Converting to fast cycling

Converting to fast cycling

Kalyanaraman, Kalyan


At the 501F & G users group meeting in San Diego (Jan. 29 – Feb. 1), Siemens introduced an upgrade package (Figure 1) that could convert existing combined cycle plants designed for base-load duty to fast cycling plants. The package could achieve a 50% reduction in the start-up time of a combined cycle plant, depending on site conditions.

Operators can select the starting mode based on a “one-push-button” control. Features of the package could include the Benson once-through Heat Recovery Steam Generator (HRSG), and stress and fatigue monitoring systems. Online monitoring evaluates trade-offs between fast starts and the consequent reduction in equipment life, helping operators make economic choices.

Achieving fast starts

Giving a presentation on the fast-start package, Norbert Henkel and Edwin Gobrecht, managers at Siemens Fossil Generation Group, Mülheim, Germany, said that most of the 501F/G units were ordered and designed as base-load plants. But many of them are operating in intermediate or cycling mode at peaking or intermediate loads. The 2006 average for the service factor (operating hours/period hours) of the fleet was slightly below 45%, making operational flexibility a key factor, said Henkel.

Features of a flexible plant are:

* High startup reliability and load predictability

* Ability to handle fast load changes and load ramps

* Fast start up and shutdown

* Frequency control and ancillary services

Several factors limit operational flexibility; chief among them is the steam cycle and its impact on gas turbine starting. Unlike the gas turbine, the steam turbine has inherent restrictions on its ramp rate, Gobrecht said. “The HRSG may have a limited ramp rate on its pressure, the steam piping requires time to warm up, and steam chemistry requires time to be established.”

The automation and control system may not be configured for quick start up, and the interfaces between the Balance of Plant, bypass systems, and steam controls may not be designed for cycling. The G-class has hold times for establishing steam cooling of the transitions.

It is widely recognized that keeping the HRSG warm during an overnight shutdown will enable a fast-start in the morning. The Siemens upgrade package includes automated drains and vents, stack damper controls, and a stand-by heating system with auxiliary steam – measures that could keep the HRSG warm.

The packages also have a HRSG stress and fatigue monitoring system to observe and regulate through feedback the effects of fast starts on the HRSG. As part of the initial assessment before designing the package, lifetime evaluation of the HRSG will be done based on operating data, and the stress limits will be re-assessed based on a future operating scenario.

While conventional drum-type boilers can be retained in the upgrade, best results are achieved by using the oncethrough Benson HRSG, said Henkel. The Benson design eliminates the thick-wall High Pressure drum and allows an unrestricted gas turbine start up. Attemperators are used in the inter stage and final stage to condition steam for start-up.

A condensate polisher is integrated into the combined cycle to eliminate the waiting time for steam purity and to fulfill the chemistry requirements of the Benson HRSG. The polisher can also be operated during shutdown to clean up condensate impurities prior to start up.

Steam supply is maintained during shutdown to seal the steam turbine’s glands and hold vacuum. This also helps to prevent air ingress into the condenser.

The steam turbine’s flexibility can be improved by optimizing turbine stress limits and start-up criteria, based on existing operating data. Unnecessary hold times can be avoided, and the ability to restart from heat-soak-speed can be added.

The Siemens package incorporates a steam turbine stress controller, which, while speeding up the start, ensures minimum reduction in component lives. The stress controller optimizes the steam turbine inlet temperature for attemperation systems, and prevents load holds due to temperature mismatches.

The controller can provide fast and flexible start-up with three different modes (Figure 2). It carries out online stress and fatigue monitoring of all critical components.

The new start-up concept is based on parallel procedures. The gas turbine is hot-started, accelerated and synchronized to the grid at minimum load. Then the load is increased at the maximum allowable ramp rate to base load.

Exhaust gas is led through the HRSG. With the first steam produced in the HRSG, the steam turbine is accelerated and loaded.

Improvement in steam turbine start-up focuses on the warm-up procedure. The steam turbine needs steam temperatures at a certain value above the metal temperature. The exact value depends on the shutdown time. A longer outage leads to lower steam turbine temperatures. The steam temperature can be adjusted through the adjustment of the gas turbine exhaust temperature – through variation of the gas turbine load – as is done in base-load plants.

But for cycling plants, this is not acceptable due to requirements for operational flexibility and the chances of increasing gas turbine emissions at part load. “Adjustments in steam temperature are performed by the attemperators, which means that steam turbine requirements can be fulfilled at gas turbine base load, and the steam turbine does not limit the gas turbine’s ramp rate,” explained Henkel (Figure 3).

Other features of the start-up include steam turbine roll off with first steam production and early closing of bypass valves. Power availability is claimed to be significantly faster. The procedure leads to a start-up time below 40 minutes.

Designing the package

The upgrade process for existing plants includes an initial evaluation based on site assessment. Siemens representatives will interview site management and plant operators, and analyze current plant configuration. They will watch start-ups, and evaluate operating data. Current control logics will be assessed, and future operating concepts will be evolved and evaluated for their economic benefits.

The second phase is the development and implementation of the upgrade package. Components are evaluated for their remaining lives, and plant-specific controls are evolved. Equipment, such as final attemperators, and stress-control and fatigue-monitoring systems are retrofitted. The final stage involves commissioning and verification.

Copyright Turbomachinery International Mar/Apr 2007

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