Irrigating Greenhouse Crops
Selecting a Well Pump
A major component of any micro-irrigation system is the pump. An irrigation pump is selected to match two important irrigation system parameters, total irrigation system flow rate and total dynamic head required.
Estimating Total Irrigation Flow Rate
The pump capacity needed, and the discharge pressure should be determined prior to purchase. The capacity is determined by the vine’s water requirements, the efficiency of the micro-irrigation system, and the largest acreage in the vineyard to be irrigated by the pump at a given time. The discharge pressure is determined by the desired operating pressure through the laterals, the pressure loss due to friction, and the various changes in elevation within the system. One of the important aspects to consider with pumping water is to consider the pump efficiency.
Total Dynamic Head
Total dynamic head (TDH) is essentially the equivalent height that a fluid is to be pumped, and one of the most important factors in the pump selection process (Figure 16.8). The pressure required for operating a given emitter or sprinkler represents only a portion of the total dynamic system head. Additional pressure must be produced by the pump to lift water from the well or other water source, to overcome friction losses in the pipe and other components of the system, and to provide velocity for the water to flow through the pipes.
Total Static Head
Static head is the vertical distance from the water level at the source to the highest point where the water must be delivered, i.e., benches with potted plants. It is the sum of static lift and static discharge. Static head is independent of the system discharge (gpm) and is constant for all values of discharge. However, it is possible that the static head may vary with time due to the changes in the system.
Well Drawdown
As a well is pumped the water level in the well declines, which is commonly called the well drawdown. The amount of the drawdown is a function of the pumping rate, the aquifer properties, well size, method of construction (well screen, etc.), and the time the pump is operated. The best way to determine the well drawdown is to test pump a well at various rates and observe the drawdown. When the water is to be pumped from the well it is important to know the drawdown to account for the additional lift.
Operating Head
Water coming out of the pump must be pressurized to overcome friction losses in the distribution lines and meet the irrigation system pressure requirement. If the pump goes directly into a pressure tank, then the pump must pressurize the water to some amount greater than the high-pressure switch of the tank. Every sprinkler or emitter has an optimum operating pressure, and the pump imparts pressure to the water to reach the required operating pressure.
Friction Loss
TWhen water flows through a pipe there is a loss of head due to friction. This loss can be calculated using hydraulic formulas or can be evaluated using friction loss tables, nomographs, or curves provided by pipe manufacturers. The pump must add energy to the water to overcome the friction losses. As the discharge of the system increases the velocity also increases, which can have a significant effect on friction loss.
Velocity Head
Velocity head is the amount of energy required to provide kinetic energy to the water. For systems with a high total head this component is very small compared with other components of the total system head. An increase of water velocity in the system will not usually result in large increases in velocity head.
Suction Head
A pump operating above a water surface is working with a suction head. The suction head includes not only the vertical suction lift, but also the friction losses through the pipe, elbows, foot valves and other fittings on the suction side of the pump. An allowable limit to the suction head on a pump and the net positive suction head (NPSH) of a pump, sets that limit. The theoretical maximum height that water can be lifted using suction is about 33 feet (10m).
Pump Power Requirements
When the term head is used, the units of measure are feet. Feet of head can be converted to pounds per square inch of pressure (psi) by dividing the feet of head by 2.3 (the conversion is 2.3 ft/psi). For example, 23 feet of head is equivalent to 10 psi.
Pump Efficiency
Manufacturers use tests to determine the operating characteristics of their pumps and publish the results in pump performance charts commonly called “pump curves.” A typical pump curve is shown in Figure 16.9. All pump curves are plotted with the flow rate on the horizontal axis and the TDH on the vertical axis. The curves in Figure 16.9 are for a centrifugal pump tested at different RPM.
Reading a Pump Curve
Pump curves typically provide three important performance relationships in one graphic display: (1) the relationship between pump capacity (gpm) and total dynamic head (TDH); (2) the relationship between pump capacity (gpm) and bowl efficiency (%); and (3) the relationship between pump capacity and brake (shaft) horsepower. Curves for different impeller diameters may also be presented in the same graphic.
Variable Frequency Drives
For all types of irrigation and not only for sprinkler irrigation, it is important to be able to control the flow and pressure if the grower is to achieve high irrigation efficiency. It is not only the pump that is important in irrigation, but even more so the regulation of the pumps. If the irrigation set-up requires unchanged flow and pressure, the most efficient pump to use is a single-speed pump operating at its best efficiency point. But if either the flow or pressure requirements are variable, or if irrigation zones are opened or closed, the most efficient way to regulate flow and pressure is to use a variable frequency drive (VFD). Not only will the uniformity be maintained at an optimal level, there will also be a great power saving for the pump, and that is probably the biggest advantage. Of course, flow and pressure can easily be regulated by means of a valve, and this is actually still the most common way to adjust pump performance. But the approach is similar to driving a car with full throttle, and then using the brakes to adjust the speed.
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