Hydrocarbon Processing - November 2021 - 82

Valves, Pumps and Turbomachinery
LOSS OF FLOW OR REDUCED
CAPACITY
There are many causes for reduced
flow or loss of flow conditions, which are
typically caused by unfavorable suction
conditions. The following outlines various
unfavorable suction condition scenarios.
Fouled suction. Most designs-including
both dirty and clear sulfur services-
utilize a suction strainer. These strainers
should be adequately sized in terms of
both surface area and perforation size. In
addition, stainless steel is recommended
to resist corrosion, which can compromise
the integrity of the strainer. The
important thing is to not assume that the
pump will only see clean molten sulfur.
Foreign material is a real possibility, depending
on the integrity and condition of
the sulfur pit. Any deterioration of the sulfur
pit that introduces foreign material to
the sulfur has a strong possibility of making
it to the pump suction boot.
FIG. 4 shows a severely fouled suction
strainer, resulting from submergence in
a foreign material. In this case, the root
cause was not pump related, but rather
the failure of the sulfur pit wall (FIG. 5).
As shown, the acid bricks collapsed, followed
by the release of a foam glass material
used for insulation behind the bricks.
This foam glass and other foreign material
continuously collected at the bottom
of the pump suction boot area, where it
eventually blocked the suction strainer.
Viscosity changes and temperature
control issues. For proper pumping
function, temperature control is critical
for maintaining the sulfur within the
required temperature range. Any issues
with the temperature control of the sulfur
pit can quickly turn into pumping
problems. Pit temperatures should be frequently
monitored for proper temperature
control. H2
S content is also a factor
to consider regarding the viscosity vs.
temperature characteristic.
High viscosity not only affects the
pump's ability to deliver flow but can also
affect the lubrication to the intermediate
bearings, due to low-flow or no-flow
conditions. The brake horsepower
required
also increases and can result in motor
overload.
FIG. 3. Sulfur viscosity vs. temperature plot.
Cavitation/submergence issues. Net
positive suction head (NPSH) and submergence
issues can also cause loss of flow
or low-flow conditions. These conditions
often arise from level control issues. These
systems are exposed to high temperatures
and possible corrosion issues; therefore,
these systems should be properly maintained
to ensure proper pump operation.
High vibration amplitudes. Due to
the inherent design of the vertically suspended
pump, the working end of the
pump is located several feet below grade
and is submerged in the molten sulfur.
Seismic vibration data is typically measured
at the motor and the pump top
bearing locations. This makes it difficult
to diagnose issues occurring at the bottom
of the pump at the suction inlet and
the line-shaft bushings. Typical failure
modes related to vibration issues include
top pump bearing failures and line-shaft
bearing/bushing failures.
The following are typical mechanisms
associated with high vibration amplitude:
* Excessive pipe strain
* Driver misalignment
* Lack of line-shaft bearing/
bushing lubrication.
Excessive pipe strain. External loads
on the pump can cause distortion of the
pump, resulting in internal misalignment
and bearing failure. Machine misalignment
can also result in elevated vibration
amplitude and shortened bearing life.
FIG. 6 shows an unloaded discharge piping
support, which resulted in external
loads on the pump. These external loads,
along with other mounting issues, resulted
in failure of the top anti-friction bearing,
along with high vibration amplitudes.
Driver misalignment and mountFIG.
4. Fouled suction strainer with
foreign debris.
FIG. 6. Unsupported pump discharge piping.
ing issues. Careful pump mounting, pipe
connections and shaft alignment should
be executed to minimize any external
loads on the pump. Shaft deflection
checks should be conducted with two
dial indicators in both orthogonal directions
(radial) during piping connections
to ensure no more than 0.002-in. pump
shaft deflection. The pump should be
rotated by hand, following piping attachment
and warm-up. This action should
be done just prior to startup to ensure
that the shaft turns freely and that there
are no issues with internal misalignment.
Line-shaft bushing lubrication isFIG.
5. Bricks collapsed, releasing foam glass.
82 NOVEMBER 2021 | HydrocarbonProcessing.com
FIG. 7. Overheated line-shaft bearing and
broken bushing.
sues. For the VS4-type pump, the lineshaft
bearings are lubricated by the sulfur;
therefore, any foreign material or abrasives
in the sulfur will compromise the bearing/bushing
life. In addition, the viscosity
of the sulfur can also have a detrimental
effect on the product lubricated bushings.
http://www.HydrocarbonProcessing.com
Hydrocarbon Processing - November 2021

Table of Contents for the Digital Edition of Hydrocarbon Processing - November 2021
