Tank Blanketting system 2

[JACOS]

In our plant tank blanket design pressure is 1.0 kPa or 2.32 oz/in2. This is very low and it is very difficult if not impossible to keep the tank Pressure/Vacuum relief valves (PVRV) from opening during flow surges. After the valve has relieved, it is very often does not reseat and the valve continues to leak gas to atmosphere. The gas contains H2S, does not smell very good and is not good for anyone. I would like to raise the tank blanket design pressure to 2.0 kPag, if possible, but any increase would be helpful.

Tank roofs are fabricated of 3/8" thick steel plate that weighs 15.3 lb/ft2.
The steel in the roof equates to a pressure of 0.73 kPa without considering the weight of the insulation and cladding. This leaves very little pressure for the frangible roof joint to handle.
It is my opinion that a higher PVRV setting would substantially reduce the release of tank vapor to atmosphere. It may be possible to do this without doing anything to the tanks. It is estimated that the load on the joint between the tank wall and roof is 5.3 lb per inch of weld, or about 27 psi (186 kPa) at the tank design pressure of 1 kPa. Even the very poorest steel should be able to handle 15,000 psi. There may be something special about the joint that makes it fail at such low loads and I would like to know what it is.

 

[ash9144]

Usually the roof joint is designed to fail first. So if you had a problem with the tank the roof joint would fail and release the pressure but not discharge the entire tank contents.

 

[hacksaw]

How is increasing the pressure going to solve the problem? It may make it more difficult.

The reseating problem is another matter, and if really critical problem, and cost is no object, you have to use automatic valves or pilot assist. engineering, oh that is free...

the displaced vapors will be there and can only be scrubbed or recycled through your process...more engineering, more cost...

 

[gr2vessels]

jacos,
It seems to me that your problem is the relief valve, rather than the design pressure. First, what kind of vacuum relief valve you have on a blanketted tank? Secondly, an oversized pressure relief valve might be operating at the extreme end of its range, where is not very stable. I bet you operate a manway size valve, when a much smaller size would operate correctly. Also, did I read correctly, are you venting H2S to atmosphere??
cheers,
gr2vessels