Power Engineering

Large Aero-Derivative Gas Turbines for Power Generation

By Amin Almasi, Rotating Equipment Engineer, WorleyParsons Services

The decision to use aero-derivative gas turbines is mainly based on economical and operational advantages. Modern aero-derivative gas turbine drivers will feature a new generation of offshore or onshore-emergency power generation trains. New innovations make these drive systems more economical, more flexible in their use and more reliable while seeking peak efficiency.

Advanced modern, compact aero-derivative gas turbines offer high performance and excellent efficiency. These modern drivers permit efficient control of torque together with potential for integrated and intelligent control/operation concepts. New aero-derivative gas turbine driver technologies will be major players to further mature the power industry.

This article discusses the latest drive concepts, major technical limits for further optimizing trains and advantages of new power solutions.

Common economic/operational advantages and benefits of the aero-derivative drivers compared to conventional heavy frame gas turbine drivers are:

  • A 10 to 15 percent improvement in efficiency.
  • Smooth, controlled start of the complete power string.
  • Higher availability and operational reliability.
  • Wide load range permits economically optimized power control.
  • Operational flexibility that allows better control.
  • Lower operating expenses due to reduced maintenance and spare part requirements.

Practical Notes On Aero-Derivative Gas Turbines

Modern aero-derivative gas turbines would be attractive options for offshore, floating or fast-response emergency power trains. Aero-derivatives gas turbine efficiency is typically around 45 percent compared to 30 to 35 percent for heavy duty industrial gas turbines (in both cases without steam recovery, in simple cycle). They usually need less than 50 percent foot-print and weigh less than 40 percent compared to heavy duty machines with same power rating.

Aero-derivative machines use most advanced aircraft engine technologies, facilitate on-condition maintenance, have numerous bore-scope ports, allow very intelligent/flexible operation and permit quick replacement of major modules (usually less than 48 hours).

One of the disadvantages for aero-derivative gas turbines are their relatively high initial cost.

Since aero-derivative machines use most advanced technologies and materials, they are expensive compared to heavy industrial gas turbines. Low initial cost was one of the main reasons for extensive applications of famous Frame 7 and Frame 9 gas turbines in traditional power generation modules. These heavy industrial large machines are typically offered at 70 to 80 percent of the initial cost per MW power, compared to famous, high performance aero-derivative models.

Within aero-derivative families, models that produced in large quantity usually are offered at more competitive prices. For example, the LM2500 is usually offered in slightly lower initial cost/MW installed power compared to other aero-derivative gas turbines. This data is based on normal/average market cases. Depending on the vendor situation, special project interest and many other complex technical, commercial and market factors exceptions could be expected.

Considering higher efficiency of aero-derivative machines, their total life cost (initial cost + fuel and operation cost) would be much less compared to heavy industrial frame gas turbines.

Low pressure air-compressor (LPC) vibration and corrosion problems have been reported as an issue for nearly all gas turbine models. Other important inspection and reliability areas are frame cracking, bearings issues, mandatory bore-scope inspection after any stall (particularly at high-pressure air-compressor (HPC)), pitting of the rotor shaft, performance loss and gas turbine transportation (including the new transportation system containers). Vibration analysis and monitoring are vital to avoid excessive dynamic loads, damages and premature failure.

The gas turbine expands when it becomes hot. This leads to axial and radial growth. It is necessary to accurately model various thermal expansions in gas turbine driven trains. Special attention should be given to coupling selection/sizing and thermal growth. The importance of coupling stiffness to manage axial, lateral and torsional movement should be emphasized.

Inlet filtration is very important. Sodium levels of inlet air should be kept below 0.003 ppm. Water content is to be kept at less than 0.5 percent of total inlet air flow weight. However, no water is to be allowed when ambient temperatures are below 5°C. Gas turbine packages should include a pre-filter (to be exchanged every six months on average), a main filter (exchanged once a year) and a HEPA (High Efficiency Particulate Air) filter 99.97 percent efficient at 0.3 microns. Uniform flow at the entrance of the radial inlet should also be emphasized. A stainless steel inlet silencer could keep noise level low (below 85 decibels).

Future Of Extra-Large Emergency Power Trains

Design of new power trains now require higher efficiencies and lower emissions as power industry continues to evolve in response to new challenges.

The key principles that have been guided and developed power generation units have always been:

  • Fitness for purpose, cost and economics. Currently minimum life cost is very important which lead to selection of high efficiency driver options.
  • High reliability and high availability.
  • Safety, health and environmental protection.

The above mentioned principles lead power train designs to "aero-derivative" concepts. This option offers better efficiencies, greater operating flexibility and higher performance trains.

All queries regarding new technologies of "aero-derivative" driven extra-large compression units should be answered before ordering equipment. Deviations to specifications must be clearly defined and understood before the contract is awarded. Changes should be kept to a minimum after a contract is awarded because they generally have a profound impact on delivery schedules and costs.

The life cycle benefits are dominated by four factors: capital cost, plant availability, project schedule and plant productions. The expectation is that the capital cost would be slightly higher for this new aero-derivative driver concept. But considerable improvements (efficiency, availability, reliability, etc.) would mitigate the impact of a slight increase in capital costs.

The trend of increasing power train size adds importance to optimal plot arrangement and equipment design. Ensuring suitable performance and lower operating costs requires a prediction of not only how gas turbine and all power plant facilities respond to changes in ambient conditions but also how they affect those ambient conditions (particularly ambient air temperature) near the power plant facilities. Modern aero-derivative gas turbines release less heat and pollution to the surrounding environment.

One of the problems with traditional heavy frame industrial gas turbines was the dependency to ambient temperature. New aero-derivative gas turbines are more flexible in this regard.

About the Author: Amin Almasi is lead rotating equipment engineer at WorleyParsons, Brisbane, Australia. He holds chartered engineer certificate from Engineers Australia and IMechE in addition to a M.Sc. and B.Sc. in mechanical engineering.

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