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Coefficient Of Discharge For Depressurizing Orifices

depressurization restriction orifice

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#1 Dieguito2


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Posted 03 March 2019 - 04:35 AM

For do depressurization calculation with hysys, I’m accustomed to specify a coefficient of discharge of 0.62 for calculate the restriction orifice (RO) diameter needed for satisfy the design constrain, e.g. reach 6.9 barg in 15 min. This value of the coefficient of discharge is coherent with the suggested value in API 520 I for a rupture disk to which a depressurization orifice can be assimilated and to Fig. 10-16 of Perry 8th Ed. Chap. 11. In a recent discussion with a RO vendor, he sustain that the correct coefficient of discharge for chocked flow is according to Miller 0.83932 (see attachment) resulting in a sensible reduction of RO diameter respect to the calculation with coefficient of discharge 0.62. Please share your opinion and experiences about.



#2 AlertO


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Posted 04 March 2019 - 01:31 AM



Vendor has suggested you correctly about RO for depressurization which usually handles with critical flow. The discharge coefficient of 0.62 coming from ISO 5167 has been proven that orifice plate sized by this method cannot create a critical flow. Hence, Cd by Miller is selected for this service. Actually, you just give the required flow rate at your condition to vendor. They will perform the calculation and select the proper orifice plate for you. This doesn't relate to RO size only, but also its thickness will be considered.


Hope this may help you.

#3 PaoloPemi


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Posted 04 March 2019 - 02:04 AM

when depressuring P,  T  (and W for state changes) are not constant, fluid properties and process change passing from critical to subcritical conditions, different procedures may include different parameters and you may contact Aspen for the details,

I have a different solution (based on Prode Properties) for piping I adopt the methods proposed by Leung and for orfices (or valves) several correlations (considering critical and subcritical conditions during depressurization), the procedure calculates the flows depending from fluid properties (process conditions) and dimensions of restriction orifice, I suppose Aspen has equivalent methods, in both cases you can specify or obtain the dimensions of restriction orifice,

I have not an extensive comparison but I do not expect large differences (from different procedures) except, perhaps, in some specific cases...

Edited by PaoloPemi, 04 March 2019 - 02:49 AM.

#4 flarenuf


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Posted 04 March 2019 - 05:53 AM

alert0 is correct in saying that 0.9 is the figure to use for critical gas flow through a flare DP orifice.
the attached file makes it all clear :-)




Attached Files

#5 Dieguito2


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Posted 05 March 2019 - 05:58 AM

Flarenuf thank you for the article,
I cite from Perry:
“…, unlike nozzles, the flow through a sharp-edged orifice continues to increase as the downstream pressure drops below that corresponding to the critical pressure ratio rc. This is due to an increase in the cross section of the vena contracta as the downstream pressure is reduced, giving a corresponding increase in the coefficient of discharge. At r = rc, C is about 0.75, while at r ≅ 0, C has increased to about 0.84.”

The coefficient of discharge 0.62 appear to be the asymptote for high Reynolds number for liquid or gas flow in non-chocking conditions. Now, what I don’t understand is why API 520 I specify 0.62 as a coefficient of discharge to be used for sizing rupture disks for critical gas flow.

#6 PaoloPemi


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Posted 20 March 2019 - 05:29 AM

those values are not constant depending in some way from inlet conditions (P,T,W),

the direct integration procedure mentioned in my previous post #3 is a possible solution,

I have tested similar procedures for more than 20 years and results are generally more accurate than selection of constant values.

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