Heating-blanket technology advances

A cold steam turbine sometimes can compromise a combined-cycle plant’s ability to start up fast enough to profit from market opportunities. Several generating facilities have reported success in eliminating cold starts by installing heating blankets capable of maintaining their steam turbines in a warm condition (nominally 500F to 600F), thereby boosting revenue with faster starts while reducing fuel consumption and emissions.

But it turns out that eliminating cold starts was just a first step. A new warming system, installed a few months ago on a D11 steam turbine for an F-class combined cycle by Arnold Group, is said to maintain the unit in hot-start condition, providing the owner maximum operating flexibility.

The warming system and insulation typically can be installed in 10 days or less when the casing is properly prepared before Arnold arrives onsite. This means the old insulation must be removed by a local contractor and all pins ground down to the casing. All insulation fiber and debris must be removed from both the casing and enclosure.

Arnold’s Pierre Ansmann told the editors the company’s goal was to design an easily removable and highly efficient warming system using its proven single-layer insulation technology to minimize heat loss and maintain sections of the turbine shell at 700F to 900F, depending on location (Fig 1). The new system can take a cold turbine to hot-start condition in 2 to 2.5 hours.

The D11, equipped with 17 zones as shown in Fig 2, has been tested successfully at low temperatures to verify both proper operation of the heaters and thermocouple response. High-temperature full-capability tests will be conducted after the summer run season.

All heating zones operate simultaneously via one on/off switch and ramp to their proper temperatures at a predetermined rate. Controls for the 110-kW (477-V) warming system are designed to prevent temperature differentials that might otherwise contribute to uneven casing expansion. Specifically, if one zone is heating too rapidly, the control system turns it off until the other zones catch up and all can again work in unison

Operation and monitoring of the warming system can be local, via the touch screen, or remote from the control room. Fig 2 is a simplified sketch of the screen. The actual screen in the Fig 3 photo provides zone temperatures and readouts for each thermocouple (two per zone). 

Ansmann said Arnold spent two years developing this new product to ensure its applicability to every make and model of steam turbine. Assembly and thermal performance tests have been performed on the most recent casing materials to simulate installation and thermal conductivity both above and below, and alongside, the split line.

The number of heating zones and their design varies with the casing arrangement and steam-turbine operating temperatures at various locations on the unit. Arnold’s engineers optimized the number of zones to satisfy warming requirements while simplifying to the degree possible the removal of blankets for maintenance. Heating elements and thermocouples are equipped with quick-release fasteners (Fig 4).

Ansmann noted that the performance of traditional multi-layer insulation systems can suffer when used in conjunction with warming systems if the pin substructure is not able to keep blankets and heating elements in place and assure uniform heat transfer across the casing—especially below the split line.

Keep in mind, too, that an ineffective insulation system contributes to high enclosure temperature, which can adversely impact electrical gear and wiring. Loss of efficiency and unsafe working conditions also result from poor-fitting and poorly installed insulation.

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