The fluid is supposed to be isothermal and incompressible, which presumes for gases; a low speed compared to a fast speed.
The flow is supposed to be steady, which means that in a given location of the valve, the fluid parameters (pressure, speed, flow,…) do not vary according to time
The frictional forces (related to viscosity) are supposed to be negligible in front of inertia force.
It is presumed that there is no cavitations (no vapour phase during the liquid phase).
