Tips on Pump Selection

Taking the challenge out of selecting a dewatering pump

Selecting a dewatering pump for an application can be a challenging enterprise. There are many different types of pumps and pump systems available on the market. Concurrently, many different pump manufacturers, dealers and distributors are vying for your business and are ready to supply the equipment for your application. As such, it’s easy to become inundated with enough information on so many different kinds of pumps to make your head spin.
Along with the multitude of models of pumps available, there are many different types of applications where pumps are needed.
Your application may be simple—such as moving unwanted, clear rainwater without any solids from a retention pond where time and output requirements aren’t an issue. On the flip side, your application may be complex like replacing an interceptor sewer line where a multi-pump system is required to handle the flow of sewage during low and peak flows. This pump must also be able to provide environmentally-safe pumping by assuring the sewage will not leak out of the pump system.
Knowing your application well and being able to report this information to pump experts when you request their assistance can help you be assured that you’re quoted the best pump for your application.
Some aspects to consider when choosing a dewatering pump for an application:

  • Type of Fluid: Pumps can handle all kinds of different fluids, such as clear water without solids; water or sewage with solids in suspension; viscous, stringy, and sludge- laden materials in suspension; and neutral, acidic or alka line fluids as well. The type of fluid may require a pump end or pump end parts made of special materials.
  • Temperature: Another aspect to consider is the temperature of the fluid. If your fluid is at higher temperatures, it may cause the pump to cavitate or not operate as well as if the fluid was at cooler temperature.
  • Flow Required: Knowing the flow required might mean the difference in a successful application and an environ- mental disaster. The pump or pumps used must have the capacity to handle the maximum expected flows. An emergency situation such as a broken sewer line can only be repaired successfully if the pump or pumps that are used can handle the flows the sewer line is experiencing at its peak. Also, knowing the flow required can allow planning for other phases of the project.
  • Suction Lift Requirements: Suction lift is the vertical distance of suction the pump has to overcome to bring the fluid into the pump. Suction lift spans from the top of the pumping fluid, into the pump casing at the eye of the impeller. The laws of physics pertaining to atmos- pheric pressure only allow a pump to lift the water approximately 28 ft in ideal conditions, which means the pump cannot lift the water into the pump if it is more than 28 ft higher than the pumping fluid.
    Depending on the type of pump you choose, you may have to take some precautions when dealing with high suction lifts. With self-priming centrifugal and end-suction centrifugal pumps, you may have to create a platform that your pump can safely rest on that is closer to the pumping fluid to reduce the suction lift. This is some times referred to as “benching down.” With submersible pumps, where the pump end is submerged directly into the pumping fluid, the suction lift issue is completely eliminated, but enough power must exist to overcome the discharge head (the vertical distance the fluid
    is discharged).
    Altitude also effects suction lift. An application in Florida, where the elevation is close to sea level, may not work as well in Denver, where the elevation is considerably higher.
  • Solids Requirements: Some applications require pump- ing where trash, debris, sewage, and other materials stand the chance of entering the pump. Trash pumps are per- fect for these types of applications. They are equipped with an impeller with vanes that are spaced out enough to allow solids to pass through and be expelled out of the pump’s discharge port. As a general rule of thumb, the solids size a pump can pass is usually half the size of the discharge port. Most pump manufacturers provide the solids capabilities on their specifications. In order to prevent larger solids from entering a pump, usually some kind of strainer is affixed to the submerged end of the suction hose. The strainer allows only solids of a certain size to enter through its holes and into the suction line.
  • Head Pressure Requirements: In some applications the pumping fluid may need to enter the pump at a cer- tain pressure. The pumping source may be at a higher ele- vation causing pressure from gravity, or may have to pass through an apparatus, such as a fire hydrant, that moves water at high pressures, which is sometimes referred to as “flooded suction” or “positive suction head” applications. Too much pressure may cause the pump’s seal configuration, be it mechanical or packing to fail. This causes the fluid to leak into the environment, which is not a desirable situation, especially when dealing with sewage.
  • High Head Requirements: When faced with an applica- tion that requires a certain pressure to be provided at the discharge point, discharging over long distances, or dis- charging at a considerable upward slope, a high head, or high pressure pump may be required. The pump is designed to discharge the fluid at high pressures and is fitted with a discharge port smaller than that of the suc- tion port to provide more pressure.
  • Air Handling Capability: Because air must be evacuated out of a pump’s priming system before suction of the fluid takes place, the air handling capability of a pump must be considered for the application. Some pumps, such as self-priming centrifugal pumps, are capable of air han- dling on their own due to their design. Others pumps, such as end-suction centrifugal pumps cannot handle air on its own and require an additional priming system to be installed to provide air handling capability. Compressor pumps, vacuum pumps, hand-operated or engine powered diaphragm pumps, and other air han- dling apparatus allow the pump to handle both air and water, and provide the capability to prime and re-prime quickly. Some pump manufacturers design their priming system integral to the pump system, which could heighten the number of repair parts needed when the priming sys- tem fails. Others offer an external, add-on priming systems, which make repairs easier to diagnose and repair. The higher the air handling capability, the more air a pump can handle. This is designated on pump literature in cubic feet per minute.
  • Power Source: There are many different types of power sources available for pumps: diesel engines; electric motors; gasoline engines; and even engines powered by propane and natural gas. The pump you choose must be able to be powered by the power source you choose and the equipment you have available. Also, most diesel-powered pumps are available with a throttle, or other variable speed control, allowing you to increase and decrease speeds at safe intervals. Pumps powered by an electric motor are usually set at one speed and one speed only. Engines and motors are selected to provide maximum power at the most efficient operating speed. Run time is a consid- eration because of the need to refill fuel tanks in remote locations. Using pumps with long run times can reduce labor costs.
  • Hoses and Piping: Hoses and piping of different diame- ters and different materials, such as PVC, galvanized steel, high-density polyethylene and many others, create different levels of friction. The pump may be introduced to already existing hose or piping at the application. Depending on the flows and discharge pressures expected, the pump may be able to produce higher flows or pressures than the existing hose or piping can handle. By knowing what kind of hose or pipe is already installed, and its capacity and pressure limitations can prevent additional unwanted repairs to the existing piping. Also, bends and adapters, such as reducers or increasers, may affect the pump’s ability to operate at peak performance because the
    resulting friction losses from the hose or piping.
  • Sound Attenuation: The application may be located in a well-populated section of town, and sound attenuation may be a requirement. More and more, municipalities across the nation are requiring the use of sound attenuated pumps in construction and other applications.

With this information known, you will be better informed as to what your dewatering application actually may entail, and you will have the information available if you decide to contact a pump expert to provide their assistance.

Joe Belli is a product marketing specialist for Thompson Pump & Manufacturing. He can be reached at 800/767-7310 or by e-mail at jbelli@thompsonpump.com.

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