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Relief Valve Sizing For Reboiler Tube Rupture Scenario

hot oil enters in column

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#1 arvind

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Posted 01 March 2014 - 05:18 AM

Dear All,

 

This is regarding relief valve sizing for stabilizer column  for its re-boiler tube rupture scenario.

due to rupture in tube side hot oil will enters into the tube and eventually in column bottom. Due to high temperature of hot oil, bottom fluid will evaporate and causes over pressurization due to direct contact with hotoil.

please refer attachment for schematic.

 

I have calculated relieving rate considering overhead system (condenser, reflux and distillate etc.) at its turn down capacity and re-boiler at design duty and vapor generated due to direct contact between hot oil and column bottom.

 

Relieving rate  = vapor generated due to re-boiler design duty + vapor generated due to hot oil enters in column bottom - turn down capacity of column overhead system*

 

*  = turn down capacity of column overhead system has been considered for conservative approach as per shell standard.

 

(I have done simulation for this case in Unisim, hot oil rate obtained from tube rupture calculation, enters at column bottom along with reboiler rated duty.)

 

Data:

 

Column operating/ design pressure:  8.7 / 11 bar g

Column relief valve set pressure: 11 barg

 

Reboiler data:

shell side:

Fluid: hot oil

inlet/outlet temp: 282°C / 160°C

operating / design pressure: 14 / 23 ba rg

 

tube side:

Fluid: column bottom

inlet/outlet temp: 130°C / 132°C

operating / design pressure: 9.2 / 12.5 barg

 

Kindly advise me whether my understanding is correct.

 

Thanks and Regards,

Arvind

 

 

Attached Files



#2 paulhorth

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Posted 02 March 2014 - 06:10 AM

Arvind,

 

I agree with your approach to calculating this relief load.

I would make a minor comment: in the relief case, the column pressure would be 11 barg plus accumulation, that is 12.1 barg, and so the temperature in the reboiler will be higher than the normal oeprating temperature. This would reduce the heat transfer from the hot oil on the shell side and so would reduce the reboiler duty. This effect would be rather small, however, because you have a large LMTD across the reboiler ( about 90 C) and this would reduce by probably about 5 C.

 

I would also comment that this relief case could be eliminated, I believe, if the maximum operating pressure of the hot oil was reduced from 14 barg to 12 barg (then there would be no leak at relief pressure). To ensure the max operating pressure was limited, you could fit a PSHH and also consider what determines this pressure (pump DP plus pressure in the expansion drum). If you can prove you could never get above 12 barg you would be OK, regardless of the system design pressure of 23 barg.

 

Paul



#3 aroon

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Posted 02 March 2014 - 08:01 PM

Arvind,

 

Generally, tube rupture scenario is considered as "Remote" scenario (Refer API 521-5.19.2, 5th edition) for which you will have to consider comparison between high pressure side maximum operating pressure to that of low pressure side corrected hydro-test pressure. If your high pressure side maximum operating pressure doesn't exceed low pressure side corrected hydro-test pressure then your scenario itself is not applicable in first place. However, you will have to consider possible design contingency as "Tube Leak".

 

In tube leak, you will have to compare high side operating to low pressure side design pressures. Many times this scenario will be applicable. The basis of calculation is the leak through 6 mm pinhole, the rate will be low and you may proceed with the calculation which you have mention in your post. Only the thing is, you have to consider reduced reboiler duty instead of rated/design duty as mentioned by Mr. Paul in above post.

 

Note that the clean UA has to be considered while calculating reduced reboiler duty, which is conservative and true picture (during start-up).



#4 arvind

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Posted 04 March 2014 - 07:16 PM

Hi,

 

Thank you very much Paul and Aroon for your valuable feedback.

 

 

Regards,

Arvind Pawashe



#5 dattatray kolte

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Posted 20 February 2015 - 12:43 AM

Arvind, 

 

Small correction, SHELL DEP say compassion should be done between high pressure side design pressure and low pressure side corrected hydro test pressure (DEP 80.45.10.11 Section 4.2.1-2.






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