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Discharge Coefficient

The document discusses the differences between sub-critical and critical flow in orifices, emphasizing that a coefficient of discharge value of ~0.6 is only suitable for sub-critical flow. For critical flow restriction orifices, engineers should refer to the Flow Measurement Engineering Handbook, which provides a constant discharge coefficient of 0.83932 for thick, square-edged orifices with a specific t/d ratio. It also notes that below a t/d ratio of 1, flow does not choke, allowing for increased flow downstream as pressure decreases.

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Swapnil Mohokar
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237 views1 page

Discharge Coefficient

The document discusses the differences between sub-critical and critical flow in orifices, emphasizing that a coefficient of discharge value of ~0.6 is only suitable for sub-critical flow. For critical flow restriction orifices, engineers should refer to the Flow Measurement Engineering Handbook, which provides a constant discharge coefficient of 0.83932 for thick, square-edged orifices with a specific t/d ratio. It also notes that below a t/d ratio of 1, flow does not choke, allowing for increased flow downstream as pressure decreases.

Uploaded by

Swapnil Mohokar
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© © All Rights Reserved
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Typically, these operate with a small differential pressure and the flow

across the orifice is sub-critical. For a restriction orifice, differential


pressure is usually large, and the flow is often critical. Therefore, the
typical value of ~0.6 for the coefficient of discharge, C, is only suitable for
sub-critical flow and is not suitable when sizing a critical flow restriction
orifice.

The primary reference that engineers are directed to for the sizing of
critical flow restriction orifices is the Flow Measurement Engineering
Handbook by R. W. Miller. This provides equations specifically for a thick,
square-edged orifice. The definition of ‘thick’ is where the plate
thickness/orifice diameter (t/d) ratio is between 1 and 7. Within this region
the flow is choked as the differential pressure is greater than the critical
pressure ratio, and the coefficient of discharge is shown to be constant
with a value of C of 0.83932.

This is based upon the work of A.J. Ward Smith and is highlighted in the
figure below.

Below a t/d ratio of 1 an interesting phenomenon occurs where the flow


does not choke irrespective of the differential pressure across the orifice.
This is explained by the vena contracta not being contained within the
orifice and exists some distance downstream of the orifice. It is therefore
free to increase in size, and so pass more flow, when downstream
pressure reduces. This effect is highlighted in the figure above by values
of C>0.83932.

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