Dear all,

am a fresher in valve field. i have a doubt, will there be change in effective Cv, for a valve connected to same size line and to a 1/2 the line size (eg: 4 inch valve to 4 inch line and 4 inch valve to 8 inch line). if there's change, how much? and why?

ragin

 

The Cv of a valve is a measured value. It will not change no matter what you hook it up to.

However, when connecting the valve in series with piping, reducers, expansion joints, elbows, etc. the total in-line resistance does change as a result of "how" it is all connected.

Using an electrical analogy: The available gain of an amplifier is a function of its individual design. When connected in series with small diameter wiring (ie, high resistance) the apparent gain is much smaller than you'd expect due to the limitations of the connected wiring.

Taking this analogy to your control valve setup, you must analyze the circuit to see if any limitations are occurring which prevent your valve from operating to its full potential.

 

dear,

am asking about the effective Cv, as per ISA standards, there will be considereable change in the effective Cv for the same flow condition, if we change the pipe size. pls check

ragin

 

To add on to what Bruce said - One of the important things to note while sizing a valve (calculating required Cv) is the line size. When the valve size is different from the line size, expanders/reducers are used and these elements introduce pressure losses to the flowing medium.

Pressure loss across the expander/reducer means the "actual" pressure dropping across the valve is lesser than what was used in the calculations when sizing the valve. As you know, volumetric flow is directly proportional to the valve Cv & square root of fluid Pressure drop. For a valve with a given Cv, the flow will be reduced, when the available pressure drop is reduced. This is what happens when using reducer/expanders. If the expander/reducers are not taken into account while sizing the valve, one might end up under-sizing the valve. This has a significant impact in the case of butterfly valves, which are typically used in low pressure drop applications. The pressure losses in the expander/reducer can be a significant % of the available pressure drop.

To account for losses across reducer/expanders ISA 75.01.01 (which is the standard for flow equations for sizing control valves), has correction factors to use when having expanders/reducers with the valve. Many of the valve manufacturers' sizing program incorporates these correction factors.

Ronald Deepak.
[email protected]

 

Dear deepak,

Thanks for such a good explanation. to what it can affect sizing of a globe type control valve.

ragin