22nd March 2026
What is 62 feet high, 45 feet wide and weighs 800 tons?
The answer is “The Lady Bessie Prescott”. None the wiser? The Lady Bessie Prescott is one of two triple expansion steam engines formerly used to pump London’s drinking water – and can be seen at the Kempton Steam Museum. On the latest Cody social (organized by Gordon K), 6 members plus friends had the pleasure of seeing a static Lady Bessie Prescott (engine number 7) and engine number 6 – The “Sir William Prescott” come into life.
Gordon had organised for members to receive a personalized tour from his contact Martin. Martin was an out an out, really knowledgeable, enthusiast and no question was too complicated or silly. He explained how the water was processed from the Thames and the journey it made (via some of our former sailing areas – Datchet and Queen Mary’s reservoirs) before arriving at the pumping station, and then the water’s onward journey via underground pipes to a number of underground reservoirs in north London. The double acting triple expansion steam system was described as well as the mechanics of starting the engine. During this part of our guided tour, we saw engine number 6 come into life for the first time as the driver opened the various valves to get the two massive 17ft diameter 32 ton flywheels to slowly move the high pressure piston to just past top dead center when the steam was applied to bring the majestic machine into life.
With No 6 engine purring along at around 25 rpm (actually just circulating water today via its three massive plunger pumps – 521 gallons per revolution!) we were walked through the 4 different levels of the adjacent engine (number 7) which was still being restored – with many of its innards exposed it provided a real insight to the scale, complexity, and shear ingenuity of these 1929 machines. The 86 inch diameter piston rings, 5ft6inch stroke, 19million gallons per day consuming 30tons of coal a day give you some idea of scale.
After a short break for coffee, it was time for our tour to resume again. On “live steam days” engine number 6 comes into life about 5 times during the day. As it started its second cycle of the day, the Cody group were stationed in prime position by one of the two steam Barring engines – these small high pressure steam engines (each about 50hp we discovered as answer to one of our questions on the tour) are used to start the large triple expansion engine.
The last part of our tour was a visit to the ground floor turbines and the mercury arc rectifiers. The turbines had none of the charm of the steam machinery but in fact were added only a few years after the steam engines to supplement the water pumping capacity – now days London’s water is pumped by electric motor pumps (boring!). The 3 mercury arc rectifiers were glowing blue ionization with dancing sparks within their glass tubes – we kind of expected Frankenstein to emerge – but these diodes were still in use to rectify the AC grid voltage to the 200V DC required for some of the electric motors.
Whether you were mechanically minded or just there to marvel at the achievements of yesteryear, there was something for everyone. As static exhibits the engines are a wow, but to see these giants fired up and pulsing with rhythmical elegance was absolutely fantastic. It was a thoroughly wonderful event – if this has whetted your appetite – make sure it’s a “live steam day” if you visit.
Thank you, Gordon and your personal contact Martin, for a truly excellent event.
– Roy