In water-bearing structures such as hydroelectric power stations, or in systems for supplying water, changes in flow rate – such as during startup and shutdown of hydraulic machines or by opening and closing shut-off elements – result in pressure fluctuations. There is a distinction to be made between rapid pressure changes that propagate with the sound velocity (water hammers) and slow pressure changes caused by mass vibrations. Pipeline systems use air vessels or surge chambers to dampen water hammers and mass vibrations.
  HM 156 is used to generate and visualise water hammers in pipes and to demonstrate how a surge chamber works. The trainer contains a pipe section with a ball valve and a surge chamber and a second pipe section with a solenoid valve.
  In the first experiment a water hammer is produced by rapidly closing the ball valve. The sudden deceleration of the water mass releases kinetic energy, which is converted into potential energy in the surge chamber. The resulting pressure vibrations are measured by a pressure sensor behind the surge chamber and displayed in the software as a pressure curve. The vibration can also be seen as oscillation of the water level in the surge chamber.
  In the second experiment a rapid closing of the solenoid valve in the second pipe section produces a strong water hammer. The water's kinetic energy is converted into pressure energy. The water hammer and the subsequent vibrations are detected by two pressure sensors in the

pipe section and displayed in the software as a pressure curve.

  The water is supplied and the flow rate measured by the supply unit.
  The well-structured instructional material sets out the fundamentals and provides a step-by-step guide through the experiments.

- demonstrating water hammers in pipes
- determining the sound velocity in water 
- understanding how a surge chamber works
- natural frequency in the surge chamber