When bad things happen to good turbines

Those of us who work on GE heavy duty gas turbines know that they are inherently good. Unfortunately, it is often the operators, maintenance people, and even the OEMs that contribute in one way or another to bad things happening to these good turbines.

Take an example, such as foreign object damage (FOD). One might ask why damage occurs so rapidly. The shape of the inlet bell mouth makes it act like a nozzle, which drops the inlet pressure and accelerates the air to a velocity of Mach 0.50; one-half the speed of sound.

(A GE heavy duty gas turbine)

The intrinsic nature of this flow characteristic is what carries foreign objects through the flow paths of the compressor and turbine sections.

What follow are examples of costly problems that could have been avoided if proper care had been taken:

1. Foreign body entering the combustion section of an MS-7001EA

2. Impact of severe boiler chemistry upon an MS6001B combined cycle operation

3. Penalty for not paying close attention to final closing clearances upon reassembly after an outage

Example 1. deals with the aftermath of a major overhaul of an MS-7001EA. Upon restart, this unit displayed a compressor discharge pressure (CPD) higher than before the outage; yet, the MW output was 9% lower than before the outage. A borescope inspection revealed anomalies at the first-stage nozzle partitions.

As the forensic disassembly progressed, it became obvious that a foreign body had

come through the combustion section and lodged between the nozzle partition trailing edge and the first-stage bucket leading edge.

The “object” appeared to have bounced around between the nozzle and buckets and then broke off a large section of a partition, which in turn acted to push several of the trailing edges of the partition upstream. This “choking” of the nozzle caused the CPD to increase beyond design parameters resulting in diminishing of the hot gas flow through the turbine section.

Read more in the Aug./Sept. 2012issue of Turbomachinery International.