Cv is the Valve Coefficient, and is a measure of the capacity of a valve, which takes account of its size and the natural restriction to flow through the valve. Using published formulae it is possible to calculate the Cv required for an application. By comparing this calculated value with the Cv capacities of different valves it is possible to select a suitable size and type of valve for the application. Common Definition of CV
The Cv of a valve is the quantity of water in US gallons at 60 °F that will pass through the valve each minute with a 1 psi pressure drop across it.
The Kv value is the metric equivalent in m3/hr with 1 bar pressure drop
Cv = 1.15 x Kv. The capacity of each valve can be expressed in terms of Cv - the value being determined experimentally in most cases. Using formulae developed empirically it is possible to calculate a Cv requirement for an application. By comparing the two figures it is possible to select the correct size of valve for the application. It is important to remember that the formulae and the valve Cv values are not exact, but are to be used as a guide. The most commonly used formulae are those supported by the Instrument Society of America (ISA). Simplified Liquid Service. C q G P v f e = ⋅ 1 1. 6 ⋅ ∆ Effective pressure drop is the smaller of (P1-P2) or ∆Pchoked P1-P2 is the actual Pressure drop ∆Pchoked = Fl²(P2-Pv) Where : Pv = Vapour pressure Fl = Pressure recovery factor Where : Cv is Valve Coefficient : Flow rate in m³/hr : Gf is Specific Gravity at the flowing temperature : ∆Pe is the effective pressure drop in Bar 1
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