API 520 is a standard, not a code. It has guidance on sizing relief devices for both liquid and vapor flow scenarios.

API 521 provides more information on determining the required relief rate for various process scenarios, which is then used in the equations in API 520.

Note, if you have an existing system/relief valve you should be able to contact the manufacturer and obtain a datasheet with the reaction forces.

If you're sizing a relief valve in addition to doing the pipe stress, note that relief valve sizing is not a simple "plug in this number and you're done" type calculation. You have to think through all process scenarios that could generate overpressure, determine the relief rates/relief area required for each, and size your device from those results. A pump deadhead or typical process flow rate will likely not be the worst case scenarios.