This UGD fitting from Ashirvad prevents the undesirable reversal of the flow of water from its intended direction in the piping system. Backflow is dangerous because it can cause plumbing systems to become contaminated and unusable. Non-return valves also known as NRV are simple and effective devices for reducing the risk of flood damage caused by back flow of surface water through the underground drainage system. It comes in 2 sizes- 4 inch and 6 inch.