Selecting the Right Trash Pump

Trash pump selection for a dewatering application means more than using what is readily available. A pump is an investment. Portable pump prices can range from a few hundred dollars to more than $100,000. All factors should be considered to achieve the greatest return on investment.

Choosing the correct pump is a science and depends on:

• Required flow
• Pressure
• Solids size
• Suction lift
• Altitude
• Specific gravity
• Viscosity
• Temperature
• Fluid type

Pump application engineers often provide a pump checklist to relay all the pertinent information to the pump manufacturer. The solids content plays a large role in dictating the proper pump type. Making a well-informed choice pays off in the long run. The goal is to select the pump that will accomplish the job most efficiently. Selecting the least expensive pump may reduce upfront costs but may increase the risk of downtime for repairs and maintenance, making the pump less efficient and ultimately more expensive.

Trash pumps are self-contained centrifugal pumps with a power source, such as a diesel engine, and a pump end mounted on a frame or trailer. Typically portable and capable of handling liquids with solids—such as small twigs, rocks, and leaves—trash pumps are often the preferred choice of contractors and the rental industry because of their versatility and ability to handle large amounts of debris. End users should consult the manufacturer’s specifications for solids-handling diameters. The majority of these pumps can handle solids from 2 to 6 inches and can move hundreds or even thousands of gallons per minute. These pumps are ideal for dewatering applications, moving water and construction applications.

High-quality trash pumps may cost more upfront, but the return on investment is worth the higher cost because they can reduce downtime for repairs and maintenance, making them more efficient to operate. To ensure pump quality when renting or purchasing, look for pump manufacturers that build their products to meet the requirements for certification as defined by the Contractors Pump Bureau (CPB).

The CPB is a group within the Association of Equipment Manufacturers that primarily provides recommended pump performance standards. The CPB uses three ratings to classify pumps: M, MT and MTC. Trash pumps typically carry the MT or MTC rating. These standards govern the materials of construction, the accessibility to the interior for clean-out purposes, the sizes of spherical solids that the pump can pass and the minimum gallons per minute that the pump can handle.

The Heart of the Pump

The pump impeller is the heart of the pump. It allows the pump to achieve the capacity, head, efficiency, net positive suction head required (NPSHR) and solids-handling capability. The impeller is a fan-like piece that rotates at a high speed and forces air out the pump end. This creates a vacuum inside the pump end and draws water in. The water is then discharged through piping or a hose at another location. Many impeller designs can achieve similar results, but deciding the best impeller type is relative to the application and the preference of the pump operator. Three common impeller types include enclosed, semi-open and open.

Enclosed impellers are the most common design found on International Organization for Standardization pumps and are preferred by oil refineries that pump hazardous content. This is the most efficient of the three impeller types. The enclosed impeller has an attached shroud and hub surface that provides strength and stability so that the vanes do not need to be as thick, resulting in increased flow. The goal of an impeller is to efficiently guide the water in through the eye and out through the vanes without recirculating or diverting it. The enclosed design accomplishes this much more efficiently because of its built-in shrouds, which guide the water and eliminate leakage. The enclosed impeller design can also compensate for shaft thermal growth, which means that the impeller never has to be adjusted.

Semi-open impellers have a single shroud, either on the back or front. Similar to the enclosed impeller, this allows for slightly narrower vanes and reduced leakage because only one leakage path over the vanes is available, making it more efficient than a fully open impeller. The semi-open design allows particulates to pass easily. However, fluid pressure builds on the one shroud, putting excess stress on the bearings and requiring balancing.

Open impellers cost less to manufacture, making pumps that have them less expensive to purchase. However, pumps equipped with an open impeller may cost more to operate and maintain. The open impeller may have any number of vanes depending on the manufacturer, but does not have a front or rear shroud, which provides the benefit of being easily accessible when unclogging is required. The vanes are generally thicker, so the impeller has the structural integrity to support itself and results in decreased flow.

The open impeller is also more sensitive to wear. As it wears, the gap between the impeller and the wear plate increases, requiring constant adjustment and may cause potential leakage, which greatly impacts pump performance. Additionally, to balance pressures on the front and rear sides of the impeller, balancing holes are drilled through the impeller’s hub. These reduce axial thrust, but they also allow fluid to pass through the impeller rather than out through the casing. This increases the NPSHR and reduces pumping capacity. The holes tend to become enlarged over time when pumping abrasive materials, further reducing efficiency.

Work with Experienced Professionals

Only trained and qualified personnel should transport, set up, operate or maintain a pump. Using factory-trained technicians is recommended, and contacting the pump provider for questions and solutions is encouraged.