The topics in this newsletter are considered to be timely and of interest. Comments and suggestions are invited. The use of trade names in this newsletter is not an endorsement of any company or product by the Maryland Cooperative Extension, University of Maryland, College Park.
David S. Ross, Extension Agricultural Engineer

Dr. Jennifer Becker joined the department in January 2000 in a Bioenvironmental Engineering/Waste Management Position as Assistant Professor. She has a 75 percent extension and 25 percent research assignment in waste management. In addition to a strong engineering background, Dr. Becker has a strong background in microbiology. Her research interest is in the area of bioremediation of polluted systems by microbes. Dr. Becker has her B.S. degree in environmental engineering from Michigan Technological University, an M.S. from the University of Illinois, and her Ph.D. from Northwestern University, both in environmental engineering. She recently taught at Lehigh University in Pennsylvania.

Dr. Gary Felton changed positions within the department when he accepted the Bioenvironmental Engineering/Water Quality position as an assistant professor last fall. Dr. Felton's assignment is 75 percent extension and 25 percent teaching. Dr. Felton had been working in water quality for three years in a 100 percent extension contract position. Gary has his B.S. and M.S. degrees from the University of Maryland and his Ph.D. from Texas A & M in Agricultural Engineering. He worked several years at the University of Kentucky and the Kentucky Geodetic Survey before returning to Maryland.

Pumps are simple, reliable ways of moving water. The typical pump is built with a motor on one end and one or more propellers attached to the motor drive shaft at the other end. The mechanical rotation of the motor shaft is transferred to the impellers, causing the hydraulic motion of moving water. The impellers are contained inside a pump casing, which directs the water flowing through it to achieve maximum energy transfer. Water flows into the casing at the suction and is pushed out of the casing at the discharge. A seal is used where the motor shaft goes into the casing to prevent water leakage. This design allows the pump to move water efficiently from a lower elevation to a higher elevation.

Although water pumps are very simple and reliable, they are damaged by running them without water or by turning them on and off at the wrong time. Most pump seals are lubricated by water. If there is inadequate water flow, these seals will be damaged and the pump will leak. Two conditions can result in this occurrence: no water flowing into the casing and no water flowing through the casing.

When more water is pushed out of the volute than is entering the suction, the pump is starved for water. This condition is called cavitation and can quickly result in damage to the pump seals, impeller, and motor.

A situation where no water is passing through or leaving the casing usually means there is a blockage on the discharge, preventing water flow through the pump. This condition is called deadhead. When this occurs, the pump discharge pressure will rise to the maximum possible for the particular pump design and the rotational energy from the motor is transferred to the water in the volute. The water rises in temperature to the point where steam is given off and high pressures develop, creating an explosion hazard.

To prevent these pump problems; every pump should have a mechanism to prevent improper pump operation. Two categories of pump controls should be considered: motor/electrical controls and pressure controls. Motor/electrical controls monitor the electrical power going to the motor to make sure that the voltage, current, and phase balance is within allowed limits. Pressure controls measure the pump discharge pressure and turn the pump on or off, or adjust the pump speed to maintain the desired water volume and/or pressure.

The pressure at the discharge end is a good indication of what is happening at the suction end. If there is no pressure at the discharge end, then there is no water at the suction end and the control shuts down the pump. The pump should stay off until the pump is fixed. On-off pressure switches turn the pump on at low pressure and turn it off at a set constant pressure. Minimum on and off timers are helpful to prevent excessive pump cycling turning the pump on and off too quickly.

Pressure sensing can be accomplished by using diaphragm spring switches or electronic pressure sensors called pressure transducers. The pressure switch diaphragm allows the water to push against a pre-loaded spring and when the water pressure on the diaphragm exceeds the spring tension, the switch position changes. The pressure transducer provides an electrical signal representing the pressure measured to a control unit that decides how the pump should operate. The pressure transducer eliminates the need for several diaphragm spring switches to measure different pressure levels.

All pumps should be fitted with the proper motor and pressure controls. Motor controls protect the motor end from overload, over and under voltage, and phase imbalance. Pressure controls will protect the pump end from excessive or low pressure, detect a line break or blockage, and provide pump run time and other valuable information.

(Adapted from Storkson, B. 1999. Pumps A Pump Primer. Irrigation Journal. Adams Business Media. Cathedral City, CA 49(6):32)

Soil compaction has been described by researchers and crop producers as a silent thief. It reduces yields, limits productivity and root growth, and makes plants more susceptible to drought stress. During heavy rains, compacted soils result in accelerated levels of runoff and erosion.

Sensing and measuring degrees and depths of soil compaction can help producers develop management options. Soil cone penetrometers have been used for years to measure soil compaction and to sense root impeding layers. Their widespread use resulted in the 1968 adoption of ASAE Standard S313.3, Soil Cone Penetrometer - to ensure soil data uniformity throughout the world. The standard describes the penetrometer unit shape and size and offers advice on construction and wear assessment. New ASAE Engineering Practice EP542, titled, "Procedures of Using and Reporting Data Obtained with the Soil Cone Penetrometer," was adopted in February 1999 to help users acquire quality data. EP542 describes penetrometers, soil measurements, and statistical treatment of problematic measurements.

Manual soil cone penetrometers are inexpensive and provide quick soil compaction assessments. The units are commonly composed of a dial gauge for pressure measurement and shaft markings for depth measurement. They are available from companies such as Dickey-John Corp. of Alihurn, Illinois; ELE International Inc. of Lake Bluff, Illinois: and Eijkelkamp based in The Netherlands. Although handy for quick assessments, these devices are difficult to use because two people are required to operate them. One person must push the unit into the soil while another reads and records pressure and depth. Typical values of cone index that stop root growth are approximately two Mega Pascals (300 psi).

More complicated soil cone penetrometers that automatically record pressures and depth have been designed and built by researchers.

Because soil compaction increases with vehicle traffic, differences in soil strength are measured across a row from a trafficked middle to an untrafficked row middle. "Middle" refers to the area between crop rows. Many replications are typically used to obtain statistical significance between compacted and uncompacted soil conditions. Data have shown variability in soil strength across rows and within fields. This information may lead to site-specific control of soil compaction.

A multiple-probe soil cone penetrometer has been used to determine both cone index and bulk density. One study investigated the clay pan soils of central Missouri to determine differences in soil strength between native prairie sod and tilled crop land. A lack of rainfall added increased soil strength.

Another study investigated tillage and weed elimination effects on pecan trees. Trenches created several years earlier with a special tillage and cellulose burial machine were also located in this study.

The most common use for cone index and bulk density data has been to determine the effect of tillage and traffic systems on soil compaction. Developing a conservation tillage system for the Tennessee Valley Region in North Alabama included determining soil strength to evaluate the effects of shallow or deep tillage and cover crops.

Studies in the Southeast helped evaluate variations in root-impeding layers and to correct it with site-specific tillage.

Modified by Gary K. Felton from an article in Resource (March 1999) by Randy L. Raper

An exciting development in water pumping since its inception is variable speed pumping. Now a pump can be operated at partial speed to better match the flow to system requirements. Pumps have been designed to handle the maximum water flow anticipated in order to prevent pump motor overload. There are many advantages to running a pump at partial speed, as most pumps do not need to operate at full capacity of the pump to satisfy the demand. When the water flow is reduced, the pump pressure goes high and pressure-limiting valves often must be installed to reduce the water pressure to usable levels. This excess pressure is energy wasted. The better alternative is to reduce the pump speed.

Two components are needed to reduce pump speed: the water pump control and the Variable Frequency Drive (VFD). The water pump control monitors the system and adjusts the pump speed to maintain the desired pressure. The VFD makes the motor spin at the desired RPM directed by the water pump control.

The water pump control measures the system pressure and speeds up the pump if the system pressure is below a user selectable target pressure, or slows down the pump if the system pressure is above the target pressure. All this happens several hundred times per second, with the actual operation being a smooth ramp up or ramp down as the pump speed is adjusted. If the water flow stops, then the pump slows down to a stop and shuts off. A safety shutoff is used to shut the pump down until manually reset if a minimum system pressure cannot be maintained for 30 seconds.

Using a VFD offers many advantages and cost savings, which range from incidental to considerable, depending on the actual installation. With a VFD, it is possible to vary the speed of any standard, off-the-shelf, three-phase motor RPM from full on to full off. This feature, combined with the water pump control, allows a pump to run at whatever speed is needed to deliver the desired flow rate and/or pressure. At partial pump capacity there is no "slam on/slam off" operation and no over pressure conditions, as is the case with constant speed pumps.

For irrigation systems on a steep hillside, the pressures can be varied to satisfy the requirements of different lateral lines at different elevations. Yet another application is the ability to use one pump for multiple duties, provided the pump is properly sized. The same pump can provide water for a low-pressure trickle system (25 - 30 psi required), as well as to a large diameter big gun system (100 psi or more).

With single-phase power, VFD's provide single phase to three-phase conversion and it is expected that VFD's will make phase convectors obsolete for water pumping applications in the near future. Since it is possible to use three-phase equipment with single-phase power, considerable savings can be realized when using submersible pumps. The single-phase submersible pump control box containing start and run capacitors and switchover equipment is eliminated and starter/contractor is not used. Three-phase output from single-phase voltage allows smaller wire sizes over long runs. For example, a 900 foot deep 5 hp submersible pump using single phase power would require #2 copper wire. For three phase operation from a VFD, #6 wire can be used a savings of nearly $2.00 per foot in wire alone ($1,800). Three phase motors are typically less expensive and more widely available than their equivalent single-phase counterparts.

Power savings are important. With an electric motor, the power required is reduced by the cube of the speed. This means that if the speed is reduced by one-half (50 percent), the power requirements are reduces by 87.5 percent. Expressed a different way, if the motor is run at « the speed then the motor requires only 1/8 of the electrical power (wattage) it would at full speed.

In dollars, if a motor requires 5.40 kilowatt hours (kW-hr) of electricity at full operation, this motor would require 0.675 kW-hr at half-speed constant operation. At a power cost of 10 cents per kW-hr, 1000 hours would cost $54.00 at full speed and $6.75 at constant half speed.

In most cases, pumping stations using VFD can eliminate pressure regulating and pressure relief valves. Controls can be customized to run a pump slowly to fill a water line and then increase the pressure once the line is full. Amperage can be managed to minimize or eliminate power company demand charges.

(Adapted from Storkson, B. 1999. New and Lower Cost Technology Provides Affordable Variable Speed Pump Operation. Irrigation Journal. 49(6):34-36.)