Griffin Dewatering

Large Diameter Impellers Make Trash Pumps More Versatile

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Courtesy of Courtesy of Griffin Dewatering

The introduction of these impellers allow for a higher range of discharge pressure.

Pumps are available in many types—diaphragm, positive displacement and progressive cavity—but the most common is a centrifugal pump. Centrifugal pumps have an impeller that spins, transferring energy to the fluid by centrifugal force and directing the water to the discharge point within the pump casing.

Trash Pumps Defined

Simply stated, trash pumps have the ability to pump solids. Despite their name, the pumps do not actually pump what most people typically identify as trash, which is deposited in trash cans. Instead, it is used for solids-handling, pumping materials such as those in a typical sewer pipeline. Perhaps the trash pump was named to provide a more attractive representation of its intended use.

The impeller and pump casing are what differentiate a standard centrifugal pump and a trash pump. The impeller has a solids-handling capacity and is sized in inches of spherical solids, which means it will pump a solid of a certain round size. Typically, trash pumps will have a capacity of 1-inch or more on a small 2- to 3-inch discharge size pump and 3 inches or more on any pump with a discharge of 4 inches or larger.

Once primed, the fluid is drawn into the pump casing and is pumped out by the impeller. Impellers are similar to fans because they are constructed of vanes (like a fan blade) that add velocity to the fluid (in the same way that a fan blade speeds up and the air flows through it). When the impeller moves, the length of the vane controls the pressure that the pump can produce, while the depth or width of the vane controls the volume and, with the impeller design, its solids handling capacity.

Impeller Design

Different types of impellers are available—including vortex and grinder—but the most common is a semi-open, trash-handling type and an enclosed, non-clog impeller.

The semi-open, trash-handling impeller design has an open face on one side of the vane that requires a wear plate to complete its design for pumping. An enclosed, non-clog impeller uses an enclosed impeller design that eliminates the need for gap adjustment. It can include replaceable wear rings to increase its useful life. The non-clog design typically has a higher efficiency, which means it offers a higher pumping capacity per horsepower than the semi-open trash pump, resulting in reduced operating costs. While a non-clog impeller pump provides higher efficiency and is more forgiving regarding tolerances and maintenance, it will be more expensive than other pumps. However, it may provide better performance and longer life. Therefore, when considering costs, all life-cycle factors should be included in the decision.

Installation Methods

When selecting a trash pump, end users need to know the maximum fluid depth and consider the installation method because this will dictate what type pump can and cannot be used. Trash pumps come in two basic installation methods—above-ground or submersed.

Typically, above-ground pumps are engine-driven or electrically driven, while submersible pumps are electrically or hydraulically driven. Either installation method will have advantages and limitations.

For most applications, above-ground pump setups are preferred for ease of installation and access to the pump, which requires maintenance or may suffer a mechanical issue. An above-ground installation involves the pump placed near the fluid. A hose or pipe is connected to the suction side and placed into the fluid that must be moved. A hose is connected to the discharge of the pump to direct the fluid to the desired discharge location. Other than the pump, this setup requires minimal space on the suction side because the only item entering the fluid is a hose or pipe.

While this setup is desirable, all pumps are bound by the laws of physics. If the fluid is below the pump, then all above-ground centrifugal pumps need a vacuum or need to create a vacuum at the pump, allowing atmospheric pressure to fill the void and push the fluid to the impeller. At sea level, the theoretical maximum suction lift for water of any above-ground pump under a perfect vacuum—regardless of whether it has a priming mechanism—is 33.9 feet. However, other factors limit the pump to a practical lift of substantially less—closer to 25 feet of suction lift with a maximum of approximately 28 feet. However, the volume greatly diminishes as the suction depth increases. Therefore, if the pumping level is approximately 25-feet deep, then submersible pumps should be installed because they do not have suction lift limits.

Submersible pumps are placed into the fluid and push the fluid up onto the pump to a connected hose or pipe that carries the fluid to the discharge point. While a submersible setup is not limited to the pumping depth, it may create access issues since the setup is submersed. If the pump requires maintenance, it must be removed and replaced.

Some submersible pumps have a maximum submergence depth (some as shallow as 50 feet or less below the fluid level), and special precautions are necessary to protect the pumps under these conditions. End users should check with the manufacturer’s recommendations when in doubt.

Pump Types

An engine or electrically driven, above-ground trash pump is available in many types with different features. A self-priming trash pump is a specially constructed pump that will prime as long as its casing is initially filled with water. These pumps are generally easiest to use because they consist of a pump and its driver (electric motor, gasoline engine or diesel engine). The pump chamber is filled, and the pump is turned on.

The impeller creates the vacuum necessary for priming and pumps the fluid out, once primed. Typically, self-priming pumps are used when priming time is not critical. The priming time is the time required from startup until the pump begins pumping by evacuating the air within the suction pipe or hose. The priming time depends on the suction lift but may take minutes with this type pump.

A prime-assisted pump will prime under dry conditions, without water in it. Generally, prime-assisted pumps prime quicker than a self-priming pump. The priming time depends on the volume of air that needs to be evacuated, which is based on the size and length of the suction pipe/hose. Prime-assisted pumps consist of a pump, driver and priming device. The priming device may be a vacuum pump, diaphragm pump or compressor/venturi, which is installed with an air/water separation chamber with its float or priming valve. As the priming device’s air-handling capacity increases, so should the size of the air/water separator.

In general, a vacuum-assisted pump will be the quickest priming pump because of the air-handling capability of the vacuum pumps. While they have the most moving parts, this type is a must for applications that require the continuous removal of large volumes of air—100 to 200 cubic feet per minute (cfm) and more—and in which priming time is critical. Typical vacuum pump types include rotary vane or liquid ring. If priming time is not as critical, the next quickest priming type is the diaphragm assist. Most diaphragm pumps will have a maximum air-handling capacity rating of 50 cfm per diaphragm. Additional diaphragm pumps could be added to increase the air-handling capacity. If the priming time is less critical, the compressor/venturi setup should be used because it generally has fewer moving parts and will likely cost less.

For pump applications with lifts of greater than 25 feet, as mentioned before, submersible pumps must be used. Since the pump is submerged, no priming device is required. Different types of submersible pumps are also offered. The most common are electric-motor driven (both the pump and motor are lowered into the effluent) and hydraulically driven, which consists of a hydraulic power unit on the surface with hydraulic hoses attached from it to the hydraulically driven submersible pump. Either type is offered with the different impeller designs described in this article.

Other Uses

While trash pumps were originally designed to have the capacity to pump solids, the design of the impellers with large vane cavities leads to higher capacities than conventional centrifugal pumps, so any project requiring a high volume can be achieved with a trash pump. In the past, trash pumps may not have been considered because of limited discharge pressure that typically could only be achieved by standard centrifugal pumps. However, the introduction of large diameter impellers has allowed for a higher range of discharge pressure. The higher discharge pressure allows for pumping a longer distance. Applications with high volumes and high pressures are now being achieved with the use of trash pumps, making them more versatile. From an owner/operator’s perspective, working on multiple projects with just one pump not only reduces the need to service different units with its inherent problems but also reduces the cost of upkeep by limiting the need for multiple replacement parts.

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