Considerations for not providing a dedicated spare pressure-relief device

Pressure-relief devices (PRDs) play a critical role in the hydrocarbon processing industry. They act as the last line of defense in protecting plant equipment from overpressure and are mandated by the American Society of Mechanical Engineers (ASME). This article provides some key factors that process engineers and plant operators must consider to determine if an installed spare PRD is necessary.

Like many other plant components, relief devices require routine maintenance to ensure safe and reliable plant operation. While most of the PRDs in hydrocarbon processing plants can be taken out for maintenance during plant turnarounds, certain PRDs may require more frequent maintenance while the plant is in operation (onstream). The term “onstream” refers to the overall plant operation. Reasons for more frequent maintenance could include safety requirements or possibly because the PRD is installed in a fluid service where fouling, corrosion or plugging could potentially impact PRD operation.

Onstream maintenance of a specific PRD in the plant can be achieved by one of the following options:

1. Providing an alternate and adequate relief protection to the protected equipment (e.g., a spare PRD, or an alternate and adequate flow path to another PRD)
2. Isolating, draining and depressurizing the protected equipment from all possible overpressure sources (taking the equipment out of service)
3. Eliminating as many overpressure sources as feasible, and implementing administrative controls for monitoring the protected equipment for the duration of the PRD maintenance.

Process engineers are required to review these options during the front-end design development step, with input from the plant operations team, to make an appropriate decision. PRDs requiring onstream maintenance are, in general, provided with isolation valves on inlet and outlet (if applicable) pipelines. ASME code requires administrative controls to be in place for proper operation of the isolation valves in the relief path so the equipment’s overpressure protection is not compromised. These requirements include providing locks or car seals in addition to administrative controls to prevent accidental closure (for more information, refer to ASME BPVC Section XIII Non-Mandatory Appendix B). Process engineers and plant operators must be familiar with the ASME code requirements, and Appendix B provides valuable guidance and requirements for determining a need for installing a dedicated spare PRD. The following will review the three options in further detail.

Option 1. Providing an installed spare PRD is generally the preferred way to provide adequate overpressure protection to the process equipment. While this option involves initial capital expenditure for the PRD and associated piping, it also provides the safest way to keep the protected equipment in continuous operation for the duration of the maintenance. Plant operators are required to follow stringent administrative procedures for isolating the PRD requiring maintenance.

In the absence of an installed spare PRD, other creative ways are available to achieve the same purpose. This includes utilizing PRDs on nearby equipment to provide the necessary protection by means of utilizing an existing pipe routing for a temporary relief path. However, process engineers should perform a thorough analysis to ensure that such a temporary and alternate relief path is adequate in terms of hydraulic requirements (inlet pipe pressure loss), PRD flow area and fluid service compatibility.

Option 2. This option is best suited for equipment that can be taken out of service while onstream plant production is maintained. A spare PRD may not be required if the entire protected system can be isolated during normal plant operation. Equipment operating in intermittent service also may not require a spare PRD. For example, consider a hydrocarbon system containing multiple catalyst bed vessels in parallel, where at least one vessel is always isolated for catalyst regeneration and is in standby mode for a prolonged duration. A spare PRD may not be provided for protecting such vessels, as maintenance can be performed when the vessel is in standby mode. In such cases, the duration of the maintenance should be given careful consideration, as the vessel cannot be returned to service without code-compliant relief protection. Plant operators generally rely on administrative procedures to open and close several valves for placing equipment back in service. Failure to properly follow these administrative procedures can compromise the overpressure protection mandated by the ASME code. One such safety incident was the ruptured heat exchanger in June 2008 that killed one worker and injured six others at the Goodyear Tire and Rubber Co. in Houston, Texas.¹ One of the root causes of this accident was failure to properly follow the procedure of opening the PRD inlet piping isolation valve before placing the heat exchanger into service.

Option 3. This is the least preferred option, as it involves operating the process equipment without overpressure protection under administrative controls. One of the first things that the plant operator is required to do in this option is to identify all potential overpressure sources beforehand and attempt to eliminate or minimize the likelihood of overpressure from such sources for the duration of the PRD maintenance. In this option, assuming that isolation valves are in place, the PRD must first be isolated by closing the inlet and outlet isolation valves before the maintenance can begin. Closure of the isolation valves would lead to blocking the relief path and to operating the protected equipment with a risk of overpressure. Plant operators are required to stay in compliance with the ASME code for proper implementation of administrative controls. ASME BPVC-XIII Appendix B-7(d) states:

“Procedures are in place to provide pressure relief protection during the time when the system is isolated from its pressure relief path. These procedures should ensure that, when the system is isolated from its pressure relief path, an authorized person should continuously monitor the pressure conditions of the vessel and should be capable of responding promptly with documented, pre‐defined actions, either stopping the source of overpressure or opening alternative means of pressure relief. This authorized person should be dedicated to this task and should have no other duties when performing this task.”

Plant operators should consider several factors to safely rely on administrative controls for monitoring unprotected equipment. One such factor is the duration of maintenance. Can the relief device be returned to service in a reasonable amount of time? Another factor is training. What kind of training would be required for authorized persons to respond promptly in an event of pressure excursion in the process service? If possible, process engineers and plant operators should steer away from this option, as this involves several factors that cannot be fully accounted for during the plant design.

Multiple overpressure scenarios. There can be more than one overpressure scenario that is applicable for a PRD. Overpressure due to an external pool fire is a well-known scenario that is applicable for process equipment at grade level. Additional overpressure scenarios include blocked outlets and auto-control failures, among others (see API 521 for more information). The process engineer should review all applicable overpressure scenarios when determining the need for a spare PRD.

It is a generally accepted industry practice not to provide spare PRDs in the following applications, as the PRD maintenance interval typically aligns with the protected system maintenance interval:

• PRDs installed exclusively for protecting equipment from external fire (fire-only PRD)
• PRDs installed in cooling water service exclusively for protecting the equipment from thermal expansion (cooling water thermal).

Multiple PRDs. Multiple PRDs are often provided when a single PRD is inadequate to handle the required relief flowrate. The ASME code allows staggering the PRD set pressures in this case. For multiple PRDs:

• A single spare PRD may be installed with the basis that administrative controls are in place to perform maintenance on one PRD at a time. When a single spare PRD is installed, performing maintenance on multiple PRDs would result in inadequate relief protection, which is a possible code violation.
• The spare PRD should have an orifice flow area identical to the operating PRD with the largest area.
• When the spare PRD is put into operation, at least one PRD should have a set pressure at 100% of the maximum allowable working pressure.

Takeaway. The ASME code does not mandate installing a spare PRD. However, in the absence of an alternate and adequate relief path, the ASME code mandates the implementation of proper administrative controls for safe isolation and subsequent monitoring of the protected system for the duration of PRD maintenance. Administrative controls play a critical role in the safe operation of any plant, and many plant operators prefer to install a spare PRD to provide an adequate and equivalent overpressure protection. Process engineers and plant operators must be conversant with the applicable regulatory requirements and must review all PRDs on a case-by-case basis to determine whether an installed spare PRD is, or is not, necessary. GP

LITERATURE CITED

1. U.S. Chemical Safety and Hazard Investigation Board, “Goodyear heat exchanger rupture,” January 2011 (report released), online: https://www.csb.gov/goodyear-heat-exchanger-rupture/
2. ASME Boiler and Pressure Vessel Code, Section XIII-2021, “Rules for overpressure protection.”