Selecting the best dewatering solution requires experience and an understanding of the physics involved. For optimum performance and equipment life, it's important to choose the correct type and size of pump, as well as set it up for the application.
Sizing pumps should be a science, not guesswork. "Trial and error is not suggested, but it is not an uncommon method in contracting pump rentals," says Mark Trumble, business development manager for RSC in LaPorte, TX.
Many parameters influence an effective pumping system. According to Dale Conway, vice president of engineering, Thompson Pump & Mfg., you need to understand the required flow, pressure, solids size, suction lift, altitude, specific gravity, viscosity, temperature and fluid type.
"If the product is water, then use suction lift and discharge elevations as a bare minimum estimate," advises Trumble. Then account for any losses in the system. "The TDH (Total Dynamic Head) is then created by adding friction losses from the distance pumped, as well as all valves, fittings and equipment downstream which have resistance to the suction and discharge."
"The longer the discharge run of the hose, the more the friction loss. The more friction loss you have, the less flow," explains Jim Widrick, construction equipment sales manager, Gorman Rupp. "So you really want to minimize how much friction loss you have through the hose." Keep in mind that all pumps are limited to a maximum suction lift of about 28 ft.
Solids content can actually dictate pump type. "Centrifugal pumps have limitations of the size and make-up (hardness) of solids," says Trumble. "Diaphragm, vane, gear, lobe, progressive cavity and submersible [pumps] all have applications that are best suited to their design and the work to be performed."
Widrick adds, "Centrifugal pumps are effective up to a maximum of 4% to 5% solids at best. If you go much higher than that, you usually have to go to a positive-displacement pump, such as a diaphragm pump."
As such, it's important to consult the manufacturer's specifications for solids-handling diameter.
Dwight Evans, engineer, Godwin Pumps, recommends using a suction screen that is slightly smaller than the solids-handling diameter to protect the pump from large debris. "Also consider the materials used in the wear parts," he says. "The higher the concentration and diameter of a solid, and the greater its hardness, the greater the potential for failure of the wear parts (impeller, wear plates, seals, volute, etc.)."
Value of wet prime
Trash pumps come in several variations with different inlet sizes, solids-handling capability and efficiencies.
"Wet prime trash pumps are one of the easiest to use and maintain as they have fewer components than other pumps - basically a pump and a motor/engine," says Jerry Soto, Griffin Dewatering Corp. "Wet prime pumps require the pump cavity to be filled for the first time, then will 'self prime' as long as there is water above the eye of the impeller. To prevent the pump casing water from siphoning the water out when the pump is shut down, there is a check valve on the suction side."
Fewer moving parts can be a big advantage. "Depending upon how it is designed, that wet prime pump can last 20 or 30 years with regular maintenance," says Widrick. "The difference in the pump end is the wet prime pump has a large volute to hold a large volume of water."
Evans notes, "The benefits of a wet priming trash pump are portability, independence from a designated power source, ability to be placed right at the water's edge and relatively inexpensive purchase price and operating cost. Disadvantages include the need to manually prime, inability to control speed and limited fuel capacity that demands more frequent monitoring and refueling."
Wet prime pumps are a little more bulky, and priming is affected by internal pump wear, adds Conway. Dry prime pumps, on the other hand, are smaller in physical size, and priming is unaffected by internal wear. However, there are more moving parts to maintain.