Use In-Line valves in locations where the use of a pressure vacuum breaker (PVB) or a double-check valve is required by city codes to protect against back flow. Check your local city codes to determine what type of valves are required. In-Line valves are installed below ground in a valve box down stream from the main line from a PVB or double-check valve. In-Line valves will not prevent back flow on their own: they need the help of a back flow prevention device.