Gas

Gas Turbine Uprates

Plant considerations and pitfalls

Plant considerations and pitfalls

BY JASON ROWELL AND INDRAJIT JASWAL

Combined cycle plants, even those built with state-of-the-art technologies at the time of commissioning, drop in dispatch order with age. Aging plants suffer the adverse effects of performance degradation and are surpassed by newer plants utilizing the latest technologies. These plants can recover performance lost to degradation and, in many cases, even surpass their original plant performance through major upgrades to their installed equipment. Gas turbine performance upgrade packages are available for most common models, and their use is one of the best means of breathing new life into an aging plant.

Gas turbine upgrade packages are available to address equipment robustness, output, heat rate, fuel and operating flexibility, or any combination of these factors. For some plants, the impact on steam cycle equipment may be negligible, and the plant can fully realize the advertised performance improvements. For many plants, however, the impact on steam cycle equipment is significant enough to limit actual performance gains. In rare cases, upgrades can even result in worse overall plant performance. An extreme example of this is unaddressed plant steam cycle limitations that prevent the upgraded gas turbine from reaching baseload.

Plant operators should consider a number of equipment and critical system evaluations prior to implementing a gas turbine performance upgrade to better ensure the end result. This pre-upgrade evaluation will allow the operator to not only avoid preventable pitfalls but also realize potentially beneficial concurrent upgrades to other systems.

Gas turbine upgrade performance impacts

The two most important combined cycle plant performance variables are electrical output and heat rate. Electrical output is the electricity generated and exported to the end users. Heat rate is a measure of the efficiency of the plant at the generating electrical output. Heat rate is defined as the energy (i.e., fuel) required to generate one kilowatt-hour of electrical output.

Gas turbines contribute to combined cycle plant performance through the following four separate variables:

  • Electrical output.
  • Heat rate.
  • Exhaust flow.
  • Exhaust temperature.

Improvements to gas turbine electrical output and heat rate directly translate into improvements in combined cycle performance, but these variables cannot be changed without impacting gas turbine exhaust flow and exhaust temperature.

Many times gas turbine exhaust flow and exhaust temperature are combined into a single variable known as gas turbine exhaust energy. When installed in an open cycle (i.e., simple cycle) configuration, gas turbine exhaust energy is merely wasted to the atmosphere via the stack; however, when installed in a combined cycle configuration, gas turbine exhaust energy may contribute more than one-third of the total plant electrical output. In a combined cycle configuration, exhaust energy is recovered in the heat recovery steam generator (HRSG) to produce steam, which is then converted to electrical output through the use of one or more steam turbine generators.

Most performance upgrades meant to improve gas turbine output and heat rate also impact the gas turbine exhaust energy available to generate power; these include compressor upgrades to increase airflow through the turbine (and subsequently the exhaust flow into the HRSG), increasing gas turbine firing temperature (and subsequently the exhaust temperature in the HRSG) or installing more efficient components (which may lower available exhaust energy). Each of these upgrades directly impacts the steam cycle performance and thus the overall plant performance, equipment and system design margins.

Older combined cycle power plants can regain SSperformance lost to degradation and, in some cases, exceed the results they were originally built to achieve. Photo courtesy: Siemens

Plant system and Equipment considerations

Heat Recovery Steam Generator Gas Side Impacts

Significant change to the HRSG gas side temperatures in the reheater, superheater, evaporator and economizer areas can occur when exhaust energy increases from the gas turbine upgrades. Typically the materials used in the HRSG can accommodate such temperature changes, but adverse effects such as increased tube and baffle vibration, tube and fin erosion, expansion joint damage and excessive hot spots may result from the upgrade. It should be noted that gas side temperature redistribution can also negatively affect HRSG carbon monoxide catalyst and selective catalytic reduction system operation.

Higher exhaust stack flow rates may exceed the combined cycle plant’s air permit emissions limits. Additionally, the higher flow rates leaving the HRSG stack could increase the noise levels, although substantial increases that would require plant modifications (e.g., stack silencers) are unlikely.

HRSG Steam Cycle Operability and Safety Valves

Increased steam flow rates resulting from higher gas turbine exhaust energy reduce the holding time of the HRSG drums, which thereby reduces the HRSGs (i.e., drum level control) ability to operate through transient events. This reduction in holding times should be evaluated to ensure that the drum levels and pressure excursions can be adequately managed by the existing hardware.

Reduced operating margins may cause safety valves to lift or simmer during plant transients such as a steam turbine trip or runback event. HRSG drum, superheater and reheater safety valve capacities should be verified for adequacy if steam flow rates increase.

Steam Turbine

The steam turbine governs the performance of the overall steam cycle. Steam turbines are volumetric flow limited machines. The volumetric flow capability of the steam turbine, known as the swallowing capacity, sets the steam cycle operating pressure for a given steam mass flow rate. As the steam mass flow rate increases, the steam cycle pressure must increase to allow the steam turbine to swallow more steam.

Combined cycle plants are generally designed so that the maximum steam cycle operating pressure is equivalent to the steam turbine maximum allowable operating pressure (i.e., rated pressure). Steam turbine manufacturers allow their units to operate above the maximum allowable operating pressure only for short durations.

If the steam flow increase is beyond the rated swallowing capacity of the steam turbine, the steam turbine must either be bypassed or the turbine steam blade path must be modified to increase the swallowing capacity and maintain the operating pressure below allowable limits. It should be noted that a continuous partial steam turbine bypass will result in a significant plant heat rate impact and increased bypass valve maintenance. Some plants have chosen to part load the gas turbines to avoid exceeding the steam cycle design limits, but this tends to defeat the purpose of the gas turbine upgrade.

Although the pressure can be maintained at the steam turbine inlet by implementing any of these modifications, higher steam flows will result in higher operating pressures in the HRSG, condensate, feedwater and steam systems.

The M501J Gas Turbine. Photo courtesy: Grand River Energy Center

Condensate, Feedwater and Steam Systems

The higher gas turbine exhaust energy yielded by many gas turbine upgrades increases the HRSG evaporator steaming rates and the heat absorption in the superheater and reheater sections. This produces an elevated demand on the boiler feedwater and condensate systems because an equivalent amount of water must be supplied to the HRSG drums and attemperators. The pumps and control valves are the most impacted components in these systems.

The condensate and boiler feed pumps are typically designed with no more than 5 percent to 10 percent margin on either flow or pressure. Any margins beyond this are unwarranted for plants designed using modern analytical tools because large margins will result in excessive pressure drops across the HRSG drum level control valves. In some cases, the condensate and boiler feed pumps may require larger impellers and potentially larger motors to accommodate the increased flow and pressure requirements.

Increased water flow can reduce pump net positive suction head available (NPSHA) margins during transient events. Low NPSHA can cause elevated vibration, increased maintenance and equipment failure. Low NPSHA can be addressed by optimizing storage tank (i.e., hotwell or drum) water levels or be addressed directly through various pump modifications.

The drum level, gland steam condenser bypass and attemperator control valves may require modifications to increase their trim flow coefficients (Cv). Control valve manufacturers typically recommend a minimum pressure drop across the valve for best performance, but this may not be possible under the uprated conditions. The original manufacturer should be consulted for available valve trim replacement options to restore the loss of control valve authority. In some circumstances, a control valve body may be too small to allow a larger trim to be installed, and a valve replacement may be the most economical option.

Steam turbine generator bypass valve capacities and operating speed verification are critical to ensure that the valves are suitable for the uprated plant requirements. Valve operating speeds may need to be upgraded to prevent safety valve popping during large transients.

Heat Rejection Systems

Increased steam generation increases condenser heat duty and, likely, steam turbine back pressure. Rupture disk capacities may need to be upgraded because of the higher steam flow into the condenser. Although condensers are typically designed to accommodate the higher heat duty and steam flow of steam turbine bypass operation, the ability of the condenser to operate properly with the higher steam turbine exhaust flow and under the steam turbine bypassed conditions should be verified by the condenser manufacturer. Potential issues include condenser internals erosion caused by higher velocities, tube flutter and vibration; decreased deaerating capability; and transient issues with hotwell level control.Downstream heat rejection equipment, such as cooling towers and once-through systems, should be confirmed to operate sufficiently at the higher heat rejection requirements.

It should be noted that higher heat rejection requirements result in increased evaporation and makeup water requirements in plant cooling towers. Cooling tower drift losses will rise as evaporation increases and may exceed the plant’s emission limits. Cooling tower performance and fill replacement options should be investigated with the original equipment manufacturer (OEM) to ensure reliable cooling tower performance.

Generators and Electrical Systems

Gas turbine generators and associated generator step-up transformers are normally provided margins to accommodate future performance uprates, but steam turbines and their associated electrical equipment are not. The capacity of the gas and steam turbine generators and the downstream power evacuation equipment and transformers should be confirmed for the higher generation capacity. In the event this equipment is found to be undersized, an adjustment to the operating power factor may be considered. When this option is not sufficient, the OEM should be consulted for potential re-ratings to the equipment.

Where Upgrades Make Sense

Gas turbine upgrades are most easily accommodated in plants where gas turbine performance is supplemented so that the steam cycle is oversized compared to the base gas turbine capability. Supplemental performance may be gained through gas turbine inlet conditioning or supplemental duct firing in the HRSG. For such installations, the maximum steam cycle operating pressure is likely set by an operating case with the gas turbine performance supplemented. The installation of a gas turbine upgrade allows for offsetting this supplemental output with improved performance from the gas turbine.

For installations without any means of gas turbine supplemental performance, the steam turbine will likely be the limiting component. Steam cycle limitation may prevent the owner from realizing the full gas turbine uprate potential without first implementing steam cycle upgrades.

Conclusions

Gas turbine uprates can recover performance lost to degradation and, in many cases, even result in surpassing the original plant performance. Plants that were originally designed with supplemental duct firing are the best candidates for gas turbine uprates, because the duct firing can be offset by the improved gas turbine performance. Even in these plants, however, the cycle must be evaluated to confirm the gas turbine uprate is not limited by the steam cycle. Plants without duct firing may still be candidates for substantial gas turbine uprates, but they need to be evaluated on a case-by-case basis.

A screening-level evaluation completed prior to committing to a gas turbine upgrade can quickly assess the feasibility, total plant cost and schedule requirements to complete this major project. Thoroughly assessing potential impacts identified in the screening study will mitigate the risk of a gas turbine upgrade yielding disappointing performance or operability on an overall plant level.

Authors

Jason Rowell is associate vice president and Gas Power Technologies manager at Black & Veatch. Indrajit Jaswal is a thermal performance engineer at Black & Veatch.

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