API 521 recommends to use a 3/4" x 1" relief valve to take care of overpressure due to hydraulic expansion, and recommends to calculate relief load if 3/4" x 1" is not sufficient.

How would one determine if a calculation is necessary ?

The 3/4" x 1" relief valve is not found in API 526 as I think it covers only flanged relief valves. Any idea where we can find info in codes about 3/4" x 1" relief valves ?