The Kv equation

Formulas behind the calculator

The Kv-calculator uses the definitions quoted in ISA-S75.01-1985 "Flow equations for sizing control valves":

pic of formula


Y=1: as long as X<<Xt where Xt is the critical pressure drop ratio. [Y = 1 - X / 3*Ft*Xt]

Fp=1: this value is the ratio between the Kv of the valve with fittings and the Kv without fittings. It is determined experimentally.

Z=1: true for ideal gasses.

Q : Volume flow  Gg: Specific gravity
N : Units constant  T1: Upstream temperature
Fp: Pipe geometry factor  Z: Compressibility factor
P1: Upstream pressure X: Pressure drop ratio DP/P1
Y : Expansion factor Ft: Specific Heat ratio

Kv measurement test procedure

ISA-S75.02-1988 "Control Valve Capacity Test Procedure" describes two test methods for determining the valve flow coefficient Kv. This section is a extract of the most practical method.


At a fixed stem travel the expansion factor Y is a linear function of the pressure drop ratio X (Y=1-X/3FtXt). As X tends to zero Y tends to 1, by plotting YKv as a function of X the value of Kv is determined by the intersection of the straight line fit with the YKv axis:

pic of formula


The pressure drop ratio and the flow can be measured with the test setup shown below. Valves V1 and V2 are used to set values of pressure drop ratio X (X=DP/P1), for constant P1 and fixed valve stem travel. P1 and P2 measure absolute pressure.

pic of setup

Specifying a Kv value for an order

When calculating the Kv of a valve usually the user specifies the values of flow rate and a pressure drop across the valve achieved at the expected stable process conditions. For an adequate regulation these characteristics should be matched by the valve's Kv at 50% stem travel. It is common practice among valve manufacturers to deliver the valve quoting the value of the Kv at 100% travel (valve fully open). Therefore it is recommended to specify that the calculated Kv must be achieved by the valve at a well defined travel position within a given uncertainty.