Orifice plate

An orifice plate is a device used for measuring flow rate, reducing pressure or restricting flow (in the latter two cases it is often called a restriction plate).

Description

An orifice plate is a thin plate with a hole in it, which is usually placed in a pipe. When a fluid (whether liquid or gaseous) passes through the orifice, its pressure builds up slightly upstream of the orifice but as the fluid is forced to converge to pass through the hole, the velocity increases and the fluid pressure decreases. A little downstream of the orifice the flow reaches its point of maximum convergence, the vena contracta (see drawing to the right) where the velocity reaches its maximum and the pressure reaches its minimum. Beyond that, the flow expands, the velocity falls and the pressure increases. By measuring the difference in fluid pressure across tappings upstream and downstream of the plate, the flow rate can be obtained from Bernoulli's equation using coefficients established from extensive research.

In general, the mass flow rate

q

m

{\displaystyle q_{m}}

measured in kg/s across an orifice can be described as

q

m

=

C

d

1

−

β

4

ϵ

π

4

d

2

2

ρ

1

Δ

p

,

{\displaystyle q_{m}={\frac {C_{d}}{\sqrt {1-\beta ^{4}}}}\epsilon {\frac {\pi }{4}}d^{2}{\sqrt {2\rho _{1}\Delta p}},}

where

The volume flow rate

q

v

{\displaystyle q_{v}}

measured in m3/s is

q

v

=

q

m

ρ

1

.

{\displaystyle q_{v}={\frac {q_{m}}{\rho _{1}}}.}

The overall pressure loss in the pipe due to an orifice plate is lower than the measured differential pressure, typically by a factor of

1

−

β

1.9

{\displaystyle 1-\beta ^{1.9}}

.

Application

Orifice plates are most commonly used to measure flow rates in pipes, when the fluid is single-phase (rather than being a mixture of gases and liquids, or of liquids and solids) and well-mixed, the flow is continuous rather than pulsating, the fluid occupies the entire pipe (precluding silt or trapped gas), the flow profile is even and well-developed and the fluid and flow rate meet certain other conditions. Under these circumstances and when the orifice plate is constructed and installed according to appropriate standards, the flow rate can easily be determined using published formulae based on substantial research and published in industry, national and international standards.