Hydrocarbon Processing - February 2022 - GP-31

PIPELINES AND UTILITIES
Case study: Seal-less caustic
circulation pump failure
D. AL DOSSARY and F. AL DOSSARY, Saudi Aramco, Dharan, Saudi Arabia
A Saudi Aramco gas plant contains two
(2 × 100%) lean caustic circulation sealless
pumps. The pumps are responsible
for delivering the lean regenerated caustic
with naphtha from the disulfides separator
to the naphtha wash settler. The pumps
are considered an essential component of
the caustic regeneration loop that ensures
removing the total sulfur from the natural
gas liquids (NGL) products.
Pump design. The pumps are 178-hp
seal-less magnetic drive pumps with a
rated flow of 294 gal/min (gpm) and a total
differential head of 638 ft. The process
fluid is totally contained inside the pump
containment shell and depended on to remove
the generated heat through internal
lubrication paths. Therefore, the slightest
restriction to the lubrication flow paths
will jeopardize the condition of the pump
internal parts, resulting in a catastrophic
failure. Additionally, the fluid runs between
the inner magnet, the containment
shell and through the shaft holes to the
rear of the pump shaft, where it returns to
the pump's suction through the thrust balance
hole in the impeller (FIG. 1)
The pump shaft connected to the inner
magnet is rotated by the action of the
magnetic flux circuit between the inner
and outer magnets. The outer magnet is
coupled to the driver motor through another
shaft to transmit the rotation to the
inner magnet.
Background. Within normal operating
conditions, the pump was tripped due
to a motor thermal overload fault signal.
All process conditions were checked and
found to be within normal parameters. The
pump was later started again and immediately
tripped on a stalled motor fault signal.
It was decided to remove the pump for
dismantling and internal inspection. Major
findings included:
* Excessive rubbing occurred between
the impeller front cover and the
pump casing, as well as thinning
on the impeller shrouds. This is an
indication of high thrust movement.
* The rear thrust pad, thrust washer
and sleeve bush were not found in
place. The silicon carbide bearings
were found to have completely
failed and their traces were found
inside the pump casing (FIG. 2).
Investigation. Based on the inspection
findings listed above, operational trends,
caustic lab sample analysis and metallurgical
analysis, the following conclusions
were reached:
* Material analysis showed that grains
of the titanium sleeve bush were
colored blue, consistent with the
newly introduced process catalyst
in 2019. This was considered a
sign of a corrosion reaction. It is
suspected that the catalyst filled the
standby pump when it was not in
operation-the pump would have
been unable to disperse the liquid.
The product would have then
attacked the front sleeve bush, as
this is mounted in the casing volute,
whereas the rear sleeve bush is in a
less accessible area of the pump.
* Occasionally, the process
is subjected to high caustic
concentration. Material analysis
shows that there should be an
accelerated rate of corrosion of
the titanium sleeve bush carrier
in instances where the pump is
exposed to higher (40%-50%)
caustic concentrations.
As shown in FIG. 3, it is suspected that
corrosion and the loss of the titanium
sleeve bush circumferential were enough
to allow the bolts to loosen their grip between
the bush carrier and holder. Consequently,
all of the bush carriers became
free to move radially and axially, leading
to the thrust pad and thrust washer silicon
carbide to shatter.
The failure of the rear thrust pad and
thrust washer (active bearing) increased
the negative thrusting and axial movement
of the whole rotor towards the pump
casing. This resulted in heavy wear on the
casing, impeller and radial bush failure.
FIG. 1. Magnetic drive pump lubrication
flow regime.
FIG. 2. Rear bush housing assembly (bush
carrier) with damaged thrust pad corrosion/
erosion impact. Additionally, signs of chemical
attack (corrosion) were found in the front NDE
titanium bush housing assembly.
Gas Processing & LNG | JANUARY/FEBRUARY 2022 31
Hydrocarbon Processing - February 2022

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