In order use a simple formula to calculate and compare the different types of valves, all variables permitting the calculation of flow have been reduced to a common denominator: the flow factor kv. This factor depends on the design of the valve and the outlet tube size.
The Kv factor given in this catalog have been set up according to the VDI/VDE 2173 standards at the following conditions :
Fluid: | water | |
Specific gravity : | 1 [kg/dm^{3}] | |
Fluid temperature: | 5° C - 30° C | |
Input P1 relative pressure: | 6 [bar] | |
$ Pressure difference P: | 1 [bar] |
For fluids, the flow factor is calculated as follows :
In practice, flow factor Kv, is almost identical for a valve carrying water or oil up to a viscosity of 3°E. Beyond that limit, pressure flow specifications are modified; one should then take into consideration an adjustment factor C:
kinematic viscosity of the fluid in centistokes [cSt] | ||
Q | flow [l/min] | |
kv_{eau} | flow factor indicated in the catalog | |
kv_{x} | corrected flow factor |
The calculation will lead to the choice of a valve with a higher flow factor value than the one which would have been chosen for a fluid with a viscosity inferior or equal to 3°E (Engler degrees).
Example : water at 20°C has a viscosity of 1 (cSt).
Conversion of the viscosity in centiStokes / Engler degrees
In the case of gases, the flow factors are calculated as follows:
subsonic
supersonic
"Q_{n}" flow factor correspond to standard conditions
P | Pressure drop in [kg/cm^{2}] or [bar] | |
P_{1}, P_{2} | Upstream, respectively downstream absolute pressure in [kg/cm^{2}] or [bar] | |
Specific gravity of fluid at standard conditions (20°C) en [kg/m^{3}] | ||
_{T} | Absolute temperature in K (0 K = -273.15°C) |
The conversions of the flow factor coefficients are:
kv = 14.28 Cv (USA)
kv = 17.11 f (UK)
When several valves are assembled in parallel, the total Kv is equal to: