2.4 HP heater drains system
The combined HP heater drains should be discharged to the de-aerator under all possible conditions. The simplest method to achieve this is to use the pressure differential between the lowest pressure HP heater and the de-aerator to overcome the static head and friction between the HP heaters and the de-aerator tank.
Figure 3.24 illustrates a typical drains system, showing the control valves and relative levels of equipment. A system with the HP heaters in a horizontal attitude is shown. As the unit load decreases, the pressure differential between the HP heater and the de-aerator decreases to a point where there is insufficient pressure head to raise the drains to the de-aerator storage tank. The drains are then diverted to the condenser. The converse applies on increasing unit load. In this instance, the level in the heater is used to determine the destination of the drain water. A rising water level in No. 6 heater opens the control valve to the No. 5 heater flashbox or to the de-aerator storage tank. The destination chosen is under the manual control of the operator and is switched according to system needs. If the level continues to rise because there is insufficient differential pressure to lift the drains to the de-aerator tank, the valve 3 in the line to the condenser is commanded to open and the drains discharge to the condenser. Backflow through valve / from the de-aerator is prevented by the NRV 75. Number 5 HP heater drains work on a similar principle but, because this is the lowest pressure HP heater, the destinations available for the drains are the storage tank or the condenser. A rising water level in the No. 5 HP heater first opens the drains control valve to the de-aerator tank; if the pressure differential is insufficient to lift the HP heater drains to the de-aerator tank, the rising water level then opens the drain control valve in the line to the condenser. On falling unit load, when the water cannot be discharged to the de-aerator, the water level continues to rise until its level controller sends a signal to open the control valve to the condenser. The drains are then discharged to the condenser until such time as the water level falls and the drain discharge route reverts to the de-aerator.
An alternative HP heater drains control system associated with vertical HP heaters is shown in Fig 3.25. The point at which drains switch from being raised to the de-aerator for discharge to the condenser flash vessel, or vice versa, is controlled by pressure switches on the main turbine. Two pressure switches are used to determine the changeover point: they are offset by about 10% between falling and rising load to prevent 'hunting'.
The No. 8 HP heater drains route is selected by use of drains route selection valves 3 and 4, working in antiphase to direct heater drains to the flash vessel or to the condenser flash vessel; the appropriate route being determined by a valve sequence initiated by the iction of the pressure switches described above. The control of the level in No. 8 HP heater is by valves 1 and 2, and that for No. 7 HP heater is by valves 5 and 6. As the drains flow path is determined by the drains route selection valves there is no need to witch signals to the control valves, which simplifies he level control system. The drains discharged to the condenser flash vessel are pumped by drains pumps into the condenser hotwell. The pumps are stopped and started, as needed, by float switches mounted in the tank.