When It's Okay to Talk Trash About Pumps

Solids content in liquid can dictate the type of pump best suited to a particular dewatering task.

Centrifugal pumps are effective up to a maximum of 4% to 5% solids. Anything higher will require a positive-displacement pump, such as a diaphragm pump.

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. 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.

The benefits of a wet-priming trash pump are portability, independence from a designated power source, the 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 models have operational limitations. They are best suited for applications with a relatively low sump. 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 involving trash-laden water found in construction applications, stream and pond dewatering and septic tanks and manholes.

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. They also can be used for bypass pumping when there is sufficient retention time to allow for the pump to prime.

While wet-prime pumps can range up to 12 in., for the most part, they have a lesser capacity than dry prime pumps due to design limitations.

Benefits of staying dry

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. The priming process is achieved by the use of an integral compressor, which creates a full vacuum through a venturi eductor. 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, but the initial cost is probably 40% more.

Generally, dry prime pumps will incorporate non-clog impellers, which are often more efficient than the open trash impeller used in the wet prime. The non-clog design allows for larger sizing. Yet, the term “non-clog” can be misleading. The impeller will clog if a large enough solid is pulled into the pump, such as a plastic bottle, towel, brick, etc.

Due to their ability to handle large volumes of air, dry prime models are often used in applications where they might 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. 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. 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.

Laying it on thick

Certain situations require positive-displacement pumps, such as a diaphragm pump. Diaphragm pumps will handle very thick materials and create full discharge pressures without regard for any certain position on the performance operating curve. Traditionally air driven, but also engine and electric fitted, they are generally of low volumes.

Diaphragm pumps are particularly suited for low-flow applications with muddy water, but they can only handle low flow and low head (pressure). The advantages are they can run dry, are relatively inexpensive, are pretty portable and are relatively small.

Because they can handle both equally, it generally doesn’t make a difference if the pumps are moving air or water. The major disadvantage is that diaphragm pumps require a lot of maintenance. The back and forth action on the diaphragm tends to wear it and other components out quickly.

Diaphragm pumps are not ideal for liquids containing abrasive solids, as they’re susceptible to a relatively short diaphragm life, depending on the product being pumped.

Be sure to consult your pump supplier to ensure you rent or purchase the most appropriate pump type for your particular dewatering task.

 

Sources: Gorman Rupp, Griffin Dewatering Corp., Thompson Pump & Mfg., Tsurumi America, Xylem Dewatering Solutions

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