Wet prime models have operational limitations, as well. "Wet priming trash pumps are best suited for applications with a relatively low sump," says Evans. "Specifically, applications with less than 200 ft. of discharge - not pumping more than 350 gpm - are the most common."
Yet, their impeller design makes them a good solution for jobs where solids are present. "They are successful in applications including trash-laden water found in construction applications, stream and pond dewatering and septic tanks and manholes," says Evans.
They work best where there is an abundance of liquid. "It would be preferred to have the pump running constantly in an application where there is plenty of water or liquid that is being pumped," Evans states, "or if the unit has an automatic float control for automatic on/off operation."
Soto adds, "The best conditions for wet priming pumps are open pit and sumping applications, where the suction lift is less than 25 ft. and where priming time is not as critical. Wet self-priming pumps normally take longer to prime, depending upon suction lift and length of suction hose/pipe.
"They also can be used for bypass pumping when there is sufficient retention time to allow for the pump to prime," he continues. "While the wet prime pump can handle some air intrusion on the suction side, if there is a significant intrusion of air, it will lose its prime and will need to re-prime itself. So for a good period of time, the pump is not pumping."
In addition, while wet prime pumps may range up to 12 in., this says little of their capacity. "For the most part, wet self-priming pumps are of a lesser capacity due to their design limitation than their cousins, the dry prime pump," says Soto.
When dry is best
Dry prime pumps prime without the need to add fluid. Prime is maintained via a priming device, which is normally either a vacuum unit, diaphragm or compressor.
Trumble explains, "The priming process is achieved by the use of an integral compressor, which creates a full vacuum through a venturi educator. This is suited for nearly all dewatering and transfer applications that require high and rapid static suction lift."
Rapid priming coupled with higher volumes results in greater efficiency over wet priming units. "A dry prime pump with vacuum assist can increase performance by 20% over a wet prime pump," Widrick estimates, "but the initial cost is probably 40% more."
According to Soto, a dry prime pump will nearly always outperform a wet prime pump due to its design. "Generally, dry prime pumps will incorporate non-clog impellers, which are often more efficient than the open trash impeller used in the wet prime," he notes. "The non-clog design allows for larger sizing, allowing the pumping capacity so you can get a much bigger dry prime."
Yet, Soto cautions that the term "non-clog" can be misleading. "The impeller will clog if a large enough solid is pulled into the pump, i.e., plastic bottles, towels, bricks, etc.," he points out.
Due to their ability to handle large volumes of air in the suction, dry prime models are often used in applications where they may experience dry running or see intermittent flow, such as sewer bypass or construction sites where the unit must constantly work to keep the area completely dry. "[Large air volumes] can occur from suction vortexing, turbulence or low fluid levels," says Trumble. "The prime-assist dry prime will pick up suction prime without any need of an operator filling the pump with fluid."
"Dry prime trash pumps can use automatic controls to start and stop pumping based on flow levels," adds Evans. "The obvious advantages include extended hours of service, increased capacity (volume), decreased dewatering time, higher lifts, independent operation, a broader range of liquids pumped, more durable wear parts and larger solids-handling capabilities."
A solution for thick fluids
Certain situations require positive-displacement pumps, such as a diaphragm pump. Diaphragm pumps are available in a wide variety of materials and elastomers. Advantages include few moving parts and simple operation.
"[Diaphragm pumps] will handle very viscous (thick) materials and create full discharge pressures without regard for any certain position on the performance operating curve," says Trumble. "Traditionally air driven, but also engine and electric fitted, they are generally of low volumes."