Common PSV for more than one piece of equipment?

[scottfhr]

FIRST SITUATION

Xylene clay treater designed in late 50's All equipment is liquid full.

Process flow is - feed filter > shell side of 150# steam heater > shell side of feed/effluent exchanger > shell side of 400# steam heater > top of vertical clay treater vessel.

There is one PSV in the system on its own nozzle on top of the clay treater.

Can the PSV on the clay treater protect the filter vessel for an external fire case? If so, does the clay treater PSV needs to be sized to relieve liquid at the volumetric rate of vapor generation in the filter? And if I can use this PSV am I correct in assuming that I'll have to set PSV sufficiently below the MAWP of filtet to allow for pressure drop in the interconnecting piping and equipment.

There is a split tube case for the 400# steam heater. Flow goes directly out the top of this exchanger to the top of the clay treater. Can the clay treater PSV provide protection? If so, can the clay treater PSV be sized for vapor for this case?

SECOND SITUATION

Distillation column - overhead line goes through an elevated condenser which drains into an overhead receiver.
There is a PSV on the overhead line before the condenser. There is no PSV on the overhead receiver. Can the overhead line PSV protect the overhead receiver for an external fire case? This would require reverse vapor flow through the condenser while at the same time liquid is condensing and trying to flow the other direction.

I haven't been able to find any guidance in ASME section VIII or API RP's that cover these situations.

 

[kkimic]

For the First Situation, as long as there is NO isolation valve between both equipment items then it should be ok for external fire case as long as it is sized correctly.

Unfortunately have not got the API RPs here with me so I cannot check where this is mentioned (if it is mentioned, which I believe it is)

Cannot really have a clear picture of you process but note that the PSV would only need be sized to relieve liquid at the volumetric rate of vapor generation if the PSV inlet is located in the liquid section - otherwise steam relief.

On the pressure drop issue remember to follow the 3% rule.

Unfortunately I cannot be of any help in your second question.

 

[CMA010]

Second situation:

It is usually assumed for evaluation of the external fire scenario that the process is blocked-in and that all heat and mass input has ceased.

You'll have to include the pressure drop over your overhead condensor in the scenario of the overhead receiver.

 

[skearse]

As kkimic mentioned, situation 1 is addressed in API 521 in Annex B. It's kind of a PITA analysis to got through, as a pressure profile needs to be developed across the entire protected system, to make sure that no single component exceeds its MAWP + allowable OP.

As for situation 2...I guess the qualified answer would be it depends. Is all the equipment located withing the same fire zone, what sort of dP would be seen if the vaporizing liquid in the receiver backflows to the PSV through the condenser, what are the MAWPs, etc. My first blush would be that you might be better off with a PSV on the receiver also, but it may be possible once you do the system analysis. From a code or standard standpoint, there are no provsions that state that the PSV must be installed on the last piece of protected equipment in the train-the PSV can be installed on any of the equipment.