4.4.4 Improvement of start-up capability
The provision of a by-pass system improves startup of the boiler and turbine. Its economic impact is most readily appreciated by considering the effect on hot starts following an overnight shutdown.
This is the duty normally seen by a unit allocated to two-shifting. Figures 1.79 (a) and (b) depict the differences in unit start-up with and without the by-pass in operation. The start-up time is roughly 15 minutes less if the by-pass is used. The saving may be considerably more if the boiler/turbine is not a flexible unit designed for two-shifting.
Depending on the intended duty of the plant, this may represent a significant saving to lifetime costs. There is also a small saving because, without a by-pass, water would be lost through the superheated vents as part of the standard start-up procedure. A further advantage of the by-pass system for start-up is that inexperienced operators are more easily able to bring the steam conditions to appropriate values before rolling-off the turbine. Against these advantages must be set the cost and maintenance burden associated with the supply of a by-pass system. In general, a small capacity (35%) by-pass system is unlikely to be justified on the grounds of improvement of start-up capability.
One notable exception is the once-through boiler, where a substantial bypass flow is required for a lengthy period during start-up. The CEGB AGR machines, which use a superheat steam cycle, are examples of this. The AGR also needs to use the by-pass for a lengthy period during shutdowns, particularly if the reactor is to be maintained at temperature in readiness for the next start-up.