The correct design of water pumping windmills hydraulic systems
is often neglected. This section will deal with the problem of
water hammer.

Water pumping windmills pump water intermittently. The water flow
in the system delivery pipes stops and starts and accordingly
transient pressure oscillations can occur.

Firstly however we deal with water hammer caused by over pumping
the water source. As stated earlier water pumping windmill
selection must be such that no more than 70 percent of the water
source yield is extracted. If the water source is over pumped air
can enter the pump during the suction stroke. During the windmill
pump down stroke this air is subjected to the full system
delivery pressure which causes the air to become compressed. The
water in the delivery pipe will then flow in a reverse direction
as the air is compressed. The reader will note that if the
windmill pump stroke rate equals the resonant frequency of the
above system (an air cushion supporting a column of water) then
dramatic things can happen. Windmill towers have been known to
shear their bolts off at the tower base etc. This phenomenon is
rare but in a device with a hoped for life of 50 years or more it
needs to happen only once. The solution as outlined earlier is to
install a weak link in the pump rod to windmill connections so
that this will fail sacrificially to protect the windmill.
Ultimately over pumping the water source must be avoided.

Water hammer can also occur when pumping water over long
distances. To prevent it air scour valves must be placed in the
delivery pipeline to remove air from the delivery pipe so as to
prevent a problem similar to that described above. Further self
draining surge tanks (Airchambers in windmill terminology) must
be installed in the delivery line to even out pressure
fluctuations. Two non return valves are used, one on either side
of the surge tank. The valve closest to the windmill serves to
unload the windmill and let it start easier at the beginning of
each stroke. The upstream valve serves to prevent harmonic
oscillations occurring due to a column of water resting on the
surge tank air cushion.