Clearing the Water

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."

Conrardy offers this rule of thumb: "A pure dewatering pump is designed to move water with small solids up to 0.25 in. in diameter. Trash pumps are capable of moving larger solids up to 3 in. based on the size of the pump. Using a dewatering pump in these situations can result in clogging and heavy wear to the components, as well as poor performance."

Decide where the pump will be
It's usually best to place a pump as near to the liquid as possible to achieve optimum performance.

"To understand why, you must realize that pumps do not suck," Rieboldt says. Although the term 'suction' is commonly used when referring to pump parts (i.e., suction inlet), pumps don't actually suck in liquid. "It's a partial vacuum created within the pump, along with atmospheric pressure acting on the liquid surface, that pushes the liquid up the suction line."

"In simple terms, the closer you can place a pump to the water or the liquid source being pumped, the more you're going to pump," says Widrick. "The elevation or distance you're pumping the water really doesn't affect the size of the pump. Rather, performance has more to do with how close you are to what you're pumping."

This has to do with Static Suction Lift (SSL), or the vertical distance in feet from the centerline of the pump to the level of the liquid to be pumped. The lower the suction lift, the more flow the pump will experience; conversely, the higher the SSL, the less flow experienced.

In addition to the distance from the pump to the water, elevation has some impact. "Engines on pumps must be derated, because as you go up in elevation, you will lose capacity," Conrardy explains. "This results in loss of performance of roughly 3% for every 1,000 ft. of elevation. And because centrifugal pumps rely on atmospheric pressure to bring water into the suction hose, suction lifts are also reduced."

Elevation requirements do not always affect the size of the pump, but you may have to look for models with oversized engines. "That's so you have enough horsepower to get the job done," says Conrardy. "Sometimes pump setup changes can accommodate for losses at elevation, like getting the pump closer to the water to compensate for losses in suction lift capacity."

"Surface-mounted portable pumps can lift up to 28 ft. from liquid to pump impeller," Snow states. "However, when long lifts occur, usually over 20 ft., flow potential is reduced significantly.

"In the case of a long suction lift, atmospheric pressure in the suction hose is reduced dramatically, which can lead to suction cavitation," he continues. "A pump experiencing suction cavitation will sound like it's 'pumping rocks.' When long suction lifts are necessary, an alternative would be an electric submersible pump."

Snow adds that faster is not always better for dewatering applications. "Many dewatering jobs can be accomplished while running the pump and diesel engine at a modest speed of 1,500 to 1,600 rpm," he says. "Higher speeds can lead to suction cavitation. If that happens, reduce the diesel engine's speed and the noise should go away. If left unattended, this situation will completely pit and destroy the impeller."

Consider discharge pressure
Discharge pressure or head is related to the elevation and distance the water must be pumped. "In selecting the proper pump for a given application, the question always must be asked, 'How high or far do we have to pump the water?' The higher the water must be moved, the greater the discharge head the pump needs to produce," Conrardy asserts.

Head is measured in feet of water, rather than the more familiar pounds per square inch (psi). You can convert psi to head in feet of water by multiplying psi x 2.31. This means a pump with 100 ft. of head pressure would equate to 43.3 psi.

"Head is a design aspect of the pump, based on the size of the engine and the diameter of the impeller," Conrardy says. "Larger diameter impellers create more pressure, but there's a balance required between the engine's limited horsepower that helps create pressure and the flow rate. This is why high-pressure pumps will have low flow rates, and pumps that have high flow rates will have lower pressures."

If you need a high head pressure at the discharge - say, to feed into a force main in a sewer bypass construction application - you will need to have a larger diameter impeller and pump casing. "Centrifugal pumps are well-suited for high head applications," adds Rieboldt.

Calculating Suction and Discharge
According to Gorman-Rupp's Jim Widrick, to match the right pump type and size to the job, you should consider the suction and the discharge independently. Here's his advice about these two considerations:

Suction: Determine the friction loss head, or the head required to overcome the resistance to flow in the pipe and fittings. Typically, this depends upon the size and type of pipe flow rate, and the nature of the liquid. Then add the static suction lift (SSL) and the friction loss (FL) together.

Discharge: Conduct similar calculations. Take a look at the static elevation difference, and add this number to the friction loss for a total dynamic discharge head - or the actual height to which water is to be pumped from the water level at the source to the delivery tank - taking into account the friction loss created by the movement of water through the delivery pipeline.

Next, add the suction and discharge calculations together to achieve Total Dynamic Head.

"Now, take a look at a pump performance curve (see below) to determine the maximum point of operation -- the point on the curve where your pumps' performance is to be achieved," says Widrick. "Keep in mind, the No. 1 thing professionals of all types frequently forget is to consider whether the pump being considered can actually deliver the minimum flow rate required for a given suction lift."

He adds, "Without these careful calculations, it's easy to put a good pump into a wrong situation. If all else fails, call a pump specialist. He or she should be able to help you calculate the right factors, as well as other relevant aspects, including pump efficiency differentials and anticipated total cost of ownership."