As discussed above in the section on design pressure, the piping system must either be designed to safely contain the maximum, expected, possible pressure, considering such factors as failure of control devices and dynamic events such as surge, or be provided with over pressure protection, such as a safety relief valve. This basic requirement is provided in para. 301.2.2, whereas requirements for relief valves are provided in para. 322.6, Pressure Relieving Systems.
For example, if a 600-psi system goes through a pressure-letdown valve (irrespective of fail-closed features or other safeguards) to a 300-psi system and then through another letdown valve to a 150-psi system, if no safety relief devices are provided, the 150-psi system would have to be designed to safely contain 600 psi. This is because possible failure of both valves must be considered. There is no provision for double contingency or other considerations relative to probability of occurrence in the Code requirements.
If a pressure-relieving device is used, ASME B31.3 refers to the requirements of ASME BPVC, Section VIII, Division 1. However, there are some exceptions. For example, design pressure is substituted for maximum allowable working pressure (MAWP), because the latter term is not used in the Piping Code.
More significantly, the set pressure requirements of ASME BPVC Section VIII, Division 1, are not followed. If they were, then the variations could never realistically be permitted, because any variation in pressure above the design pressure would result in opening of the pressure relief device. Rather than limit the set pressure to the design pressure, the Piping Code allows the set pressure to be any value, as long as the maximum pressure during the relieving event, including consideration of potential accumulation (additional pressure buildup beyond the pressure at which the valve opens), does not exceed one of the following two alternatives a) the maximum relieving pressure permitted by BPVC Section VIII, Division 1, or b) the maximum pressure permitted in the allowances for variations provisions of ASME B31.3. However, the owner's approval is required for the set pressure to exceed the design pressure.
For liquid thermal expansion relief devices, the set pressure may simply be established at 120% of the design pressure, without any requirement for owner's approval nor any need to comply with the additional requirements necessary to use the allowances for variations. The set pressure can be set at higher than 120%; however, then the other requirements of justifications and approvals explained above must be satisfied.
Block valves are permitted in the inlet and discharge lines to the relief valve, with certain limitations. These requirements are outlined in para. 322.6.1. #Little_PEng