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Dear All

I have carried out a process safety review on a number of circa 1978 vertical storage tanks with capacity 60m3, which are used for storage of organic solvent for use on plant. These tanks are operated at atmospheric temperature/pressure and are open vented via flame arrestors to provide normal breathing requirements for loading/off-loading. The tanks are also fitted with 600mm bursting disc with burst pressure 3PSIg +/-20% (approx.0.25barg). The tanks are all marked with Design pressure 0.5 bar and test pressure 0.7 bar , which is also repeated on the technical drawings along with reference BS2654 as design standard.

Vessel height is 7.6m and there is a note on drawing stating actual test pressure is 0.76bar, so it looks like test pressure is just water fill.

My problem is that BS2654 states maximum design pressure of 56mbar which is well below the stated design pressure and that of the bursting disc set pressure. My concern is therefore that the design pressure stated is not correct and in case of overpressure, the tank would fail before the bursting disc would relieve.

Tanks are mild steel construction with 6mm flat base, 6mm walls and 6mm domed top. 3.4m dia * 7.6m height.

Should I believe the design pressure and stick with the existing bursting discs, or should I reduce the design pressure in line with the standard and replace discs with lower burst pressure discs?

Thanks in advance for your responses
Andrew