A check valve on the discharge line of a pump should have a fast closing time to prevent flow reversal through the pump and the valve slam that can occur with delayed valve closure, or where surge tanks are incorporated into the pump station design. Valve slam can damage the valve, pump, or system piping. If it is not possible to have a check valve that closes before the surge tank responds and slams the valve, some type of dampening device, such as a dash pot, is necessary to control valve closure during the last 5 to 10 percent of the valve travel.