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Sulphur Systems - Sulphur Pumps
January 16, 2003
Introduction
Sulphur pumps are specially designed pumps
to handle molten sulphur. The physical
properties of sulphur are such that its viscosity can change rapidly over a small
temperature change that can cause a pump motor to overload.
Lewis offers a specially designed vertical
submerged centrifugal pump for molten sulphur service.
In North America and elsewhere the world, Lewis pumps have virtually become
the standard in the industry for the pumping of molten sulphur.
The impeller housing, shaft housing and
discharge pipe are all steam jacketed to maintain the sulphur at its optimum pumping
temperature. The sulphur temperature should
never 158°C (317°F) at which point it begins to caramelize and becomes impossible to
pump. Some impurities in sulphur can modify
its viscosity characteristics so the optimum pumping temperature may be different from the
recommend temperatures.
Lewis recommend that steam jacket be
supplied with saturated dry steam at a maximum pressure of 35 psig (2.41 bar g). This corresponds to a steam temperature of 138°C
(280°F). As the sulphur passes through the
close internal running clearances of the pump its temperature will rise 14°C (28°F) due
to friction. This results in a molten sulphur
temperature of 152C (306F) which is still below the maximum allowable temperature.
Lewis pumps are available in a large range
of sizes. Nearly any size pump available for
sulphuric acid service can be supplied as a sulphur pump.
The vendor should be contacted for large pump services but pumps as large as
a Lewis size 8 are known to be available (see Appendix B).
Typical Lewis pump lengths are 84",
114" and 144" although standard units are available in lengths ranging from 7'
to 22'.
Prior to putting a pump in service steam
should be on the jacket for several hours which will allow all the pump parts to attain
the normal operating temperature. The sulphur
inside and outside the pump should be molten at the normal operating temperature.
Prior to starting the pump, the pump shaft
should be rotated by hand to ensure that it is free to turn. Sulphur may freeze the pump shaft causing the
motor to overload on startup.
The pump must be fully supplied with
sulphur otherwise extensive damage may occur to the pump.
During operation, a small amount of molten sulphur is pumped upward through
and around the internal bearings lubricating them. Bypass
passages are provided to return this sulphur back to the tank or pit.
Steam should always be supplied to any
installed stand-by pumps to ensure they are ready to operate as required. The pump shafts should be rotated by hand on a
regular basis to ensure it is free to turn and not frozen by sulphur. Pump shafts may freeze if the tank or pit is
operated at low levels. Raising the sulphur
level will sometimes be sufficient to melt the sulphur and free up the pump shaft.
When a pump is removed from service for
maintenance it should be removed from the pit or tank and all sulphur should be drained
from the pump casing. Attaching flexible stem
hoses to the pump steam jackets will allow sulphur to drain freely from the pump as it is
being lifted.
Once in the shop solid sulphur can be
removed from the pump surfaces using a steam lance, wire brush or sandblasting. Never use a direct flame to remove sulphur as this
will generate SO2 gas.
Distortion or changes to the metal properties may occur if the part are heat
by direct flame.
Sizing
The following factors must be considered
when sizing a sulphur pump.
- Flow rate
- Depth of sulphur pit or height of tank
- Line pressure drop, fittings, spray nozzle, valves, etc.
- Vertical distance from supply point to highest point in piping
- Tracing steam conditions
- Electrical current characteristics
- Operating pressure inside furnace
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