Page 490 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Gas Turbines 467

Any control system faces its most severe test during the start cycle.?
Consequently more problems are encountered at this time and these prob-
lems are the most difficult to troubleshoot. It is critical to understand the
sequence of events which the control system is trying to execute.
The normal sequence of events in the starting of a gas turbine is:

1. The starter is energized and the gas turbine accelerates to ignition
speed.
2. The fuel valve is opened and the ignition system energized. Some
period of time is alotted to confirm that lightoff has occurred.
3. The turbine is accelerated using an acceleration fuel schedule along
with help from the starting device.
4. At some point the gas turbine achieves a self sustaining speed and
the starting device drops out. The turbine continues to accelerate
until it reaches the speed setting of the governer.
5. After a suitable warm-up period, load is applied to the turbine.

All control systems, no matter how complex, receive information
about the turbine from devices mounted on the machine such as thermo-
couples, transmitters, probes, W flame detectors etc. These devices are
critical and should be checked at every service outage. Sensors which
have failed should be replaced and those which can be adjusted recali-
brated.

Lube OH System
As in other process plant machinery trains, the quality of the lube oil
system which is supplied with the train has a definite impact on the ser-
vice factor of the unit. Gas turbines are normally supplied with a lube oil
package mounted in the bedplate of the turbine. These packages do not
normally meet the type of standards (such as API 614) which have been
developed to ensure the reliability desired for refinery and process plant
application. The gas turbine purchaser may have a number of options
from which to choose.

1. Accept the vendor’s standard. In some cases this is all that can be
done and as long as vendor and user agree on its adequacy, this will
be the least expensive and easiest option.
2. Review and propose changes to the vendor’s standard package. This
method is the most common. Care should be taken to avoid making
the vendor’s design less reliable and unnecessarily expensive.