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Pressure Relief Valve For Fire Case


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#1 CheAmine

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Posted 10 October 2016 - 11:22 PM

Hello,

 

We are having two parallel filters (F1 is online and F2 is stand-by) located downstream of Mercury removal beds. These filters are equipped with inlet motor operated isolation valves and each filter is equipped with one PSV sized for fire case as it is the credible scenario (see sketch).

 

PSV2 is due for testing date however we are not able to do so because M3 is passing. Hence in the current situation, PSV 2 might not pop up in case of fire in F2. M3 requires total train shutdown so that it can be fixed as per which the management would not like to do a shutdown.

 

As a mitigation for that, I would like to put both F1 and F2 online (M1 and M3 to be fully open), so that in case of fire case in F2, the overpressure can be released by PSV1 located in F1. since PSV2 is no more reliable and PSV1 was recently tested.

 

Note that the calculated orifice size required for the calculated relief load for both PSV 1 and PSV 2 is 0.2 cm2 each and the installed orifice size for PSV1 and PSV2 is 0.7 cm2 each. Hence I assume that even in the case of external fire for both F1 and F2, PSV1 is sufficient to release the required relief load of both filters.

 

Kindly let me know if the above mitigation is correct.

 

Regards,
 

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#2 Pilesar

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Posted 12 October 2016 - 05:29 AM

The PSVs are likely sized for a fire case with the vessel blocked in. With your inlet valve open the situation is different in that the inlet line can also be used as an escape route for fire-generated vapor. If you lock open both inlet valves, you may decide that the blocked-in fire case is no longer credible. Calculate the relief area required if the inlet valves were open to see if your vessel would be adequately protected. It may help the calcs to also lock open the outlet valve on the filter that is in service. Whether or not to operate the plant with a relief valve overdue for inspection is a management decision. Many companies would grant a 'special extension' for this particular relief valve testing period if the time were relatively short. Identify the hazards, assess the risks, and document the calculations so that responsible management can make an informed decision. If the plant is to operate with modifications to the design, this should all be included in your 'management of change' process.



#3 CheAmine

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Posted 16 October 2016 - 04:12 AM

Thank you Pilesar,

 

I am not sure how the calculation of fire relief case when the inlet valve is open will differ from the normal fire relief case. Can you please advice on that.



#4 Pilesar

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Posted 16 October 2016 - 07:00 AM

   The safe operation of your pressure vessel requires that it be designed to withstand the strain due to the forces of operation. For normal operating conditions, the vessel was manufactured with specific types of materials, physical dimensions, thicknesses, weld procedures, etc., that allow it to withstand normal operation. It was certified to a maximum allowable working pressure (MAWP) based on its construction and tested at a pressure higher than MAWP under controlled conditions. There may be abnormal situations in actual operation when the strain on the pressure vessel would be greater than the vessel is designed to withstand. All of these abnormal potential situations should be considered to determine if they are a credible concern and what steps would be required to keep the strain on the pressure vessel within design parameters. This is one of the most serious tasks for an engineer since the consequences of a pressure vessel failure may include death for people in the vicinity. 
   There are guides developed from experience to help engineers evaluate the possible overpressure scenarios. There are checklists of scenarios to consider and calculation methods which have become standard practice. But these guides and calculation methods do not size pressure relief systems... they are just tools to be used by a human engineer who is responsible for designing the safety systems to deal with abnormal operations. That engineer should completely understand the credible scenarios and their consequences. He must document his findings and calculations so that they can be understood by others. What if the vessel catastrophically fails? Is there enough documentation to help with your defense in front of a court jury? These questions provide extra motivation for me to provide accurate evaluations and thorough documentation.
   In your situation, you must determine the credible scenarios where pressure will build in your vessel above its MAWP. If you disable the inlet valve so that it is absolutely certain to be open, then what will happen if external heat is applied to the vessel? The volume of the fluid inside the vessel will expand. Will the expanding volume build pressure in the vessel or will it expand out through the inlet line? The engineer who knows the details of the situation should be the one to evaluate whether the safety system is adequate.





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