How to Find the Right Pump

Pump suppliers often hear their customers say, "I need a six-inch pump." This is a typical request but lacks the vital information necessary for choosing the correct pump for a particular application. While the same pump can be used for dewatering, mining or other related applications, end users must evaluate several factors to determine the best fit for their pumping needs. These factors include the pumped material, the distance from the pump and the elevation difference from the source to the discharge point.

Pumping Basics

Pumps move water or other fluids from point A to point B. They come in different types, including diaphragm pumps and positive displacement pumps. Diaphragm pumps work like the human heart, where valves allow blood to flow into a chamber. From there, the muscle flexes and forces the blood out through the arteries. Positive displacement pumps are similar to a water cannon toy and consist of a cylinder and a plunger. The plunger forces the water out. The most common type of pump is the centrifugal pump. These use an impeller that spins and adds energy to the fluid by centrifugal force and directs the water to the discharge point. Just as fans contain fan blades that speed the air passing through them, impellers are constructed of vanes that add velocity to the pumped fluid.

Above-Ground or Submersible?

All pumps are bound by the laws of physics. If the pumped fluid is below the pump, a centrifugal pump must be able to create a vacuum to allow atmospheric pressure to fill the void and push the fluid to the impeller. At sea level, the theoretical maximum suction lift for water is approximately 33.9 feet; however, other factors limit pumps to a practical pumping suction lift that is closer to 25 feet of suction lift with a maximum of approximately 28 feet. The volume diminishes as the suction depth increases, so users should consider submersible options as suction lift increases. For most installations, above-ground pump setups are more desirable because of their ease of installation and access to the pump for maintenance. An above-ground installation will include the pump placed near the fluid, a hose or pipe connected to the suction side and placed into the fluid, and hose or piping connected to the discharge of the pump to direct the fluid to the discharge point. This setup requires minimal space on the suction side since the only item going into the fluid is a hose or pipe. If the suction lift is 25 feet or more, a submersible pump should be used. This type of pump is partially or entirely placed in the fluid and pushes the fluid up. It is not bound by a suction lift. A hose or piping is connected to the pump and lowered into the fluid, and additional hose or piping is connected to the discharge point. While a submersible setup is not limited by pumping depth, it may create access issues. The entire unit needs to be placed into the fluid because of the pump size. If maintenance is required on the pump, it must be removed and replaced. Some submersible pumps have a maximum submergence depth below water—some as shallow as 50 feet or less below the fluid level. Special precautions are necessary to protect the pumps under these conditions. Users should check with the manufacturer's recommendations.

Priming Systems

If the pumping level of the fluid is below the pump, a standard centrifugal pump requires a priming system to evacuate the air and allow atmospheric pressure to push the fluid into the impeller. This can be achieved by a hand primer—typically a manually operated diaphragm pump—in combination with a foot valve strainer on the suction hose or other priming mechanism. Once primed, the centrifugal pump will continue to pump as long as the suction lift does not exceed the suction lift capacity of the pump or have an air leak that the pump itself cannot overcome. A self-priming pump is a specially constructed centrifugal pump that will prime itself as long as its casing is filled with water. This type generally consists of a pump and a driver, which could be an electric motor, a gasoline or diesel engine. These are typically the easiest to use. When the pump chamber is filled with water and the pump driver—which spins the impeller—is turned on, the impeller and casing create the vacuum and evacuation of air necessary for priming and pump the fluid out. Self-priming pumps are typically used in applications where priming time is not critical, because the priming time is dependent on suction length and lift and can take several minutes. A prime-assisted pump is a centrifugal pump that has an automatic priming device and will prime itself under dry conditions. Priming can occur whether the pump has water in it or not. Prime-assisted pumps prime substantially faster than self-priming pumps. The priming time is highly dependent 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 include a pump, driver and priming device. The priming device may be a vacuum pump (using rotary vane, liquid ring or other type of vacuum), a diaphragm pump (which is connected to the pump or engine shaft) or a compressor and venturi combination. All are installed in conjunction with an air/water separation chamber with its float or priming valve. In general, a vacuum-assisted pump will be the quickest priming pump, followed by the diaphragm prime and then the compressor with venturi setup. Once primed—or submerged in the case of submersible pumps—the fluid is drawn into the casing and is pumped out by the impeller.

Clean Water, Solids Handling

Impellers are constructed of vanes that add velocity to the pumped fluid. When the impeller is spinning, the length of the vane generally controls the pressure the pump can produce, while the depth or width of the vane controls volume. The most common types are enclosed standard centrifugal impellers for clean or semi-clean non-solids fluid, semi-open and enclosed non-clog impellers. The impeller design and its pump casing are the difference between a standard centrifugal pump and a solids handling pump. Typically, the impeller for clean water will be a highly efficient enclosed impeller that will be thinner and have more vanes than an impeller meant for solids handling. The impeller of a solids-handling pump is wider with fewer vanes to allow passage of a solid. The solids-handling impeller has a capacity sized in inches of spherical solids that it will pass. Semi-open trash handling impeller designs have an open face on one side of the vane and require a wear plate on the other. This design will require periodic adjustment. The wear plates are replaceable, which is a cost benefit as long as the impeller is not also worn. The enclosed non-clog impeller design combines the efficiency of an enclosed impeller and the solids-handling capability of a semi-open impeller. It can include replaceable wear rings to increase its useful life. The non-clog impeller pump has a higher efficiency and higher price tag than other types, but it will cost less to operate for a given pump volume or head.

Dewatering, Stormwater

Typical dewatering or contractor pumps deal with water, dirty water or trash-laden effluent, which could come from pumping out a pond, ditch or other source. These are not to be confused with groundwater dewatering, which is a more specific type of pump and system. A dewatering pump can be a small gasoline-driven self-priming pump or electric submersible pump. They can also be more sophisticated dry-priming pumps or larger submersible pumps that will generally only be available from a pump specialty store (Image 2). Stormwater pumps are generally larger. Sometimes they will be of higher pressure than a standard dewatering pump, since they may need to pump the fluid further. Stormwater pumps may be submersible or diesel-driven and will likely need automatic controls for automatic startup. Since they are likely handling large volumes and may have to pump debris along with water, they are nearly always solids-handling pumps. Mining pumps are more specialized than dewatering or stormwater pumps. They may be required to handle high pressure and high volumes, and the pump end may also need to be constructed of abrasive or corrosive-resistant materials, such as hardened iron or stainless steel. Depending on the application and access at the site, the pumps could be submersible or diesel-driven. The term jet pump usually refers to a higher pressure pump that uses the pressure to "jet" piles, wells or caisson into the ground. They typically use clean water impellers and may have multiple impellers that increase the pressure capacity of the pump. Most of the time, these pumps are diesel-driven, but some submersible pumps can be used under certain conditions.

Conclusion

Whether the pump is used for dewatering, stormwater, mining or other applications, several factors designate which is the proper pump. These include the material being pumped in combination with the distance from the pump and the elevation difference from the source to the discharge point. Many times, it pays to consult with a pump expert to make sure the best pump is chosen. Only those companies that routinely use pumps of all types will have the experience to see beyond the request for a six-inch pump.