The main purpose of the steam turbine bypass system is to virtually duplicate the expansion and heat transfer normally undertaken in the operating steam turbine. By doing so, the steam turbine bypass system enables faster plant startups, continued operation of the gas turbine generators (GTGs) and heat recovery steam generators (HRSGs) following a steam turbine generator (STG) trip, and simple-cycle operation (if designed with the condenser for this service) of the GTGs with the STG out of service.
Bypass systems in a combined-cycle power plant
Steam Turbine (ST) bypass systems may be categorized into cascading and parallel (non-cascading) type. For a three pressure level steam system, ST bypass is usually cascading type with the cascade flow going from high pressure (HP) steam to cold re-heat (CRH) steam line, through the HRSG re-heat section, exiting with hot re-heat (HRH) steam bypass to the condenser. Low pressure (LP) steam from the HRSG bypasses directly to the condenser.
In the parallel bypass arrangement, HP steam bypasses directly to the condenser through the HP bypass valve. Steam generated by the IP drum is either admitted to the cold reheat section (CRH) or bypassed directly to the condenser. IP admission to the CRH is only allowed when the generator breaker is closed. With this system, there is no flow through the re-heater when the ST is not in operation. Boiler feed for any configuration typically relies on condensate collected from the condenser hotwell, with boiler feedwater make up made through the condenser.
Throttling devices are used to reduce the pressure from inlet throttle pressures to the desired output conditions. The throttling process is isenthalpic since no work is accomplished and little or no heat transfer occurs across the system boundaries, therefore the outlet enthalpy is virtually the same as the inlet enthalpy. Without some additional conditioning, the discharging steam would most likely exceed the thermal limits of the downstream piping and system equipment. To prevent this from occurring, the bypass system is normally supplied with a water spray system that injects a controlled quantity of water into the steam flow. The actual volume of water injected is dependent on the actual operating conditions being exhibited and a simple heat balance. This water then mixes with the steam, absorbs heat via various heat and mass transfer processes, evaporates, and cools the steam to an enthalpy level that is more representative of the actual turbine discharge and acceptable to the plant equipment. This is in the range of less than 1,200 BTU/lbm, or as otherwise determined by the condenser manufacturer.
Steam bypass systems are employed in many ways. The two most prevalent operating modes are between the Main Steam and the Cold Reheat Line, when the plant design incorporates a reheat steam cycle, and the hot reheat and low-pressure steam bypass(es) to condenser. See Figure 1 for a basic steam cycle flow diagram, which illustrates the HP, hot reheat, and LP bypasses for a 2 x 1 or 3 x 1 configuration. The diagram illustrates a triple pressure reheat unit since that is the most common combined-cycle commercially used today. There is a separate set of bypass valves per HRSG-in normal practice-to assist with individual CTG/HRSG startup.
Figure 1: Condenser Bypass Diagram. Note: Bypass may be configured in one condenser admission line or up to three per service, depending on configuration (2x1 vs. 3x1) and control philosophy.
The HP bypass has fairly simple design and implementation requirements. The pressure drop, flow requirements, rangeability, and quantity of spray water injected are minimal by valve engineering standards. The hot reheat bypass to the condenser, on the other hand, is far more complex due to a number of factors, including:
- Large quantity of water injected
- Minimal distance for vaporization and thermal equilibrium
- High-pressure drop ratios due to condenser vacuum conditions
- High rangeability and turndown requirements
- Speed of operation
- Noise limits
- Protection of the condenser (operating permissives in place)