Page 447 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Steam Turbines and Turboexpanders 425

Another possible cause is the application of excessive pipe loads on the
unit flanges, causing possible internal rubs or bearing malfunction, or
shaft coupling misalignment.
Higher than normal expected readings may also be produced by the
inherent resonant vibration of a stationary component, often aggravated
by low oil viscosity.

Excessive Thrust Load
Excessive pressure on the bearing thrust face could exceed the design
thrust force, causing damage and rapid deterioration. Protection against
this is normally provided by the use of a differential pressure switch for
alarm or shutdown.
The thrust control system should be checked to eliminate errors in indi-
cations and in the control system. The oil lines to the meters should be
checked and purged of dirt and gas pockets. Excessive thrust loading can
be caused by a higher than normal pressure behind the expander rotor
which the balance system is unable to control. This suggests rotor back
seal deterioration or icing.
The plant pressures and flows should also be checked. If the pressures
and flows are not within normal limits, the machine may not be able to
run properly. This situation should be checked with the manufacturer.

Filter Collapse
Failure to open the pump and filter bypass valves during the prestart
check can result in trapped gas crushing the filter cartridge when the oil
pumps are started. The filter cartridge should be checked for the correct
part number and micron size for its usage. After a long period of down-
time, the cartridge could become contaminated with water, causing
weakening and reduced permeability.

Oil Loss in the System
Oil loss can be determined by the reservoir level. One condition that
can cause oil loss is high oil level in the reservoir with a high seal gas
flow rate. This causes flooding of the mist eliminator in the reservoir
vent line, sending the oil into the compressor. To correct the problem, the
oil level should be maintained between the specified minimum and maxi-
mum levels, and the seal gas should be maintained at the minimum re-
quired flow rate.
With hundreds of turboexpanders in service worldwide, an increasing
demand for a working knowledge of the machines and supporting sys-
tems has developed. The “Three-Step” precheck described earlier is in-