Gas Turbine Compressor Degradation

Contamination and Erosion

All gas turbines experience losses in performance with time and the compressor has a significant impact. In a typical heavy duty axial compressor a 1.0% loss in compressor efficiency will create a 1.1% loss in output.

Compressor fouling is a serious concern and can be mitigated or recovered through proper operational practices. Dirt, oil and debris in the front stages of the compressor can result in a loss of mass airflow, and contamination in the last stages can result in power-robbing drops in the pressure ratio. These foreign objects can also erode or damage surface finishes and airfoil geometry, resulting in reductions in airflow and pressure ratios. Water-washing can partially clean contaminated blades, but recapturing full efficiency can only be achieved by opening the unit and mechanically cleaning the surfaces and replacing damaged components.

While the unit is open, the compressor can be coated to make the airfoils less susceptible to dirt and debris and increase the ability of the water wash to thoroughly clean the airfoils. Adding filtration to the inlet also helps maintain a clean compressor.


In a typical heavy duty gas turbine compressor section, air is compressed to many atmospheres pressure by the means of a multiple-stage axial flow compressor. The compressor design requires highly sophisticated aerodynamics so that the work required to compress the air is held to an absolute minimum in order to maximize work generated in the turbine. Any changes to this precise geometry can materially affect performance.

Air leakage through and around components significantly rob performance. One example is a bleed valve which remains open during operation. Another would be a leak at the 4 way joint. While sealing the horizontal and vertical joints are necessary as the machine ages and the casing warps, sometimes all that can be done without purchasing new casings is to manage the leakage. Leaks are more costly to the aerodynamic cycle at stages further down the axial compressor. A leak at an early stage might not be worth the cost of repair.

At the tail end of the compressor rotor is the inner barrel, which provides the inner diameter flow path and the internal support for the exit guide vane (EGV’s). On the internal surface of the inner barrel there is a labyrinth seal called the high-pressure packing seal. On field inspections we often find significant rubbing of the rotor to the labyrinth seals of up to 90 mils. This excess clearance and thus increased airflow results in a loss in performance.

This leakage can be minimized by retrofitting the high-pressure packing seal area with a wire brush seal. The wire brush seal is flexible and will deflect (not wear) if it does contact the rotor. The bristles of the brush deflect in the direction of rotation so that a closer effective clearance can be maintained. The seal even remains intact during transient events where some vibration occurs. Also, there will be less performance degradation over time since the wire brush will bounce back to the original configuration after contact. These losses can only be repaired during an overhaul.


Air temperature and pressure can seriously affect performance. Since the gas turbine is an air-breathing engine, its performance is changed by anything that affects the density and/or mass flow of the air intake to the compressor. When measuring performance degradation over time, remember to correct for changes to the reference conditions of 59 F/15 C and 14.7 psia/1.013 bar. Differing ambient air temperatures affect the heat rate. Correction for barometric pressure is more straightforward. A reduction in air density reduces the resulting airflow and output proportionately, but the heat rate and other cycle parameters are not affected.

Humidity is an often overlooked factor affecting performance. Humid air, which is less dense than dry air, also affects output and heat rate. In the past, this effect was thought to be too small to be considered. However, with the increasing size of gas turbines and the utilization of humidity to bias water and steam injection for NOx control, this effect has greater significance.

TGM can help you assess your unit’s existing performance versus its original design and establish a performance measurement process to accurately capture decreases which could indicate the onset of serious problems.

Turbine Generator Maintenance can help you achieve your goals in restoring your machine to its new and clean condition or upgrading its performance to achieve higher output, lower emissions or both.

Set IGVs for Maximum Efficiency

Have your combustion turbines been losing power after overhauls? There are many factors which can affect power production, but the IGV settings are one area that even the OEM can overlook.

We recently helped a plant with three GE Frame 6B gas turbines that had lost power over successive overhauls. Units 1 and 2 had been overhauled by the OEM in the last few years and Unit 3 had been overhauled in 2006 by a competitor. All three units had been experiencing an unexplained loss of power.

A gas turbine needs to breathe air – and a lot of it – to make horsepower. The inlet guide vanes (IGVs) on a heavy duty gas turbine are designed to modulate (open and close) in response to commands from the control system to regulate this air flow. These commands control turbine exhaust temperature, protect against a compressor “stall” or “surge” (extremely damaging to the compressor blading), and other controlling functions. The IGVs look like little airplane wings that rotate or pivot to allow more or less air into the compressor. They are calibrated to the turbine control system by measuring the actual vane angles with a machinist’s protractor and inputting the readings into the control system. This lets the electronic controls know physically where they are so that the system can properly control the unit.

On checking Unit 1 and 2, the IGV calibration was significantly out of calibration. Unit 3 wasn’t as bad but it was also slightly out of calibration., We accurately calibrated the IGV’s to the OEM control specifications on all three units.

The results of this work were impressive. The heat rate (fuel efficiency) improved on Unit 1 by about 2%, gained 2.5 mw on Unit 2 and 0.5 mw on Unit 3. This made a significant contribution to the customer’s bottom line at the expense of just a few days of work.

The IGV calibrations are just one of the critical instruments or calibrations that could affect power production. Contact PSG® for a full analysis of any reductions to the heat rate or power output of your turbine

Power Services Group (PSG) – the Best Alternative to the OEM for Alstom GT24 Gas Turbine Maintenance

During the first half of 2017, PSG successfully performed a number of maintenance activities at combined cycle power plants with Alstom GT 24 Integrated Cycle System (ICS) turbine-generator equipment.  GT24 work executed by the PSG team for multiple customers throughout the continental U.S. and Latin America:

  • Generator field assembly and owner oversight
  • SSS clutch replacement
  • Bearing # 12 investigation and replacement
  • Owner Oversight during triple C-B-B-Inspection
  • Owner Oversight of HP and IP-LP rotor rework at OEM shop
  • GT 24 assembly supervision
  • Balancing and vibration engineering services

PSG’s leading Subject Matter Expert on Alstom GT 24 ICS equipment, Bob Fischer, comments “The demand for GT24 maintenance services is dramatically increasing as the OEM experiences a shortage of skilled and experienced resources and customers are looking for cost effective alternatives.   With the GT24 fleet reaching maturity, more owners are searching to reduce their OEM dependency.”

If you are interested in starting a discussion on how PSG can help you be successful with our gas turbine maintenance options, please visit our website to request more information, or email us direct at

Proper Bearing Loading Helps Reduce Turbine Equipment Failure

The main reasons for bearings to wipe are:

  1. Insufficient lube oil supply
  2. Low lube oil pressure
  3. Water in the lube oil.

Every once in a while a fourth cause appears: High bearing loading.

Proper bearing loading is calculated by the elevations of the bearings, component weights and shaft alignments (bending moments, lateral, torsional). These forces have a direct impact on the stress of the equipment and thus, need to be a part of the equation.

The OEM calculates the elevations and coupling alignments during the design process, based on the catenary curve (or sag chart). Calculations of bearing loadings and alignment are usually accurate based on the design engineers’ mathematical calculations and computer model for the rotor’s geometry, speed, weight, and bearing design.

The following Catenary Curve graphic depicts bearing loading:

Proper Bearing Loading


Most of the time, high bearing loading is caused by misalignment of the turbine power train from the original design. That is, some force has moved the components from their original alignments. This may include temperature, rotor vibration and other forces associated with the wear-and-tear of the operation.

The source of the bearing failure can be eliminated by carefully measuring and re-aligning to the original specifications. We have also seen examples where the original calculations were not accurate or after years of operation, the bearing pedestals had moved.

Recalculating bearing loading is an arduous and potentially expensive process, so all other contributing factors should be eliminated before attempting this course.

It is important to maintain proper bearing loading to avoid the worst-case-scenario of any impact damage, equipment fatigue, or even catastrophic malfunction. Preventative maintenance can help you be proactive and reduce any bearing challenges that can cause a more severe event in your operation.  On three bearing units, it is not uncommon to utilize a dynamometer to check bearing loading during alignment and their adjustments.

PSG can perform the recalculation and re-alignment without the participation of the OEM.  If you believe your turbine may be affected by any of the above mentioned reasons for bearing wipe, please contact us today for a free assessment of your current situation.


Safely Check for Gas Turbine Fuel/Air Leaks

Traditionally operations and maintenance personnel have used gas detectors to check the fuel pig tail flanges for leaks during the startup process. This is to ensure that no fuel is present that could cause a fire in the compartment. An less risky alternative to this process is to perform a soapy water check while the unit is on crank. A pressurized garden sprayer is filled with soapy water and while the unit is being cranked and sufficient air pressure is present the entire unit is sprayed down with the soapy water mixture. Not only will this produce evidence of leakage at the fuel flanges but will reveal any air leaks on the can bases, the wrapper four way joints, the primary fuel covers or anywhere else air can escape.

Since this a relatively simple and safe procedure TGM® recommends performing this check prior to any maintenance being performed (after the unit is shut down and just before the LOTO is initiated). Any and all air leaks can be identified and they can be addressed during the scheduled maintenance cycle if feasible.

There have been several TIL’s released on the care and quality of the flexible metal hoses for CT’s. The end user has gotten into the practice of having the flexible metal hose pressure checked during the hot gas path cycle. If the soapy water checks are performed prior to any work being performed and any of the flexible metal hoses are leaking then why bother to have them pressure checked. This approach can save expensive labor cost by eliminating a test on a hose that will fall out. Plus wouldn’t it be better to know you have a leak at the wrapper four way joint before it is removed to perform a hot gas path than finding out during the inspection at start up?

Compressor Failures – Stator Vane Lock-up

Some compressor failures have been attributed to “lock-up” of the stator vanes. The vane roots are designed to rock slightly at their roots when moved with your hand. Rust and debris can inhibit this movement. An immobile vane changes the stress profile on the vane which can cause cracking and potential failure. Mechanics should check for proper movement at each inspection.

Removing locked-up stator vanes has been a challenge for service providers. The OEM has come up with material upgrades to address the corrosion problems in these areas. The compressor upgrades are designed to reduce the potential for compressor failures and lock-up of the stator vanes. Some providers use destructive methods to cut out the vanes, potentially damaging the compressor casing. The OEM recommends the use of their (expensive) special tooling to reduce and hopefully prevent casing damage. An alternate method that TGM®has used very successfully is a process of heating and quenching. Heating the locked-up stator vane segments with a torch and quickly quenching the vane with cold water will typically free the vane segment for removal without damaging the casing. The process of heating and quenching (applied by an experienced team) can save time and money while reducing and hopefully eliminating damage to the compressor casing. TGM® can provide its expertise in removing stubborn stator vane segments and, if needed, quickly replace damaged stator vanes to get the customer back on line as soon as possible.

TGM®‘s experienced combustion turbine Technical Directors and crews often delight customers by providing innovative methods that can efficiently and permanently solve issues and get the customer back on line to make power. Early detection of shim migration and stator vane problems can be performed by TGM®through borescope inspections and eddy current NDE.

TGM®‘s comprehensive borescope diagnostics can provide recommendations and solutions to address many other compressor issues. Contact Us to find out how we can help.

Protect your turbine with a good coat

Thermal Barrier Coatings (TBCs) protect the first several rows of hot gas path parts from the high combustion temperatures in many advanced large frame turbines. Not all coating applications are equal, and some can even reduce the efficiency of your unit.
TBCs are designed to reduce the temperature of the buckets and stators while providing resistance to corrosion and reducing oxidation of the component. TBCs form aluminum-oxide and chromium-oxide scales and act as a physical barrier to reduce component temperatures, extending the life of the parts. These TBCs are subjected to mechanical stresses, and spallation (coating separation) can occur. If a significant amount of TBC has separated from the metal, the parent metal will be exposed to the hot combustion gases and component degradation will be accelerated.
Hot gas path components should be inspected for TBC spallation at every Hot Gas Path or Major outage. Streaks of brown lines that appear to be coming from the cooling holes are a good sign that the component is receiving adequate cooling air, and spallation is at a minimum. When receiving components from the repair shop it is important to carefully inspect the components for an even coat of TBC. Also inspect the cooling air passages for debris and proper sizing to ensure that proper air flow can pass through the tiny passages for designed cooling.

When a water wash fails to recover efficiency…

If a combustion turbine experiences reduced power output and heat rate, the usual suspect is compressor fouling. But what if cleaning the compressor through on-line and off-line wash is not enough to recover lost compressor efficiency?

Some users experience no gain in gas turbine output or improvement in heat rate even after a Major outage. A likely suspect is the first row nozzles; refurbished or degraded nozzles may be contributing to the problem. In many large combustion turbine frames such as the GE 7FA, the amount of compressed air used for cooling has a significant impact on gas turbine efficiency. The first row nozzles in a 7FA can use up to 20% of the air developed by the compressor for cooling. Turbine efficiency can be greatly reduced if these cooling holes are enlarged and use too much air for cooling. First stage nozzle cooling holes may not have been checked for hole dimensions during the refurbishment process. This can also affect your ability to tune low NOx combustion systems. The amount of air being used by first row nozzles affects the fuel/air ratio, making it more difficult to tune the unit.

The only solution is to remove the first row nozzles and replace with new or refurbished first row nozzles with correct cooling hole dimensions. It is also worthwhile to check the first row nozzles when they come back from the repair shop to ensure the cooling holes are not blocked and are the correct dimension before installing them into the unit.

Your fuel bill (and possibly your EPA permit) will thank you for it.