PTQ Q3 2022 Issue

gauge, mounted on a nearby available ¾” vent, to monitor the system pressure. Discussion The following sections describe several considerations that should be addressed before implementing the practice of stationing an operator at the bypass when the PSV is removed. Regulatory requirements S ome codes restrict the use of valves between the protected equipment and PSV. For example, ASME BPVC Section I PG-71.3 prohibits installation of any valve between the boiler and PSV, making it impossible to isolate the PSV while the unit is operating: “No valve of any description shall be placed between the required pressure relief valve or valves and the boiler, nor on the discharge pipe between the pressure relief valve and the atmosphere.” 7 While ASME BPVC XIII nonmandatory Appendix B 6 allows the use of PSV isolation valves, other local jurisdic - tions can have more stringent requirements. They may pro - hibit operation without a PSV or require special approval for alternative methods of overpressure protection when a PSV is not used. If there is any doubt, it is best to confirm with the local authority that the practice of stationing an operator at the PSV bypass is allowed when the PSV is removed. Operator response time Traditionally, between 10 to 30 minutes are assumed for an operator to recognise and respond to an overpressure upset. Typically, response via a control panel is considered to occur within 10 minutes. However, the same action is sometimes assumed to take longer (20 to 30 minutes) if there is a global upset (such as power failure, fire) to account for the alarm overload. Thirty minutes is some - times used for the time it takes an operator to travel to the field and manually manipulate a valve or turn off a pump. In any case, if the overpressure occurs faster than the opera - tor can recognise and respond, operator intervention is not a valid mitigation. See API-521 section 4.2.5 for more dis - cussion on operator interventions.⁴ PSVs have much faster response times than the time for operator interventions, with some opening times reported to be in the order of milliseconds. 7 When operator interven - tion is credible to prevent overpressure, the PSVs are some - times not sized for those cases, and operator intervention (together with adequate SIS systems) is used as the last line of defence. These cases are typically limited since many overpressure upsets occur much quicker than between 10 to 30 minutes. Liquid overfill is one case where operator intervention to stop flow to a vessel may be acceptable if the time from the first alarm (usually the liquid alarm high [LAH] alarm point) to overfill is greater than the operator response time. Where the PSV is not sized for a particular case, and instead reliance on operator intervention/SIS was used, then those original mitigations can be considered instead of the PSV bypass.

Outlet valve CSO/LO

PSV

To are

Bypass globe valve, NC

Inlet valve CSO/LO

From protected vessel

Figure 2 Typical PSV installation with bypass valve. NC = normally closed, CSO = car-sealed open, LO = locked open

Sections a) and b) are typically satisfied by implement - ing a rigorous site lock or car-seal programme. The isola - tion valves are under strict administrative control. Proper authorisation is required before any administratively con - trolled valve is manipulated. Section c) is typically addressed by installation of the gate valve stems horizontal or at 45 degrees to minimise the chance of a broken gate falling and restricting the open path. The requirements under sections d) and e) are the topic in the sequel of this document. It is sometimes proposed to sat - isfy requirement d) by stationing an operator at the PSV bypass. The intent is that the operator will manually open the PSV bypass valve to prevent overpressure. In lieu of the PSV, the manual operation of the bypass valve becomes the last line of defence against overpressure. Stationing an operator at the PSV bypass appears to have been proposed more frequently 15+ years ago. However, it is still occasionally proposed today, perhaps at times with - out considering all the implications. This practice must be approached cautiously, especially in hazardous services where failure of the operator to open the bypass valve in time results in severe consequences. In the sections below, a critical review of the practice is presented, along with sev - eral cautions. Operators should be afforded flexibility in the operation of their plant, and operator interventions will always be an important part of day-to-day operations. The consider - ations in this discussion are limited to cases where the last line of defence (PSV) is removed and replaced with manual operation of the PSV bypass valve. Typical PSV installation A typical installation of a PSV on a vessel is shown in Figure 2 . The inlet piping is sized to ensure the PSV does not chatter, and the outlet pipe is sized to ensure the back pressure does not negatively affect the operation/capacity of the PSV. 1 The bypass valve is normally closed. When the PSV is removed, the inlet/outlet car-sealed/locked (CSO/ LO) isolation valves will be closed and blinded. The expectation upon the operator is that they will be able to open the bypass valve in time to relieve the impend - ing overpressure. It is likely the operator would be provided with a radio and be in communication with the operations group at the control panel who are monitoring the system. The operator may also be provided with a local pressure

PTQ Q3 2022