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How Pumps Work: Operation & Performance

Instructor: Darren Grant

Darren has 27 years experience as a certified HS science teacher and college professor. He holds a M.S. in Science Ed. and B.S.Ed. in chemistry

A pump allows us to move liquids or gases from one place to another. Whether it is to pump some hand sanitizer onto your hands to protect you from germs, or to pump gasoline in your vehicle to get you down the road. Suppose you had a flat tire on your bicycle, how would you fix it without a pump? Pumps are all around us, let us take a look at how they work!

Pump Basics

A pump is a device for moving fluids from one place to another, or for pressurizing gases. Fluids tend to flow from an an area of higher pressure to an area of lower pressure. Most simple pumps make use of a moving piston and one way valves. One way valves allow the fluid —which is anything that can flow be it gas or liquid— to move in only one direction in the system. These valves prevent the fluid from flowing backwards. Pistons apply pressure to the fluid, moving the fluid to where we want it to go. Liquids are incompressible, meaning they cannot be compressed. Therefore, pumps can just move liquids around. Because gases are compressible, pumps can actually force more and more of a gas into a closed space, increasing the pressure of the gas.

Basic pump operation.

simple pump diagram
pump diagram

Consider a simple bicycle pump pictured above. It is composed of a piston which is a one way valve sealed inside a cylinder equipped with two, one way valves. The one way valve on top allows air to enter the cylinder when the piston is moved down by the attached rod. The piston seals against the cylinder acting as a one way valve and and presses down on the air below it. This downward movement causes two things to happen: the piston is forced down by the attached rod, and air is forced out of the bottom of the pump under pressure. At the same time, air rushes into the top part of the pump to fill the area of lower pressure created by increased volume. When this happens, both one way valves are open allowing air to move.

When the rod is pulled up both one way valves close, effectively sealing the cylinder off from its surroundings. As the piston is pulled up, the portion of air in the cylinder above the piston is squeezed into a smaller volume and is pressurized. At the same time, the portion of the cylinder below the piston increases in volume effectively decreasing the pressure. The air is forced from the high pressure upper portion to the lower pressure section of the cylinder below, past the one way valve piston.

foot pump
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Pump efficiency

To efficiently operate this simple pump, it is imperative to pull the piston all of the way to the top of the cylinder on the upward stroke, and press the piston completely to the bottom of the cylinder on the downward stroke. Failure to do so will result in inefficient movement of air by the pump. Failure to compress the piston completely against the bottom of the cylinder will result in some air remaining in the bottom portion of the cylinder. During the following upward stroke less pressure difference between the top and the bottom of the cylinder will limit the quantity of air transferred to the bottom of the cylinder. This will leave some pressurized air in the top of the cylinder, and will draw less air in through the top one way valve during the next downward stroke. So, an incomplete upward or downward stroke will limit the quantity of air moved by the pump.

In most other pumps the same forces are at work. Consider the rotary pump pictured below.

rotary pump
rp

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