13.5.6 Reheater drains
The location of the drains receivers, control valves and the routing of the drain pipes, requires careful consideration to ensure stable and safe operation of the system.
Condensate from each bled-steam tubenest header is drained by gravity to a bled-steam reheater drains tank. One tank per vessel is the usual arrangement. In the case of multiple headers, as in NEI-Parsons type vertical reheaters, drains pipes are not commoned but individual pipes lead the condensate from the headers to the drains tank, thus avoiding the problems of instability mentioned above. Similarly, drainage from the tubenests in the MSR vessels are kept quite separate and are not cross-connected.
Condensate from the drains tanks is normally routed to a heater flash vessel or drains cooler for ther-modynamic gain. If however the heater bank is out of service, either a three-way valve routes the drains to the condenser flashbox, or a motorised valve in the line to the heaters closes and an emergency drain valve in the line to the condenser flashbox opens in response to a high level signal from the level controller in the drains tank. This emergency drain valve also opens in response to a high water level in the heaters. All valves and drain pipework must be sized to pass the bled-steam drains at maximum turbine capacity. Drains tanks are sized such that the time taken to fill the volume bounded by the tankwalls, and the upper and lower limits of the control band, will be several times the control valve stroking time.
Non-return valves are usually provided in the line to the heater flashbox to prevent possible refluxing of water during transient operation, or backflow to the condenser flash vessel from the heaters in the event of valve malfunction. Since MSR vessels are situated at or near engine room floor level, sufficient height is available underneath the vessel to accommodate adequately-sized drain tanks and provide a falling drain to all possible destinations. The base of a vertical reheater, however, may be low in the turbine hall and a positive static head may not be possible for all operating conditions. In this case, it might be necessary to provide a pump which would have to be sited in the basement, or even in a pit, in order to provide the appropriate net positive suction head. Otherwise particular drains might have to drive against an adverse static head.
The arrangement and control of the live steam drains is very similar to the bled-steam drains except that the condensate is finally routed to a heater flash vessel further along the feed train.