Part 3: Gas Turbine Operation – Troubleshooting Support
Summary: This is Part 3 of a discussion on Gas Turbine Operation with my guest, Robert Hawkins. In Part 1 we covered concepts related to the basic operation of Gas Turbines, which are important to understand for effective troubleshooting when issues arise. In Part 2 we covered issues that can occur with startup, shutdown, and combustion. In Part 3 we cover considerations related to support systems, compressor performance and water wash, turbine performance, and vibration monitoring
- Operational concerns with support systems such as lube oil, fuel, hydraulic oil, and more
- Compressor performance including online and offline water wash considerations
- Turbine performance and vibration monitoring
Guest Bio: Robert is a Lead Technical Support Specialist for Nexus Controls Remote Diagnostic Service team and has over 30 years of experience with steam and gas turbines, mechanical, and control systems. His past roles include Product Services Engineer, Instructor, and Field Engineer.
Why is it important to understand gas turbine troubleshooting of support systems?
In the first podcast, we covered the basic operation of gas turbines. Our second podcast covered common issues with startup, shutdown and combustion. This podcast, our third and final in the series, discusses troubleshooting of support systems – important components that have necessary applications but often receive less focus. Thus, we review support systems (compressor systems and water wash, turbine performance, and vibration monitoring) to provide the final piece of information needed for a broad understanding of gas turbine operation. Armed with this information, gas turbine operation can be better optimized while troubleshooting can be handled safely and quickly to mitigate risks and minimize operational disruptions.
In gas turbine operations, high pressure hydraulic oil operates the control valves on the fuel and air systems. If the hydraulic oil does not operate properly, then the control components will not operate properly, resulting in the gas turbine not operating properly. Thus, hydraulic oil system performance is crucial to proper gas turbine operation.
There are three areas of the hydraulic oil system that affect its performance:
- Pressure. Electric motor-driven pumps produce high pressure oil for the hydraulic system (approximately 1200 psi pressure). Pump operation is important in achieving and maintaining pressure, making it vital for proper valve operation.
- Accumulator. Pressure is maintained through transients by an accumulator, which is pre-charged and similar to a hydraulic spring. The accumulator is often overlooked by operators, but has an important role in maintaining pressure. For example, if there is a momentary lag in the motor-driven pump in producing pressure, problems can occur in the hydraulic oil system.
- Filters. The hydraulic oil system filters can become plugged and create second-order problems, so filter condition is also important to proper operation.
Did you know? Efforts are underway to phase out the use of hydraulic oil in gas turbine operations. Since oil is flammable, the intent is to eliminate its use. The future direction is to use electric motor driven valves. To-date, only fuel valves have been engineered to use an electric motor. While progress is being made, full replacement of hydraulic oil is not yet viable.
The compressor function is fundamental to the performance of the gas turbine. Compressor performance dictates unit efficiency, fuel economy, combustion system performance and overall operational parameters throughout the gas turbine operation. Let’s look at the components of the compressor that impact its proper operation.
Airfoils or air blades
Internal compressor components are known as airfoils or blades. The airfoils can become dirty, damaged or worn, resulting in decreased efficiency and performance. This manifests itself as a decrease in output pressure (compressor discharge pressure). Damaged or worn blading requires repair work to restore performance but in the case of dirty blading, to proactively mitigate dirt build-up and prevent baked on deposits that are more difficult to remove, water wash systems can be used to periodically clean the compressor airfoils.
Water wash systems
The two forms of water wash systems are Online and Offline. The online method is intended for use during unit operation. If air temperatures allow, the online wash can be used by reducing unit load, opening up the Inlet Guide Vanes (IGV) and then spraying demineralized water into the air path at the compressor bellmouth. The dirt is washed off and pushed further down into the gas turbine engine and becomes part of the exhaust gas.
Relative to the online water wash, the offline method is a far more effective way to clean the airfoils. As the name implies, it is used when the gas turbine is offline and cranking at slow speed. The cleaning fluid is water and chemical solvent (serving as the ‘detergent’) that is mixed into an emulsion and then sprayed into the compressor. When a deluge of emulsion is sprayed into the compressor, it travels all the way through the system since the unit is offline and not at high temperature, allowing the water to remain in liquid form rather than turning to steam as it eventually does in the online method. Thus, the detergent emulsion reaches all the stages of the compressor and cleans all of the compressor blades, by saturating the deposits to achieve more effective washing. The four steps of the offline water wash are:
- Wash - Detergent spray is used to help to remove surface deposits
- Soak - Detergent soak helps saturate and then loosen deposits
- Rinse - Demineralized water washes the deposits down through the casing and out the false start drain valves, which channel the dirt and deposits out of the unit. The rinse phase continues until the water begins to appear clean.
- Dry - To dry the compressor, the unit is simply started and begins to operate normally. Alternatively, the unit can be fired for 10-15 minutes to dry the water out of the system.
To achieve optimal levels of cleaning and more effectively restore compressor performance, a combination of periodic online and offline washing is recommended.
In terms of water wash frequency, it varies. For example, location-specific air quality can influence the wash schedule as industrially-compromised air quality necessitates more frequent washing.
For operations that leverage both online and offline water wash systems, the online water washes can be done on a more frequent basis (daily, weekly, bi-weekly, etc.) to help decrease baked on debris. To determine the timing of online water washes, operators can monitor compressor discharge pressure, which declines over time. Operators can select conditional pressure set-points to trigger the need for an online wash. After an online water wash is completed, the compressor discharge pressure should increase to standard operating levels. If it does not, then the offline water wash system should be deployed to remove deposits and more comprehensively and thoroughly clean the unit. Offline water washes can also be completed routinely each time a planned shutdown is scheduled.
As noted above, another condition that can impact compressor performance is the degradation of airfoil profiles over time or due to mechanical damage. While airfoil aging cannot be prevented, it can be monitored and evaluated so that operators can make an economical decision on when the airfoils should be replaced to fully restore compressor efficiency. The same methodology can be used to evaluate if mechanical damage is suspected and appropriate maintenance plans can be made.
Operationally, turbine performance is important. To proactively maintain turbine performance, older gas turbines may utilize a turbine wash, similar in concept to the compressor wash. This, however, is only necessary when the unit is operating exclusively on liquid fuel or burning a lot of liquid fuel, which puts deposits on the blade units.
More broadly, turbine performance can be checked using a performance monitoring support system. These auxiliary systems measure the exhaust pressure out of the machine, enabling operators to understand how the turbine is performing. Exhaust pressure needs to be as low as possible, with vacuum pressure being ideal. This ensures the gases are being drawn out of the turbine vs. putting up a pressure wall to reduce the expansion of gases through the turbine. If exhaust pressure climbs too high, a safe shutdown may be initiated for operational protection.
Rotating equipment such as gas turbines, use vibration monitoring to indicate performance of the unit. In gas turbines, seismic probes placed on the bearing housing are used to monitor vibration signals. When vibration levels increase, it is a direct indication that machine performance is changing. Alarm levels are used to alert operators to potential problem areas and help to provide operational protection. If vibration levels increase above the alarm level there is a protective trip.
We hope you enjoyed this conclusion to our three-part series on gas turbine operation. In summary, this series has covered the fundamentals of gas turbine operation, troubleshooting of primary functions such as startup, shut down, and combustion, along with troubleshooting of secondary-but-crucial support systems. Leveraging these knowledge areas helps to ensure optimal gas turbine operation and health. At Nexus Controls, we hope that our insight, based on years of proven experience, will be helpful to gas turbine operators around the world.