# Pneumatic/​Hydraulic/​Thermal Liquid domain - flow discharge coefficient vs. flow coefficient

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Andreas on 31 Aug 2015
Answered: Andreas on 13 Oct 2015
Hi everyone,
we are using thermal liquid, hydraulic and pneumatic domain a lot, but face problems with the flow discharge coefficient Parameter all the time. Example: I would like to parameterize the "variable area pneumatic orifice" block like the "Constant Area Pneumatic Orifice (ISO 6358)" block, i.e. using Kv/Cv coefficient instead of flow discharge coefficient (Cd). Alternatively a conversion formula from cd to Kv/Cv would be helpful. Most formulas I found are incorrect or use specific gravity or effective orifice area and thus are not helpful.
You could use this one for liquids:
Cd=Kv*sqrt(rho/2)*(1/(DN^2*pi/4))
DN=inner pipe diameter
But this one won't work for gases, since pressure and density after the orifice is not known.
In the German/European market all valve datasheets only contain Kv values for gas and liquid valves.
(this concerns thermal liquid and hydraulic orifice blocks as well).

Andreas on 13 Oct 2015
Suggestion for thermal liquid domain:
The local restriction block contains
delta_p == mdot_A_square / (Cd^2 * 2 * restriction_area ^ 2 * rho_u);
Where Cd is the flow coefficient.
For the European market you could copy the ssc-file and change the formula to:
delta_p == mdot_A_square * rho_u / 1000 * 1e5 / Kv^2;
Output: Pa (without 1e5: bar)
Thus you won't need the restriction area parameter anymore.

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