ORIFIC Pressure Drop Calculation

[ilan7]

We are desing 3 orifices one after the other to reduce steam pressure.
The question is: when calculating the pressure drop on the second orifice, do we need to add the recovery pressure of the first one in order to estimate the inlet pressure to the second orifice? same with the thierd orifice? The total pressure drop will be each orifice DP minus the recovery pressure of each orifice?
Thanks
ilan

 

[hacksaw]

the amount of recovery depends on the distance between the plates. for example, if you place an orifice in the "jet" formed by the first all of your discharge coefficients are off.

there are multiple hole orifies designed to accomplish what you want and deal with noise issues to boot.

you need to be cautious about using flow meter methods that distinguis between orifice drop and net pressure loss.

 

[MariusChE]

Is there a neat and quick calculation available somewhere (formulae)?

remove.marius@mailbox.co.za

 

[Cockroach]

Ilan7, 1) depends on the spacing between orifice fittings; ideally you would want FULL pressure recovery, 2)Yes, see #1, 3) Yes, assuming #1 & #2 are met.

Consult the AGA Report Number 3 for orifice fitting calculations and proper modelling parameters. Also, the Handbook of Measurement, Meuller is the bible of the industry and probably the best textbook on the subject.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[hacksaw]

Ken,

full pressure recovery? I think the plates are being used in a pressure reduction attempt (without a valve). Pressumably critical flow conditions are occurring in at least one of the plates so full recovery is not an option.

 

[Cockroach]

I disagree HackSaw. Typically we are talking dP around 100 inches of water, 3.617 psig. This hardly creates conditions for critical flow.

For example, in my design of Valor Orifice Fittings utilized in natural gas pipeline metering applications, NPS 6 ANSI Class 600, upstream pressures are no greater than 1440 psig and typical dP are 75% full scale deflection of 100 inches of water. In this regard the AGA Report Number 3 specification calls for 6' 10" in upstream displacement between the front face of the orifice plate and backside of the tube bundle (Type II Straightening Vane) and an additional 7' 10" of pipe in front of the tube bundle. Full pressure recovery is at 2' 3.5" minimal distance from backside of the orifice plate so that any densiometer, transducer, thermocouple weldalet must be BEYOND that downstream distance. I would expect the same type of design to hold valid for buddy's application.

Orifice plates are NOT flow control devices, they measure velocity of the fluid stream. In this regard, pressure reduction attempts to critical flow conditions are absolutely out of the question. Full pressure recovery would be evident by simply cranking the above numbers for natural gas. (i.e. I use thermodynamic methane tables)

Therefore I stand by me previous posting.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[hacksaw]

Ken,

No disagreement on this either in Canada or in the US, but the original query deals with use of restriction devices. related, yes, but really a completely different application and design requirement.

 

[ilan7]

Hi Fellows
Thanks to all. I did not mean to have you gentlemen take it so serious...
The orifices we design are actually for pressure drop and not for flow measuring, and that's the reason I did not write at the instrumentation and control forum but here.
We have a line with steam pressure of about 18 bars, and in case of emergency we have a valve which open and relese the pressure to a 1.3 bar abs. (about 20 psi ABS)line. In order to make the valve life easy, we are adding an Orifice which will take most of the pressure drop. We calculated the orifice and then we noticed that we have two version of calculation: one when considering the recovery pressure and one with out it.
How is right?

 

[hacksaw]

the orifice (discharge) coefficients of the plate are based on fully developed turbulent flow. this precludes the placement of a second plate immediately up stream if using the usually restriction plate calculations.

physically you can do it, but your d/p estimates will be off by more than suggested by incomplete pressure recovery.

good luck

 

[25362]

Have a look at the O&GJ of April 15, 1985 Technology article titled Program gives design data for multiple-orifice system to avoid cavitation by Goksem and Patel.

If a single reducing orifice is intended then the Chemical Engineering issue of August 17, 1987, under the heading of Plant Notebook brings an article (with example) titled Quick sizing of restrictive orifices by Herman E. Waisvisz.

Hopefully, these articles would provide answers to your queries.