Kempton is home to two 1,000-ton triple-expansion steam pumping engines, known as Triples. One of these massive engines has been restored and is now the largest working example in the world, while the second Triple has been kept as a static display for guided tours. In their day, these engines each pumped 19 million gallons of water to North London and, apart from periods of maintenance, worked 24 hours a day, seven days a week, from their installation in 1927 to their decommissioning in 1980. The Triples are known as No.6 and No.7, following on from the original five Lilleshall triples installed in an adjacent building in 1906. The Lilleshalls carried on working in tandem with the new Triples until 1968, when they were scrapped. Engine No.6 was officially named after Sir William Prescott, the then-chairman of the Metropolitan Water Board, and No.7 was named for his wife, Lady Bessie. The plaques bearing their names can still be seen on each Triple. The engines were built by Worthington Simpson Ltd at the company’s works at Newark on Trent, near Nottingham. Known originally as James Simpson & Co Ltd, the firm merged with the Worthington Pump Company in 1903 and subsequently changed its name to Worthington Simpson in 1917. The order for the two Triples was placed on 25 January 1924, with a delivery date two years later in 1926. The contract was valued at £94,750, with a late-delivery penalty of £20 per unit, per week. No record has been found of any penalty being paid, although No.6 was not completed and commissioned until mid-1929 – well after its agreed delivery date.
Each Triple stands 62ft 6in (19 metres) high from the basement to the tops of the valve casings, and is 45ft (13.7 metres) long. Including the ram pumps, each engine weighs 1,000 tons (1016 tonnes) and is capable of producing 1008hp. They are of the inverted vertical triple expansion type, in which steam is expanded through three cylinders arranged in line directly over the crankshaft, with the plunger pumps below, before passing to a condenser. The layout originated in the USA where Edward P Allis introduced it to a waterworks in 1886, although ships’ engines of a similar design had already been in use for many years. Kempton’s Triples are thought to be the biggest ever built in the UK. Engines of this type were hard to beat for efficiency and reliability, although the initial cost – which included building an engine house tall enough to accommodate them – was high. At one time, about a third of all pumping engines in UK waterworks were of this design. A much later and smaller example, built by Worthington Simpson and installed in 1938-40, can still be seen in action at Brede in Kent, along with the surviving member of a pair of superb 1904 Tangye triples. When the Kempton engines were finally shut down in 1980, they were last working Triples in the UK.
Efficient by design
Mechanically, the arrangement of the Triples is attractive by virtue of their 120° crank setting, which not only helps to balance the moving parts, but provides a steady input of power from the engine and a steady discharge of water into the pressure main. There are never less than two steam cylinders driving and one plunger pumping at any one time, as our animation diagram shows. The Triples were supplied with steam by a battery of water-tube boilers at a pressure of 200psi, at 150°F (66°C) of superheat. The engines were designed to operate 24 hours a day, seven days a week, although in practice they were run on an overlapping basis, nine months on and three months off, depending on demand. Steam for the Triples was produced by six Babcock and Wilcox boilers, consuming up to 13 tons (13.2 tonnes) of coal a day and equal to the best electricity generating stations of the time. In 1945, two John Thompson boilers were added, one on each side of the existing Babcocks. In 1933, the output of the engine house was further enhanced by the addition of two Frazer and Chalmers steam turbines, built in Erith, in Kent. They were installed where a third Triple had originally been planned and were numbered No.8 and No.9.