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A fluid passing though an orifice constriction will experience a drop in pressure across the

orifice. This change can be used to measure the flowrate of the fluid.

To calculate the flowrate of a fluid passing through an orifice plate, enter the parameters

below. (The default calculation involves air passing through a medium-sized orifice in a 4"pipe, with answers rounded to 3 significant figures.)

Inputs

Pipe (inlet) diameter upstream of orifice, Di:10

cm

Orifice diameter (less than the inlet diameter), Do:8

cm

Pressure difference across the orifice, p:10

Pa

Fluid density, :1.29

kg/m^3

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Flow Coefficient, Cf:0.7

Answers

Velocity at the inlet, Vi: 1.76 m/s

m/s

Volumetric Flowrate, Q: 13.9 l/s

l/s

Mass Flowrate: 0.0179 kg/skg/s

Calculate Again

Default Values

Equations used in this Calculator

As long as the fluid speed is sufficiently subsonic (V< mach 0.3),

theincompressibleBernoulli's equation describes the flow reasonably well. Applying this

equation to a streamline traveling down the axis of the horizontal tube gives,

where location 1 is upstream of the orifice, and location 2 is slightly behind the orifice. It is

recommended that location 1 be positioned one pipe diameter upstream of the orifice, andlocation 2 be positioned one-half pipe diameter downstream of the orifice. Since the

pressure at 1 will be higher than the pressure at 2 (for flow moving from 1 to 2), the

pressure difference as defined will be a positive quantity.

Fromcontinuity,the velocities can be replaced by cross-sectional areas of the flow and the

http://www.efunda.com/formulae/fluids/glossary.cfm?ref=incomp#incomphttp://www.efunda.com/formulae/fluids/bernoulli.cfmhttp://www.efunda.com/formulae/fluids/navier_stokes.cfm#continuityhttp://www.efunda.com/formulae/fluids/navier_stokes.cfm#continuityhttp://www.efunda.com/formulae/fluids/bernoulli.cfmhttp://www.efunda.com/formulae/fluids/glossary.cfm?ref=incomp#incomp

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volumetric flowrate Q,

Solving for the volumetric flowrate Qgives,

The above equation applies only to perfectlylaminar,inviscid flows. For real flows (such as

water or air), viscosity and turbulence are present and act to convert kinetic flow energyinto heat. To account for this effect, a discharge coefficientCdis introduced into the

above equation to marginally reduce the flowrateQ,

Since the actual flow profile at location 2 downstream of the orifice is quite complex,

thereby making the effective value of A2uncertain, the following substitution introducing

a flow coefficientCfis made,

where Aois the area of the orifice. As a result, the volumetric flowrate Qfor real flows is

given by the equation,

The flow coefficient Cfis found from experiments and is tabulated in reference books; it

ranges from 0.6 to 0.9 for most orifices. Since it depends on the orifice and pipe diameters

http://www.efunda.com/formulae/fluids/glossary.cfm?ref=lam#lamhttp://www.efunda.com/formulae/fluids/glossary.cfm?ref=invis#invishttp://www.efunda.com/formulae/fluids/glossary.cfm?ref=invis#invishttp://www.efunda.com/formulae/fluids/glossary.cfm?ref=lam#lam

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(as well as theReynolds Number), one will often find Cftabulated versus the ratio of orifice

diameter to inlet diameter, sometimes defined as ,

The mass flowrate can be found by multiplying Qwith the fluid density,

http://www.efunda.com/formulae/fluids/overview.cfm#reynoldshttp://www.efunda.com/formulae/fluids/overview.cfm#reynolds