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In accordance with the characteristics of a long water pipe supply system with water hammer, the analysis of the protecting devices combined with the air valve, pressure relief valve and slowly closed plate valve was carried out for the transient process caused by accidental pump-stop based on the theory of water hammer and the method of characteristics. The pressure of water hammer and relative cavitating volume along the pipeline were obtained simultaneously. The evolvement of pressure of water hammer against time was achieved. The numerical results showed that the present water hammer protections to the pipeline system were necessary, which restrained not only the continuously rising up but also the excessively decreasing of water hammer pressure and were helpful to suppress the occurrence of water column separation. However, the results also showed the cavitations in local raised position of pipeline. The optimization of the position about air valves should be implemented further. 