The first code to look is your local regulator code. That has preeminence over any code of practice.
Then you have to verify to what code is the equipment built, may be American ASME, European PED, Japanese, Chinese codes...
Now, assuming that is built to ASME (a big IF)
You have to demonstrate that the existing valve is able to relieve the demand of a fire case. If it is already built and operating, it should have been done by the designer. The cases considered for design should be available (although in many places that information is buried in a project binder in the back of a documentation room, where you have to clear the spiderwebs before opening them)
If you find that the valve is not capable of relieving the required capacity in case of fire, you may have to add a supplementary valve exclusively for the fire case
See API Section VIII Div 1 UG-125 and followings, the Appendix M of the same Code, API 520 Parts 1 and 2, and API 521.
The first one (ASME) if the actual code requirements (where that code is used)
The last one is where you will find information on how to get there.
Edited by Saml, 08 June 2018 - 06:50 AM.