K = (1.59923 * P * d4 * P) / W2
Where W = Mass Flow Rate, P = Fluid Density, d = Pipe Diameter, P = Change in Pressure.
For example, when the change in pressure is 12 bars, pipe diameter is 2 mm, Fluid density is 3 kg/m3, mass flow rate is 4 kg/Hr, Then the differential pressure is 57.57227999999999.