Required vs Rated Flow for Pressure Drop Calculations
Pressure changes from protected equipment to a pressure relief valve (PRV) and from a PRV to the final relief effluent location are determined to allow an evaluation of the effect of the pressures on the relief valve capacity and stability. A choice of flow rate is made to evaluate the pressure changes, and the capacity of the PRV at relieving conditions is the common choice for both inlet and outlet piping, at least for piping immediately attached to the PRV.
Additional guidance was incorporated into the 6th edition of the API Standard 520 Part II1 on the use of the required relief rate versus capacity of a PRV in the inlet and outlet pressure drop calculations. Specifically, the use of the required relief rate is possible for modulating type PRVs and the evaluation of pressure drops are not required for specific cases.
As stated in both API Standard 520 Part II 5th Edition §5.32 and 6th Edition §6.3.11, the rated capacity of a PRV should typically be used to size the discharge line from the PRV to atmosphere or to the relief header, with the exception for modulating type pilot-operated valves in which the required relief rate can be used to size the discharge piping. For sizing the common relief header, the required relief rates can be used. However, it is important to note that whenever the discharge piping or common relief header is sized using the required relief rate, the outlet pressure drop would need to be re-evaluated for any process changes that are expected to affect the protected system resulting in a change in the required relief rate of a particular overpressure scenario.
The following summarizes the guidance provided in API Standard 520 Part II 6th Edition §6.3.2 and §7.3.8 for thermal relief valves1:
For evaluation of the inlet and outlet piping for ambient heating scenarios (including solar radiation):
- Generally not needed if designed solely for liquid hydraulic expansion where the required rate is significantly lower than the rated capacity of the relief valve and a steady flow at the rated capacity cannot be reached
- May need to be considered for a long pipeline or large liquid-filled vessel, where the required rate due to thermal expansion could be high
- Should be considered for liquids where blocking in with ambient heating may result in overpressure due to vaporization (for example, refrigerated liquids, LPG or LNG)
Evaluation of the inlet and outlet piping should always be considered where pressure within the protected system can be generated by process heat:
- Cold side of exchangers when blocked in with continued heat from the hot side fluid resulting in liquid thermal expansion or even vaporization at relief pressure
- Heat traced piping or vessels where the blocked-in liquid can be vaporized at relief pressure
The following §126.96.36.199 pertains to the selection of flow rates for inlet losses in general:
A good design practice is to use the rated capacity for inlet pressure drop calculations since doing so does not constrain future operations and knowledge of the valve’s modulating behavior is not required. The required relief rate may be used where the PRV has modulating characteristics. Modulating pilot-operated PRVs are considered to have these characteristics; pop-acting pilot-operated PRVs do not have these characteristics. Some direct spring-loaded PRVs may exhibit modulating characteristics; the valve manufacturer should be consulted for guidance on this question. The user is cautioned that valves that exhibit modulating characteristics relieving liquids may not exhibit those same characteristics relieving vapors and vice versa.
It is common practice to use the required relief rate of a liquid relief scenario in the piping pressure drop calculations for a liquid capacity certified relief valve (for example, a relief valve protecting a blocked pump discharge), where the liquid relief stream does not flash in the relief valve or in its discharge piping and there is no vessel or surge capacity upstream of the relief valve.
It is also noteworthy to point out Note 3 of Table 2 of API Standard 520 Part II 6th Edition which states that while the rated capacity of a vapor certified PRV for a liquid relief scenario needs to be checked using the non-certified liquid PRV sizing equation, the inlet line loss should be evaluated based on the rated capacity calculated using Kp = 1.
 American Petroleum Institute. “API Standard 520 Part II-Installation: Sizing, Selection, and Installation of Pressure-relieving Devices”. 6th Edition, March 2015.
 American Petroleum Institute. “API Standard 520 Part II-Installation: Sizing, Selection, and Installation of Pressure-relieving Devices”. 5th Edition, August 2003.