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Manual Depressurisation - How To Determine Restriction Orifice Size

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    Junior Member

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Posted 14 October 2020 - 03:23 AM

Good Day everyone,


Please I will need your help/input on how to determine the restriction orifice size for a manual globe valve to be used for manual depressurization during blowdown for maintenance.


Many thanks.

#2 breizh


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Posted 14 October 2020 - 03:42 AM


You may use the documents attached to support your work.

Hope this is helping you. 




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Posted 14 October 2020 - 04:56 AM

Thank you so much Breizh, I will check through the attached documents.

#4 TheHague


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Posted 11 August 2021 - 09:18 AM



Can I also have the copy of the attached files?


Thank you!

#5 breizh


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Posted 11 August 2021 - 09:56 AM


For you and others . 



Attached Files

#6 astro


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Posted 12 September 2021 - 08:48 PM

For those interested, a trip in the Wayback machine yields good background (courtesy of mbeychok) to the posted spreadsheet:



When sizing a restriction orifice for critical flow, Ward-Smith, Critical Flowmetering: The Characteristics of Cylindrical Nozzles with Sharp Upstream Edges, Int J Heat Fluid Flow, 1(3):123–132, 1979, investigated and demonstrated the importance of orifice bore to plate thickness in determining the appropriate discharge coefficient. Ward-Smith's work is referenced by Miller, Flow Measurement Engineering Handbook, 3rd Ed. (Eqn. 13.23) and I've seen Miller's position on critical flow orifice sizing widely referenced. Unfortunately for my tastes, Miller doesn't delve into the detail of the source material enough.


Ward-Smith provided a meta-study by drawing on 6 published data sources, as well as doing his own empirical work. For me, this offers a firm basis that provides confidence in his findings. Miller summarised matters with the following statement:

Square-Edged Orifice. Cunningham (1951 first drew attention to the fact that choked flow will not occur across a standard, thin, square-edged orifice. Recently Ward-Smith (1979) showed theoretically that, for choking to occur, the discharge coefficient must approach unity. His highest measured value was approximately 0.9, and during his work the orifice remained unchoked down to a choking pressure ratio of 0.2. However, choked flow is achieved by increasing the plate thickness.

For a sharp edge and for ratios of plate thickness to bore diameter [t/d] between 1 and 6, the discharge coefficient is:
C = 0.83932 (13.23)

Ward-Smith doesn't specify a defining number but instead presents a graph that presumably Miller has quantised and interpolated.


From Ward-Smith's paper, between 0.3 < t/d < 1.0, Cd rises roughly linearly to a maximum. The highest value reported is 0.9. Bear in mind Ward-Smith's cautionary advice, "slight variations in the sharpness of the leading edge undoubtedly lead to variations in Cd".


My take from this is that judgement is required when applying critical discharge coefficient and I, personally, consider the application involved when considering design margin and the process need on the upstream side (where the driver can be to quickly depressure to avoid vessel rupture due to fire) and downstream side (where the driver can be to avoid overloading a vent/flare &/or preventing excessive low temperatures and brittle fracture) of the restriction.


AFT mention Ward-Smith in their paper on critical flow through an orifice here:


But for reasons I don't understand, AFT appear to reference a webpage for information rather than read the source material. What the AFT paper does offer though, is a sense of the pitfalls in appropriate determination of discharge coefficient and the influence of plate thickness.


All a bit murky but if there's no other message, ensure that you understand the basis behind your source of discharge coefficient and whether that source considers plate thickness in a robust manner.

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