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PROPER CARE CAN PREVENT CATASTROPHIC RESULTS
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 theOEMthat contribute in one way or another to bad things happening to these good turbines.
Take an example, such as foreign object damage (FOD). Onemight ask why damage occurs so rapidly. The shape of the inlet bellmouth 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. 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 anMS-7001EA
2. Impact of severe boiler chemistry upon anMS6001B combined cycle operation
3. Penalty for not paying close attention to final closing clearances upon reassembly after an outage
Example 1. This deals with the aftermath of amajor overhaul of anMS-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 (Figure 1).
The onsite visual forensic analysis revealed what appeared to be the initial impact zone, a shiny spot. Additionally, several first-stage buckets exhibited “unusual” deposits. Spectral analysis from the GE Metallurgy Laboratory revealed significant traces of AISI 4140 tool steel in these deposits; this material is not used on a GE MS-7001EA unit.
When advising the disassembly contractor of this finding, they noted the use of tool steel feather wedges to help alignment of the combustion liners during installation.
Example 2. This deals with the impact of a severe boiler chemistry event upon an MS6001B in combined cycle application. The unit uses attemperated steam injection to achieve compliance with regulated NOX level exhaust emissions.
Essentially, the boiler chemistry event involved carryover of hydrazine into the flow stream of condensate used for steam injection attemperation. The deep blue coloring of the first stage turbine nozzle is an oxide of cobalt. This transmogrification changed the characteristics of the alloys used for the first stage nozzle and the first-stage buckets such that they were no longer suitable for use in a GE turbine. The end result being that both the first stage nozzle and the first stage buckets could not be refurbished and were only suitable for scrapping (Figure 2).
Example 3. This involves the penalty for not paying close attention to final closing clearances upon reassembly after an outage. Power plant users often believe that since they are going to change out turbine nozzles and buckets during an outage, it is not important to take all of the internal clearances when opening the unit, and again when closing the unit. They believe that they can save outage time by not expending the effort to take accurate readings and comparing them to what they were at the last outage; such a belief can quickly become a false economy.
Turbine, compressor and bearing clearances are where the “rubber meets the road.” The OEM’s design engineers view the clearance diagrams with their specific tolerances as the “bible” for setting the performance and reliability parameters for any heavy duty gas turbine.
Rubs and consequential damage are most often a low-speed event initiated with the first fired start. Whether it is a standard, simple, high-low tooth seal on the shroud blocks or the newer honeycombed seal surfaces, the shroud material is tougher and harder than the bucket material. The wearing away of the shroud rails on the Z-form shrouded tip buckets not only creates a performance loss, but also affects bucket mechanical integrity as well as damping.
It is crucial that the person taking the clearances not only have a good understanding of how and where to take the clearances accurately, but also understands that this task is one of the most important of the outage. Adherence to the designed tolerance range affects optimum performance. It is equally important to ensure that one fully understands the true cost of trade offs during reassembly, and reviews all of the before and after clearances. The turbine is speaking to you.
If one understands the inseparable dimensional relationship between the rotor and the stator during various phases of operation, he or she can determine where tomake judgments as to where to go outside of the optimum clearance envelope.
Most of the time, we do good things to good turbines. We operate them within design limits and maintain them “as if” they were our own. On occasion, though, we do some “not so good” things to our turbines that result in bad consequences.
We are human, so sometimes we forget things (like the famous roll of paper towels or the aluminum ladder in the compressor inlet) and suffer the catastrophic results of such negligence. If we are careful, count our tools and check once, twice, thrice for foreign objects being left inside the turbine, the success of the outage will be positive. Nobody wants bad things to happen to good turbines.
Alfred P. Shuman is Senior Staff Technical Advisor at PAL Turbine Services, LLC. For more information visit: www.pondlucier.com