Bursting disc exit pressure queries - reaction forces

[chemks2012]

Hello all,

I have a bit of confusion and need your help please.
A reactor has got an independent bursting disc [designed for two phase release] and a relief valve [designed for vapour release] both relieving to atmosphere.

Reactor design pressure: 6barg
Maximum accumulation pressure: 6.6barg [i.e. 10% accumulation]
Bursting disc size and inlet/outlet line size: 8”
Burst pressure specified: 2.75barg
Overpressure allowed: 91% [i.e. 2.5barg]
Relieving pressure: 5.25barg [i.e. 2.75 + 2.5 barg]
Estimated two phase flow release rate: 100kg/sec
Estimated two phase flow pressure drop during release or back pressure : 1.35bar

Bursting disc inlet line: negligible height
Bursting disc discharge line: 8m straight long pipe without any bends/fittings [discharged at some angle though]

Check sum – see below

Vessel accumulation pressure >= relieving pressure + back pressure
6.6barg >= 5.25 + 1.35 = 6.6barg


My queries:

1) I have selected overpressure of 91% [it was selected by trial and error method]such that the sum of relieving pressure + back pressure stays equal to or below vessel accumulation pressure. Is this correct?
2) What would be my exit pressure here? Would it be 5.25barg – 1.35barg = 3.9barg? Please let me know if my understanding is correct.
3) If my understanding on point#2 above correct, I believe I can use 3.9barg as Pe [~71psia] in the reaction force estimation equation given by API 520 [See attached file -taken from Pentair book].

Thanks very much in advance.
KS

 

[Duwe6]

1) yes - you stay below MAWP + 10%.

2) Exit where? Vessel calc's to be 6.6 barg; end of vent pipe is 0.0 barg.

3) I would use 6.6; 5.25 ignores the pressure accumulation. 3.9 assumes that after ignoring the accumulation, the tailpipe will still be there 20-years later. Steel is damn strong in compression; you will be surprised at the small size of the bracing for the reaction force from the 6.6 barg. This bracing really only HAS to work once; testing & inspx of the bracing can be made part of the Test & Reset of the PRV after it has 'blown'.

 

[chemks2012]

Duwe6,

Thank you very much for your input, much appreciated.

1) Thanks
2) Pressure at the relief discharge pipe
3) Agree, it’s better to design pipe for higher pressure say at maximum vessel accumulation pressure of 6.6barg but I want to understand pressure profile during pressure relief event.
Please let me know if my below understanding is correct
Vessel shell will see maximum pressure of 6.6barg when bursting disc relieves at 5.25barg. In this case, pressure drop in the discharge line is 1.35barg and hence the pipe exist will only see 3.9barg [i.e. 5.25-1.35 = 3.9barg]. Have I missed anything here?

Thanks

 

[Duwe6]

Looks good to me. But there is a 'language' problem.

Pipe exit = open end of pipe = zero psi. Thrust on PRV generated by the flow is caused by the 3.9barg at the discharge of the PRV. And I'm sure that is exactly what you meant.

 

[chemks2012]

Duwe6
Thanks once again.
Yes, agree with you. Pipe exit pressure of course atmospheric [0 psig or 0 barg] but I believe the PE [i.e. PE given in the two phase flow reaction force equation shown in the attachment of the first post] is this thrust pressure of 3.9 barg NOT 0 barg.
Thanks
KS