Orifice plate thickness to achieve critical flow

• Apr 2, 2003

• #2

hacksaw

Mechanical

Dec 7, 2002

2,587

US

you are correct, neither thin or thick plates exhibit critical flow characteristics because of the entrance losses.

Thick plates do offer a discharge coefficient that is nearly independent of reynolds number and tend to approach critical flow behavior better than a thin plate. You only approach critical flow in really thick plates i.e. T/d>>>D, because you achieve pipe flow prior to the final expansion, but you still have to deal with the entrance losses.

There are corrections for the "critical flow" behavior of thin/thick plates (T/d~1) in Millers Flow Handbook, but they only increase the complexity of the calcualtion rather than improve performance.

A flow nozzle does achieve true critical flow. Some of the meter manf. can supply them.

 

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• Apr 2, 2003

• #3

JoelleLaw

Chemical

Mar 3, 2003

3

AU

Choking must occur, as the vena contracta has a minimum size equivalent to that of the orifice.

As pointed out to me, the following link supports this idea:

http://www.optimal-systems.demon.co.uk/flow-orifice.htm

 

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• Apr 4, 2003

• #4

hacksaw

Mechanical

Dec 7, 2002

2,587

US

Joel,

For thin plates it only occurs as a limiting condition as acknowledged in the web-site you've referenced.

The issue raised by Mark is that critical flow does not generally occur with plates that fall in the thin-plate category. True critical flow conditions can occur only for true nozzles with a contoured inlet.

 

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• Feb 1, 2004

• #5

25362

Chemical

Jan 5, 2003

4,826

CA

To Hacksaw, if I'm not mistaken the "0.5-0.6 pressure ratio" ROT doesn't apply to flow through nozzles which may reach critical (sonic) flow already at a 5-10% pressure drop. Right or wrong ? Please comment.

 

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• Feb 2, 2004

• #6

davefitz

Mechanical

Jan 27, 2003

2,927

US

A good discussion on this topic is thread798-51260, or get a copy of the following thechnical paper on choked flow thru square edged orifices:
author A.J. Ward-Smith , Int J Heat Fluid V1 no 3 pp123-132 1979.

Basically the discharge coeficient is related to the ratio of the plate thickness divided by the orifice diameter. For a truly sharp edge t/d=0, the Cd=1.0 . For a t/d in the range 1<t/d<3, Cd=0.81 . For very thick plates t/d >7 the Cd is calculated using the Fanno relationships for frictionally choked flow in piping.

 

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