Orifice Plate Critical Pressure Ratio 2

[denniskb]

Can anyone advise how to determine the Critical Pressure Ratio at which a Square Edge Orifice Plate with compressible fliud reaches critical (sonic) flow and the flows at pressure ratios beyond the critical ratio.

I need to correctly predict flowrates through pressurising and blowdown restriction orifices from high pressures where dP/P1 ratios are as high as 0.98 and the readily available texts do not provide the solution.

Almost all of the orifice plate calculations identified are for accurate measurement flows below the critical range and limit dP/P1 to say 0.35 or 0.6 to ensure the flow is subsonic.

Crane TP410 provides the formula for critical flow but first the Net Expansion Factor Y chart on A-21 is only provided up to dP/P1 = 0.6 and second the Critical Pressure Ratio chart on the same page is only provided for Nozzles and Venturi Meters and not for Square Edge Orifices.

I understand that the flow will continue to rise as the downstream pressure falls further because the vena contracta grows in diamter.
Dennis Kirk Engineering

http://www.ozemail.com.au/~denniskb

 

[olef]

Statoil (Norway) uses API 520 (Part 1) for sizing of orifices, utilizing the same equation as for PSV's (which really just is an orifice once lifted - correct me if I'm wrong). See API 520, Chap. 3.6, which provides equations for critical pressure ratio, and sonic and subsonic flow.

However API 520 specifies a discharge coefficient Kd of .975, which seems to be too high. We recommend using a factor of .81 for blowdown.

What confuses me however, is that there seems to be some confusion around the discharge factor. Which, as far as I can see, is not the same as the one used in e.g. Crane.

If anyone can help me with a good source on the API discharge factor, I'll be grateful.

I can also recommend the neat little shareware program Orifice from

http://www.norcraft.com

(free to try for 1 month), which does both critical and non-critical flow.

Regards
OleF

 

[davefitz]

The paper referenced above, by Ward-Smith, shows that a Cd = 1.0 can be ustilized in the theroretical case of t/d=0, so a knife edge might use Cd= 0.97. For thicker plates and after erosion, a Cd=0.81 wold be recommended.

 

[olef]

davefitz,

I have been reading up on this topic now, and I must say your postings have helped me a lot.

Quote from an earlier post:

&quot;thin plate (0<t/d<1)Cd varies smoothly from 1 to 0.81 as function of t/d.&quot;

By smoothly, do you mean linear, or is there a function?

If so, could you please post it?

Regards
OleF

 

[davefitz]

The case of t/d is practically impossible to meet , even a razor edge brand new the Cd= 0.97.

The curve suggests a semi log relationship of Cd(linear) vs log,10 (t/d)where Cd=0.97 at t/d = 0.01 and Cd= 0.81 at t/d=1.0

 

[olef]

Thanks davefitz

Cd = 0,81 - 0,0347 Ln(t/d)
for square edged orifices with t/d < 1

ok with you?

Regards
OleF

 

[davefitz]

OK, but you should get a copy of the paper and review fig 7 and the tables of test data to get an idea of the accuracy.

I had been using a Cd=0.84 for a t/d = 0.5; a better plot on semi log paper would use the 2 points t/d=1, Cd = 0.81 and t/d=0.5 Cd=0.84. There iia lot of data that supports the Cd=0;84 at t/d=0.5