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# Restriction Orifice sized but choke

## Restriction Orifice sized but choke

(OP)
I've selected a 2" Restriction Orifice RO bore size 0.54" with process condition P1=790kPag, P2=110kPag, Q=18350 Sm3/d, T=15C on Utility Air. The sizing tells me I have critical dP=418 kPa meaning my predicted P2=372kPag. Is it true even though the RO is at choke flow and can only handle press drop of 418 kPa, the pressure at the RO outlet is going to be 110 kPag because 110 kPag is my available process back pressure? Or is my P2=372 kPag then eventually gets to 110 kPag further downstream from the RO? Downstream of the RO goes into a vessel and out into a pond so estimate distance between RO and pond is 100ft.
sinhock
Replies continue below

### RE: Restriction Orifice sized but choke

sinhock,

It’s not clear to me if you have a thin plate orifice, or a thick plate orifice. Which do you have, or what is it’s thickness?

A thin plate orifice will NOT choke. A thick plate orifice will choke.

Good Luck,
Latexman

### RE: Restriction Orifice sized but choke

Your downstream pressure will be set by the downstream system based on flow and frictional resistance.

The critical dp of 418kPa is just the min DP you need to get to critical flow. Your upstream pressure can be higher than this and the flow will then depend on density.

The RO can handle anything you throw at it.

You don't really seem to understand choked flow and what it means. Look that up first

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Restriction Orifice sized but choke

#### Quote (LittleInch)

You don't really seem to understand choked flow and what it means. Look that up first

Good Luck,
Latexman

### RE: Restriction Orifice sized but choke

(OP)
Thank you Pierreick, Latexman and LittleInch for feedback. The RO selected has a thickness of 0.125" and thank you for the advise.
jess

### RE: Restriction Orifice sized but choke

That's a thin plate orifice. It will not choke. Your thinking in the original post (OP) is not 100% correct.

Good Luck,
Latexman

### RE: Restriction Orifice sized but choke

(OP)
Thank you Latexman.

### RE: Restriction Orifice sized but choke

Thin plates do restrict flow,but the discharge coefficient being less than unity it is not precise flow restriction.

Precision flow restrictions rely on thick plates, reduced bore pipe segments (as a really thick plate), or for precise requirements critical flow venturis or flow nozzles.

A thin plate will limit your flow but not precisely, often that is all you need.

No restriction determines the downstream pressure, that is determined by your process.

### RE: Restriction Orifice sized but choke

(OP)
thank you hacksaw. good to know the process downstream press dictates the RO downstream press.
jc

### RE: Restriction Orifice sized but choke

In my opinion the RO thin plates also may choke but a very high relative pressure drops, around values greater than 80%. In the paper of Cunningham, tittled: "Orifice meters with supercritical compressible flow" published in Transactions of the Asme, July 1951, the critical flow through the RO is practically for (P1 - P2)/P1 between 0.8 and 0.9
Also, it´s very significative to see the changes of the Crane Technical Paper No.410 "Flow of Fluids Through Valves, Fittings and Pipe" along the years. For example in the edition of 1965, page A-20, clearly the RO thin plates (square edge orifices),don´t choke. In the edition of 1979, page A-21, the maximum relative pressure drop is 60%. In the edition of 2010, is 20% because is more focused to the RO as flow meter.

### RE: Restriction Orifice sized but choke

casflo,

I think Cunningham would disagree with you:

Good Luck,
Latexman

### RE: Restriction Orifice sized but choke

Latexman, the part of the Cunningham´s article you attach is clear in the conclusion, but it´s necessary to analyze in deep all the document to see the limits of the tests, before extend the results to whichever pressure, pipe and RO size.
The article has as Reference 1 the paper of T. Stanton which states that the tested RO plate with a hole diameter of 6.15 mm, choke at P2/P1 = 0.2
Cunningham tested with air at P1 = 113 psia a RO plate of do/D = 0.15 installed in a 2 in pipe and the minimum measured value of P2/P1 was 0.13. Also tested with steam a similar plate, installed in a 3 in pipe and the results were the same.
Additionally tested with air at P1 = 90 psia, 7 RO plates with values of do/D from 0.2 to 0.8, installed in a tubing of only D = 0.53 in.The minimum tested value of P2/P1 was 0.14. The Figures 2, 3 and 7 of the article show the expansion factors Y obtained in the tests, that are extrapolated from P2/P1 = 0.13 to 0, assuming that the RO don´t choke.
In my opinion, considering only this article, seems risky to assume that the RO 1 hole thin plates, installed in pipes of more than 3 in and for pressures greater than 100 psia, don´t choke.
I think it would be necessary more tests with higher pressures and greater pipe sizes to know if the RO thin plates choke or not.
In the design of the nuclear power plants, there are RO plates to limit the steam discharged in the case of a pipe break and usually the calculations assume they choke.

### RE: Restriction Orifice sized but choke

casflo,

I do not disagree with you. This could very well be unsettled science, where more data is needed to shine light on the subject. Cunningham cited 3 pro-choking references and 3 con-choking references (one being his). 50/50 is not overwhelming evidence, right? And, yes, Cunniham's article makes it clear which side of the argument he is on. When I need to limit flow for safety reasons, I settle the uncertainty by using a thick plate orifice, usually a short section of smaller pipe.

I'd love to see other technical articles on this, or hear other's opinions.

Good Luck,
Latexman

### RE: Restriction Orifice sized but choke

Latexman,
I agree with you. The thick orifice plates choke. But, there is no unanimity to state when a RO plate is thin or thick. I consider that the RO plate is thin if t/do < 1, but I have seen papers that assume values equal or less than 1 and others with a value of
t/do < 0.5
When t/do > 7, the thick plate changes to a long thick orifice or insert orifice, used in the nuclear power plants for very high pressures.
casflo

### RE: Restriction Orifice sized but choke

Dear All,

It is generally understood that t/do is inherently attributed to the need to make the square edge orifices thick enough to withstand the acoustic reaction forces associated with high pressure drop. This is why we rarely find t/d= 1 in such applications. The corresponding calculated choked flow comes at the expense of such a reduction in t/d i.e. need to increase the RO thickness and we find that as per ward smith analysis of Cylindrical sharp edge nozzles the maximum obtained coefficient of discharge for a choked flow is 0.84. On the other hand, Crane paper is only discussing non-choked flow which has a Cd = 0.65. But in all cases, defining the case as choked or non choked depending on the calculated critical flow pressure which is a function of the gas properties and how the pressure drop across the RO varies with respect to it i.e. R>Rc.

### RE: Restriction Orifice sized but choke

The pressure downstream the RO is the available back pressure plus the frictional losses between the orifice and downstream pressure source, calculated for the specified flow rate. The additional drop in pressure energy beyond the choked flow condition will be converted in other forms of energy like noise and vibration. It is not 'available' for inducing a further increase in flow rate due to the choked flow condition.

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