System Sensor WFD 20 Water Flow Detector – 2 Inch shall be installed on system piping as designated on the drawing and/or as specified herein. Detectors shall mount on any clear pipe span of the appropriate nominal size, either a vertical upflow or horizontal run, at least 2˝ from any fittings that may change water direction, flow rate, or pipe diameter or no closer than 24˝ from a valve or drain.