Principles Of Operation

The operation of a suction pump is dependent on and its efficiency limited by atmospheric pressure. If there were no atmospheric pressure there could be no suction lift to a pump. This is shown by a reference to the suction pump, Fig. 97, which consists of a piston, a, in a pump barrel or cylinder, b, a valve, c, that opens on the down stroke of the piston and closes on the up stroke, and a valve, d, that opens on the up stroke of the piston and closes on the down stroke. The operation of the pump is as follows: When the piston, a, makes an up stroke it exhausts some air from the suction pipe, e, and a sufficient quantity of water flows in to replace the exhausted air and balance the atmospheric pressure on the water outside. On the down stroke of the piston the exhausted air which has been confined in the pump cylinder escapes through the valve, c, which opens on the down stroke. The next up stroke of the piston still further exhausts air from the suction pipe and a still higher column of water flows in to replace the exhausted air. Repeated strokes of the piston exhaust all air from the suction pipe and pump cylinder, which then fill with water which is pumped out as was the air.

Fig. 97

* Distributing Main - A pipe extending from distributing manifold to a group of fixtures or to a number of groups.

+ Distributing Branch - A pipe extending from a distributing main to the several fixtures in a group.

# Fixture Branch - A pipe connecting a distributing branch to fixture, cock or faucet.

Lift Of A Pump

Theoretically a pump will raise water a distance equal to the height that atmospheric pressure will balance a column of water in a perfect vacuum. Experience and experiment, however, have demonstrated that a pump will raise water only about .75 of the theoretical height. This difference between the theoretical and the actual lift of a pump is due to the loss of head caused by friction in the pipe, and the impossibility of securing a perfect vacuum on account of mechanical imperfections in the pump and connections, air in the water and vaporization of the water itself. The constant .75 holds true, however, only for water at ordinary temperatures. Any appreciable raise in the temperature of water will cause a corresponding loss of lift. This is due to the fact that in a vacuum water vaporizes at a lower temperature than when under pressure, and when air is exhausted from the suction pipe of a pump connected with a hot water tank or receiver, the water instantly flashes into vapor and fills the suction pipe, preventing the formation of a vacuum. Water of temperatures higher than 180 degrees Fahrenheit cannot be raised by suction but must flow into a pump by gravity. Waters of lower temperatures but over 100 degrees Fahrenheit are much easier handled when they flow by gravity into the pump cylinder. The suction lift for waters of about 100 degrees Fahrenheit should never be over 7 feet, and for 160 degrees Fahrenheit not over 1 foot.

Atmospheric pressure varies with the elevation, that is, the distance above or the depth below sea level; hence on the side or top of a mountain the atmospheric pressure and consequently the lift of a pump will be less than at the sea level. Also, the atmospheric pressure and lift of a pump in a deep pit or mine will be greater than at sea level. The atmospheric pressure at sea level varies with the conditions of weather, but for practical purposes is taken as 14.7 pounds per square inch, and as 1 pound pressure will balance a column of water 2.309 feet high, it follows that in a perfect vacuum atmospheric pressure should balance a column of water 14.7X2.309=33.95 feet high. Atmospheric pressure at different altitudes with equivalent head of water and the vertical suction lift of pumps can be found in the following table:

Table XXXVII - Suction Lifts Of Pumps

In addition to the vertical lift, a suction pump will draw water horizontally an almost unlimited distance; nevertheless, when water must be conveyed any great distance, better results are obtained by using a force pump and placing it close to the source of supply.