Health management systems can be a great tool for saving your company money. A wide variety of industries use health management systems. Over the past fifteen years, pipeline companies introduced gas turbine health management systems to manage the high costs associated with compression equipment maintenance and repair. Some of these health management systems are site-based, while others are based centrally in the company’s head office.
Similarly, the power generation industry also adopted health management systems. There is a growing need for gas turbine power plants to operate in a load-following mode, particularly with the adoption of intermittent green energy sources such as wind and solar. In this mode, gas turbines may spend a significant portion of their life underfired. But when market conditions are favourable, an operator may elect to up-fire their gas turbine to obtain additional output during peak demand. In either case, maintenance intervals predicted by health monitoring to optimize the life cycle are more accurate than the assumptions of the OEM’s traditional schedules, allowing the operators to make sound business decisions.
Most often, by the time damage or wear becomes apparent, it may already be too late. If there is a step change in vibration or if unexpected alarms suddenly appear, then the damage may already be done. Even if the turbine is stopped before any severe damage occurs, operators can still face an unplanned outage resulting in high outage costs and potential performance penalties. OEMs design their maintenance cycles so planned maintenance takes place before unplanned maintenance is required. To cover their risks, they come up with maintenance cycles that can be overly conservative and require maintenance more often than what the gas turbine requires. The OEM assumes worst-case scenarios for engine power levels and the severity of starts and stops.
Liburdi's Gas Turbine Health Management (GTHM) system provides a more sophisticated approach to identifying signals characterizing equipment problems. By utilizing historical data gathered by the system, signals compare trends in previous readings and analyze shifts indicating a potential developing equipment problem. The GTHM program can identify performance trends which can help plan optimal times for compressor cleaning or indicate severe deterioration issues in the flow path over time.
GTHM also allows customers to plan gas turbine maintenance by calculating the rate of component deterioration (expected for the specific operation history seen by that turbine). This is particularly beneficial for gas turbines that do not always operate at full power. These turbines see substantially lower rates of component deterioration and could benefit from increased operational hours between maintenance cycles.
The GTHM system also has the flexibility to improve predictive capability over time. You can analyze historical data gathered by the GTHM system after encountering equipment problems. You can review data to indicate which changes in parameters correlate to the problem. These changes can then be trended on other units to prevent similar damage.
The GTHM system also identifies and automatically alerts operators of equipment operations outside of defined limits. These limits are based on calculations derived from numerous instrument readings. For example, the GTHM system sends alerts if driven compressors operate near choke or surge points.
Optimization of Maintenance Cycles
There are two ways a GTHM system can optimize maintenance cycles. Firstly, the system uses real-time operation monitoring data to calculate the units in actual operating conditions. It then stores collected data and compares the result with design conditions. Differences between the two are flagged for operators’ intervention. Secondly, the system accurately predicts the life of critical components through the damage accumulation leading to each potential failure mechanism. Thus it adjusts for on-condition maintenance to provide safe operation intervals for wide load swing.
Even if the critical function of the equipment is not in question, it may still be economically beneficial to perform gas turbine maintenance based on the condition of the equipment. For example, suppose that degradation in compressor performance was identified by the GTHM. Further investigation reveals the cause to be degradation of seal clearances. The GTHM system allows you to compare the cost of the performance degradation in terms of lost power and/or additional fuel consumption to the cost of performing the maintenance to correct the seals immediately.
The rate of damage accumulated in components of the hot section of the gas turbine establishes the maintenance intervals and component lives for gas generators. In pipeline applications, the limiting damage is typically caused by mechanisms such as oxidation and creep. These are the result of extended exposure to elevated operating temperatures. The rate at which damage accumulates is a function of the component temperature, which is directly related to firing temperature.
Engine manufacturers set overhaul and component replacement intervals based on the hours of operation with the assumption of anticipated damage at full load. Typically, these intervals are not adjusted for variable operating loads. In pipeline applications, many engines in the fleet operate at a fraction of base load. As a result, those components take on lower operating temperatures and stresses. This makes it possible to operate for longer intervals before replacement. In particular, damage rates are a strong function of temperature. For example, at the temperatures typical of first-stage blades, a 35 Celsius degrees decrease in metal temperature will increase the creep life by approximately 10x. To safely extend the life of hot section components, we must first establish the component's useable life under baseline operating conditions. It's also crucial to identify active damage mechanisms. The manufacturer’s recommended life can be a good starting point to determine useful life. Then destructive metallurgical examinations of components are required to accurately determine useful life in actual operation (and to identify which damage mechanisms are life-limiting).
Typically, a sample of each component is examined at the first several overhauls for the fleet leading engines of each design. The samples identify the damage mechanism and which components set the overhaul interval (typically the high pressure turbine components). Once the active damage mechanisms and safe operating life under baseline conditions are known, you can create a GTHM life prediction module that relates gas turbine engine operating conditions to component damage.
A health management system can be one of your best assets in maximizing your company's budget. A company can realize significant savings by delaying gas turbine maintenance activities until the life of pacing components is effectively consumed while also ensuring the components will be repairable at the time of removal.
If you're curious about optimizing the life of your engines and components and want to learn how to get the most life out of your repairs, contact dnagy@liburdi.com to start a conversation today.
