When you have water where you don't want it on your construction site, you need a pump to move it. Deciding what size, power and type of pump, however, can be a challenge.

To help you understand pumps better, we contacted four leading pump manufacturers. These, they say, are the major factors in successful pump selection.

Know the flow
"Required flow has everything to do with sizing a pump system," says Peter Snow, training manager at Godwin Pumps. "Pumps are sized by flow, and a supplier can't provide the right one unless you know how much flow you require."

Mark Conrardy, sales engineering manager for Wacker Corp., adds, "When selecting a pump for a given dewatering application, there are two important questions that a contractor needs to ask: 'How much water do I have to move, and how much time do I have to move it?' This is where flow rate comes into play."

Flow is measured in gallons per minute (gpm). Pumps offer different flow rates, depending on the design. For example, high-flow pumps are capable of moving large volumes of water, but do not create a lot of pressure. High-pressure or high-head pumps can move water over longer distances, but require more horsepower to achieve the same flow rates.

"By knowing the needed flow rate, you can determine if you need a small 2-in. electric submersible pump (to produce a 50-gpm average flow rate) or a large 12-in. diesel-powered portable pump to move more than 5,000 gpm," Snow points out.

"Generally, if a contractor requires a 500-gpm flow rate, he needs a 4-in. pump. A 6-in. pump can handle a 1,300-gpm flow rate, while for 4,000 gpm, enlist a 12-in. pump," advises Jim Widrick, manager of construction equipment sales for The Gorman-Rupp Co. "If the pump size doesn't adequately align with the job, the pump will pump at the rate it was designed for, and the contractor will likely be frustrated."

Ben Rieboldt of Tsurumi (America) Inc. cites the following analogy: "Just as a person uses more energy to carry a 5-gal. bucket of water than a glass of water, a pump uses more energy to move a larger volume of water than a smaller volume. Therefore, the higher the required flow rate, the greater the power required."

To move a large volume of water, the pump will need an impeller with a long vane moving within a large volute or pump casing. "You also need a larger suction and discharge bore, as well as a larger engine or motor to provide the increased power required," says Rieboldt.

Determine viscosity
"Viscosity is a measure of thickness or flowability of a liquid," notes Conrardy.

If you're just pumping water, viscosity is not a problem. If the water contains mud, sand, silt or stones, it takes more horsepower to pump the liquid.

"Most pumps used in the construction industry are meant for pumping water found on jobsites, not heavy, thick materials," he says. "But if you have such viscous materials, a positive displacement-type pump is required. In this industry, usually it means a diaphragm pump. These are meant to pump slurries or low volumes of water with a lot of heavy mud in them - and they don't create high flow rates."

"Because a high-viscosity liquid is more resistant to flow, these liquids will reduce the efficiency of a centrifugal pump and increase the power required," says Rieboldt. "In general, an application requiring a relatively low flow rate, and with small or no solids, will be well-served by a submersible dewatering pump or end-suction centrifugal dewatering pump."

That changes if you need a relatively high flow rate for a liquid that may contain larger solids. "Then, a submersible solids-handling pump or end-suction centrifugal trash pump is better," says Rieboldt. "The liquid's specific gravity must be accounted for when pumping solids. It is a measure of the relative weight of a substance - in this case, a liquid that may contain solids - compared to an equal volume of clear water at standard temperature and pressure. You also need to maintain a sufficient carrying velocity so the solids do not settle out from the liquid within the pump and discharge line."