Flow Calculation from Pressure Differential Devices
When installation requirements and flow conditions from ISO 5167 are met, standardized flow devices can provide flow rate results with good accuracy.
The diameter ratio is defined
β = d/D (1)
The Reynolds number, expressing the ratio between the inertia and viscous forces in the upstream pipe is
ReD = m/0.25 π μ1 D (2)
The expansion factor takes into account the compressibility of the fluid.
Nozzles, Venturi nozzles, Venturi tubes
Discharge Coefficient and Flow Coefficient
The discharge coefficient is defined for an incompressible fluid flow and relates actual vs. theoretical flowrate.
Corner tappings: L1 = L'2 = 0
Flange tappings: L1 = L'2 = 25.4/D with D [mm]
D and D/2 tappings: L1 = 1; L'2 = 0.47
ISA 1932 Nozzles
Long radius nozzles
Classical venturi tubes with an »as cast« convergent section
Classical venturi tubes with a machined convergent section
Classical venturi tubes with a rough-welded sheet-iron convergent section
Discharge coefficient and flow coefficient are related
Where the discharge coefficient depends on the Reynolds number the mass flow can only be found by iteration.