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Bechtel engineers point out that maintaining condenser vacuum and preventing the hotwell from subcooling dramatically decreases the morning startup time and almost eliminates the huge swings in chemistry associated with a cold startup.
To minimize corrosion potential due to dissolved oxygen (DO), the Bechtel engineers recommend:
1. Using an auxiliary boiler to sustain condenser vacuum by maintaining steam turbine seals in units that shut completely down at night. This auxiliary steam source is also used to sparge the condenser hotwell, thus preventing the condensate from sub-cooling and picking up dissolved oxygen; to ensure, during part-load conditions, so that the hotwell does not sub-cool; and to minimize the amount of makeup de-mineralized water required to support operation of the sky valves.
2. Designing the condenser vacuum system to use low-pressure motive steam.
for powering the steam jet air ejectors. This will ensure, when using vacuum pumps, that the vacuum pump cooling water supply is cold enough to stay ahead of flashing due to low vacuum conditions. An alternative approach would be to configure a hybrid system where the vacuum pumps take suction from the steam jet air ejectors.
3. Putting the steam-turbine gland, steam condenser on the closed cooling water loop. This measure precludes having to operate the large condensate pumps during shutdown.
4. Maintaining a positive pressure at all times in the HRSG drums during shutdown by closing the stack damper and all outlet valves to the HRSG after the gas turbine is secured. Cooling the high-pressure drum to saturated conditions at 400°F to 500°F (205°C to 260°C) will take from eight to 16 hours, or longer. After the boiler has cooled to within the range of the auxiliary steam source pressure, open the high-pressure evaporator spargers and keep the drum as close as possible to drum soak conditions, in order to provide a warm start of the HRSG with no ramp-up restrictions. Because heat migrating from the boiler's high-pressure section to its intermediate-pressure and low-pressure sections will tend to keep the drums at significant pressure, it is necessary to vent the low-pressure drum throughout the night to prevent over-pressurization.
5. Applying and monitoring a nitrogen blanket above all water spaces, if the HRSG is taken off-line for maintenance under cold conditions. As the steam space pressure decreases, align the nitrogen system to push and replace the steam, thus minimizing air in-leakage. However, always follow the manufacturer's recommendations for chemical dosing the drums.
Proper system integration, with regard to balance of plant equipment and interconnecting support systems, will enable the cogeneration plant's gas turbine, HRSG, and steam turbine to generate power output and heat rate reliably at all design loads and configurations, in accordance with specifications.
References: Bechtel Power Corp., Frederick, M.D.; Black & Veatch, Kansas City, Miss.; Vogt-NEM, Louisville, K.Y.
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