Humphrey pump

The Humphrey pump is a large internal combustion gas-fuelled liquid-piston pump. They were used for large-scale water supply projects. They were only popular for a short time, from around 1910 to the outbreak of World War I, but they continued in service for a long period afterwards, some lasting into the 1970s.

The pump was invented by H A Humphrey, MIMechE. It was described in a paper presented by him to the Institution of Mechanical Engineers on 19 November 1909. A pump capable of pumping 250,000 gallons per hour to a head of 35 feet was exhibited at the 1910 Brussels Exhibition, where it was awarded two Grands Prix, for both engines and pumps.

The pumps are an internal-combustion fluidyne engine. Although fluidynes are now considered to be a form of the external combustion Stirling engine, the Humphrey pump was an early example of one using internal combustion. A fluidyne has few moving parts: although they may include various valves, their key feature is that a mass of liquid acts as their piston, rather than the more usual mechanical piston. They are normally used for pumping liquids and use the pumped liquid as its own piston.

Humphrey's pump consists of a large U-shaped tube, filled with water. One end is sealed by a heavy cast iron 'combustion head'. The combustion head contains the inlet and exhaust valves, the ignition source and withstands the pressure of the combustion chamber. Water enters the pump through sprung inlet valves below the combustion chamber. A distinctive feature of the Humphrey pump, compared to others, is that the water only has to flow through this single set of valves: there are no non-return outlet valves, only the inertia of the delivered water within the outlet pipe is used to control its flow. As there are no delivery valves opening and closing suddenly, the water flow changes smoothly, avoiding water hammer and the usual problems for this type of pump.

Combustible gas, mixed with air, is supplied into the combustion chamber and then fired by an electric spark. The force of the expanding gas drives water downwards from the combustion chamber and accelerates it along the delivery pipe. Once the chamber pressure drops to that of the inlet water, the delivered water is no longer being accelerated but the large mass of water in the long pipe continues to flow by its own inertia. This reduces the pressure in the chamber below atmospheric, which causes the inlet air and gas valves to open, drawing in fresh air as a scavenging effect. As the water begins to flow back down the U, this increases the chamber pressure and causes the exhaust valves to open.

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