Irrigation Water Pumps

Pumps used for irrigation include centrifugal, deep-well turbine, submersible nd propeller pumps. Actually, turbine, submersible and propeller pumps are special forms of a centrifugal pump. However, their names are common in the industry. In this publication, the term centrifugal pump refers to any pump that’s above the water surface and uses a suction pipe.

Before selecting an irrigation pump, you must make a careful and complete inventory of the conditions under which the pump will operate. The inventory must include:

• Source of water (well, river, pond, etc.)
• Required pumping flow rate
• Total suction head
• Total dynamic head

You usually have no choice about the source of the water; it is either surface water or well water, and the local geology and hydrologic conditions will determine the availability. However, 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.

“Head” is a term commonly used with pumps. Head refers to the height of a vertical column of water. Pressure and head are interchangeable concepts in irrigation because a column of water 2.31 feet high is equivalent to 1 pound per square inch (PSI) of pressure. The total head of a pump is composed of several types of heads that help define the pump’s operating characteristics.

The power added to water as it moves through a pump can be calculated with the following formula:

WHP = Q x TDH/ 3,960 (1)

where:

WHP = water horse power

Q = flow rate in gallons
per minute (GPM)

TDH = total dynamic head (feet)

However, the actual power required to run a pump will be higher than this because pumps and drives are not 100 percent efficient. The horsepower required at the pump shaft to pump a specified flow rate against a specified TDH is the brake horsepower (BHP), which is calculated with the following formula:

BHP =WHP/Pump Eff. x Drive Eff. (2)

BHP – brake horsepower (continuous
horsepower rating of the power unit)

Pump eff. – efficiency of the pump
usually read from a pump curve
and having a value between 0 and 1

Drive Eff. – efficiency of the drive unit
between the power source and the
pump. For direct connection, this
value is 1; for right-angle drives,
the value is 0.95; for belt drives,
it can vary from 0.7 to 0.85

Centrifugal pumps are used to pump from reservoirs, lakes, streams and shallow wells. They also are used as booster pumps in irrigation pipelines. All centrifugal pumps must be filled completely with water or “primed” before they can operate.

The suction line, as well as the pump, has to be filled with water and free of air. Airtight joints and connections are extremely important on the suction pipe. Priming a pump can be done by hand-operated vacuum pumps, an internal combustion engine vacuum, motor-powered vacuum pumps or small water pumps that fill the pump and suction pipe with water.

Centrifugal pumps are designed for horizontal or vertical operation. The horizontal centrifugal has a vertical impeller connected to a horizontal drive shaft, as shown in Figure 3.

Figure 3. A horizontal centrifugal pump.

Horizontal centrifugal pumps are the most common in irrigation systems. They are generally less costly, require less maintenance, and are easier to install and more accessible for inspection and maintenance than a vertical centrifugal. Self-priming horizontal centrifugal pumps are available, but they are special-purpose pumps and not normally used with irrigation systems.

Vertical centrifugal pumps may be mounted so the impeller is under water at all times. (See floating pump on cover.) This makes priming unnecessary, which makes the vertical centrifugal desirable for floating applications. Also, a self-priming feature is very desirable in areas with frequent electrical power outages or off-peak electrical price reductions.

Self-priming also lends itself to the new control panels for center pivots where automatic restart is a programmable function.

A note of caution

Because the bearings are constantly under water, these pumps may require a higher level of maintenance.

Deep-well turbine pumps are adapted for use in cased wells or where the water surface is below the practical limits of a centrifugal pump. Turbine pumps also are used with surface water systems.

Because the intake for the turbine pump is continuously under water, priming is not a concern. Turbine pump efficiencies are comparable to or greater than most centrifugal pumps. They usually are more expensive than centrifugal pumps and more difficult to inspect and repair.

The turbine pump has three main parts: head assembly, shaft and column assembly and pump bowl assembly, as shown in Figure 4. The head normally is cast iron and designed to be installed on a foundation. It supports the column, shaft and bowl assemblies and provides a discharge for the water. It also will support an electric motor, a right-angle gear drive or a belt drive.

Figure 4. A deep-well turbine pump.

The shaft and column assembly provides a connection between the head and pump bowls. The line shaft transfers the power from the motor to the impellers, and the column carries the water to the surface. The line shaft on a turbine pump may be water- or oil-lubricated.

The oil-lubricated pump has a hollow shaft into which oil drips, lubricating the bearings. The water-lubricated pump has an open shaft. The bearings are lubricated by the pumped water. If pumping fine sand is a possibility, select the oil-lubricated pump because it will keep the sand out of the bearings.

If the water is for domestic or livestock use, it must be free of oil, and a water-lubricated pump must be used. In some states, such as Minnesota, you have no choice; water-lubricated pumps are required in all new irrigation wells.

Line shaft bearings commonly are placed on 10-foot centers for water- lubricated pumps operating at speeds under 2,200 RPM and at 5-foot centers for pumps operating at higher speeds. Oil-lubricated bearings commonly are placed on 5-foot centers.

A pump bowl encloses the impeller. Due to its limited diameter, each impeller develops a relatively low head. In most deep-well turbine installations, several bowls are stacked in series one above the other. This is called staging. A four-stage bowl assembly contains four impellers all attached to a common shaft and will operate at four times the discharge head of a single-stage pump.

Impellers used in turbine pumps may be semi-open or enclosed, as shown in Figure 5. The vanes on semi-open impellers are open on the bottom and they rotate with a close tolerance to the bottom of the pump bowl.

Figure 5. Cutaway view of two enclosed impellers within their pump bowls.

The tolerance is critical and must be adjusted when the pump is new. During the initial break-in period, the line shaft couplings will tighten; therefore, after about 100 hours of operation, the impeller adjustments should be checked. After break-in, the tolerance must be checked and adjusted every three to five years or more often if pumping sand.

Both types of impellers may cause inefficient pump operation if they are not adjusted properly. Mechanical damage will result if the semi-open impellers are set too low and the vanes rub against the bottom of the bowls. The adjustment of enclosed impellers is not as critical; however, they still must be checked and adjusted.

Impeller adjustments are made by tightening or loosening a nut on the top of the head assembly. Impeller adjustments normally are made by lowering the impellers to the bottom of the bowls and adjusting them upward. The amount of upward adjustment is determined by how much the line shaft will stretch during pumping. The adjustment must be made based on the lowest possible pumping level in the well.

The pump manufacturer often provides the proper adjustment procedure. The adjustment procedure for many of the common deep-well turbine brands is outlined in Nebraska Cooperative Extension Service publication EC 81-760, titled “How to adjust vertical turbine pumps for maximum efficiency.”

A submersible pump is a turbine pump close-coupled to a submersible electric motor, as shown in Figure 8. Both pump and motor are suspended in the water, thereby eliminating the long drive shaft and bearing retainers required for a deep-well turbine pump. Because the pump is above the motor, water enters the pump through a screen between the pump and motor.

Figure 8. A submersible pump installed in a well.

The submersible pump uses enclosed impellers because the shaft from the electric motor expands when it becomes hot and pushes up on the impellers. If semi-open impellers were used, the pump would lose efficiency. The pump curve for a submersible pump is very similar to the curve for a deep-well turbine pump.

Submersible motors are smaller in diameter and much longer than ordinary motors. Because of their smaller diameter, they are lower-efficiency motors than those used for centrifugal or deep-well turbine pumps.

Submersible motors generally are referred to as dry or wet motors. Dry motors are hermetically sealed with a high-dielectric oil to exclude water from the motor. Wet motors are open to the well water, with the rotor and bearings operating in the water.

If the circulation of water past the motor is restricted or inadequate, the motor may overheat and burn out. Therefore, the length of riser pipe must be sufficient to keep the bowl assembly and motor completely submerged at all times. In addition, the well casing must be large enough to allow water to flow past the motor easily.

Small submersible pumps (under 5 horsepower) use single-phase power. However, most submersible pumps used for irrigation need three-phase electrical power. Electrical wiring from the pump to the surface must be watertight and all connections sealed. The electrical line should be attached to the column pipe every 20 feet to prevent it from wrapping around the column pipe.

Voltage at the motor leads must be within plus or minus 10 percent of the motor nameplate voltage. If a 5 percent voltage drop occurs in the submersible pump cable, voltage at the surface must not be less than 95 percent of rated voltage.

Because the pump is in the well, lightning protection should be wired into the control box. Lightning striking on wells with submersible pumps is a leading cause of pump failures.

You can select submersible pumps to provide a wide range of flow rate and TDH combinations. Submersible pumps more than 10 inches in diameter generally cost more than comparably sized deep-well turbines because the motors are more expensive.

Many manufacturers make submersible booster pumps. These pumps usually are mounted horizontally in a pipeline. An advantage of using a submersible as a booster pump instead of a centrifugal is noise reduction. This is a desirable attribute in residential settings and near golf courses.

Submersibles also have been used as booster pumps in the suction lines of centrifugal pumps. This application is used in situations where the water level will fluctuate a considerable amount during the season. Having a submersible in the suction line will change the head at the inlet of the centrifugal pump from a suction head to a positive head.

Propeller pumps are used for low-lift, high-flow rate conditions. They come in two types, axial flow and mixed flow. The difference between the two is the type of impeller. The axial flow pump uses an impeller that looks like a common boat motor screw and is essentially a very low-head pump.

A single-stage propeller pump typically will lift water no more than 20 feet. By adding another stage, heads from 30 to 40 feet are obtainable. The mixed-flow pump uses semi-open or closed impellers similar to turbine pumps.

In permanent installations, propeller pumps are mounted vertically, as shown in Figure 9a . For portable pumping platforms, they are mounted on trailers or on pontoons for use as floating intakes.

Figure 9a. A power-take-off (PTO) driven propeller pump used to move large volumes of water in low lift conditions.

Figure 9b. A propeller pump.

Portable propeller pumps commonly are mounted in almost horizontal positions (low angles) to allow them to pump into pipelines easily, as well as to be backed into a water source. Portable propeller pumps commonly are powered by the power take-off (PTO) on tractors. On many farms, propeller pumps are used to pump out waste storage lagoons.

Power requirements of the propeller pump increase directly with the TDH, so adequate power must be provided to drive the pump at maximum lift. Propeller pumps are not suitable under conditions where the discharge must be throttled to reduce the flow rate. Accurately determining the maximum TDH against which this type of pump will operate is important.

Propeller pumps are not suitable for suction lift. The impeller must be submerged and the pump operated at the proper submergence depth. The depth of submergence will vary according to various manufacturers’ recommendations, but generally, the greater the diameter of pump, the deeper the submergence.

Following recommended submergence depths will ensure that the flow rate is not reduced due to vortices. Also, failure to observe required submergence depth may cause severe mechanical vibrations and rapid deterioration of the propeller blades.

The selection of an irrigation water pump is based almost entirely on the relationship between pump efficiency and the TDH the pump will provide at a specific flow rate. As shown before, these parameters also are the basis of the pump characteristic curve. Use Table 2 to narrow the selection of a pump type for a broad range of flow rates and total dynamic heads.

One item not included in the TDH values in Table 2 is the suction lift. If your application needs to lift the water to the pump, then you will have to use a centrifugal pump.

Care and Maintenance of Irrigation Wells - an NDSU Extension publication

“Center Pivot Design,” Irrigation Association, Falls Church, Va., www.irrigation.org

MWPS-30, Sprinkler Irrigation Systems, MWPS, Iowa State University, Ames