WIND ENERGY SYSTEMS - Cd3wd

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