RO Sizing and Back Pressure 1

[dash81]

Hi everybody, I'm a junior process engineer and I'm facing my first RO sizing and specification, it is a restriction orifice for gas blowdown from a pressure vessel. The blowdown is from 1400 psig to 30 psig in 15 minutes according to client's standards. The RO has a block valve downstream in which a pipe spec break occurs (600#-150#). I performed a HYSYS dynamic simulation including the vessel (with actual dimensions), the RO, the relief network pipe and flare, and found the peak value of back pressure at the RO to be around 100 psig, so I specified it with this value and develop a Datasheet for it. It was sent to a manufacturer who says the pressure drop across the RO will be only around 700 psig (which is the critical pressure). So I am concerned about the difference between the RO manufacturer and the simulation results, knowing that the relief system downstream the RO is 150#. I would really appreciate any advise. Thanks in advance. Best regards.

 

[JLSeagull]

Instead of a restriction orifice, consider a fixed or adjustable choke for dropping the high pressure to low pressure. Adjust the downstream piping as required. The fixed choke performs somewhat like a restriction orifice and some can handle your 1400 psig. The adjustable choke includes a needle and seat. Your application sounds like an offshore requirements (perhaps API RP 14C etc.) thus a fixed choke seems like a potential solution instead of a restriction orifice.

 

[casflo]

When the RO is a sharp edge orifice there are not critical conditions, so the downstream pressure could be reduced until the 100 psig value that you say. The 700 psig that the manufacturer says would be in the case of an orifice with rounded edge, a nozzle or a venturi. See the graph to obtain the net expansion factor Y in the Crane thecnical paper and you can verify that for square edge orifices there is not limit for the pressure ratio.
Best regards

 

[katmar]

We can argue over the intricacies of shock waves and critical pressure drops and adiabatic expansions and so on, but your real problem is to know what pipe spec to use. It may well be that at peak flow the pressure drop from the RO to the flare is only 100 psi, but this is the situation under flowing conditions. Seeing that your block valve is downstream of the RO, under no-flow conditions you will have the full vessel pressure all the way to the block valve. You should check whether a 600# system can take 1400 psig at your actual temperature.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[casflo]

When the block valve is closed the problem is as katmar says to check if the 600# pipe can take 1400 psig, but the pipe spec change to 150# is downstream the block valve, so when it opens the question is if in the 150# pipe the pressure is 100 psig or 700 psig.
Regards

 

[dash81]

Thanks all for your answers. I forgot to mention that there is a blowdown valve (closed during normal operation) upstream the RO, so in no-flow condition the 150# pipe is not at 1400 psig. Thanks again, I really appreciate your comments. Regards.