13.5.5 Separator drains
Both cyclone separators and MSR internal separators are normally situated close to engine room floor level. This provides sufficient height to accommodate the drains tank, sufficient net positive suction head for a separator drains pump (if fitted) and a falling drain from the drains tank to the condenser.
Separated water from each separator vessel, or from each MSR shell, and from the cold reheat pipes is drained by gravity into the separator drains tank. Commoning of drains pipes before the drains tank is not acceptable, since pressure differences between individual pipes and vessels can cause water to back-up in the pipework. Water levels can become unstable and oscillation or hunting can occur in the drains system leading to vibration and sometimes failure, particularly of the pipework supports. It is normal practice therefore to provide an individual separator drains tank and drains control system for each vessel containing a separator.
Horizontal drains tanks are often chosen for economy but vertical tanks have the advantage of a constant surface area and have greater hydraulic stability in level control. The layout of the drains pipework to the drains tanks should be chosen with care, particularly where loop seals are included. The lowest point of the loop must be far enough below the floor of the vessel to permit free drainage during any tank transient pressure rise. Any transition in the bore of the drains pipework should also be well below the floor of the vessel to inhibit any tendency to flashing at this point of sudden pressure reduction. Drains tanks are normally fitted with vent pipework, level indicators, high level switches and pressure indicators.
Water from the drains tanks may be discharged to a heater in the feed train via a flashbox, or pumped to the de-aerator, in which case the pump flow is regulated by a level controller on the drains tank. A recirculation valve is then necessary to satisfy the minimum flow requirements of the pump and to guard against cavitation. Forward flow may be prevented completely in the event of high level in the de-aerator by a motor-operated isolating valve in the pump discharge line. In this case, drains are routed to the condenser. Drains are also normally routed to the condenser during start-up to prevent contamination of the de-aerator inventory. Pump discharge lines are always provided with isolating valves and non-return valves.