The pressure loss coefficient uses the dynamic pressure concept. If VS is the average fluid speed in the section S, the dynamic pressure in this section is written:
where m is the per volume ratio (in kg/m3).
Note :
The dynamic pressure is created by the fluid speed. It is, for example, the pressure we feel when passing an arm through the car window (the ambient pressure is always around 1 bar, but it is the relative speed between your arm and the ambient air that creates a pressure on it.
The pressure loss ∆P of a valve assembled on this piping is then written :
where K is the pressure loss coefficient of the valve calculated for the section S. It will be written KS :
- KS is a coefficient without dimension, uniquely depending on the form and the condition of the valve surface, and the form and the dimension of the internal mobile organs. Then, if the fluid vein form is perfectly conserved, KS does not depend on the valve DN.
- KS is the same (for a given machine, with a given opening) for all fluids, whatever is the viscosity.
- KS does not depend neither from the fluid pressure nor the speed or the debit.
- KS varies with the valve opening factor (it is bigger when the valve closes)
Unfortunately, KS can not be theoricly calculated and it is necessary to make measurements on a specific test bench in order to evaluate its value according to the valve opening factor. All SAFI’s valves and materials pressure losses have then been experimentally measured.
The KV debit ratio expresses in M3/h the water flow circulating in a machine by creating a 1 bar pressure loss.
Where :
- d is the fluid density compared to water
- ∆P is the pressure loss in bars (and not in Pa anymore)
- Q is the fluid volume flow rate in m3/h
Note :
The US coefficient of the CV flow expresses in Gallon US per minute the water flow going through a device and creating a 1 psi pressure loss (1 gallon US = 3.78533.10-3 m3 ; 1 psi = 0.0689476 bar)
then :
The flow coefficient can be calculated from the pressure loss coefficient with the following formula :
Where S is the reference section expressed in m2
