The CARB-certified OPW 71SO vapor-tight Overfill Prevention Valve is designed to prevent the overfill of underground storage tanks by providing a positive shut-off of product delivery. The shut-off valve is an integral part of the drop tube used for gravity filling. The OPW 71SO-4000 allows easy installation (without breaking concrete) and requires no special manholes. Retrofits directly for both new and existing tanks with 4" fill risers.