13.5.2   Second-stage reheat

 

The live steam supply to the second-stage reheater tube banks is obtained direct from the main steam pipework immediately upstream of the HP stop valves.

To prevent excessive LP turbine back-end temperatures during low-load operation when the hot reheat steam flow is low, the live steam reheat supply is controlled by a valve which regulates the live steam flow according to the temperature of the hot reheat steam. In certain designs, this control valve does not open until the machine has reached a particular load. At full-load, the valve is fully open and could be subject to wear. A by-pass valve is therefore sometimes provided so that the temperature control valve is not in use during full-load operation. At a hot reheat temperature slightly below the normal full load temperature the control valve closes as the by-pass valve opens. A small warming connection may also be provided to enable the pipework to be pre-warmed to reduce the degree of thermal shock to the reheater and pipework when the temperature control valve opens.

In common with the bled-steam supply, isolating valves are always included in the live steam pipework, but non-return valves are generally not fitted.

 

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