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Pump Performance And Selection For Irrigation Systems

By Daniel Scaliter, ASIC






A Triplex Vertical Turbine Pump by Peerless, installed at Hazeltine National Golf Club in Chaska, MN. Hazeltine is the host for the 1991 U.S. Open.


The primary function of pumps is to add hydraulic energy to a fluid. This occurs when the mechanical energy passed through a power source to the pump is transferred to the fluid. In other words, the pump is the mechanical device that is able to transfer the mechanical energy of a rotating motor, for example, into pressure, head and velocity energy. The amount of each type of hydraulic energy (pressure, head, velocity) varies from place to place in a system. For example, when water is at rest in a storage tank, it possesses head energy but not velocity or pressure energy. Once water starts to flow from the tank, it has head pressure and velocity energy. As water flows from the end of a pipe, the head and pressure energy are transformed to velocity energy alone.

Pumps can be divided into two main categories: Dynamic--in which energy is continuously added to increase the fluid velocity within the machine to values in excess of those occurring at the discharge, such that subsequent velocity reduction within or beyond the pump produces a pressure increase; Displacement--in which energy is periodically added by application of force to one or more movable boundaries of any desired number of enclosed, fluid-containing volumes, resulting in a direct increase in pressure up to the value required to move the fluid through valves or ports into the discharge line. Displacement pumps will not be discussed here, because those are rarely, if ever, used in irrigation systems.

Dynamic pumps are subdivided into different varieties, such as centrifugal and special effects. The pumps used for irrigation systems are of the centrifugal type. Because of the large selection of centrifugal pumps available, there is little difficulty in finding a pump to meet the conditions encountered in an irrigation system installation.

Types of centrifugal pumps include propeller, volute, deep-well turbines and submersible turbines. Propeller pumps are widely used in situations when low head pumping is needed. Volute pumps are used when pumping from surface sources and, in general, when the total head exceeds approximately 45 feet. Such pumps are also available in many types: vertical and horizontal shafts, end, bottom and double suction with semi-open or closed impellers, and so forth.

Deep-well turbine pumps are used, as the name implies, to pump water from a subsurface source into a distribution system. Most commonly, they come built in stages. The number of stages varies depending on the necessary head and discharge pressure, as each stage adds a certain amount of pressure. The head and discharge performance graphs provided by manufacturers I indicate the operating situation for one stage e only. The specifier must take this into consideration to provide the correct number of stages to enable the pump to provide sufficient pressure at the end of the system (e.g., if 200 feet of head is needed, and each stage provides 20 feet of head, then a 10-stage pump will be required).






A Variable Frequency Drive (VFD) Vertical Turbine by Peerless, installed at Ingersoll Farms in Oconomowoc, Wl. Photos courtesy of Watertronics Electronically Controlled Pumping Systems


Submersible turbine pumps are used for high head applications where long shafts are undesirable.

A discussion of pumps must include hydraulics. In order to facilitate an understanding of both, following is a list of commonly used terms and their definitions:

HEAD – Energy per unit of weight, commonly measured in feet. Most unit measurement in pumping is specified in feet rather than PSI. To convert feet of head to PSI, multiply the given head in feet by 0.433. To convert PSI to feet of head, divide the given pressure in PSI by 0~433.

STATIC HEAD – is the vertical distance between the water level of the source of supply and the point of free discharge or to the level of the free surface of the discharged water. For example, when supplying water from a reservoir with an elevation of 1650 feet to a pipe with an elevation of 1500 feet, the static pressure at the pipe will be (1650-1500) x 0.433 = 65 PSI (0.433 is a constant for conversion of feet of head to PSI).

VELOCITY HEAD – The velocity head of water moving with a given velocity is the equivalent head through which it will have to fall to acquire the same velocity. It is the head necessary to accelerate the water. Knowing the velocity of the fluid, one can easily figure the velocity head from the formula:

2 v
H=__ 2g

in which ‘g’ is acceleration due to gravity and is equal to 32.16 feet per second. The velocity head is a factor in figuring the total head, but the value is usually small and almost always negligible; however, it should be considered when the total head is low, and also when the suction lift is high.

SUCTION LIFT – is the vertical distance from the free level of source of supply to the discharge nozzle of pump. Suction lift exists when the source of supply is below the discharge nozzle of pump.

DYNAMIC SUCTION LIFT – is the vertical distance from the source of supply when pumping at required capacity to discharge nozzle of pump, plus velocity head, entrance, friction and velocity losses, but not including internal pump losses.

STATIC SUCTION HEAD – is the vertical distance from the free level source of supply to the discharge nozzle of pump. Suction head exists when the source of supply is above discharge nozzle of pump.

DYNAMIC SUCTION HEAD – is the vertical distance from the source of supply, when pumping at required capacity, to discharge nozzle of pump, minus velocity head, entrance, friction and velocity losses, but minus internal pump losses.

TOTAL DYNAMIC HEAD – is the vertical distance between the source of supply and point of discharge when pumping at required capacity plus velocity head, friction, velocity, entrance and exit losses. (By referring to Figure 1, it is possible to see that the total dynamic head will be equal to the sum of suction entrance loss, suction pipe friction, suction lift, discharge lift, miscellaneous losses, discharge pipe friction.)

NET POSITIVE SUCTION HEAD – A centrifugal pump will operate only if a liquid can enter the first stage impeller under a pressure usually equivalent to atmospheric pressure (14.7 PSI). The pressure required to operate a pump is referred to as net positive suction head (NPSH) and must be sufficient to prevent evaporation of water as it enters the pump. All liquids have a tendency to change to a vapor state. Liquids are said to have a high vapor state when their tendency to vaporize is great. Many remain as liquids only at a pressure greater than atmospheric pressure. Others have extremely low vapor pressure and demonstrate only a slight tendency to vaporize even under vacuum. Water is somewhere at a midpoint between these extremes. Pump manufacturers specify the required NPSH for the various pumps. The designer has to compute the available NPSH based on the local altitude, operating temperature, atmospheric pressure change and other minor considerations. For operational safety, the available NPSH should exceed the required NPSH by the manufacturer by at least 2-3 feet.

CAVITATION – A condition that occurs when the pressure acting on a stream of liquid falls to or below the vapor pressure of that liquid. When the water enters the impeller, there is an increase in velocity due to a reduction of size in the water passages, thereby causing a reduction in pressure. If the reduction in pressure falls below the vapor pressure of the liquid, the liquid will begin to boil and subsequently vaporize, forming bubbles to collapse at such a rapid rate that a rumbling noise can be heard. The collapse of these pockets is so violent that it causes pitting on the impeller and bowl surface. Protection against cavitation should start with the hydraulic design of the system in order to avoid the low pressures, if practical. Otherwise, use of special cavitation-resistant materials or coating may be effective. Small amounts of air entrained into water system have markedly reduced cavitation damage.

A pump study permits selection of the type of pump and discharge system best suited to the project. The study should include such general data as: 1. site condition – to determine the pump location and electrical power; 2. quality of water – to determine the type of impeller to use, e.g., sandy water will require a semi-open impeller rather than a closed one; 3. total dynamic head needed – which will include the system design pressure, which is equal to all losses in piping in the irrigation system, plus difference in elevations, plus the sprinkler operating pressure; 4. peak irrigation demand; 5. performance curves from different pump manufacturers; 6. cost and maintenance.

With all this data, the performance curves (graphs that describe the working condition of a pump with regard to GPM and head in feet or meters) can be reviewed to obtain a pump that will fit our requirements at the highest possible efficiency. The most efficient operating point for a centrifugal pump can be selected from the manufacturer’s pump performance curves. It is important to note that the efficiency indicated in the performance curves is for the bowl assembly only. Overall pumping plant efficiency can be obtained by multiplying the bowl efficiency by the motor efficiency. A typical pumping plant efficiency is about 70-75%.

In some cases, especially in landscape irrigation, there is great variation in system discharge. One valve can use 70-80 GPM, while another will have to use 30 GPM, and both of them will need the same pressure – 30 psi. It is important then to choose a pump that has a more or less flat curve that will give a flexibility of changing GPM but still keeping a more constant operating pressure. Also, the irrigation controller should be programmed so that the total GPM going through the point of connection is kept at a relatively constant rate, enabling the pump to work at its highest efficiency.

In evaluating cost for a pump, keep in mind that frequently a more expensive, quality pump may reduce expenses in the long run by requiring less maintenance, possess greater durability and operate more efficiently.

In conclusion, it is hoped that this article has clarified some points regarding pumps and their selection. However, pumps is an extensive subject, and for further study, we recommend additional reading (see references).

References:

Cornell Pumps, Engineering Data. Portland: 1985

Driscoll, Fletcher, Ph.D. Groundwater and Wells, 2nd Ed. St. Paul: Johnson Division, 1987.

Karassik, Igor, etal. Pump Handbook 2nd Ed New York: McGraw-Hill, 1985

Schwab, Glenn O., et al. Soil and Water Conservation Engineering 3rd Ed. New York: John Wiley & Sons, 1981 Streeter, Victor L. and E. Benjamin Wylie.

Fluid Mechanics 7th Ed. New York: McGraw-Hill, 1979.

Daniel Scaliter is a graduate of Ruppin Agricultural Institute, Israel. He is a Soil and Water Engineer and a member of the American Society of Agricultural Engineers and the American Society of Irrigation Consultants. He is President of Scaliter Irrigation Engineering, Inc. in Redlands, California.



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