Page 473 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

450 Major Process Equipment Maintenance and Repair

For an application where downtime is costly, the purchasing of spare
rotors is imperative. Consideration should also be given to stocking of
stationary vanes for the hot gas path, axial compressor inlet guide vanes,
transition pieces, and other components along the hot gas path.
Considerable savings can be achieved by ordering these parts with the
original purchase order for the machine. This is due to the added lever-
age enjoyed by the customer before he places an order, and by allowing
the vendor to minimize his cost of production by manufacturing the parts
in parallel with the machine.
Once a level of spares is established adequate enough to protect the
operation, the decision can be made to either replenish this inventory
with new OEM parts when they are consumed, or to try some of the re-
pair techniques currently available. In recent years techniques for the re-
pair of alloys once unrepairable have advanced and will continue to ad-
vance as long as there is a market for this type of service.4- s The lead
time required for the purchase of new hot parts can at times be unaccep-
tably long. In this case remanufacturing or repairing parts may be the
only acceptable alternative.
The level of spares required for a gas turbine control system varies
with the type of system and the complexity of the design. Currently sys-
tems vary from older relay-based systems, through analog and hybrid an-
alog/digital, to fully redundant digital systems. In the redundant systems
the spares are on-line. In solid state analog systems (which are generally
not redundant) a decision has to be made whether to stock individual
cards, or a whole spare system and whether it should be kept powered-up
or wrapped on a warehouse shelf. Generally a powered-up complete
spare is the most reliable approach, but is also the most costly. Again, an
assessment must be made as to the relative costs versus the reliability ob-
jectives for the unit.
Spare parts for aircraft derivative units are generally kept as complete
gas generator assemblies. Other external assemblies associated with the
engine are stocked as well. These include fuel manifolds, thermocouples,
bleed valves, speed pickups, and guide vane actuators. Each operator
must assess how many spare engines should be stocked to service his op-
eration. This may mean none, or a share of a pooled spare engine or in
the case of a major Canadian pipeline one spare for every four running
engines.
Special tools are normally required to perform routine maintenance on
both industrial and aircraft derivative engines. They are usually identi-
fied in the maintenance manual for fabrication by the user, or supplied
with the engine by the manufacturer. The responsibility for ensuring that
these tools are available for an overhaul rests with the user.