As described above for inlet lines, the principal hydraulic performance requirements of a pressure relief valve outlet line are also specified in the ASME Boiler and Pressure Vessel Code, Section VIII, paragraph UG-135:
The size of the discharge lines (from pressure relief devices) shall be such that any pressure that may exist or develop will not reduce the relieving capacity of the pressure relief devices below that required to properly protect the vessel, or adversely affect the proper operation of the pressure relief devices.
Once again, more specific guidance concerning acceptable outlet pressure drop is provided in Non-mandatory Appendix M of Section VIII,
The flow characteristics of the discharge system of high lift, top guided safety, safety relief, and pilot-operated pressure relief valves in compressible fluid service shall be such that the static pressure developed at the discharge flange of a conventional direct spring loaded valve will not exceed 10% of the set pressure when flowing at stamp capacity. Other valve types exhibit various degrees of tolerance to back pressure and the manufacturer’s recommendation should be followed.
API Recommended Practice 520 reiterates this guidance that the built-up back pressure on a conventional valve should be limited to 10% of set pressure when flowing at the rated capacity – i.e., at 10% overpressure. However, acknowledging the connection between the valve’s allowable overpressure and the back pressure at which the valve may begin to reclose, RP 520 Part I extends this guidance,
Conventional pressure relief valves should typically not be used when the built-up back pressure is greater than 10 percent of the set pressure at 10 percent overpressure. A higher maximum allowable built-up back pressure may be used for overpressure greater than 10 percent.
These guidelines are specific only for conventional, spring-loaded pressure relief valves. For other types of valves, the manufacturer should always be consulted for values of the built-up back pressure at which a specific valve suffers loss in flow capacity and/or in stable performance. The generic-use data for the back pressure capacity correction factor for balanced pressure relief valves presented in API RP 520 Part I indicate that such valves suffer a loss in flow capacity at built-up back pressures exceeding 17%, and 30% of the set pressure for liquid, and vapor services, respectively. In general, pilot-operated pressure relief valves are not affected by built-up back pressure until the total back pressure causes a transition from critical to sub-critical flow conditions, which occurs when the total back pressure exceeds roughly 50% of the relief pressure.
The sizing of relief device outlet lines is generally subjected to two separate types of back pressure development evaluations. First, the built-up back pressure is calculated for each pressure relief valve, for each contingency in which that valve alone is flowing. In this calculation, the valve is assumed to be flowing at its full rated capacity of the fluid flowing in each contingency (i.e., the stamped capacity, adjusted to the properties of the flowing fluid). The back pressure developed must not exceed the acceptable value for the type of valve to be installed; if the calculation predicts an unacceptably high back pressure, the discharge line should be modified or a more tolerant valve should be selected.
For valves discharging directly to atmosphere, this is the only outlet-line hydraulic performance calculation required. For valves discharging into a common collection system, however, an additional back pressure development evaluation is carried out. As discussed in more detail in Section 1250, the back pressure developed at each flowing pressure relief valve is calculated for each relief contingency. In this calculation, per the recommendation of API Recommended Practice 521, each relieving device is assumed to be flowing at the capacity required to prevent more than the allowable overpressure in the equipment it protects in the contingency under consideration. That is, the required relief flow rates, as discussed in Section 1222, are used in calculating the back pressures developed during simultaneous release from relief valves opening under the same contingency. The acceptance criteria for these back pressures are the same as those for the analysis in which one valve at a time is assumed to be flowing.
As in the evaluation of inlet pressure drop, standard piping flow equations are used to calculate the pressure drop through the discharge line, with each piping segment and each fitting accounted for by a flow resistance factor or an equivalent length of pipe. Unlike the inlet line loss calculation, however, the outlet pressure drop calculation should include all contributions to the pressure drop. Sonic limitations and kinetic energy effects can significantly increase the calculated pressure drops. Potential energy effects can be particularly significant for liquid and two phase releases.