Stationing an operator at a pressure safety valve bypass Caution is recommended before stationing an operator at the PSV bypass for overpressure protection
Jonathan R Webber Fluor Canada Ltd
P reventing overpressure in process plants is critical for the protection of the public, operators, environ- ment, and equipment. Sufficient overpressure above the design pressure can result in failures of pressure-con- taining parts, leading to potential injuries and contamina- tion of the environment. Significant effort is made in the design phase to ensure adequate overpressure protection is provided. Overpressure protection is principally provided by pres- sure safety/relief valves (PSVs), which are self-actuated devices designed to open when the operating pressure at the PSV inlet reaches the PSV set pressure. The opening of the valve allows the process fluids to be safely released to atmosphere or a flare system where they are combusted at a flare tip. The three standard PSV types are conventional, balanced bellows, and pilot-operated valves. A schematic of a conventional type of PSV is provided in Figure 1 . Other references can provide explanations of how these valves and flare systems work. 1, 2, 3 PSVs are typically designed to be the last line of defence to prevent overpressure after operational oversight, basic process controls, and safety instrumented systems (SIS) have failed to intercede. The set pressure of the PSV is typi- cally set at or below the protected system design pressure. The PSV orifice is sized according to the upsets to ensure the pressure in the protected equipment/piping does not exceed code-specified allowable overpressure. PSV testing frequency API-521 is a common standard used for determining the required relief loads that the PSVs must pass to prevent overpressure. 4 The PSV orifice must be sized to allow the required flow to pass. Preliminary PSV sizing of the ori - fice is typically performed using API-520, 1 which is then validated by the PSV vendor as part of the procurement process. PSVs must occasionally be removed to undergo testing and/or maintenance. If the frequency of the PSV testing/ maintenance is higher than the frequency of the unit turn- around (i.e., when the unit is shut down) then it is occa- sionally proposed to remove the PSV while the unit is in operation by closing the PSV inlet/outlet isolation valves. Adequate overpressure protection is still required while the unit is in operation. Nonmandatory Section ASME BPVC
Set pressure adjusting screw
Bonnet
Spring
Bonnet plug
Body
Seat disk
Figure 1 Typical conventional PSV
XIII B-7 states that full area stop valves are permitted upstream/downstream for isolation of a PSV if: “ (a) Administrative controls are provided to prevent unau- thorised valve operation. (b) Valves are provided with mechanical locking elements. (c) Valve failure controls are provided to prevent accidental valve closure due to mechanical failure. (d) Procedures are in place to provide pressure relief pro- tection during the time when the system is isolated from its pressure relief path. These procedures shall ensure that when the system is isolated from its pressure relief path, an authorised person shall continuously monitor the pressure conditions of the vessel and shall be capable of respond- ing promptly with documented, predefined actions, either stopping the source of overpressure or opening alternative means of pressure relief. This authorised person shall be dedicated to this task and shall have no other duties when performing this task. (e) The system shall be isolated from its pressure relief path only for the time required to test, repair, and/or replace the pressure relief device.” 6
55
PTQ Q3 2022
www.digitalrefining.com
Powered by FlippingBook