Rainbird DPUV4E Drainage Pop-Up Relief Valve with 4" PVC Elbow. This is a Drainage Pop-UP Relief Valve with a 4" PVC Elbow, that is generally shut except when there is a build up of pressure. As the pipes used for drainage fill and build pressure, the cover rises to allow excess air to escape. As the water escapes and is drained, the pressure drops and the cover in closed back into shut, flush position. Like the other Rain Bird Drainage Products, these Valves are made of High Density Polyethylene, a high quality plastic material, with UV inhibitor to avoid sun ware. Can be installed horizontally or vertically.