The TDC 12 valve is an adjustable check valve that is used in irrigation systems. It is installed inline with the mainline pipe and has a lever that can be used to adjust the flow of water through the valve. When the lever is in the closed position, the valve will block the flow of water. When the lever is in the open position, the valve will allow water to flow through. The amount of water that flows through the valve can be adjusted by turning the knob on the top of the valve.