Hydrocarbon Processing - August 2021 - 19

Special Focus Valves, Pumps and Turbomachinery
M. BADUGHAISH, Saudi Aramco, Dhahran, Saudi Arabia
Case study: Reliability of low-flow, high-head pumps
Pumps are one of the key elements
used in different industries, including oil
and gas, petrochemicals and water treatment.
Their range of applications is very
wide and process requirements mandate
the use of specific types and designs. Due
to that wide range, it sometimes becomes
challenging to select the right pump design
that fits some applications.
One such application is the low-flow,
high-head (LFHH) pump, which requires
the use of low specific speed designs,
which are commonly known by their
sensitivity to operate within a wide operating
range due to the narrow operating
window from their best efficiency point
(BEP). This makes the reliability of low
specific speed pumps questionable. However,
acknowledging the reliability concerns
of these pumps does not relieve end
users from the responsibility of carefully
specifying the right process parameters
prior to purchasing the pump and then
operating it properly.
This article discusses an investigation
into high-speed integrally geared pumps
in a gas processing plant. These pumps
are used to transfer hydrocarbon condensate-since
commissioning, they suffered
from high vibration and frequent
premature mechanical seal failures. Consequently,
the end user was seriously considering
replacing the pumps due to their
low reliability and mean time between repairs
(MTBR).
Before diving into the investigation
findings and recommendations, it is important
to provide the reader with the design
parameters of these pumps to better
understand the case. The three integrally
geared pumps (OH6) are designed to
pump 63 gal/min of light HC condensate
(0.51 relative density and 383 psia
vapor pressure) and discharge with a
pressure of approximately 600 psia (the
pump rated differential head is 1,058 ft).
The pump is rotating at 6,980 rpm. Two
pumps are normally in parallel operation
and the third is used as a standby. The
pumps are designed with a dual unpressurized
seal with API flushing Plan 11
and leak detection Plan 76.
Repair shop findings. One pump was
removed to the repair shop for internal
inspection. It was observed that the
seal flush Plan 11 had multiple orifices
(orifice pack) with a small orifice size
of about 1 mm, as shown in FIG. 1. This
is considered a small size to provide sufficient
cooling for the seal. In addition,
a noticeable amount of dirt upstream
of the orifice was found that could have
blocked the flushing line. The elasticity
of all seal and pump O-rings was affected.
FIG. 2 shows that they were found to be
hard and cracked, which is an indication
of elastomers overheating. The O-ring of
the primary seal was also found to be cut,
shown in FIG. 3. An extrusion indicated
an incorrect installation. The last observation
detailed in the internal inspection
was the orifice size of the seal leak detection
line. It was found to be very small
(< 0.5 mm). According to API 682, all
orifices must have a minimum bore of 3
mm. FIG. 4 shows the small orifice size of
the seal leak detection line.
Site findings. As mentioned earlier,
these pumps were designed for light
flashing hydrocarbon, so the seal and seal
leak detection system had been selected
on that basis. The pumps were designed
with dry running backup seals and API
Plan 76, assuming that only gas would
be seen by the seal cavity. However, it
turned out that the process was not only
light hydrocarbons, but contained traces
of heavy products, as well.
Part of the leaked product through the
primary seal to the seal cavity remained
in liquid form and continued accumulating.
Leak detection Plan 76 for these
FIG. 2. Pump and seal O-rings.
Hydrocarbon Processing | AUGUST 2021 19
pumps is supposed to direct the gas to
the flare header and does not account for
liquid presence to be drained-the liquid
level was increasing, and it was noticed
that liquid was leaking from the seal leak
pressure transmitter port of Plan 76. During
another site observation, it was determined
that individual permanent suction
strainers were installed on the suction
lines with 1.5-mm opening sizes. There
was no differential pressure transmitter
across the strainers to alert plant personnel
of any blockage during operation.
Furthermore, the pumps' operating
data were reviewed over an extended period
of time (7 mos). The data showed
that each pump was operated between
15 gal/min and 120 gal/min, which is
a very wide operating range for a pump
FIG. 1. Orifice size for API flushing Plan 11.
Hydrocarbon Processing - August 2021

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