The present invention provides a fluid flow prevention apparatus for a wet barrel fire hydrant which is simple and inexpensive to install and effectively cuts off water flow while reducing water hammer. The fluid flow apparatus includes an in-line or non-hinged valve member for closing a flow passageway connected to the hydrant that is operably movable from an open to closed position upon damage to the hydrant. An adjustable damping member controls the rate of closing of the valve member to reduce the effects of water hammer.