The science of pumping

Pumps are a common item in many rental inventories. In a lot of cases, renting them to customers is a simple matter of determining the right type of pump and the correct size. When your customer has a large or complex project, however, it's helpful to understand a little about hydraulic science.

The start to any successful pump rental begins with asking your customer what they are trying to do, says Pete Snow, training manager at Godwin Pumps of America. "Getting a big picture view of the pumping application helps determine the follow-up questions and, ultimately, the design of the portable pumping system. Is it a small dewatering job at a construction site for a few hours or days, or is it a large sewer bypass requiring continuous pumping for weeks? The level of complexity dictates the course of action."

Bill Thompson, president of Thompson Pump & Manufacturing, notes some of the questions that the rental counter person should ask initially:

• Where is the application/pumping taking place?
• What are you pumping?
• How much needs to be pumped?
• How high does the liquid need to be pumped up/how much does the liquid need to be lifted/is the liquid in a large ditch (in other words, what is the lift pressure)?
• How long will you be pumping?

According to Snow, small dewatering jobs can be handled by gasoline-powered wet-priming pumps with suction and discharge sizes of 2 to 3 in. in diameter. These pumps can run for several hours on a tank of gas and will move up to 250 gallons per minute. "The system design usually includes 20 to 30 feet of suction hose and 100 to 200 feet of layflat discharge hose," Snow says. "General rental businesses can meet these application requirements."

On the other hand, large dewatering or sewer bypass jobs are far more complex and can involve 24-hour pumping operation and flows up to 4,000 gallons per minute and more.

Know the basics
To fully serve customers with large dewatering projects, you need to first understand the fundamentals of pumping. According to Snow, the simplest way to approach a customer's pumping application is to identify three elements: flow, lift and distance.

Flow - The amount of flow (usually measured in gpm or gallons per minute) dictates the size of the pump and hose. Flow is the horizontal axis on a pump performance chart.

Lift - This means the vertical elevation difference from the water to the pump suction and from the pump discharge to the end of the hose or pipe. The more lift, or elevation difference, the stronger the pump required to overcome gravity resistance when moving water.

Distance - This refers to the space between the pump and the discharge point. This determines the amount of friction resistance encountered when water travels through the hose or pipe.

"Lift, or gravity resistance, and distance, or friction resistance, are combined to determine the total amount of resistance the pump will see during the application," Snow explains. "This is referred to as total dynamic head and is the vertical axis on a pump performance curve."

He continues, "Each pump is rated capable of achieving a certain amount of flow and overcoming a certain amount of resistance. Ideally, the pump system will be designed so that the pump operates in the middle area of flow and head, known as the 'sweet spot' of the curve."

To best serve customers with large pumping projects, Thompson notes that rental personnel need to understand that all jobs are different, they must have a working knowledge of pumps, the different types of pumps and which is best for certain applications. They also need to know about various accessories and how to properly set the pump up on the jobsite.

Hydraulics 101
While a bit academic, some of the principles governing the behavior of water are interesting and can help in the understanding of what a pump is capable of.