As a gas turbine accumulates fired hours, some performance degradation will occur in both output and efficiency. Non-recoverable performance degradation is defined as performance that cannot be recovered without significant maintenance (Hot Gas Path inspection or Major Inspection). Recoverable performance degradation is defined as that which can be recovered without significant maintenance (on-line compressor water wash, off-line compressor water wash, rice cleaning the compressor, etc.). Typical non-recoverable performance degradation is approximately 5% for output, and 2.5% for efficiency at approximately 20,000 fired hours. Of course, this varies somewhat for different site conditions, different operating conditions, and different fuels, but these are good averages for non-recoverable performance degradation for most operators. Approximately 2/3rd’s of this performance degradation is due to compressor issues, and 1/3rd is due to hot gas path issues. About half of this non-recoverable performance loss can be recovered during a Major inspection by proper maintenance and repair processes. This article will discuss compressor and turbine section performance degradation in separate sections.
Compressor Performance Degradation
Compressor performance degradation has many different causes:
- Dirt/Oil Fouling the Compressor Blades. Dirt from the inlet air due to inadequate air filtration will collect on various parts of the compressor resulting in rougher surface finish, and more power being required to pump the air thru the compressor, and less air flow. This gets compounded by frequent oil leaks from the #1 bearing, resulting in a sticky film that retains more dirt. This performance loss is referred to as recoverable performance loss, as it can be regained by frequent off-line and on-line water washes. On-line wash would typically be done with cold water (potable water) and at a flow rate of 7 to 10 GPM thru small spray nozzles mounted in the inlet casings. This would be done during normal operation, at any load. Off-line wash would be done with the unit on crank mode (approximately 20% speed), with hot water (potable water would be ok, but, some operators use boiler condensate water) some operators also use a soap compound mixed with the water, especially if oil deposits are present. Care should be taken to make sure the recoverable compressor performance degradation does not exceed 5% so as to avoid compressor issues.
- Compressor Blade and Casing Surface Finish Degradation. Frequently local environment air will include different types of chemicals that may cause corrosion on the compressor blade surfaces. Such corrosion will result in non-recoverable performance degradation that can only be recovered during a major overhaul (new blades, and/or polishing each blade). Typical atmospheric corrosion causes include ocean salt air, chemical plants, fertilizer, etc. Most of this can be controlled with proper inlet filtration.
- Wear on the Labyrinth Seals at the back end of the compressor (this seal is intended to provide a measured air flow from the compressor discharge, to the forward wheel space area for cooling). This is harder for most operators to fully understand. Average new unit clearances for the High Pressure Packing (HPP) labyrinth seal is approximately 40 mils, but, average clearance when units are opened up for the first Major Inspection (MI) is about 100 mils. The performance derivative for HPP clearance is 20 mils increase in clearance will be a loss in output of 1% (or 3% for the increase from 40 mils, to 100 mils). This performance loss would be classed as non-recoverable performance degradation as it cannot be corrected without a Major Inspection overhaul. This increase in HPP clearance can be due to many factors including rotor alignment or significant vibration. But, the major source of high HPP clearances is the over speed testing that is frequently done during commissioning to calibrate the over speed instrumentation. The sudden trip when the unit reaches the over speed limit will frequently result in significant rotor vibration (and consequently rubs on the HPP seal). All of the above can be addressed during commissioning and operation with much care by the operators, but, otherwise, significant non-recoverable performance degradation will result
Compressor Blade Coatings. Smooth compressor blade coatings are offered by some OEM’s and also by third parties. Such coatings are good, for a short time, but, will also suffer from the same types of compressor blade fowling and corrosion as the coatings get removed by erosion, due to dirt in the inlet air. So, if the operator needs short term performance recovery after an outage, such coatings may be of use. But, if the customer needs a permanent performance improvement, the cost of the coating may not be a good investment.
Compressor Clearances. All gas turbines some with a set of recommended clearances for all blades and seals. All these clearances should be carefully measured when the turbine is opened for maintenance, and again, just before closing the turbine after the maintenance. All clearances should be carefully recorded in the maintenance reports. The frequent answer that “we took all the clearances and did not record then, because they were all within spec.” usually means that no one actually took the clearances, and/or that many were out of spec. After the unit is closed up, and operating, it is too late to correct any issues related to out-of-spec clearances. Typical performance loss when all compressor stages have a 20 mil increase in clearance is 1% in output, and efficiency.
Foreign Object Damage. Foreign object damage is when solid objects (nuts, bolts, parts of silencers or inlet filters, etc.) enter thee compressor during operation. As these objects bounce around going thru the compressor, they can cause considerable damage to airfoil shape, and in some cases cause liberation of compressor blades. Actual damage and impact on performance are dependent on the extent of damage. A significant outage would be required to correct the damage.
Compressor Bleed Valve leakage. Most gas turbines designed since the early 1990’s have bleed valves to bypass some compressor air during unit start up. This is to minimize possibility of compressor issues such as surge, stall, and stress during start up. On older units, sometimes these valves stick in the open position, and continue to allow air to bleed off during operation. This should be easy to check, as the air going thru these valves usually creates a significant noise. If the sticking valves cannot be corrected by impact, the unit may need to be shut down to fix the valves.
Hot Section Clearance Performance Degradation
All hot section performance degradation would be classed as non-recoverable performance degradation. Different types of hot section performance degradation would include the following:
- Air Foil Surface Degradation. Similar to the compressor blades, the blade surface finish is critical to optimum performance. Typical reasons for degradation in turbine blade surface finish are ash deposits from using heavy fuels, corrosion from contaminants in heavy fuels, and liquids in gas fuels. Significant surface finish roughness can result in up to 2% loss in output. Repair, replacement, or polishing of hot section airfoils should be considered during each HGP or M inspection to recover lost performance, as well as to maintain mechanical capability
- Turbine Bucket Tip Clearance. Turbine bucket tip clearance is the easiest to measure when taking opening clearance checks. Typical bucket tip clearances are large—up to 80 mils in many cases—so as to avoid rubs during operation. Thus, by design, a lot of air flows over the tips of the buckets—a necessary performance loss, as bucket tip rubs sometimes result in significant damage to the unit. But, any rubs over the OEM’s recommendation, will lose significant additional performance. Increase in stage one bucket tip clearance can result in output loss up to 1% and efficiency loss of 0.5%. Typical reasons for bucket tip rubs are rotor alignment and high vibration events.
- Labyrinth Seal Wear. Labyrinth seals are used in several locations in the hot turbine section: stage 2 nozzle inner diaphragm, #2 bearing, and the bucket tip clearance for stage 2 & 3 buckets. These seals can be damaged during high vibration events, or due to rotor alignment issues. Performance loss can be up to 0.5% for 40 mils rub on the stage 2 nozzle diaphragm or on the stage 2 & 3 bucket tips. These seals can only be repaired during a HGP of MI maintenance activity.
- Stage One Nozzle Throat Area Increases. Stage one nozzle area is critical to performance as any change will not only hurt performance due to surface finish issues, but, it will also impact compressor discharge pressure for a given air flow. The relationship between compressor discharge pressure and exhaust temperature is typically how the control curves are set up. So, any change in stage 1 nozzle throat area will result in performance loss.
- Bearing Seal Wear. Bearing seal wear will result in additional air flow leaking into the oil drain lines and thus, less air going thru the turbine to generate useful work.
- Foreign Objet Damage (FOD). Similar to FOD n the compressor, FOD in the hot gas path section will result in performance loss. FOD in the hot section can be from pieces of combustion hardware upstream that was liberated due to a failure. So FOD of stage 2 and 3 HGP parts can be from failed parts of stage 1 HGP components. Any FOD in the HGP section usually results in unit shut down, and significant repair.
Summary
Many of the above performance degradation issues can be controlled by the operators with care during operation. But, many others can only be addressed during Hot Gas Path of Major inspections, with proper repair processes. There are also numerous advance tech sealing improvements available (brush seals, cloth seals, honeycomb seals, abradable coatings, etc. that can have a significant impact on reducing performance loses due to rubs. The Keck Group International is available to help any customers evaluate their performance degradation, evaluate condition of all turbine parts during maintenance activities, to provide training to operating personnel on best operating practices to minimize performance loss, and/or to help with repair practices aimed to minimize performance loss. For contact information: