We have a fractionator overhead receiver in the hydrocracker unit without any PSV on it. We want to know why the designer didn't put any PSV on the receiver. Please introduce a good and practical refrence for this matter, if it is possible.
Answers
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14/09/2007
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KengYong Chan, Air Products & Chemicals, Inc, chanky@apci.com
It is unusual to have a coded pressure vessel without any overpressure relief protection. Below could be the possible reasons:
1) The receiver is opened (with any block valves in between) to the column such that the column PSV also protects the receiver from overpressure demands.
2) If the receiver could be blocked in or overpressure, it is possible that there are (redundant) critical safety instrumented systems (such as emergency depressurizing valve to flare etc), along with normal pressure control system to protect the receiver from overpressure. The safety instrumented system will then be deemed safety critical.
3) The receiver could be designed such that the MAWP is greater than all the possible high pressure overpressure demands. For external fire blocked in overpressure scenario, the receiver could be installed at an elevated height such that a pool fire or the height of fire is unlikely to impact the receiver.
It is best to review the various overpressure scenarios on this receiver and then evaluate what are the safeguards (SIF or relief devices) present on the receiver. In some cases, a Fault Tree Analysis may have been done to indicate that the overpressure frequency is reasonably low or meets the company target frequency.......
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13/09/2007
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Bob Kalaria, KBR, bhavdip.kalaria@kbr.com
The fractionator itself probabaly has PSV to protect itself from over pressure. Usually there is no block valve in between fractionator and the fractionator overhead receiver which makes it just one single system. This means that these two equipments cannot be isolated even accidently. Per ASME SEC VII, such a system would not require separate relief valve for fractionator and overhead receiver. The PSV on fractionator protects the overhead receiver too.......
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