Question: “What is the correct burst pressure for a 4 bar tank? Should we install a 4.4 bar rupture disc?”
Tank Guru: The Pressure Relief Requirements for most common tank containers are stated in Section 178.275(g) of Title 49 of the Code of Federal Regulations (also known as ‘49 CFR’). This covers tank types T12, T14, and T20, as well as older IM-101/IMO Type 1 tanks.
178.275 (g) 3 states “The frangible disc must rupture at a nominal pressure 10% above the start-to-discharge pressure of the reclosable pressure relief device.”
178.275 (g) 6 Setting of pressure relief devices… states “and 110% of two-thirds of the test pressure for shells having a test pressure of more than 4.5 bar… A self-closing relief device must close at a pressure not more than 10% below the pressure at which the discharge starts.”
So in the typical example where a normal “fleet-type” T11 tank has Test Pressure of 6 bar (or MAWP of 4 bar), and the Pressure Relief Device has a set-to-discharge pressure of 4.4 bar (110% of two-thirds of the test pressure); when a bursting disc is installed (actually making it become a T12 tank) the burst pressure of the rupture disc should be 10% higher than the 4.4 bar set pressure of the PRV. Therefore you should actually be installing rupture discs with a burst pressure of 4.84 bar, not 4.4 bar.