13.4.2 Steam-to-steam reheaters - part 7
The temperature difference between the cold and reheated steam, as it flows across the hot tubes of the reheater, produces temperature differentials which are many times greater in the first tube row than in the final tube row at full-load, with even greater differentials at part-load.
By arranging the heating steam flow through a hairpin tubenest such that one leg has a low rate of condensation and the corresponding leg a high rate, it is possible to achieve a reasonable balance in the steam condensed per tube. Even so, a sufficient differential exists to induce the steam, which has not been condensed by the inner hairpin, to flow via the drain section of the header into the lower leg of the outer hairpin. Obviously, whilst operating in such a manner, the outer tubes are not adequately drained or vented and a section of tube is intermittently deprived of heating steam. This section would then be transiently cooled by the external steam until the heating steam flow is re-established. Such a cyclic operation has been shown to be capable of causing fatigue failure on MSR tubes.
In early designs, this mode of operation was avoided by venting sufficient steam from the drain section of the header to ensure a continuous flow through all the tubes. To vent such a flow external to the reheater would result in large thermal losses, so the reheaters were fitted with a vent element. This comprised a layer of tubes, located prior to the main tubenest, which condensed the necessary flow of steam. The vent was independently vented and drained.
Current designs incorporate the simpler solution of controlling the distribution of steam into the tubes. Suitable restrictions are placed at the inlet to appropriate tubes, to balance the pressure drop through the nest to the extent that, with a small vent flow, the tube temperature differentials are maintained at an acceptable level and thermal strains are minimised. Vent lines are normally led to heaters in the feed trains for thermodynamic recovery, but on start-up and low-loads it is sometimes necessary to provide a slightly enhanced vent flow. An additional line is then provided which is usually routed to the condenser. It is possible for conditions to arise which cause a pressure rise in the HP turbine and consequently in the MSR, these are:
- A turbine trip from full-load, resulting in the entrained steam between the stop valves and the interceptor valves settling at a pressure level between normal pressure and the HP cylinder inlet pressure.
- Operating with HP feedheaters by-passed and MSR reheating tubenests isolated.
- Leaking tubes in a live steam reheater tubenest allowing steam to by-pass the turbine stop valves.
- Leakage across the turbine stop valves in excess of corresponding leakage through the LP turbine valves.
- Closure of all six LP turbine interceptor valves, whilst an HP turbine stop valve and governing valve remain open.
The MSR is, therefore, fitted with a relief capacity sufficient for the full reactor steam output. Relief valve capacity is provided to cater for a nominal number of burst tubes, with a setpoint above the maximum normal operating pressure, and bursting diaphragms to cater for the total steam flow at a suitable higher pressure.