Why you must prime a centrifugal pump 6-6
Although the term “pressure” is not normally a part of a centrifugal pump man’s vocabulary, we are going to have to discuss it for a couple of minutes.
The earth’s atmosphere extends approximately fifty miles (80 km.) above the earth, and rests on the earth with a weight equivalent to a layer of fresh water thirty four feet (10 meters) deep at sea level. To remove air from the pump cavities and the suction piping, the pump must develop enough head to equal the equivalent of this 14.7 psi., or one bar pressure. In another paper we learned how to convert this height (head) to a pressure reading by use of the following formulas:
34 ft./2.31 = 14.7 psi.
10 meters/10 = 1 bar
Unlike a positive displacement pump that can pump a liquid to any head as long as the pump body is strong enough, and there is enough horsepower available, the centrifugal pump can only pump a liquid to its rated head. You’ll recall that this head was determined by, and limited to the diameter of the impeller and the impeller speed (rpm.)
Since the weight of water is approximately 8000 times that of air (50 miles vs. 34 feet or 80 Km. vs. 10 meters) the centrifugal pump can produce only 1/8000 of its rated liquid pressure. In other words, for every one foot water has to be raised to prime the pump, the centrifugal pump must produce a discharge head of approximately 8000 feet (each meter requires a head of 8000 meters) and that is impossible with conventional impeller diameters and speeds.
All of this means that if you intend to use a centrifugal pump you’re going to have to come up with some sensible method of priming it. Your choices will include :
- Install a foot valve in the suction piping to insure the liquid will not drain from the pump casing and suction piping when the pump stops. Keep in mind that these valves have a nasty habit of leaking.
- Evacuate the air in the system with a positive displacement priming pump operating between the pump and a closed discharge valve.
- Fill the pump with liquid prior to starting it.
- Convert the application to a self priming pump that maintains a reservoir of liquid at its suction.
One of my readers, George Mathew, offers another explanation:
A centrifugal pump is a rotodynamic pump. This means that the head developed (in meters of liquid that is pumped) depends on the velocities determined by diameter of the impeller and the impeller speed (rpm.). As the pressure developed is related to the head by the equation head = pr / sp. weight, the pressure available will be proportional to the specific weight of the liquid. This means that the pressure (or pressure difference) created with air will be only around 1/800 times that with water ( density of water = 1000 kg/ m3 and dry air at S.T.P has a density of 1.2 kg/m3 ). Therefore, if the pump is not primed, the suction pressure created will not be sufficient to lift water.