STEAM TURBINE PUMPING EQUIPMENT OF RAND, SOUTH AFRICA, WATER BOARD
Supplies Principal Industrial District of the Section—Adopts Steam-Driven Centrifugal Pumps—The Power Plant
THE works of the Rand (South African) Water Board are always of interest to water works men seeing that this plant handles large volumes of water for the principal industrial district of South Africa. The latest works completed by this concern consist of an extensive barrage across the Vaal river and the pumps herein described are for the purpose of pumping some of the water a distance of 25 miles to Zwaarkopies. At this later place are high lift pumps. Some steam driven units here are worthy of passing notice. They supply Johannesburg, which as possibly some readers are aware is not only the principal city of the rand but is situated at a very high elevation and on the summit of a hill. The pumps therefore have to deal with some 100,000 gallons per hour against a total head of 960 feet. Although now 20 years old these four engines one of which is shown in Fig. 1 and may be taken as typical of the best British practice in water works engines showed under test the remarkable steam consumption of 10.904 lbs per pump h. p. hour which, so far as the writer is aware, has only once been exceeded.
Pumps of the New Plant
Turning to the new plant one sees the different conditions which pertain after 20 years and 5 years war in between. Engines of the class just outlined while practically inapproachable in the matter of steam consumption are almost prohibitive in cost at the present day and for the purpose of dealing with 250,000 gallons per hour against a head of 565 feet steam turbine driven centrifugals have been installed. The plant may be regarded as typical of good British practice. The pumps shown in section in Fig. 2 are four stage pumps incorporating the usual British practice of ring casings. These are of cast iron while the impellers, guides, passages and diffusers are of bronze.
The impellers, 23 1/3 in. in diameter, run at 955 r. p. m. On the right in a chamber just outside the main casting of the pump casing is seen the hydraulic balancing disk while at the extreme end of the shaft is the auxiliary Michell thrust bearing. This is now quite usual practice in high lift pump practice and the spring combined with it should be noted. This is special to the pumps in question and takes into account the surges that are likely to exist in a long pipe line, its purpose being to allow the hydraulic balance to function, yet at the same time to relieve it of a constant proportion of the total thrust.
The Power Unit
Here advantage is taken of using the relatively inexpensive high speed impulse turbine and incorporating gearing between turbine and pump so as to insure both running at the most economical speed, an arrangement now seen on most oil engine driven sets. The turbines supplied with steam at 200 lbs pressure and 5000 superheat and exhausting into 28 in. vacuum are of quite simple design and as both high pressure and temperature are confined to the cast steel nozzle box, the casings are of cast iron, while the wheels are of high tensile with phosphor bronze blading. Turbine, gear box and pump are mounted on one bedplate. Test curves given in Fig. 3 show a pump efficiency practically constant at 80% with a steam consumption at normal load of 13 lbs per pump h. p. hour.