Orifice (diaphragm) calculation

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Orifice calculations

1. Introduction

Orifices are often used in fluid mechanics to limit mechanically the flow. It is important to be able to size them.
The restriction orifice mentioned here is a thin perforated plate, also called diapgragm.

2. Pressure drop through an orifice

The following correlation is given by Gibson, it is reproduced in many books.

Orifice pressure drop calculation
Equation 1 : singularity pressure drop coefficient for an orifice

With :
K = Pressure drop coefficient
D = Pipe diameter (m)
d = orifice diameter (m)
um = fluid velocity, before diaphragm, (m/s)
ΔHs = Pressure drop in m

The coefficient K can also be calculated thanks to the following graph :

Graph 1 : K coefficient as a function of d/D

Abascus for calculation of coefficient K for an orifice

Source :
Gibson et al., Hydraulics and its applications, 5th edition, Constable London, 1952