not sure what the MAWP of the vessel or what style rupture disc you are looking to use but this is a bad application for a rupture disc.
some RD's fail "early" if installed backwards and having that much back-pressure is effectively the same thing. That is if you have near 0 psig on the vessel and you have the 65 psig back-pressure, that is the essentially the same as installing it backwards with 65 psig on the vessel.
MAWP is 270 psig.
Tank is having pressure = 0 psig
65 psig back pressure is created because of flow of liquid in another pipe where the RD will be discharging, which is 2000 ft long line going to tank.
This is unusual but problem is client does not like to lay another pipeline of 2000 ft long.
I do not know, how to define the burst pressure for this?
To be safe, the burst pressure should be 205 psig. However, can your process live with this low burst pressure when the other pipe is not flowing? If not, then another technology will be needed. If a relief device can be used, either a balanced bellows or pilot relief valve can be used. If a positive seal is needed, a rupture pin device may be applicable. Rupture Pin Technologies has a balanced version of their Model C valve.
Just a note. When sizing the rupture disk in this situation the standard vapor or liquid sizing equations are not applicable. A full hydraulic analysis of the relief piping, including to 2000 feet of piping to the other tank while the other pipe is flowing is required.
