There is blindness and randomness in using air valves in engineering application at present, hence water hammer protection in pressurized pipelines is simulated numerically for various types of air valves to establish some theoretical guidance for selection of air valve in application. According to theory of water hammer, a hydraulic transient mathematical model is built for pressurized pipelines. The effects of air valves on water hammer protection are predicted numerically in pressured pipelines by using the method of characteristics and by employing a few the examples. The results show that a traditional air valve installed at the pipeline high point can reduce negative pressure but can cause excessive pressure as well. The set of air valves with orifice area ratio ranged in 0.05-0.20 can reduce the negative pressure effectively but also can lower the excessive pressure. The anti-slam air valve with 0.05-0.20 orifice area ratio and nearly 1.0 floating pressure coefficient can reduce the negative pressure and can decrease the excessive pressure. Properly selecting air valves and optimizing their parameters can reduce the negative pressure in a pressurized pipeline significantly and prevent column separation to occur.
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