2.3 System configuration part 3
The medium-actuated feed isolating valves installed on the HP heaters at Grain power station are examples of these valves and are designed to give by-pass and isolation in about 12 s from receipt of a signal to isolate the heater bank. Figure 3.21 shows the diagrammatic arrangement of the system and gives a brief explanation of how it works.
Figure 3.22 is a more detailed drawing of the isolation valve with a brief description of the salient design features. The most important features to note from Figs 3.21 and 3.22 are the provision for the system to fail safe (i.e., to isolate the feedwater side of the heaters), and the slowing down of the closing stroke over the last few millimetres to ensure that no hydraulic shock can occur due to sudden valve closure. As the feedwater isolating valves have a protective function, high integrity is important. The use of the feedwater as a medium to close these valves, ensures that a pressure source to actuate the valves is always available and the actuator can be made to produce whatever closing force is needed to close the valve as swiftly as the system hydraulics will allow. However, the system is complicated and costly, and is only needed if heater flooding times are measured in seconds, as with some designs of vertical heaters. The cost benefits of increased reliability must be included.
The bled-steam lines to the heaters are isolated by motorised parallel slide valves. The power-assisted nonreturn valves are given the signal to close but will remain open until the closing spring can overcome the reducing steam flow tending to keep the valve open.
When the heater bank is isolated on the steam and feedwater sides, small heater shell drains are operated which discharge to the condenser. Due to the action of the heater vents, assisted by the shell drains, the pressure in heaters and the associated steam and drain pipework within the isolation valves will fall to condenser vacuum.
The duplicate water level sensing devices which initiate a bank trip due to a HWL are float switches manufactured by Mobrey to meet the plant manufacturers' and CEGB needs (see Fig 3.23). Also illustrated are the methods used for testing the duplicate float switches with the heaters in operation.
The overall electrical control scheme is described in Volume F. This gives full details of the duplicated electrical supplies from the 110 V DC system which powers the duplicate high integrity master tripping relays. The master tripping relays initiate all protection valve closures except for the autonomous action of the bled-steam non-return valve due to a pressure reversal. In the event of a turbine trip, a separate signal trips the master tripping relays which shut all the protection valves on both banks.