Does the Flow Coefficient Cv have any units?

[Rahul Baro]

TL;DR Summary

Why Flow coefficient doesn't have any unit ?

Flow coefficient in valves doesn't have any units. why?

 

[Lnewqban]

It can have units, if you want: (gallons / minute) * sqrt (1/p.s.i.)

Please, see:
https://en.wikipedia.org/wiki/Flow_coefficient

 

[Rahul Baro]

It can have units, if you want: (gallons / minute) * sqrt (1/p.s.i.)

Please, see:
https://en.wikipedia.org/wiki/Flow_coefficient

in what cases it does not have any unit?

 

[Lnewqban]

In practical terms, the coefficient is expressed only as a number.
For example, the manufacturer of a valve tells an engineer that he offers two valves, one with Cv=20 and another one with Cv=10.
Those numbers by themselves, help the engineer select the valve that best fits its system and/or control modulation.
The first valve is able to carry 20 US gallons per minute for a pressure drop of 1 p.s.i., while the second valve, for the same pressure drop, will carry 10 US gallons per minute.

 

[Rahul Baro]

o

In practical terms, the coefficient is expressed only as a number.
For example, the manufacturer of a valve tells an engineer that he offers two valves, one with Cv=20 and another one with Cv=10.
Those numbers by themselves, help the engineer select the valve that best fits its system and/or control modulation.
The first valve is able to carry 20 US gallons per minute for a pressure drop of 1 p.s.i., while the second valve, for the same pressure drop, will carry 10 US gallons per minute.

okay! i understood the idea.. thanks alot!

 

[Lnewqban]

In Europe, they use the Kv instead (SI units).
You are welcome!

 

[russ_watters]

In practical terms, the coefficient is expressed only as a number.

To expand/generalize, by definition a coefficient is a unitless multiplication factor, like a percentage. For example, a friction coefficient is a percentage/fraction of normal force "converted" to friction. Similarly, a flow coefficient is a fraction of velocity pressure lost in a valve or other device. The way we often use it in IP units, I think it is a bastardization that includes some rolled-up constants.

So usually if you see units, it's for clarification; eg, N/N (friction).

 

[Dullard]

Where Cv is 'expressed' as a unitless coefficient, imprecision is present. See post #2. Dimensions are a gift from God - respect them. Or else. There are cases where true coefficients exist (see post #7). Cv isn't one of them.

 

[Rahul Baro]

To expand/generalize, by definition a coefficient is a unitless multiplication factor, like a percentage. For example, a friction coefficient is a percentage/fraction of normal force "converted" to friction. Similarly, a flow coefficient is a fraction of velocity pressure lost in a valve or other device. The way we often use it in IP units, I think it is a bastardization that includes some rolled-up constants.

So usually if you see units, it's for clarification; eg, N/N (friction).

okay! thank you so much for the help :)

 

[gmax137]

Careful. People say "flow coefficient" and this should mean "Cv" as used by valve manufacturers and discussed above. But sloppy people might mean "k" or "resistance coefficient" sometimes also called zeta. This goes into the Darcy equation as fL/D, a dimensionless multiplier on the velocity-squared term.