4.4.3 System effects
In operation, other problems arise from the introduction of the by-pass systems. Following a load rejection, the HP governor valves close fully and once the by-pass is operating, the non-return valves in the HP turbine exhaust prevent reverse flow into the HP turbine. However, the steam already in the turbine cylinder is effectively 'bottled up5 and the continuing rotation of the turbine causes rapid overheating of the blading due to turbulence and frictional heating of the contained steam.
A similar problem arises during start-up. Without a by-pass, the HP turbine exhaust pressure prior to synchronisation is at a very low value, controlled by the flow through the IP and LP turbines. With a by-pass, there will be a much higher flow through the by-pass and the turbine exhaust pressure is considerably higher. Normally this value of exhaust pressure would only be attained with a much higher flow through the turbine. The HP turbine is therefore operating with its efficiency considerably reduced and in addition is producing some rotation Toss heating. A typical condition line for the HP turbine is shown on the Mollier diagram in Fig 1.78.
These problems are effectively dealt with by providing a connection from the HP turbine exhaust to the condenser, sized to take approximately the HP flow that obtains prior to synchronisation and arranged to open only when the by-pass is in operation.
During by-pass operation, there is a transient loss in the water stored in the de-aerator due to the re-circulation of spray water. An assessment of the worst case transient is normally made to ensure that feed system stability is maintained.
The remaining problems concern the malfunction of the by-pass system and associated plant. To protect the LP turbine, it is necessary to trip the by-pass valves closed in the event that loss of condenser cooling water causes a high LP turbine exhaust pressure. Failure of spray water valves and other aspects are covered in more detail in Chapter 2.