4.4.1 Superheat plant
The superheater and reheaters are an integral part of the boiler. The basic system for a single reheat turbine is shown in Fig 1.76.
The system depicted permits completely independent boiler operation. Although the system operation seems quite straightforward, the function of reducing the steam pressure from turbine inlet conditions down to exhaust conditions involves a major change in the energy available which imposes a heavy duty on the components involved. The need to supply a by-pass system must therefore be carefully evaluated against the benefits.
The prime reason why UK manufacturers have supplied by-pass systems for certain applications is to permit continued operation of the boiler and turbine following a large load reduction or a circuit-breaker trip to the house load. In a weakly-connected supply network, this may be an overriding consideration, since the ability to run-through such a transient would allow rapid reconnection of the unit following fault clearance. Without a by-pass, the power mismatch following opening of the circuit-breaker causes the turbine governing and interceptor valves to close rapidly. To accommodate the reduction in steam flow, the boiler firing rate must be rapidly reduced to a minimum. On most boilers, with the possible exception of some oil-fired units, this involves a firing trip which necessitates a purging operation for several hours prior to reloading. If a by-pass is supplied, a large proportion of the excess steam can be diverted through the by-pass system. If necessary, the boiler can be slowly reduced in load to a low level, ready for reloading when required. If the boiler load is maintained and the system fault cleared, the unit may be reloaded to a generator load corresponding to the capacity of the by-pass within a few minutes. Subsequent loading to full load is then accomplished at the normal rate.
If provided for this basic reason, the turbine bypass system may also provide other benefits, although these are not always significant. A full list of the other possible benefits are summarised as follows:
- For hot starts, following less than a 1 h shutdown, the boiler flow through the by-pass can be adjusted to raise the main and reheat steam temperatures to the values required by the turbine. When the turbine valves are opened, the thermal stresses in turbine components is minimal. The subsequent run-up and loading may then be selected to minimise either the time to load or the thermal stress, or some compromise between the two.
- On warm starts, the by-pass may be used to bring the main and reheat steam temperatures to a positive mismatch in order to minimise the time to load within available stress margins.
- In general, the by-pass provides for a smooth warm-up of boiler components, including the reheater and the main steam pipework. A more rapid boiler run-up may be possible, thus saving fuel.
- Water loss during start-up is minimised since, if no by-pass is fitted, superheater vents are used to discharge steam to provide a flow of steam through the superheater.
- Independent operation of the boiler is more readily achieved with the full capability of de-aeration and condensate polishing.