Do Pumps Suck?
I will begin this blog by posing a question: Does suction really exist? Take a moment, decide upon your answer and let's go on this journey together.
A majority of people in the pumping industry would say yes, suction has to exist. If you were to ask any ten year old child with a bottle of cola and a straw he will state “Of course it does! How could I drink from my straw if it didn’t?” If the child was asked to elaborate, he would explain how he uses his mouth and the straw to pull the liquid into his mouth. However, the physics of how he gets his drink to his mouth is slightly different. If you can understand this difference then you will understand why pumps don’t actually pull liquid in which will help you to understand all of the resulting limitations that exist.
To delve further into this subject let me provide you with an example. The photo below illustrates a simple horizontal end suction pump set-up to “suck” water up and out of the pond. The basic forces in play here are centrifugal force and atmospheric pressure.
Atmospheric pressure, as illustrated below, is basically the weight of the air above us pushing down on the earth’s surface. At sea level the column of air above us is approximately 50 miles high. If one could isolate a column that was a one inch square, you would find that the weight of the air in that one inch square column would be approximately 14.7 lbs. Hence, the pressure exerted by the atmosphere on the earth’s surface at sea level is said to be 14.7 pounds per square inch (14.7 PSI). See image below:
The other force pertinent to our example was centrifugal force. Centrifugal force is an apparent force that acts outward on a body moving around a center and arising from the body's inertia. Simply put, it is the force that caused you as a child to fly off the merry-go-round in the neighbourhood park when the big kids made it go too fast. Within a centrifugal pump this is the force that throws the liquid out, or the rotating impeller creating a pressure in the discharge casing. See image below:
Now that we have a basic understanding of the two forces that are relevant to our initial pump example, let us examine how those two forces interact. Assuming that our pump is primed(full of liquid) as the impeller starts to spin, centrifugal force will throw the liquid out of the impeller and into the pump casing. As this liquid departs the impeller it leaves behind a void, or more accurately it induces a low pressure zone. With the inlet side of the pump being connected and sealed to a non-collapsible hose that extends below the ponds surface, that low pressure zone, in effect, extends down the hose into the pond.
Atmospheric pressure being higher than the reduced pressure in the hose then pushes the liquid up the hose and into the pump where that liquid can be thrown out creating a continues cycle. The key point here is that the liquid is being pushed by atmospheric pressure and not actually pulled by the pump!
This brings us back to my initial question: Does suction really exist? If atmospheric pressure is responsible for pushing the liquid into the low pressure zone and the pump is responsible for creating the low pressure zone, then is the pump really sucking the liquid up the pipe?
My answer is no. Atmospheric pressure pushes the liquid so it is directly responsible for the movement of the liquid. It is important to look at it this way as it makes it easier to understand why pumps cannot “suck” water out of a well where the water surface is more than 34 ft. below the pump. Next week’s discussion on deep wells and NPSH will elaborate further on this subject, but until then I would avoid arguing with any ten year old about colas, straws and suction.
Until next time,