PSV sizing scenario

[Jack Nicholson]

Hi everyone. I have a question. Would you please help me?
On the attached PFD, we have a circulation drum (310-V-4) which transfer and get refrigerant ethane to/from coldbox heat exchanger.
In the case of accidentally closing of PV and LV, all liquid ethane in circulation drum could vaporize due to heat transfer with atmosphere, so that at 35 C, the vessel pressure would be 45 barg (design pressure: 13 barg).
What would be the PSV sizing scenario? We have thermal expansion scenario, but it is applicable to line with full of liquid. Also we have blocked outlet scenario, but it is applicable to pump discharge or wellhead pipelines.
Thanks in advance

 

[Latexman]

It is a heat imbalance case with vent closed. No cold ethane coming in (loss of cooling). Heat gains from ambient and cold box loads continue. The PSV load will be similar to the normal design load of PV plus loss of cooling.

Good Luck,
Latexman

 

[LittleInch]

Not quite sure what the question is here?

Your max gas flow rate would appear to be boil off gas from the ethane.

Now what that rate is might be quite difficult to calculate as it could be a mixture of heat coming into the drum plus the heat presumably coming in from that HX on the RHS.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

There may be 2 relief scenarios here:
1) Max operating non fire case
This may be the total max design case vapor leaving this flash drum, with LV still open, recirculation through cold box, and PV closed, with correction made for pressure to be relieving pressure and corresponding boiling temp for ethane. With high boiling temp for C2 at relieving temp, heat pickup through thermosyphon loop through cold box may be very little or non existent. Flash vapor in feed stream 330-29 will also be much less at relieving pressure.
2) Blocked in Firecase
If this drum is located outside the cold box, this scenario will be applicable if drum base is at less than 7m above grade. Presumably , there will be no heat pickup through thermosyphon loop in this scenario, as in (1), since C2 boiling temp at relieving pressure will be much higher. Take credit for any insulation that can withstand 15minutes of firecase exposure, else take no credit. During a fire, presume F&G system at this area would have shutdown feed through LV.

Other relief scenarios would present much smaller loads on PSV.

 

[Jack Nicholson]

Dear georgeverghese
Drum is 15 m above the ground, with 110 mm insulation and F&G protection system.

So I don't think we could use fire case.

 

[TiCl4]

Scenario 1: BPCS failure, LV fail open. Result depends on max incoming flow rate. Likely overfill of drum and liquid carryover to next stage through PV line. Not immediately an overpressure scenario, but evaluation needed.

Scenario 2: BPCS failure, PV fail closed. LV working as normal. Overpressure case, size for normal thermosyphon hear input, vapor relief.

Sceanrio 3: BPCS failure, PV fail close. Contributing cause: LV fail open. Drum will go to feed pressure quickly, then have heat input from thermosyphon. Same relief as Scenario 2. As the system increases pressure, need to evaluate backflow from this drum to feed source if drum MAWP > source pressure. Potential upstream overpressure consequences.

Scenario 4: BPCS failure, both PV and LV fail closed. Same relief as Scenario 2.

Scenario 5: Leak in thermosyphon heat exhanger. Evaluate relative pressures of tube/shell sides to determine consequences. If the heating medium is higher than the drum MAWP, it will begin to fill the drum and cause significant flashing - likely a much higher relief rate than previous scenarios. If drum pressure > heating medium, evaluate downstream consequences. Not sure what a “coldbox” HX is - perhaps someone could explain it to me. Is this an finned vaporizer like on N2 systems? Or does it actually have a shell side?

My $0.02

 

[georgeverghese]

Firecase at this drum at 15m elevation may still be applicable if there is a dedicated liquid collection deck directly under this drum (pool fire case) or deck anywhere within 7m below this drum.
Agreed, tube rupture case as pointed out by @TiCl4 would be applicable if thermosyphon pass design pressure of 13barg at cold box MCHE (probably compact all welded plate HE) is less than the max operating pressure(up to PAHH) of any other pass at the MCHE for the passes bundled together with this thermosyphon pass - check the assembly drawings for the MCHE - rupture may either be within any interpass plate or header box.