Hydrocarbon Processing - April 2022 - GP-37

MAINTENANCE/LEAK DETECTION
Case study: Sour condensate NGL
export pumps seal failure
W. KHALAF ALLAH, Saudi Aramco, Dhahran, Saudi Arabia
In a facility, a frequent seal failure occurred
on two sour condensate natural
gas liquids (NGL) export pumps. After
dismantling and inspecting one of the
seals, it was determined that the seal had
high contamination on the inner and the
outer seals in addition to high-temperature
failure for the outer seal elastomer
(dry running seal, mating ring O-ring).
The pump owner reported frequent
seal failures and requested pump specialists
to oversee a comprehensive investigation.
A site visit was conducted by the
pump specialists to gain a better understanding
of the pump and seal systems.
Additionally, previous seal failure reports
were reviewed covering the period
from 2014-2017. This case was considered
to be an exceedingly high seal failure
rate, with a mean time between failures
(MTBF) of ~7 mos, considering that
only one of the two pumps is typically in
operation. This is much lower than the
seal design life of 36 mos.
Unit A non-drive end (NDE) seal leak
incident investigation. As per the provided
data, the Unit A NDE seal failed
after less than 1 mos of operation while
the pump was not running and resulted in
leakage to the atmosphere, triggering the
lower explosive limit (LEL) alarm in the
plant. The pump was shutdown 7 hr before
the alarm. FIG. 1 shows the pump discharge
pressure and the seal leakage detection
from the time of the pump shutdown
to the time where the alarm was triggered.
FIG. 2 shows the pressure trend for the
drive end (DE) and the NDE seal leakage
detection pressure for the complete
23-d duration of the seal operation.
The inspection of the seal at a local
facility showed the following:
* High contamination on inner
and outer seals (FIG. 3)
FIG. 1. Pump depressurization due to seal leakage.
Gas Processing & LNG | MARCH/APRIL 2022 37
* Contamination material
was partially metallic
(i.e., attracted to magnets)
* Inner seal dynamic O-ring
had a significant amount
of contaminated materials
stuck to it
* Outer seal mating ring O-ring
was hardened and cut due to
high temperature
* Inner and outer seal faces were
showing heavy wear marks.
Seal failure analysis. Based on the collected
data, the most logical scenarios
for the failure of the Unit A NDE mechanical
seal are listed here:
1. The assessment of the pump's
23 d of operation shown in FIG. 2
shows that the NDE seal leakage
detection pressure maintained an
approximate average of 20 psig,
which exceeds the alarm limit set
at 10 psig. This could
be caused by one or a
combination of the below:
a. A stuck check valve as per
American Petroleum Institute
(API) Plan 76. This is highly
probable as it might create
back pressure in the cavity
between the two seals.
b. A passing API Plan 76 drain
valve or a passing mechanical
seal drain valve. This might
result in flow entering the
seal cavity.
c. A passing pump drain valve
or passing API Plan 23 drain
valve. This is also possible and
might result in pressurizing
the seal cavity if one of its
drain valves is passing.
2. The high seal cavity pressure
had a significant effect on the
outer seal. This pressure, as
well as the high contamination
in the process liquid, are believed
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