In the essential service water system (SEC) of the nuclear power plant (NPP), opening and closing devices, pipeline fracture, etc., may induce water hammer. Studying the theory and calculation of water hammers in SEC is very crucial for the safe operation of the NPP. This paper focuses on the SEC of NPP, employing RELAP5 to model the operation of the SEC. The accuracy of the model is verified by comparing the experimental results with those from the RELAP5 calculations. The time step is reduced to 0.001 s to predict the pressure peak under the actual operating conditions. This study divides the water hammer into two types: (1) void-induced water hammer; and (2) rapid operation of the valve-induced water hammer. In the void-induced water hammer scenario, the pressure peak is not so high and so severe, and the pressure waves propagate continuously, gradually diminish. In contrast, when the water hammer is caused by the closure of the valve, the pressure peak is larger. Upon reaching air-containing section, the pressure waves caused by the valve closure can superimpose, resulting in irregular pressure fluctuations. By observing the variations of pressure and flow, we can study the variation law of the water hammer phenomenon better, and provide a reference for future research of water hammer phenomenon in the SECs.