Low Pressure Storage Tank PSV Discharge Destination

[Scall]

Hi Folks,
I'm designing a tank farm to be located in Texas. There are three low pressure, blanketed tanks connected to a flare stack. The emergency venting requirements (fire case) would lead to a very large flare header. My question is can I design the tanks with a weak roof-to-shell joint, or a weighted thief hatch, so that they relieve to atmosphere in the event of a fire? or do I need to connect them to the flare header? API 2000 seems to imply the former. NFPA 30 and OSHA 1910.106 seem to imply the latter. These three tanks would sit in the same dike as several other atmospheric tanks holding stable (high flash point) hydrocarbons. In the event of a pool fire, those tanks will relieve to atmosphere though open vents and weighted thief hatches.

Apologies if this has been answered elsewhere.

Thanks

Scall

 

[don1980]

There are no requirements to route low pressure tank vents (normal load or emergency load) to a flare header. As illustrated by your preliminary sizing calculations, it's almost impossible (and certainly impractical) to attempt to route a fire relief loads (from low pressure tanks) to a flare header.

For emergency loads such as fire exposure, you can choose to protect the tank using a frangible roof seam or by using an emergency vent valve routed to the atm.

 

[Scall]

Thanks Don,
Appreciate the help.
Scall

 

[MJCronin]

Recall that Frangible Roof Design (FRD) is meant to prevent shell uplift during over pressurization which may cause failure of the bottom to shell joint. Rupture of the shell to roof joint is judged always preferable to rupture of the shell to bottom joint

FRD is also meant to prevent damage to piping, sumps and appurtenances resulting in a release of a tank’s contents.

Be aware that there are design limitations to storage tank frangible roofs !!!!
Your preferred selection of roof type and anchorage design must also be evaluated

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.660.1074&rep=rep1&type=pdf

For the roofs of smaller diameter tanks they are not permitted by API-650 (Section 5.10.2.6)

Frangible roof designs have been discussed repeatedly over the years on "eng-tips"

https://www.eng-tips.com/viewthread.cfm?qid=338992

https://www.eng-tips.com/viewthread.cfm?qid=394841

https://www.eng-tips.com/viewthread.cfm?qid=373545

Be aware ....

There has been, over the years, some challenges to the basic concepts and fundamental methodologies of "frangible roof design" for API-650 Tanks

http://docshare02.docshare.tips/files/4813/48135181.pdf

MJCronin
Sr. Process Engineer