API 14C - SAFE Chart - Pressure Vessel 1

[jcaiken]

API 14C SAC A.4.c.5 seems to indicate that a PSV is required on a pressure vessel (max pressure >5 psig) unless it is the final vessel on a flare header (i.e. Flare KO Drum). However, this implies that any other vessel venting into the flare header (e.g. Closed Drain Sump) needs a PSV to protect against blockage of the final vessel. So where do I relieve the PSV? to atmosphere? Presumably each PSV needs to be sized for the blocked outlet flows from the associated vessel.

If locked-open valves are provided round a PSV to enable maintenance then does this mean that the vessel can be isolated from the PSV in the context of SAC A.4.c.3 and A.4.c.4?

Also, does the presence of a water-seal vessel downstream of the Flare KO Drum mean that the Flare Drum also needs a PSV to comply with API 14C.

I seem to be going round in circles so any comments & advice would be grastefully received.

 

[MortenA]

You should not be able to block in your "final vessel". This means not valves in the flare header. Shoul work be rfequired on the flare header then the plant must be shut down.

There are some cases also mentioned in API 521 the possibility of having a flare gas recovery system where valves in the flare is allowed. But this requires some carefull consideration.

Normally all PSV's must then relieve to the flare header. Some sites will however use local vents instead of a flare header but its either or. You dont need a special "if the flare header is out of service PSV".

Best regards

Morten

 

[JoeWong88]

jcaiken,

As usual, when we work too much and it end-up we are questioning our believe like a dog chasing his own tail...

API 14C SAC A.4.c.5 seems to indicate that a PSV is required on a pressure vessel (max pressure >5 psig) unless it is the final vessel on a flare header (i.e. Flare KO Drum). However, this implies that any other vessel venting into the flare header (e.g. Closed Drain Sump) needs a PSV to protect against blockage of the final vessel. So where do I relieve the PSV? to atmosphere? Presumably each PSV needs to be sized for the blocked outlet flows from the associated vessel.

No matter how we interprete the statement in any code, ultimately is there is NO possibility existent of potential blockage results overpressure scenario.

Let say a degassing drum with vent line and flame arrester discharge to atmosphere directly. This drum is also consider "final vessel". However there is a potential of flame arrester blockage and an overpressure protection device is required.

Another example is two drum connected together can be protected with a PSV if there is NO possibility existent of potential blockage on the interconnecting line.

Another example is a Flare KOD with Flare Recovery system (e.g. compressor, eductor, etc). The flare line connected to flare stack may have an on-off valve to facilitate plant maintenance. Under this case, eventhough the Flare KOD is the "final vessel" but an overpressure protection device (i.e. rupture disk) would be installed across the on-off valve to ensure a clear relief path.

In your "closed drain sump connected to flare KOD case", i would consider a.4.c.4.

If locked-open valves are provided round a PSV to enable maintenance then does this mean that the vessel can be isolated from the PSV in the context of SAC A.4.c.3 and A.4.c.4?

Do you mean the isolation valves on the inlet and outlet of PSV or on the PSV bypass line ? I guess you mean the maintanance isolation valves on the inlet & outlet of PSV.

Strict compliance to code, there should be NO devices (i.e. valves) potential creating blockage between the protected equipment, PSV and disposal path. Nevertheless this does not allow PSV routine inspectionand change-out. The plant may require partial shutdown and blowdown for PSV maintenance and lossing inventory is not cost-effective. Thus, these valves are considered highest critical valves and handle by strict working procedures...This case possibly a deviation.

Also, does the presence of a water-seal vessel downstream of the Flare KO Drum mean that the Flare Drum also needs a PSV to comply with API 14C.

NO PSV is required.

Whenever a large relief, the water-seal will be destroyed and there is NO possibility existent of potential blockage results overpressure scenario.

Hope above help.

Best regards,

JoeWong

 

[jcaiken]

Thanks for your insights.

If the Flare KO Drum has no internals, isolation valves, or other potential causes of blockage then do upstream vessels that cannot be isolated from the Flare KO Drum need a PSV. JoeWong88 suggests a SAC of A.4.c.4 which says that you can avoid the need for a PSV if a suitable downstream PSV (or flare?) cannot be isolated from the vessel. However, I think that if such a vessel could be isolated (using locked-open valves) then a PSV would still be required for fire/thermal expansion case.

I agree that locked-open isolation valves used to allow maintenance of a PSV would not strictly comply with API 14C but this has become acceptable, even though I have heard of occassions where equipment has been returned to operation after such maintenance without de-isolation of the PSV.

 

[Longinthetooth]

It is accepted practice to have 2 PSVs in parallel with 3 way valves on the inlet and outlet. Each PSV is rated for the full relief requirement. These 3 way valves have only 2 operating positions so that either one PSV or the other has to be on line. (you can gang the 3 ways together so that they move in tandem. This allows maintenance of the off line PSV. This has been common practice for many years. Some valve manufacturers make 3 ways especialy for this purpose.