How to Choose the Right Agricultural Pump

In 2015, irrigation accounted for almost 42 percent of all freshwater withdrawals in the United States, according to the U.S. Geological Survey (USGS) (1). This amounted to about 118 billion gallons per day. In addition to agricultural use, irrigation of golf courses, parks, nurseries, turf farms, cemeteries and other self-supplied landscape-watering uses also are included in the estimate. Agricultural irrigation accounted for about 81 percent or 95.6 billion gallons per day of the total. Surface water sources supplied about 52 percent and groundwater sources supplied about 48 percent of the total for agriculture. Many irrigation systems are pressurized and therefore need a pump. Because the heart of most irrigation systems is the pump, it is important to select the correct type for the irrigation system and it must be as efficient as possible.

Considerations for Choosing a Pump

Choosing an irrigation pump is based on the type of pump and also on the relationship between pump efficiency and the total dynamic head (TDH) at a specific flow rate. Types of pumps used in agriculture irrigation include:

• centrifugal (above ground)
• deep-well turbine
• submersible
• propeller

When selecting an irrigation pump, consider the pump’s operating conditions, including:

• the type of water source
• the required pumping flow rate (determined by the irrigation system requirements)
• total suction head (lift from the water surface to pump intake)
• total dynamic head (total pressure supplied by the pump)

Surface water or well water will give you the source of water, and its availability will be determined by the local geology and hydrologic conditions. The type of irrigation system, distance from the water source and size of the piping system will determine the flow rate and total dynamic head. Use Tables 1 and 2 to help with the selection process for a broad range of flow rates and total dynamic heads. In Table 2, suction lift is not included. If the application needs to lift water to the pump, a centrifugal pump must be used.

Surface Centrifugal Pumps

Above-ground centrifugal pumps are commonly used with surface water sources or shallow groundwater sources. They are also used as booster pumps in irrigation pipelines. They must be filled with water, or “primed,” before they can operate. The suction line and pump must be filled with water and free of air, so airtight joints and connections are important on the suction pipe. Centrifugal pumps are designed for horizontal or vertical operation. Horizontal or vertical refers to the orientation of the motor and drive shaft, not the impeller. Horizontal centrifugal pumps are the most common for irrigation. They generally offer lower cost, require less maintenance and are easier to install and inspect for maintenance than other types. While self-priming horizontal centrifugal pumps are available, they are usually special-purpose pumps and not used in agricultural irrigation. Vertical centrifugal pumps can be mounted so the intake and impeller are always under water. Priming is unnecessary in this case, making it useful for floating applications. A self-priming feature is helpful in areas with frequent power outages or with center pivots where automatic restart is a programmable function. However, more maintenance may be needed because the bearings are constantly under water.

Deep-Well Turbine Pumps

Used in cased wells or where the water surface is below the usual limits of a centrifugal pump, deep-well turbine pumps do not have to be primed because their intake is always under water. These pumps are generally more expensive and more difficult to inspect and repair than centrifugal pumps, but they are usually more efficient. The turbine pump includes three main parts: the pump head assembly that supports the motor, usually cast iron and installed on a foundation; the shaft and column assembly; and the pump bowl assembly. The line shaft on a turbine pump may be water- or oil-lubricated, but if it is used for a domestic water supply or livestock, a water-lubricated pump must be used. Some states, such as Minnesota, require use of only water-lubricated pumps in new irrigation wells. The pump head assembly provides alignment between the pump and the power unit. It is important to ensure that the well is straight and the pump column assembly is aligned vertically. It should not touch the well casing. If it does touch the well casing, vibration can wear holes in the casing, and may also cause excessive bearing wear. A concrete foundation that is large enough to securely fasten the head assembly helps provide trouble-free installation.' The foundation should have 12 inches of bearing surface on all sides of the well. If the well is gravel-packed, the 12 inches must be measured from the outside edge of the gravel packing.

Submersible Pumps

In agricultural irrigation applications, a submersible pump is a turbine pump close-coupled to a water-tight electric motor, which eliminates the long drive shaft and bearing retainers needed for a deep-well turbine pump. The pump is above the motor and water enters the pump through a screen between the pump and motor. Because submersible motors are smaller in diameter, they are lower efficiency than those used for centrifugal or deep-well turbine pumps. The motor may overheat or burn out if the circulation of water past the motor is restricted or inadequate. The length of riser pipe must be enough to keep the bowl assembly and motor submerged at all times. Also, the well casing must be large enough to allow water to flow past the motor. Submersible pumps used for irrigation must have three-phase electrical power and wiring from the pump to the surface must be watertight with all connections sealed. Lightning is a common cause of submersible pump failures, so protection should be wired into the control box. Specially designed submersible booster pumps can be mounted horizontally in a pipeline and are good for noise reduction if used near a residential setting. Another use of submersibles is as a booster in a centrifugal pumps’ suction line where the water level may fluctuate a lot.

Propeller Pumps

Low-lift, high-flow rate conditions call for propeller pumps, which come in two types: axial and mixed flow. An axial flow pump uses an impeller similar looking to a boat motor screw. Mixed-flow pumps use semi-open or closed impellers that are similar to turbine pumps. Propeller pumps can be oriented vertically such as on a pontoon for a moveable pumping platform, but for agricultural applications they are typically mounted horizontally on trailers. This allows them to pump into pipelines easily and be backed into a water source. On many farms, propeller pumps are used to pump out waste storage lagoons and are commonly powered by the power take-off on tractors. Propeller pumps cannot be used for suction lift as the impeller must be submerged and the pump operated at the proper submergence depth. In general, the larger the diameter of the pump, the deeper the submergence needed to prevent sucking air.