January_1968 — Page 41

A sheet-pile cut-off has been driven. below the foundation slab of the Indus Fabridam to prevent piping through the alluvium on which the foundation is built.

Pipes and Valves

Only a small proportion of the quantity pumped from the Indus to Tau Pass will be at the maximum rate of flow (see fig. 3:5). It is thus econo- mic to accept extra running costs to overcome occasional high pipe-friction losses, even though the resulting large variation in head renders variable-

speed pumps necessary. The maxi- mum velocity through the 54 in. pipes is 11.5 ft./sec, and maximum friction loss 105 ft., which, with a static head of 50 ft.. gives a head range of rough- ly 3:1.

To protect the pipes against exces- sive surge pressures in the event of a power failure causing all the pumps to stop suddenly, air loaded pressure vessels (see fig. 3:7) are connected to the pipelines. These will act as a reservoir to maintain the flow and pre- vent the separation of the water column that would otherwise occur. Four tanks having a total capacity of 18,000 cu. ft. are connected to the two 54 in. mains; there is also a 2,000 cu. ft. tank for the 48 in. Tai Lam Chung main.

The

The major cross-connection and isolating valves on the station pipe- work are of the butterfly type. advantages of butterfly valves over the more usual sluice valves are their cheaper cost, their ease of operation.

their relatively light weight and their

suitability for flow control. The par- ticular valves used at the Indus in- corporate a patented inflatable rubber seal which is expanded against the valve disc after closure, thus ensuring a watertight seal without the wear normally experienced with fixed seals. It is understood that this is the first occasion that large butterfly valves have been used in a waterworks in- stallation in Hong Kong.

Electrical & Mechanical Plant

The main pumping plant, delivering to Tau Pass, consists of ten vertically- mounted double-suction single-stage centrifugal pumps. The nominal out- put of each pump is 20 m.g.d. and this can be maintained against the variable head by altering the speed of rotation. The output from one unit operating on its own can be re- duced to about 7 m.g.d. by the same

means.

A flow-duration curve similar to that shown in fig. 3:5 was used in assessing the quantity that could be extracted from the river with various pumping capacities, although other factors such as the probability of the Plover Cove reservoir being full (pumping therefore being unnecessary) also had to be considered.

With the capacity selected, the es- timated unit cost of the water deliver- ed to Plover Cove taking capital and operational costs into account is roughly 25 cents per 1000 gallons, and is, thus, comparable with the unit cost of water collected from other in- direct catchments by catchwaters and

tunnels. The pumps are expected to operate at their full capacity for an average of 12 days per year.

The pumps are close-coupled through solid shafts 20 ft. long to vertically-mounted patented N-S vari- able-speed 6,600 V a.c. motors hav- ing a maximum output of 880 h.p., manufactured by Laurence Scott and Electromotors Ltd. of Norwich. The synchronous speed is 750 r.p.m. but the actual speed can be varied be- tween 445 and 960 r.p.m.

motors

While variable-speed a.c. with a large power output have not always been found very satisfactory, the N-S type, which incorporates a number of special features, has al- ready proved its reliability. The al- ternative of a D.C. system was con- sidered but was found to be appreci- ably more expensive.

In an induction motor some mea- sure of speed control can be obtained by connecting resistances in the rotor circuit through slip-rings, although this is inefficient and results in a waste of energy.

In the N-S motor the surplus energy is fed back into the supply. Before this can be done, however, the frequency and voltage of the rotor current must be altered to those of the supply. The frequency change is achieved automatically by the use of a commutator, while the voltage is adjusted in a variable- voltage transformer known as an in- duction regulator.

As well as removing energy from the rotor to reduce the speed below the synchronous speed, speeds greater than synchronous can be obtained by

KOWLOON

CANTON

RAILWAY

SURGE VESSEL

2000 CU. FT.

FABRIQAM

FEED TANKK

LL. I. TRANSPORI

BOOSTER PUNE

CHINA LIGHT

AND

POWER CO LTD'S COMPOUND

ELECTRODES FOR

AUTOMATIC CONTROL

PUMPING STATION {PART SECTIONED AT +5 AND +30 3

R:VER

INDUS

-

Pat

Fig. 3:3.

River Indus pumping station layout

Far East Architect & Builder January, 1968

/SEPTIC TANK

PAVED

PRESSURE RELIEF VALVES

BUILDING A

DISCHARGE CHAMBER

WELL

NOTE

LEVELS ARE IN FEET ABOVE PRINCIPAL DATUA

55

SECTION_A_A