January_1968 — Page 43

allow for the increasing pipeline losses. The process operates in reverse when a pump is stopped. The call order of the pumps can be set on a plug board and will be changed from time to time so that the running time for each pump is made roughly equal.

The pumps can be started and stopped, of course, directly through the switchgear and the motor speed can be changed by turning a handle on the induction regulator; normally, however, if non-automatic control is required, this will be effected remote- ly from the control room, so avoiding the danger to personnel associated with the manual operation of the circuit breakers.

The control panel also indicates motor current and speed, the quan- tity of water being delivered, the delivery head and fault conditions. The majority of the pipeline valves in the vicinity of the station can also be operated remotely from the con- trol room.

In addition to indications on the local panel, information such as the number of pumps running, the flow and the occurrence of faults will be transmitted over a telemetry cable- link to the Sha Tin treatment works where the performance of the station will be monitored. The telemetry equipment is fully transistorised and works on the time-division principle whereby each function is scanned at regular intervals. At set intervals flows and other data will be logged at Sha Tin by automatic typewriters, as well as changes of state, such as the start of an extra pump, as they occur.

Apart from an emergency stop button for each pump, direct control is not possible from Sha Tin. Monitoring facilities are also available at the Tai Po Tau pumping station and will be brought into use in the event of a break in the cable between Tai Po Tau and Sha Tin.

Pumping Station Building

The design of the pumping station building (see fig. 3:3 and 3:8) was dictated by three main factors firstly the pump design which requires the pumps to be set at low level to ensure that the suctions are always flooded and do not require priming, secondly the large difference in pos- sible river levels and the necessity of keeping all vulnerable electrical and control equipment above any possible flood level and thirdly the importance of having some pumps operational as soon as possible after the start of corstruction.

The last requirement complicated the construction procedure and it is considered that, in similar circum- stances in future projects, it would be preferable to create a temporary installation completely separate from the permanent station.

Reinforced concrete has been used throughout the building with the lower parts designed in accordance

Far East Architect & Builder January, 1968

Fig. 3:7.

18,000 cu. ft. surge vessels

with the B.S. Code of Practice No. 2007 for water-retaining structures. The pumps are mounted in enclosed sumps, which are founded on in situ weathered meta-sediments. The sumps carry a substructure of shafts, walls, columns and beams to raise the switchroom and motor-room floor to +25 ft. P.D. Above this rises the superstructure comprising the motor- room, switch room, control room and store.

The part of the structure between the pump sumps and the motor-room floor is, in effect, dead space as it is subject to flooding. This design was adopted to avoid flotation of the sumps in time of flood, in preference to having an open sump and adding substantially to the weight of the

structure.

The superstructure is a series of portal frames, which allows a clear span for the travelling gantry-crane, high headroom for ventilation and minimum obstruction from heavy columns at floor level. The portal frames are bottom hinged to reduce bending moments in the lower parts of the structure.

The construction of the sump for the booster pumps and the corresponding structure

up to the

Fig. 3:8. Pumping station

=

motor-room floor was completed by May, 1964 together with the delivery pipework and the suction pipework from the intake to the west of the building. The west sump for three variable-speed pumps together with its integral intake-structure and motor floor were constructed next. Mean- while the delivery pipelines to Tau Pass were completed by April, 1965. The east sump for the remaining seven variable-speed pumps, the re- mainder of the substructure and the entire superstructure were then con- structed whilst these pumps were operational, the motors being housed in temporary wooden sheds.

Tau Pass Culvert

The Tau Pass culvert which is 111⁄2 ft. in diameter and 2,400 yd, long was constructed partly in cut and cover and partly as a tunnel. The cut and cover section was designed as a rein- forced concrete arch in section, and longitudinally it was divided into sections to avoid cracking due to differential settlement. Considerable difficulty was experienced in the construction of the cut and cover section due to the ingress of ground- water from the alluvial soil.

57

57