full close valves used for positive shut-off, such as solenoid valves and ball valves to name a few.

Another common term is Delta P, with a typical symbol of ∆P.

Delta P is defined as the pressure drop developed across a device that has flow through it.

To determine Delta P, subtract the pressure measured at the outlet side of the device (P2) from the pressure measure at the inlet side of the device (P1).

Therefore: P1 – P2 = Delta P (∆P)

Pressure drop must be known at the design phase of any project, so that the engineer can select the correct size devices, valves, pumps, etc.

Pressure drop (∆P) and flow rate (GPM) are dependent upon each other, the smaller the orifice, or the greater the flow rate, the greater the pressure drop across a device.

Another factor that must be known to mathematically determine flow rate (GPM), Pressure drop (∆P), and Cv, is the fluid specific gravity (G).
The specific gravity of water is 1.0

Therefore:

There are many other contributing factors to be considered when selecting the right type, and right size of valve for your application, such as velocity, friction loss, viscosity, etc.
Remember, no two manufacturers valves are exactly the same, and just because your pipe size is 2”, that does not necessarily mean that a 2” valve is the proper choice.
Please contact Jomar International Ltd for assistance, and the security of knowing that you have the right valve for the application.