Hydrocarbon Processing - November 2021 - 81

Valves, Pumps and
Turbomachinery
K. BRASHLER and W. K. ALLAH,
Saudi Aramco, Dhahran, Saudi Arabia
Vertically suspended molten sulfur pumping
challenges and best practices
The objective of this article is to share
industry experience related to the reliability
challenges with vertically suspended
molten sulfur pumps. Due to the operating
temperatures and the unique chemical
properties of sulfur, several operational
and reliability challenges are inherently
present across the industry. The following
will discuss common pump field troubles
reported by operations, pump failure
modes, and case studies that demonstrate
typical failure modes and mechanisms.
Vertically suspended molten sulfur
pumps are used in oil and gas facilities
to transfer molten sulfur from melting
plants. Both horizontal and vertically suspended
pumps cover the broad variety of
molten sulfur applications; however, this
article will focus on the vertically suspended
pumps.
The typical vertical design is an API
VS4 or VS5 type. The VS4 is a vertically
suspended, single-casing, volute, lineshaft-driven
sump pump. The VS5 is the
same pump design as the VS4, except that
the pump is cantilevered rather than utilizing
a line shaft, with intermediate bearings
to support the rotor. The VS5 cantilevered
design is restricted to shorter submergence
depths (approximately 6 ft) and is
typically applied in dirty sulfur service. By
design, the cantilevered VS5 pump does
not require product lubrication to intermediate
bearings; therefore, this design is
better suited for dirty service. FIG. 1 shows
both VS4 and VS5 pump types.
Pumping molten sulfur can be a very
challenging application due to the inherently
high temperatures and the unique
chemical properties of sulfur. These
vertically suspended pumping applications
require special attention in terms of
both operation and maintenance-i.e., to
maintain reliability and minimize maintenance
costs.
One of the most critical aspects to
consider involves the unique properties
of sulfur. Sulfur experiences an abnormal
variation in viscosity with changes in
temperature, which only allows sulfur to
be pumped successfully within the range
of 135°C-155°C (275°F-311°F). Most
pump designs incorporate a steam-heating
jacket to maintain a constant temperature
throughout the entire pump. FIG. 2
shows a typical cross-sectional drawing
for an API VS4-type pump with lineshaft
bearings, top anti-friction bearings
and steam jacketing.
The percentage of hydrogen sulfide
S) content also influences the viscos(H2
ity
vs. temperature characteristic, which
can also change the temperature range for
successful pumping. The pump material
selection is also dependent on H2
S content.
FIG. 3 shows a graphical representation
of sulfur viscosity vs. temperature.
Due to polymerization, the liquid sulfur
experiences an exponential increase in
viscosity as it passes through the transitional
zone of above 155°C (311°F).
Typical pump-related troubles reported
in the field include the following:
* Loss of flow or reduced capacity
* High vibration
* Overamping of the motor driver
* Pump seizures or broken shaft
* Corrosion issues.
These reported field troubles or failure
modes can have many different causes.
Each reported field trouble is discussed
in the following sections, with the possible
failure mechanisms. Case examples
are shown of similar failure modes
and mechanisms.
FIG. 1. API vertically suspended VS4 and VS5
pump types.
FIG. 2. Cross-sectional API VS4-type pump.
Source: Lewis Pumps.
Hydrocarbon Processing | NOVEMBER 2021 81
Hydrocarbon Processing - November 2021

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