Impact forces can be generated by quickly stopping the motion of a moving stream of liquid in a pipe. An initial estimate of the magnitude of the pressure spike can be made by visualizing a plug of liquid in motion, as in Fig. 13.3. When the valve closes, a pressure must build up on the liquid cross section at the valve (and thus throughout the liquid) such that the force equals the rate of change of momentum of the moving rod of liquid.
The mass is the volume of the liquid rod multiplied by the density of the liquid, so:
The high-pressure pulse that first develops at the valve transmits back upstream through the liquid refrigerant at a sonic velocity12. For ammonia liquid at -34°C-(-29°F), the sonic velocity is 1715 m/s (5625 ft/s). The associated noise is attributable to the metal pipes becoming sound generators. The pressure pulse calculated in Example 13.1 is not sufficient to rupture the pipe. However, in plants where the liquid velocity is high or a solenoid valve closes quickly, pipes may shake each time a valve closes, which is an indication to the operator that an abnormal situation prevails. Also, the pressure-relief valve might open momentarily at each pressure pulse and eventually start leaking.
Designing for low velocities in the liquid lines is one method of reducing this impact force.