自动把超过设定值的管道压力通过旁通管道泄压；
主管道压力可以根据现场要求调节，如要增加管道压力，就顺时针旋转向导阀的调节螺钉；反之，则逆时针旋转。
持压阀：
维持用户端管道的压力；当管道压力超过设定值时，阀门自动打开，把多余的压力释放到下游；当管道压力低于设定值时，阀门关闭；
主管道压力可以根据现场要求调节，如要增加管道压力，就顺时针旋转向导阀的调节螺钉；反之，则逆时针旋转。
The Model 500 Pressure Sustaining/Relief Valve
is a hydraulically operated, pilot-controlled, modulating valve designed to maintain constant upstream pressure within close limits. This valve can be used for pressure relief, pressure sustaining, back pressure, or unloading functions in a by-pass system.
In operation, the valve is actuated by line pressure through a pilot control system, opening fast to maintain steady line pressure but closing gradually to prevent surges. Operation is completely automatic and pressure settings may be easily changed by adjusting screw on top of the pilot.
用于维持稳定的上游压力及避免过高的上游压力，大部分灌溉，民用建筑及工业设施均需这个保护功能。WAF50 可用于中等压力的供水系统，可提供平稳和精确的压力控制。WAF53 带塑料导阀，特别适用于灌溉、过滤、水处理及低压家用工况。
