Arcing
repair
PL
Sahu and N Pradhan discuss the application of electric arc spraying
technology for repairing a bull gear shaft of a centrifugal compressor
Oxygen,
nitrogen and argon gases required in the process of steel making in
Visakhapatnam Steel Plant are produced at the air separation plant.
Four feed air compressors are installed to supply feed air to air separation
units for production of these gases. These compressors were originally
manufactured by Mannesmann Demag, Germany. The rated capacity of each
compressor is 100000 Nm3/hr at a discharge pressure of 7 bar. By configuration,
it is an integrally geared centrifugal compressor. As the name suggests,
it is a combination of a centrifugal compressor and gearing, each end
of the two pinion shafts carries shrink fitted overhung impellers. The
low speed pinion shaft carries stage 1 and stage 2 impellers and the
high-speed pinion shaft carries stage 3 and stage 4 impellers. The bull
gear rotor drives these two pinions. An electric motor of 11000 kW capacity
is coupled with the bull gear shaft to drive this four-stage compressor.
The bull gear rotor
The bull gear shaft with a shrink fitted huge gearwheel
of 2022 mm diameter constitutes the bull gear rotor. It rotates at 1500
rpm, with a very high pitch circle velocity of 157 m/s. Like other rotor
assemblies of the centrifugal compressor, it is dynamically balanced.
One side of the bull gear shaft is coupled with an electric motor with
the help of 2250 mm long coupling spacer. The other side is geared to
the main oil pump and is supported on two journal bearings. The tri-metal
bearings are hydro dynamically lubricated. The coupling side bearing
has thrust faces also on both the sides. Correspondingly the shaft has
two thrust bearing shoulder surfaces (thrust collars) opposed to each
other as shown in Figure 1.
Details of failure
Feed air compressor-3 tripped on bull gear shaft
position too high during the start-up. When the machine was opened for
inspection, it was found that the thrust bearing face on gearwheel side
was fully damaged, A melted babbitt metal layer blocked the passages
for lubricating oil on the thrust face. The thrust collar of the shaft
on gear wheel side was badly damaged due to the metal-to-metal contact
while the wear out was about 0.3 mm. It was a major failure as thrust
collar is an integral part of the bull gear shaft, The cost of a new
bull gear rotor is about Rs 20 (Figure 1)
....CONTD
