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Calculate the pressure drop of air across an orifice?

  1. Oct 12, 2015 #1
    Hi guys,

    I don't know if you can help me, or if I am asking my question correctly but your feedback will be greatly appreciated. I have a 2" piping system that connects a scrubber that is rated for 675 psi (36" OD x 7'-4") to another scrubber rated for 75 psi (84"ID X 24'- 0") ; there is a .3/8" orifice plate in the system and roughly 100 ft of schedule 40 steel pipe between the vessels. The system does have high and low pressure alarms but I am trying to prove that in the remote case that the high pressure scrubber reaches 675 psi there will be enough pressure drop at the currently installed orifice plate to make the 75 psi scrubber safe. I know it will be safe because the low pressure scrubber has a 12" pipe going to a Flare but I want to make sure that we will never reach 75 psi if the high pressure scrubber reached operating limits.

    How would you guys approach this situation? The gas flowing through the system is methane at 60F, I have estimated the flow by using a quick formula that estimates the amount of gas blown through a line puncture and it gave me 96,989 scf/Hr; I calculated this by assuming that the orifice was a line puncture of a system at 675 psi. This will be the choked flow rate at the orifice so this it should be the most flow rate that the system will see. I can then use that flow rate to calculate the Pressure drop due to piping friction but I am having a struggle calculating the pressure drop across the orifice.

    Am I approaching the problem wrong? Or any ideas on how to calculate the DP?

    In advance, thank you!
     
  2. jcsd
  3. Oct 12, 2015 #2

    JBA

    User Avatar

    I have a three successive orifice flow program that I have; and, using this program with the 2" sch 40 pipe ID as the first orifice, the .375 orifice as second orifice and the 2" sch 40 pipe ID again for third orifice with 60F air and an upstream pressure of 675 psig.

    This program does not calculate any piping pressure loss for the 2" pipe and without that effect the program calculated the pressure at the .375 orifice entrance to be 674.82 psig and the discharge pressure in the 2" pipe after the .375 orifice to be 8.56 psig with a flow rate of 1394 scfm (83,640 SCFH) which is very close to your calculated flow for the methane at 60F.

    Clearly this is not a duplicate representation of your piping with a .375 Dia orifice because it does not include any piping losses but any added piping losses on the upstream 2" pipe will only serve to reduce the .375 dia orifice inlet pressure; and, piping losses downstream of the .375 orifice will have little effect due the fact the pressure ratio across the .375 orifice is far above the critical pressure ratio required to insure choked flow through the .375 orifice.

    Just for interest I made a second calculation with the 12" pipe as the third orifice and the program calculated a .375 orifice discharge pressure of .01 psig for that final orifice diameter.

    It is up to you to decide whether this three orifice flow result give you any comfort regarding the safety of your system; but, based upon the first calculation using the 2" pipe ID as the first and third orifices, it appears to be reasonable estimate the effect of the .375 orifice in terms of both pressure drop and flow rate on your system with the 675 psig upstream supply pressure.

    One reassurance I can give you is that the program I used is based upon solid classical ideal gas flow equations and has proven to be accurate for predicting air three stage flow pressures through the inlet, nozzle and valve outlet combination on high pressure relief valves.
     
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