Hydrocarbon Processing - June 2022 - GP-20

INFRASTRUCTURE & EQUIPMENT
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.
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. Over20
MAY/JUNE 2022 | GasProcessingNews.com
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. "
PRANAY K. NAMILAKONDA is a Process Engineer with Samsung Engineering
America, a global EPC firm based in Houston, Texas. He is a licensed Professional
Engineer and has more than 15 yr of experience in front-end engineering
execution of various hydrocarbon projects. He also leads the process engineering
group, with an emphasis on engineering automation. Mr. Namilakonda earned
an MS degree in chemical engineering from Oklahoma State University, and a
BTech degree in chemical engineering from Osmania University, India.
pressure 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 caseby-case
basis to determine whether an installed spare PRD is,
or is not, necessary.
https://www.csb.gov/goodyear-heat-exchanger-rupture/ https://www.csb.gov/goodyear-heat-exchanger-rupture/ http://www.GasProcessingNews.com
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