sizing for relief valve

The methods and equations detailed in API 520 and 521 is used as basis for sizing
relief valves.
Relief valve size and material selection is in accordance with API RP 526.

clarification on sizing method for relieving
1 blocked outlet
Assume other control valves renain in their normal operating position if the outlet valves closed
2 external fire
Relieving loads are based on wetted surface area with liquid at HLL in vessel.
Fire case may be ignored provided all parts of blocked in system are located above a nominal flame height of 25 feet above grade.
3 control valve malfunction
For control valve, assume the valve is fully open ,but take credit for normal downstream off-take.
The relief flow shall be calculated on the basis of both the control valve and bypass valve
4 thermal relief
Piping containing flammable liquids , which have a risk/likelihood of being left isolated during normal operation, resulting in revief due to hydraulic expansion due to heat tracing ,ambient and fire shall be provided with a single thermal relief valve.