# CHOCKED FLOW IN RESTRICTION ORIFICE.docx

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CHOCKED FLOW IN RESTRICTION ORIFICE How to rectify chocked flow in the restriction orifice located downstream of the Blow down valve. Actually we want to reduce pressure using restriction orifice from 10.3 barg to 0.7 barg. But chocking occurs at 6 barg pressure. RO located on the 2 line and the flow rate is 4.4 MMSCFD. In the situations such as you mentioned above,chocked flow downstream of RO is a common occurance. Size the RO for dP=9.6 bar and maximum flow of 4.4 MMSCFD. I find th
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CHOCKED FLOW IN RESTRICTION ORIFICE How to rectify chocked flow in the restriction orifice located downstream of the Blow down valve. Actually we want to reduce pressure using restriction orifice from 10.3 barg to 0.7 barg. But chocking occurs at 6 barg pressure. RO located on the 2 line and the flow rate is 4.4 MMSCFD. In the situations such as you mentioned above,chocked flow downstream of RO is a common occurance. Size the RO for dP=9.6 bar and maximum flow of 4.4 MMSCFD. I find this is an interesting question. Indeed, given the critical pressure ratio: Pc/P1 = (2/(k+1))^(k/(k-1)), with Pc= critical pressure and P1 = upstream pressure (10.3 barg in this case), gas flow becomes chocked i.e velocity becomes sonic (mach nº = 1) at vena contracta when the pressure downstream the orifice (P2) reaches Pc. OK. But, like the OP, i am wondering how the downstream pressure can further decrease from Pc to the required backpressure (0.7 barg in this case) while the velocity cannot further increase? by what mechanism? by form factors i.e. by changes in flow direction? N.B: When the fluid is steam and the restriction orifice is a sharp edge orifice, there is not critical pressure. Thanks a lot. As fluid passing the RO,velocity is increased till maximum at VC and decreased after it passed VC.The velocity at VC is at maximum due to minimum cross sectional area for flow. The increased pressure from VC is called pressure recovery. If the velocity at VC is at Sonic velocity (Mach no=1), the downstream pressure,increased by pressure recovery, is called Critical pressure (Pc).Recovered pressure is accompanied by velocity increase above Sonic Velocity. Keeping downstream pressure lower than Pc,means amount of pressure recovery dictated by amount of downstream pressure,but anyway after increasing velocity in downstream it can again increase may lead to second choking. Sorry, i don't understand this part of the explanation. After VC pressure will be recovered up to back pressure (may be higher than/equal to/lower than  Pc) already existed in downstream. If downstream pressure (Pd) would be higher than Pc there is no chocked flow and fluid pressure to be recovered up to Pd along with velocity decrease.After pressure recovery the velocity could be increased may lead to chocking in downstream piping. If downstream pressure (Pd) would be equal to/lower than Pc,then chocked flow occured and fluid pressure (corresponding to sonic velocity in VC) being recovered up to Pd along with velocity decrease.After pressure recovery the velocity could be increased may lead to second  chocking in downstream piping. Size the pipe and fittings downstream of the RO for 0.7 barg pressure drop (0.7 barg backpressure downstream of RO). The expansion wave (shock wave) will take care of the 6 barg ---> 0.7 barg. Fallah, I mean that the flow rate of 4.4 MMSCFD(Blow down rate of the system) should pass through the RO. For me the Choking occurs at 6 barg(this indicates only 2-3 MMSCFD of flow rate only pass through the RO) as i mentioned earlier. How can i release remaining flow through the RO. Actually in your case (as per what you mentioned) chocked flow would be occured when downstream pressure is equal to Pc (here 6 barg) or lower (here 0.7 barg).  As long as upstream and downstream pressure are fixed in 10.3 barg and 0.7 barg,for all Beta values calculated by RO sizing software we have chocked flow through RO but with different flowrates.When you fix folwrate=4.4 MMSCFD for software then it gives you corresponding Beta value be able to handle that fixed flowrate. As you can see in attached after VC pressure is recovered up to downstream pressure (P2=0.7 barg) that is lower than Pc (6 barg).Therefore you shall size the RO for your maximum dP (9.6 bar) and flowrate of 4.4 MMSCFD. Fallah, I have some doubt on the statement ...you shall size the RO for your maximum dP (9.6 bar) and flowrate of 4.4 MMSCFD... . As Latexman highlighted, shock wave will take care the pressure decrease from critical pressure (6 barg ??) to 0.7 barg. Should the RO be sized for dP of (10.3 - 6) bar and flowrate of 4.4 mmscfd ? Sheiko, ...N.B: When the fluid is steam and the restriction orifice is a sharp edge orifice, there is not critical pressure... I am interested in this statement. Do you mind to share with us why no critical pressure in this scenario or advise the source of this statement ? RO shall be sized for dP of (10.3-0.7) bar and flowrate of 4.4 mmscfd.RO sizing software will recognize Pc (critical pressure) and consider this value with respect to downstream pressure in its sizing condition. My opinion is : Real driving force in generating maximum mass flux passing RO (or sonic flow at vena contracta VC) is differential pressure between RO upstream pressure and critical pressure. This critical pressure occur close to the VC. Shock wave (highly irregular flow pattern) results critical pressure decrease to backpressure (0.7 barg). The driving force may be marginally change as backpressure increased (or decreased). Generally it is ignore. It could be assumed driving force in generating maximum mass flux passing RO (or sonic flow at vena contracta VC) is proportional to differential pressure between RO upstream pressure and corresponding pressure (lowest pressure along RO) of sonic velocity at VC.  When RO downstream pressure being decreased below critical pressure,velocity at VC (sonic) and its corresponding pressure (lowest) don't change and as per above statement mass flux passing RO dosen't change and pressure after VC increased just up to downstream pressure. Obviously,additional dP beyond decreasing downstream pressure below critical pressure that previously caused mass flux passing RO was increased,now with constant mass flux,converts to shock wave and high internal turbulence in RO downstream zone. Dear JW, The reference is in this thread: http://www.eng-tips....d.cfm?qid=51260

Jul 23, 2017

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