Look through the beginning parts of API 520, it's actually quite helpful.
Relief valves are used in the worst case pressure scenario to keep your vessel from failing. In this high pressure situation, it may not be ideal to have a full bore valve suddenly open to relieve pressure; the forces and velocities of this relief event would be very high. Since the high pressure helps push things out of the system, you can use a smaller orifice area and moderate the flow to what you need it to be.
Sometimes you do want that full bore relief; in that case a rupture disc is likely your best option or possibly a pilot operated relief valve.
The J Series you refer to are the "Crosby" range of spring operated pressure relief valves. These are designs meeting API-520, API 526, etc. These specifications do not cover "Full Bore" nozzles for spring operated designs, but established "letter orifice" sizes D, E......R, T" each letter having a distinct effective area.
There are other spring operated PRV designs where you can have "Full Bore" dependant on application and manufacturer, but these will generally have a lower coefficient of discharge. Many manufacturers working to European DIN/TUV specs have such designs.
Full Bore nozzles are available for pilot operated pressure relief valves and used in standard body designs.
Per ISO, only the term Safety Valve is used regardless of application or design.
It'd be better to focus on the relief calculations and ensuring you've analyzed all scenarios properly and determined the required relief rates & orifice sizes. Once you're confident that information is correct, you can look at specific valves.
Dear Sir,
Usually u would like to use the full bore when the largest orifice is not enough to relieve the required capacity and To be honest, although i am not very experienced person, I have seen rarely full bore.
