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Chapter 7—Asynchronous Loads 7–3 prepared by Aermotor, now a Division of Valley Industries. At one time, the Aermotor turbines accounted for 80-90 percent of all water pumper sales in the United States, hence are likely to appear in old photographs. They are now manufactured in Argentina. Their 1980 sales in the United states were on the order of 3500 units, which was more than the combined production of all electric generating wind turbines in that year. Figure 1: Aermotor illustration of key water pumping terms. Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
Chapter 7—Asynchronous Loads 7–4 This sketch shows the normal installation with the wind turbine located directly above the well. The turbine is connected to a gear box and crankshaft which converts rotary motion into reciprocating motion on the pump rod. The pump rod enters the well pipe through a packer head which allows the motion of the rod but blocks the water from leaking out. The pump rod then connects to the piston in the pump at the bottom of the well pipe. The height of the equivalent column of water which is raised by the piston is referred to as the total head. It includes the distance from the water level in the well to ground level, from ground level to the height of discharge, and a quantity for friction losses of water flowing in the pipe. The pumping level is seen to be greater than the standing water level by the amount of drawdown. This refers to the decrease in water level during pumping and may vary from an insignificant amount to several meters. Water has to flow back into the well from the subsurface water bearing strata of sand and gravel, called aquifers, so the drawdown will be generally proportional to the rate of pumping. The pump is normally located below the maximum draw down level by an amount adequate to ensure proper pump-suction operating conditions. This varies with the piston size, operating speed, flow rate, and pressure, but can be as much as 2 or 3 m. A picture of a piston pump is shown in Fig. 2. Both the piston and the bottom of the pump have check valves which only allow water to flow in the upward direction. When the piston is lifted by the piston rod, the piston valve closes and the piston lifts the entire column of water above it, until water overflows out of the discharge pipe at the top. At the same time, a slight suction is formed under the piston, causing the suction valve to open and water to flow in under the piston. During the next half of the cycle, the piston moves down, causing the suction valve to close and the piston valve to open, so water flows through the piston into position to be lifted during the next half-cycle. The flow of water will be inherently pulsating due to this reciprocal action. This poses little or no problem in filling a tank, but may not be suitable in those applications requiring more uniform pressures and flows. The piston packing must fit tightly to the cylinder liner to prevent leakage around the piston during the up stroke. The packing will often wear rapidly if the piston moves at a linear speed well above rated, so pump speeds must be limited to reasonable values. Other problems associated with overspeed operation are improper valve action and low suction pressure. If the suction pressure drops too low, the water will vaporize under the piston. This limits the flow and also causes vibration in the pump rod. The pump size is normally described in terms of the piston diameter, which is the same as the diameter of the inside of the cylinder. The terms piston diameter, cylinder diameter, and pump size are all used interchangeably. The actual flow to the discharge system is termed the pump capacity. The theoretical flow under ideal conditions is called the pump displacement. The displacement of the simple pump in Fig. 2 is given by Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
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