Restriction Orifice

[sniperel]

Dear friends,

I need to depressurize a vessel containing natural gas at 270 barg. When sizing the Restriction Orifice ( downstream pressure 1 atm) I got the message that critical flow will occur and maximum pressure drop is 123 bar. I mention that before the Restriction Orifice the pipe class is 2500 # and after the RO is 150 #.Do I need to specify a higher pipe class because of the downstream pressure calculated by the program (147 bar) ?

Thank you for your help!

[Pedro_Suarez]

270 barg... This appears to be serious.
RO is the cheapest solution when it can be used.
I am not an expert and I never did or saw something like that.
Why don't you reduce the pressure in stages using pressure regulators?
Are you taking into account that at 270 barg NG is liquid and at 1 bar NG is gas?
I don't know what you want to do exactly, but if you want to vent the vessel, be careful with the temperature (anti-freeze systems), the noise and the ATEX zone that you are going to generate.

[ponnusj]

The message that you get is that with the huge pressure drop, the flow is choked (or critical flow). The pressure drop of 123 bar is the pressure drop above which there will be no increase in flow. It does not mean that the pressure downstream of the RO is 147 bar. The pressure rating downstream of the RO is based on the flare/vent backpressure + line losses. You need not specify a higher pipe class based on 147 bar.

[matajaldin123]

Chocked flow will accrued when a flowing fluid at a given pressure and temperature passes through a restriction into a lower pressure environment the fluid velocity increases to sonic. For natural gas the minimum differential pressure ratio for choked flow is around 1.9 so 270/1.9 is 142 that is the pressure downstream of orifice which mass flow will chock lower than it. This is not downstream pressure, downstream pressure depends on downstream system.
Regard
Taj