Orifice plate flow calculation - theory
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Calculation of flow rate using orifice plate calculator is for incompressible flow,
based on the Bernoulli principle:
where is:
p - pressure
rho - density view table
V - velocity
g - gravitational constant (9.81 m/s2)
z - geodetic height
Assumption that pressure lost is negligible (pressure drop is obvious and included with coefficient
of discharge which is introduced bellow):
and:
and if velocities substituted with flow rate:
where is:
Q - volumetric flow rate
D - diameter
Pressure drop through the orifice because of velocity increase can be calculated as follows:
Expressing flow rate from the previous equation leads to:
Substituting:
flow rate can be determined as:
where is:
C - coefficient of discharge
e - expansion coefficient
Coefficient of discharge can be calculated using following equation (ISO):
where is:
beta - diameter relation D2/D1
ReD - Reynolds number which can be calculated as follows:
where is:
ni - kinematic viscosity view table
mi - dynamic viscosity view table
L1 and L2 are functions on tap type and it is:
L1=L2=0 for corner taps
L1=1 L2=0.47 for D & D/2 taps
L1=L2=0.0254/D D[m] for 1" taps
Expansion coefficient e can be calculated (for gases only):
where is:
kappa - isentropic coefficient; kappa = 1.4 for air and other two atom
gas molecules view table
Other values are calculated using following equations:
mass flow:
velocities:
If flowing fluid is gas, then it is considered as incompressible and ideal. Equation for ideal gas:
can be used for calculation of temperature T:
as well as density rho:
where R is gas constant (R=287 J/kgK for air). view table
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