Built-duckbill valve 
 XH41A type built-in, duckbill valve http://www.jkyzf.com nested structure of an all-rubber, was beak-like shape, has a "forward flow, reverse backstop" feature new to the drain valve product, which according to stress analysis and stress calculation for different conditions, http://www.jkgssb.com different design mechanisms, use different rubber formulations, made according to special technology. Positive work, there are inlet water flow, small pressure, able to open the valve, the greater the pressure, the greater the valve open, water flow greater http://www.jkgssb.com/Product-20110666 . html. Reverse operation, the natural valve port closed, sealed against the pressure, the greater the back pressure, valve port and more tightly sealed, the better, it is widely used in municipal, water, sewage treatment and other water discharge to prevent intrusion, back , is the ideal product to replace door.