Rain Bird® DPUV4EHUB Drainage Pop-Up Relief Valve with 4" PVC Elbow and Hub Fitting Adapter is a pop-up water relief valve (also commonly known as a pop-up drainage emitter) that is normally closed except when there is a build up of hydrostatic pressure. As the drainage pipes fill and build up slight pressure, the cover rises to allow the excess water to escape. Once the water is discharged the pressure drops and a stainless steel spring automatically retracts the cover to its closed flush position. The flush design of the product ensures that it blends well into the surrounding area. It is manufactured from a structurally foamed High Density Polyethylene (HDPE) with a UV inhibitor to reduce sun damage. Typical application is at the discharge end of the drainage pipe. Can be installed in either a vertical or horizontal position.