Check the listing for details. Zurn Wilkins 3/4” 34-350XL Double Check Backflow Preventer Valve. Condition: New. Listed at 156.99 USD. The 3/4 in. 350XL Double Check Valve Assembly is used in low health hazard, potable water applications for protection against both backsiphonage and backpressure of polluted water. Its stainless steel shut-off valve handles, hardware, and struts are corrosion-resistant and the durable UV resistant reinforced nylon EZSwap Pressure Vessel resists corrosion and mineral build-up to better withstand harsh outdoor conditions.The 3/4 in. 350XL Double Check Valve Assembly is used in low health hazard, potable water applications for protection against both backsiphonage and backpressure of polluted water. Its stainless steel shut-off valve handles, hardware, and struts are corrosion-resistant and the durable UV resistant reinforced nylon EZSwap Pressure Vessel resists corrosion and mineral build-up to better withstand harsh outdoor conditions.