By Jim MacRae, Project Specialist, Ketek Group
If you’ve ever thought about it, you might have thought pumping water is simple: Hook a pump to a couple of hoses, turn it on and away you go.
Nothing could be further from the truth. Pumping water, especially large volumes over long distances, is a science that involves calculations of atmospheric pressure, volume, flow, friction loss, and more.
The basic premise is that pumps work by creating a vacuum. Nature, which abhors a vacuum, searches for a way to fill it. If the pump is connected to a suction hose and the other end of that hose is immersed in water, the weight of the atmosphere pushing down on the surface of the water will force it into the hose and up to the pump. The impeller in the pump will then push the water along the discharge hose.
The calculation of atmospheric pressure is crucial to choosing the right pumps. As you may remember from high school science class, the weight of the atmosphere pushes on the surface of the Earth. If the planet’s surface were perfectly round, the pressure (at sea level) would be 14.7 pounds per square inch (PSI) everywhere. But it’s not. So atmospheric pressure increases at low elevations (where there is more air above you) and decreases at high elevations.
Atmospheric pressure of 14.7 PSI will force a column of distilled water to rise 33.9 feet in a vacuum. Of course, water to be pumped is never distilled – it’s usually coming from a river or lake or from an underground well. So the rule of thumb is that most pumps can lift water 20 feet. But even that calculation is just a beginning, as the pumps actual efficiency will be affected by temperature, humidity, suspended solids, and the inability of the equipment to create a perfect vacuum.
Then there’s friction loss and “head” to consider. Friction loss is the loss of pressure that occurs because, as the water tumbles through the pipe, it encounters resistance from other water molecules and from the sides of the pipe, which might be rough. “Head” is the height to which a pump can raise water. If you have to pump water up 30 feet and your pump doesn’t have at least 30 feet of head, it won’t work.
Many pumps need to be primed before operation. Prime refers to the suction hose being water tight. Air bubbles in the water can implode and cause a shockwave known as cavitation, which damages the pump surface. This is a potentially dangerous problem as cavitation can cause the impeller casing to explode and send metal shards flying.
Each pump will have an operational sweet spot which can be seen on a pump curve, which is a graphical representation of the pump’s performance characteristics. For complex jobs, where multiple pumps are required, it’s best to prepare a profile of the entire job. Think of each pump as a runner in a relay race. As one pump is almost exhausted, it passes the water to the next pump, which is fresh! You don’t want the pumps to be too close together or you’re wasting energy. If they’re too far apart, the water won’t get there.
Even with all the calculations, it’s important to remember that every pump is unique and requires a knowledgeable operator. Improper operation can lead to property damage and serious personal injury. Pump operation can be particularly tricky in cold climates, where snow and ice add to the challenges. Be sure to use a company with the experience to determine which pump, or pumps, are right for the job.
Visit Ketek.ca to learn more and to see the variety of pumps Ketek offers.