The Savery Pump

The first successful steam pump was patented by Thomas Savery in 1698, and in his words provided an "engine to raise water by fire". The artist rendering here is understood to use artistic liberty; it is unlikely the egg-shaped vessels existed. The unit had two boilers, D and L, connected by pipe E. The device operation will be described using vessel P for discussion. Valves r and M were both closed. Vessel P was filled with steam through pipe O. The valve between the boiler and the vessel was closed using handle Z. Water was showered on the vessel from reservoir X, cooling the vessel, condensing the steam, creating a vacuum, and valve M was opened to suck in the water from below. Then valve M was closed, valve r opened. Handle Z was switched back and the water was expelled upwards through pipe s using steam pressure.

While vessel P was expelling water upwards through pipe s, the vessel Pr was sucking water upwards. All the valves were then switched and the cycle is repeated.

The Savery pump required pressurized steam to force the water upwards. Water could be pushed upwards limited only by pressure of the steam. Savery writes: "my engine at 60, 70, or 80 feet raises a full bore of water with much ease." The boiler would have needed to hold 35 psig pressure to raise water 80 feet- similar to the pressure in an automobile tire. It is likely that this use of such pressure was a reason that the Savery pump had a reputation for boiler explosions. Zealous operators undoubtedly increased the boiler pressure to pump water upwards further, and thus created some of the accidents by overpressurization.

Water will rise up about 33 feet in a laboratory vacuum. This is because water vaporizes to create a pressure equal to the vapor pressure inside the vacuum, and the maximum pressure difference is Patm - Psat. Trying to create a lower vacuum simply causes more water to evaporate, limiting the pressure differential. Savery's device was limited in practical usage to 20-25 feet of suction. (It was difficult to remove all the air from boiler feed water, so a perfect vacuum did not exist after all steam was condensed. Also, the well water contained some dissolved gases.) Therefore, the pump needed to be located within about 25 feet of the water. Imagine the difficulties of placing a boiler with a raging fire down in a deep well!

The later steam pump designs by Newcomen and Watt used only atmospheric pressure steam to avoid pressurized steam. Also, they used liquid pumps that were compact and could be driven by steam. The compact pumps could be placed more easily in a well or mine within 25 feet of the water. Savery's pump is described in more detail in a manuscript that he wrote to advertise the pump, The Miner's Friend, as listed in the Bibliography of this web site. It is also available online.