11.3 By-pass systems for fossil-fired plant
The importance of system by-pass in fossil-fired plant is as follows:
- After large load decreases, the turbine can still run at the much reduced load while the boiler conditions can be maintained. This permits rapid restarting when load is restored.
- The required steam conditions can be established when initially starting the boiler.
- Steam flow is provided through the reheater circuit during start-up but before steam is admitted to the turbine.
The by-pass scheme comprises two systems — an HP system and an LP system. A schematic representation of the HP system can be seen in Fig 2.86.
The HP steam is taken from each of the main steam lines after the main steam stop valves. The steam passes into the cold reheat inlet via four pressure control valves and their associated isolating valves. The correct temperature to the cold reheat is achieved through the employment of desuperheater spraywater. The spraywater supply to the pressure control valve is taken from the main feed pumps and has its own set of pressure control and isolating valves. The presence of warming piping and drains in Fig 2.86 will be discussed later in this section.
During start-up the pressure control valves are opened manually until the valves are more than 10% open, at which point the system is controlled automatically. When the boiler steam pressure reaches a specified value, the by-pass system automatically keeps the boiler at that value. In the event of a turbine trip, a signal is sent to the control valves to open rapidly. Alternatively, in order to protect the cold reheat piping system, the pressure control valves can be ordered to close rapidly under the following conditions:
- When the spraywater pressure is low.
- If any spraywater valve fails to open within three seconds of its associated pressure control valve.
- If the steam entering the cold reheat pipes is at too high a temperature.
- If the pressure in the cold reheat system is too high.
- If condenser pressure is too high.
Unlike the nuclear by-pass system, the pressure control valves will fail locked in their last-held position if there is a loss in electrical power. The spraywater pressure control valves also fail locked in their last-held position.
The LP by-pass system consists of four lines taken from the four hot reheat pipes (upstream of the IP turbine control valves). These four lines are then converged into two. The steam in each line flows through an isolating valve, a pressure control valve and then into the condenser dump tube. The LP by-pass system, therefore, is very similar to the by-pass system on a nuclear plant: it behaves in the same way as the HP system, the pressure control valves opening or closing under exactly the same circumstances.