Hydrocarbon Processing - April 2022 - GP-40

MAINTENANCE/LEAK DETECTION
Pumping light hydrocarbons such as NGL is challenging.
Repetitive mechanical seals failures can occur due to
the very low relative density (specific gravity) of such
fluids. Arrangement 3 of API Plan 53B is designed with
a seal barrier liquid different than the pumping fluid.
Specific seal design recommendations.
Recommended solutions for
the seal design can be divided into two
main options:
* Option 1: Keep the
existing Arrangement 2 seal
(dual-unpressurized seal)
and attempt to resolve any
deficiencies in it.
* Option 2: Use an Arrangement 3
seal (dual-pressurized seal) rather
than the current Arrangement 2
seal, as recommended by API 682
for applications with a specific
gravity < 0.5.
Option 1: Dual-unpressurized seal
(Arrangement 2). To enhance the seal
performance, remove the restriction
caused by one of the two seal API Plan
21 orifices. This can be done by opening
one orifice to full bore size (i.e.,
no flow restriction)-as indicated in
FIG. 5-to avoid piping modifications
that may be needed if the orifices were
removed completely.
If significant contamination is obFIG.
6. API 682 seal Plan 75 and 76 configurations. Source: API.
75. This can have considerable
cost impact. A simplified Plan
75 may be considered based on a
level indicator rather than a level
transmitter required by company
standards. In such a case, the
operating procedure should
ensure daily level checks by
the plant operator.
5. The visual inspection of the seal
auxiliary system at the plant
showed that the seal chamber
vent line was blinded. This means
that the seal will not be properly
vented, which can cause seal
failures especially after pump
priming. Therefore, the inboard
and outboard seal vent lines
must be connected to the closed
drain system or the flare header
to ensure venting of the stuffing
box when needed. Further
enhancement may be achieved
if pressure indicators can be
added at these two vent lines
to show the stuffing box pressure.
This will aid in troubleshooting
future mechanical seal failures.
40 MARCH/APRIL 2022 | GasProcessingNews.com
RECOMMENDATIONS
AND ACTION PLAN
The required actions to resolve the
fundamental pump and seal system design
deficiencies include:
* Remove the pump suction
strainers. If a large foreign object
or high quantity of sludge
is expected, the plant may
keep a large-opening strainer
(> 0.25 in.) to ensure the
strainer will be cleaned when the
differential pressure reaches 0.5 psi.
* If an increase in the current
process flowrate is not expected,
consideration should be given to
re-rate the current pump for the
actual operating conditions.
* Connect the inboard and outboard
seal vent lines to the closed drain
system or the flare header to
ensure venting of the stuffing
box when needed. A further
enhancement may be achieved if
pressure indicators can be added
at the vent lines to show the
stuffing box pressure.
served or expected, consider adding a filter
to API Plan 21. Re-install the flush line
temperature measurement downstream
of the API Plan 21 cooler. If corrosion pitting
is still observed for future seals, verify
the cause of the pitting and upgrade
the seal material, as needed. Consider
changing seal manufacturers to other seal
vendors if issues are not fully resolved. In
such cases, ensure the new seal is selected
based on actual site operating conditions
and request that the seal vendor provide
its experience list for seals in similar operating
conditions with satisfactory performance
(i.e., at least 3 yr of MTBF).
Option 2: Dual-pressurized seal
(Arrangement 3). As stated previously,
low specific gravity liquids are difficult to
seal. Therefore, API 682 recommends arrangement
3 seals for liquids < 0.5 specific
gravity. This can be a very viable option,
particularly if the seal material is upgraded
from 316SS to duplex or super duplex.
The recommended seal plan for such
a case will be API Plan 53B. The main
benefit for an Arrangement 3 seal is that
it relies on the barrier liquid to lubricate
the seal faces. In such a case, a high-quality
barrier liquid can be used to ensure
high seal reliability. GP
WESAM KHALAF ALLAH works within the
Consulting Services Department for Saudi Aramco.
http://www.GasProcessingNews.com
Hydrocarbon Processing - April 2022

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