Page 474 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Gas Turbines 451

Maintenance OrganizationlPlannlng

The key to all satisfactory maintenance activity is planning.’ This is
especially true when dealing with gas turbines. Even the simplest of
maintenance tasks, whether it is recognized or not, requires some mea-
sure of preplanning. A maintenance policy for gas turbines must address
the increased importance of inspection and overhaul planning if un-
planned outages are to be avoided. Each user regardless of the number or
type of the engines must institute the following basic types of planning to
be successful in gas turbine maintenance:

1. Material/Spare Parts Planning
2. Manpower/Training
3. Technical Analysis/Historical Record keeping
4. Contingency Planning

Each of these basic types come together into a formal turbine overhaul
and inspection plan. Without any one of them, the overall effort will
never achieve maximum effectiveness. This type of planning is funda-
mentally no different from other types of rotating machinery planning.

Condition Monitoring

Advances in condition monitoring over the last few years along with
the high cost of inspections and overhauls are gradually shifting the em-
phasis in gas turbine maintenance from a preventive to a more predictive
approach. More and more users of both industrial and aircraft derivative
engines are challenging the manufacturer’s recommended inspection
schedule in favor of a “more scientific approach.” In process plants, the
manufacturer’s schedule often just does not mesh very well with the man-
ufacturing plan for the process unit. In other cases units are shut down
due to a problem in another part of the plant. This gives an opportunity
for some minor inspection, such as a borescope inspection, and based on
this data run lengths are extended. It must be remembered that many
parts of a gas turbine are designed for a finite life, typically 100,OOO
hrs., and that there is a statistical probability that the part will not last
that long. This is one of the main factors which sets gas turbines apart
from other types of rotating equipment. Whereas many companies are
extremely successful in operating compressors, motors, and to a certain
extent steam turbines based on observed condition only, this is not yet
feasible with gas turbines. Most users rely on a “home-brewed” mixture
of fired hours, historical data, and condition monitoring to determine gas
turbine run lengths.