Steam cooling hits the mark

Nearly a decade of operating experience with Mitsubishi Heavy Industries’ G Series gas turbines has quelled operators’ concerns over the reliability of large framed industrial gas turbines using steam cooling technology.

Brent MacDonell, Paul McDonough, Boston Generating Co., Charlestown, MA, USA
Ken Boral, Mitsubishi Power Systems, Orlando, FL, USA; and
Yasushi Fukuizumi, Mitsubishi Heavy Industries, Takasago, Japan

Commercial implementation of steam cooling technology for large frame industrial gas turbines has cleared power plant users’ initial concerns on the plant’s reliability and availability. A considerable number of large scale combined cycle plants built on the steam cooling concept are benefiting from higher combined cycle efficiencies and reliable operation even in cases where economic considerations impose daily start and stop operation.

Mitsubishi’s G class steam cooled gas turbine, which applies steam cooling to stationary components, is approaching the industry milestone of one decade in commercial operation. Since the first unit’s commercial operation verification test in 1997, at the MHI T-point combined cycle power plant, an additional 17 units are operating in both 60 Hz (M501G) and 50 Hz (M701G) regimes. The 18-unit fleet has accumulated more than 280 000 actual hours operating in various ambient conditions ranging from the tropical climate of the Philippines to extreme cold weather conditions in New England. The two 50 Hz lead units operating in Japan have accumulated more than 42 000 actual operating hours each.

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Fifteen of these units have completed their first scheduled inspections, with the hot gas path parts found in good condition exhibiting expected wear and tear.

In the USA there are 11 Mitsubishi M501G units in operation in four different plants. These plants dispatch under very different operating requirements ranging from base load type regimes to daily start and stop operation. The high availability reported on MHI G gas turbine fleet has been a prime contributor in relieving the concerns the industry had regarding steam cooling technology applied to gas turbine stationary components. In the case of the M501G, it is applied to cool the combustor liner. In more recent upgrades the row 1 blade rings are also steam cooled.

Operational experience

The six MHI G units installed in Boston Generating Co’s plants in New England, USA, were the first to be installed in the United States. These are located at the Mystic plant and the Fore River plant. All six units began commercial operation in 2003 and the leaders have accumulated around 18 500 actual operating hours.

Boston Generating operation and maintenance teams consist of highly motivated professionals that actively participated in the construction and commissioning of the plant. Through active on-the-job training, the staff gained detailed knowledge of the operation and maintenance of the plant, contributing to the very efficient operation of the power station.


Figure 1. Schematic representation of the steam cooling circuit within the BOP circuit
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The staff’s strong desire to effectively run this plant is evident by their implementation of modifications to the original DCS operating screens that increases functionality and ease of operation. This includes a clear identification of start-up permissives and quick and simple access to manually operated load runbacks in case an operational disturbance requires quick action.

The advantage

Steam cooling provides a dual benefit over air as a cooling medium. In addition to better heat transfer characteristics, the use of steam cooling reduces the air usage for cooling purposes. As a result more air is available for the combustion process, which contributes to improving emissions. Additionally, reduction in cooling air results in less temperature dilution of the hot gas caused while mixing with the cooling air.

The cooling steam is reheated as it flows through the steam-cooled components in the gas turbine. The reheated steam is then directed through the intermediate pressure (IP) steam turbine where the energy collected is used to generate power, increasing the overall combined cycle efficiency.

In order to avoid thermal shock of the steam cooled components, there is no transition from air cooling to steam cooling in Mitsubishi’s steam cooling system. The steam cooling circuit is warmed before start-up by means of a fully automatic sequence. A unit start permissive is imposed on the system to ensure the steam cooling circuit has been evenly heated and drained of any condensate. The condition of the steam-cooled components during inspections confirms the advantages of avoiding thermal shock.

An auxiliary steam supply is used to provide this warm-up steam in addition to steam for the ejectors and steam turbine gland seals. After starting one of the machines, the plant provides auxiliary steam without external source requirements. At Mystic power station, the initial auxiliary steam supply is provided from an existing conventional plant and at Fore River there is a 28 120 kg/hr intermediate pressure superheated auxiliary boiler.

Mitsubishi steam and water purity requirements are the same for the steam turbine and the gas turbine cooling steam. These requirements were established after extensive evaluation of the effect of impurity build-up in the gas turbine steam cooling circuit and the compatibility of the material to steam atmospheres.

Start-up reliability

A look inside the gas turbine enclosure, especially around the combustors, reveals a relatively simple system. Fuel staging is not required, resulting in only two fuel gas supply circuits. The combustion dynamics condition is monitored using Mitsubishi’s CPFM system (Combustor Pressure Fluctuation Monitoring), eliminating the need for flashback thermocouples.

The starting reliability of Mitsubishi’s G class gas turbines has proven excellent. The ignition system relies on two very dependable igniters, proven during years of service in previous generation models. The key to high availability and high starting reliability of MHI G turbines lies in the robust and simple design.

As the number of operating units in the fleet grows and design improvements are validated at the T-point validation power plant, upgraded parts are retrofitted to benefit from proven higher durability designs.

Availability

The US fleet availability numbers recorded for the combined cycle power train equipment ranges from 99.9 per cent to 93.7 per cent. The 99.9 per cent corresponds to a plant that does not operate continuously and has not undergone scheduled inspections to date. The 93.7 per cent relates to operation under a heavy cycling regime.

Gas turbine steam cooling involves the integration of the cooling circuit within the combined cycle balance of plant (BOP). For this reason, the plant design must consider disturbances in the BOP and the resulting potential disruption of steam cooling supply. A redundant steam cooling supply is included to provide back-up steam in case of interruption in the steam supply. On several occasions, both of Boston Generating Co’s plants have experienced steam supply disruptions due to problems in steam by-pass valve operation. These incidents could have been prevented by using better quality valves.

The Boston Generating Mystic and Fore River power stations have Long Term Service agreements with Mitsubishi, which includes availability warranties for the gas turbines, gas turbine generators, steam turbines, steam turbine generators and their respective auxiliaries.

The gas turbines and steam turbines of both power stations have achieved high availability while operating under strict stack emission limits. Allowable stack emission values in the Boston area are based on the stringent hourly average and include NOx and CO corrected to 15 per cent O2 at no greater than 2 PPMVD. These low stack emissions levels are achieved by SCR systems for NOx and CO oxidation catalysts installed in each heat recovery steam generator. Low emission values are also imposed on ammonia slip of no greater than 2 ppm on an hourly average basis These emissions limits are achieved between 56 per cent to 100 per cent gas turbine load.

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The availability of the power train based on the IEEE definition has been as high as or higher than industry reported availability for non steam-cooled units. Table 1 shows the specific values for both Boston Generating units. The availability of all units during the first year in operation was close to 99.8 per cent. The second year for Mystic plant including the scheduled inspection time, the availability presented an average higher than 95 per cent.

Remote support

The first and second scheduled inspections of the gas turbines were completed ahead of the LTSA scheduled timing to take advantage of seasonal load demand cycles. During the first and second scheduled inspections the combustor and hot gas path components were inspected and found in good condition. A few standard weld repairs were performed on the combustor and turbine parts. The steam-cooled transitions did not present any steam leaks or obstruction of the cooling passages.

From day one, these plants were connected to Mitsubishi’s 24×7 remote support center facilities in Orlando, Florida and Takasago, Japan. Boston Generating operating staff maintain excellent communication with the remote center staff and benefit from RMC quick support.

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The standard combustion dynamics protection system (CPFM) originally installed in Boston Generating plants is also connected to Mitsubishi’s remote monitoring center. This system has maintained closed surveillance of the combustion dynamic behaviour and has also been effectively used for combustion tuning at site.

Improved components

The continuous operation of Mitsubishi’s validation plant at T-Point has resulted in improvements that are retrofitted to fleet units, including Boston Generating Mystic and Fore River power stations. The introduction of such improvements is carefully coordinated by Mitsubishi LTSA team and Boston Generating Maintenance staff for implementation during scheduled inspections.

The M501G has been upgraded to the M501G1. This upgraded design includes steam cooling application to blade rings to optimize blade tip clearance during start-up and steady state, improved aerodynamic blade and vane profiles and MHI’s latest low NOx combustor technology which includes new combustor fuel nozzles and swirler holders.

A 1×1 combined cycle plant based on the M501G1, which is the 60 Hz upgraded version, is currently under construction in Portland, Oregon, USA. It is scheduled to start commercial operation in May 2007.

Reference

Brent MacDonell, Paul McDonough, Ken Boral, and Yasushi Fukuizumi: “Operational Experience of MHI G Series Steam Cooled Gas Turbine in the USA” Presented at POWER-GEN International 2005, Las Vegas, USA, December 2005.

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