The main pumping plant at Claymills consists of four Woolf compound, rotative, beam pumping engines. These are arranged in mirror image pairs, in two separate engine houses, with a central boiler house and chimney. The engines were built in 1885 by Gimson and Company of Leicester. All the engines are similar and the following description is limited to only one, but applicable to all.
The high pressure cylinder is 24 inch bore by 6 feet stroke and the low pressure cylinder is 38 inch bore by 8 feet stroke. Steam is distributed by means of double beat 'Cornish' valves, mounted in upper and lower valve chests. These are actuated from an underfloor camshaft and will be worked manually during starting. The camshaft is driven from the crankshaft by means of three pairs of bevel gears; the Watt type governor is also driven from a point in this gear train. The cylinders act on one end of the beam, via Watt's parallel motion. The beam itself is of an interesting box section construction; consisting of wrought iron plates and angles, joined together by rivets. The beam is 26 feet 4 inches between end centres, 4 feet deep at the centre, weighs 13 tons and is carried on 12 inch diameter bearings.
The rather plain connecting rod drives on to a crank with a radius of 4 feet, mounted on a 12 inch diameter crankshaft. This latter carries a flywheel with a most impressive diameter of 24 feet and weighing in at 24 tons. This flywheel merely served to maintain smooth rotary motion; the engine actually actuated a pair of reciprocating ram pumps. These are each of 21 inch bore by 6 feet stroke. The crank-half pump is connected to the beam, via the earlier form of Watts's parallel motion, whilst the engine-half pump is directly connected to a tail rod from the HP piston.
The engines are run condensing and are fitted with jet condensers. The condensate and incondensible gases are removed by a vertical, single acting air pump driven from the innermost suspension link of the engine-half parallel motion. A small proportion of the condensate was filtered and passed to a hot well beneath the boiler house. It was pumped from there, to a water softening plant, by a small pump (4 inch by 2 feet) driven directly from the crank-half of the beam.
The cooling water was drawn from lagoons about 40 yards from the engine houses and returned there to cool after use. When the sludge digestion plant was constructed on the lagoon area, a modern cascade cooling tower was erected on a temporary basis to enable the engines to continue running until the new plant was completed; the cooling tower being dismantled once the steam plant was withdrawn from use.
The engines were normally run in pairs, never all together, although a third engine was occasionally employed during times of heavy load. These operated at about 10 rpm and two engines had a combined pumping rate of 5.5 million gallons per day. Each engine house contains a single cylinder horizontal air compressing engine, for charging the air vessels (anti surge vessels) on the pump discharge mains. At a later date, horizontal single cylinder barring engines were added (one for each pair of engines).
Steam was supplied at 80 psi, by a range of five Lancashire boilers. The originals were replaced in 1937 by the current five. These are fitted with Green's economisers and Meldrum's mechanical stokers. There is one horizontal single cylinder, rotative boiler feed pump by Buxton and Thornley of Burton on Trent and a vertical by Halls. The boilers also supplied steam to over thirty auxiliary engines. Most of these survive on, and off, the site. Three of these are housed in a more modern block behind the engine house. The best are the pair in the dynamo house. These are a horizontal single cylinder engine, by Buxton and Thornley, dating from 1889, and an inverted vertical duplex engine, built at Claymills circa 1906.