I'd suggest you refer to ...

I'd suggest you refer to "Fluid Mechanics for Civil Engineers" by Weber pp133 onwards to fully appreciate this. Basically you are arresting suddenly a column of water in a pipe. Many of the network modelling packages have facility to calculate water hammer but I'd suggest an understanding is essential rather than believing a number from a black box. I've seen field measurements using elaborate sensors and loggers and some truly alarming forces can be generated. The transient wave is followed immediately by an equal negative vacuum wave and this could be catastrophic for a PE pipe. Once you have a better understanding of the forces you may consider other materials or provide surge vessels  on the line to reduce the effect. Operationally you should ensure that valves are open and closed slowly and if pumps are present that they cannot fail suddenly. 

Hope this helps. It's a good question and a topic that is often not given sufficient thought.