An orifice plate is a device which measures rate of fluid flow. It uses the same principle as the Venturi nozzles, namely Bernoulli's principle. It basically says that there is a relationship between the pressure of the fluid and the velocity of the fluid - velocity increases, the pressure decreases and vice versa. This article or section should be merged with Fluid mechanics Fluid dynamics is the study of fluids (liquids and gases) in motion, and the effect of the fluid motion on fluid boundaries, such as solid containers or other fluids. ... Bernoullis principle states that in fluid flow, an increase in velocity occurs simultaneously with decrease in pressure. ...

An orifice plate is basically a thin plate with a hole in the middle. It is usually placed in a pipe in which fluid flows.

As fluid flows through the pipe it has a certain velocity (which we want to measure) and a certain pressure (which is quite easily measured). When the fluid reaches the orifice plate, with the hole in the middle, the fluid is forced to converge to go through the small hole; the point of maximum convergence is actually just after the physical orifice, at the so-called "vena contracta" point (see diagram). As it does so, the velocity and the pressure changes. By measuring the difference in fluid pressure between the normal pipe section and at the vena contracta, we can find the velocity of the fluid flow by applying Bernoulli's equation. This article is about velocity in physics. ... Pressure (symbol: p) is the force per unit area acting on a surface in a direction perpendicular to that surface. ... In fluid dynamics, Bernoullis equation, derived by Daniel Bernoulli, describes the behavior of a fluid moving along a streamline. ...

With negligible frictional losses or change of elevation, Bernoulli's equation reduces to an equation relating the conservation of energy at two points in the fluid flow:

or

with

Q = A * V or V = Q / A

and Q₁ = Q₂

Solving for Q₁:

And finally introducing terms to account for fluid compressibility and orifice geometry:

Q₁ is the volumetric flow (e.g. gallons/hour)

V₁ is the fluid velocity

A₁ is the pipe area

A₂ is the orifice area

A₂ / A₁ is known as the β ratio.

P₁ is the inlet pressure

P₂ is the outlet pressure

ρ is the density of the fluid

The meter coefficient, C_meter, accounts for all non-idealities, including friction losses, and depends on the type of meter, the ratio of cross sectional areas and the Reynolds number. Equations for this exist (for example BS EN ISO 516). For rough calculations C_meter can be approximated to 0.62.

The compressibility factor, Y, accounts for the expansion of compressible gases; it is 1.0 for incompressible fluids. These two factors can be estimated from correlations.

This type of meter only works well when supplied with a fully developed flow profile. This is achieved by a long upstream length (20 to 40 diameters, depending on Reynolds number) or the use of a flow conditioner.

Orifice plates are small and cheap to install, but impose a significant energy loss on the fluid due to friction. If space permits, a venturi meter is more efficient. A fluid passing through smoothly varying constrictions is subject to changes in velocity and pressure, as described by Bernoullis principle. ...

External links

• Flow through Orifice Plates in Compressible Fluid Service at High Pressure Drop
• Small Diameter Orifice Flowmeter Calculation for Liquid Flow
• Liquid Orifice Meter Plate Calculation
• How to do an orifice calculation
• Flow Measurement
• http://www.efunda.com/formulae/smc_fluids/calc_orifice_flowmeter.cfm