How to prevent start-up delays in ‘new generation’ gas turbines

July 31 2013 - William E. Forsthoffer

The gas turbine, at first glance, appears to be a very complicated piece of rotating equipment. Part of the reason for this perception is due to the complexity and number of the various support systems involved. The accessory gearbox is a very critical piece of equipment, since it provides power take-offs to the majority of support system pumps, starters and blowers.

The availability of the gas turbine is a direct function of the support systems. Particular attention must be paid to the preventive maintenance (PM) and predictive maintenance (PDM) requirements of all the support systems to achieve optimum gas turbine reliability.

The actual systems present in a specific gas turbine design are a function of vendor design preferences, customer requirements and local environmental requirements. The details concerning each type of gas turbine support system are:

  • Inlet and exhaust system
  • Engine noise abatement
  • Control and protection
  • Lube and hydraulic systems
  • Cooling (engine external, internal and auxiliary systems)
  • Injection systems
  • Fire protection
  • Internal component cleaning
  • Accessory gear box

The inlet and exhaust systems provide the engine with an acceptable level of inlet air filtration, moisture removal and noise reduction. There are two basic types of gas turbine air filters in current use – pulse air type and conventional stage type. Pulse type air filters have gained wide acceptance in regions of excessive dust (desert regions) and in regions of very low temperature conditions. They are highly efficient and can be changed on line. Regardless of the type of air filter (pulse or conventional), filters are often ‘staged’ to meet local conditions.

Engine noise abatement

The inherent result of energy input and extraction from a gas along with gas velocities (in excess of 960 kilometers or 600 miles per hour) result in high engine noise levels. A highly sophisticated noise abatement system, along with an engine enclosure, is required to reduce the generated noise to acceptable levels.

The control system is the heart of the gas turbine, and is responsible for safe and reliable start-up, at-speed operation, shutdown, monitoring and protection. The lubrication and hydraulic systems continuously provide clean, cool lubrication and hydraulic fluid to the components at the proper pressure, temperature and flow. The lubrication system used for aero-derivative type gas turbines is different from that in an industrial gas turbine, in that a scavenge (vacuum) system is added in the latter, to return lube oil to the sump under flight conditions. The system is retained on mechanical drive applications of gas turbines. Industrial gas turbines use gravity drain methods for returning lube oil to the reservoir.

Cooling (engine external, internal and auxiliary systems) plays a very important role in engine reliability because of the high temperatures generated within the engine (in excess of 1,100 degrees C or 2,000 degrees F). Injection systems are required for pollution control and/or additional power (power augmentation).

High engine temperatures, and the close proximity of fuel and potential ignition sources within an enclosure, provide a potentially hazardous environment for the engine. As a result, a fire protection system is required in the engine enclosure. Moreover, available power can be significantly reduced by engine fouling (accumulation of dirt on air compressor blades and stators). Most gas turbines incorporate some type of crank and/or on-line cleaning system.

Accessory gear box

The typical accessory gear box connections for both industrial and aero-derivative gas turbines include an engine starter, main lube oil pump, hydraulic (control oil pump) for both types of turbines. Other connections are an automization air compressor and main fuel pump for liquid fuels (only industrial turbines), and scavenge pumps and air/oil separator (only aero-derivative turbines). Cooling water pump is optional for industrial turbines.

Remember, the engine reliability is a direct function of the reliability of each individual system component! Required PM and an effective PDM program is a must! Do not assume that a proven vendor’s gas turbine model will have sufficient field operating experience. Model generations (higher operating temperatures, new control systems and modified support systems) can and will change.

Failure to determine experience of gas turbine models and their support systems has led to significant start-up delays and reliability issues that can last for the life of the operating plant. Start-up delays can result in significant revenue losses. In addition, reliability issues resulting from unproven gas turbine components and/or their support systems can add to revenue opportunity losses for the life of the process unit. These issues can expose the end user to revenue losses in the hundreds of millions of USD.

Screening for proven model experience (including experience for all support systems) during the pre-FEED phase of the project to determine the acceptable vendors list is a must. This is a new best practice, added in 2010 as a result of the selection of a proven gas turbine from a vendor that had recently combined forces with a larger company. While the gas turbine design was maintained during the takeover, the proven control system was replaced by a ‘new generation’ system that was used for the first time. The result was a start-up delay of three months to correct the problems.