Hydrocarbon Processing - June 2022 - GP-19

INFRASTRUCTURE & EQUIPMENT
Considerations for not providing
a dedicated spare pressure-relief device
P. K. NAMILAKONDA, Samsung Engineering, Houston, Texas
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.
Gas Processing & LNG | MAY/JUNE 2022 19
Hydrocarbon Processing - June 2022

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