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MythBusters: Gas Turbine Surge Can't Happen to Me
Industrial gas turbines are designed to operate safely during startup, shutdown, and over a wide range of normal load and speed conditions. Within the gas turbine, the axial compressor does have aerodynamic surge and choke stability limits and, clearly, operation beyond these limits must be avoided.
The choke limit is unlikely to be a problem in a two-shaft gas turbine because its fixed first-stage turbine nozzle will assure a positive pressure ratio across the compressor for all running cases while single shaft engines at part load may operate in choke. However, the speed lines of an axial compressor map are relatively steep and, although the compressor is typically designed with sufficient surge margin at full load, any significant impedance to the flow through the gas turbine can move the compressor’s operating point into surge.
Thus, there are scenarios when the axial compressor can potentially surge and, although this is not a common event, when it happens it results in violent low-frequency axial vibrations that are often catastrophic to the gas turbine. Specifically, during axial compressor surge, significant damage potential exists for the seals, thrust bearings and even the blades.
In a surge event, hot combustion gases may flow backwards through the compressor, which is not designed for the resulting temperatures. These surge events can usually be avoided through proper maintenance practices and diagnostic monitoring of the gas turbine performance.
Thus, the most prominent causes for gas turbine axial compressor surge are discussed below:
- Degradation of the compressor from blade fouling, erosion, and increased blade tip clearances reduces its ability to overcome the head imposed by the gas turbine system and results in decreased surge margin. Compressor degradation can be limited using online and offline washing and proper inlet air filtration by reducing surface deposits, blade edge degradation, and rotor imbalance. Additionally, visual and borescope inspections of the rotor blades, tip clearance opening, and online trending of the pressure ratio should be regular performed to monitor the state of the degradation inside the gas turbine's axial compressor.
- A saturated air inlet filter creates a significant pressure drop. Almost all gas turbine air inlet filters are equipped with a pressure differential sensor and operators generally do a good job of monitoring and replacing filters as needed. However, sudden ingestion of solid particles, such as sand from a windstorm or dirt or cement dust from a nearby construction zone, can lead to rapid plugging of the inlet filter and a high pressure differential rise. To avoid having this type of event result in axial compressor surge, an underpressure protection device, such as an implosion door, should always be installed in the inlet system. This implosion door should have a switch that is wired into the unit control system and triggers an alarm and shutdown when actuated.
- When the hot section turbine first stage nozzles or blades degrade due to high-temperature corrosion or oxidation their ability to extract work from the gas mixture exiting the gas turbine combustor is reduced. Consequently, the gas entering the second or third turbine stage has a higher temperature and increased volume flow which can result in choking of the nozzles in these stages. This results in an increased back-pressure and a reduction in the surge margin of the compressor. Borescope inspections can usually provide a good indication of the condition of the nozzles and blades in the hot section. Clearly, surface corrosion should be carefully monitored and the root cause of it, such as fuel or air contaminants or fuel liquid drop out, identified and eliminated. Trending of the gas turbine exhaust temperature is also a good indicator of the performance loss of the turbine section.
- The use of fuel with a low heating value or the injection of steam or water into the combustor for NOx control results in an increased volume flow across the gas turbines first-stage nozzle and, thus, in a higher combustor pressure and reduced compressor surge margin. Since the overall air-to-fuel ratio in a gas turbine is very high, the impact on volume flow from using a low-heatingvalue fuel or combustor injection is typically not sufficient to directly result in axial compressor surge. However, in combination with other surge-margin reductions, such as compressor degradation, this may need to be considered. Thus, when switching to a new fuel with a lower Wobbe Index or retrofitting the combustor to steam and water injection for NOx control, the operator should always perform an analysis of how this may impact the axial compressor the surge margin.
- During startup the gas turbine’s starter motor speed, inlet guide vane positioning and combustion firing temperature must be carefully sequenced to avoid crossing the surge line. For example, overfiring at low starting speeds does not just cause overheating of the hot section, but can also result in an excessive part speed combustor pressure and, thus, reduced surge margin. The proper startup sequencing to avoid surge is programmed into the unit control by the manufacturer and should not be changed by the operator. If a faster startup sequence is required for operational reasons, the manufacturer should always be consulted before making any changes to the control settings. Also, because the proper inlet guide vane schedule is critical for surge avoidance during startup and at part load conditions, the functioning of the actuators and their correct angle settings should be regularly verified. Stuck inlet guide vanes, due to, for example, corroded guide brackets, arms, or actuator malfunction, can easily lead to severe gas turbine damage.
- Combustion oscillation, commonly called humming or rumble, is the result of periodic flame instability excitations being amplified by acoustic resonance into large pressure pulses. These pressure pulses can travel upstream into the compressor and cause periodic axial compressor surge. Combustor oscillations can be caused by many factors (Mythbuster, March/April 2009) and is usually associated with lean premixed combustion systems. In almost all modern gas turbines combustor oscillations can be completely avoided by proper combustor tuning.
The above are the primary reasons that cause gas turbine surges. By properly maintaining the gas turbine, using appropriate inlet filtration, and monitoring the degradation of compressor, turbine, and overall performance of the gas turbine, these dangerous and damaging surge events can be almost completely eliminated.
As a minimum the overall gas turbine power loss due to performance degradation should be monitored. If the output power is reduced by more than 7% to 8% when compared to its original performance, the gas turbine is not just operating inefficiently, but there is a good chance that the axial compressor’s surge margin is severely reduced.
