<P> A pneumatic non-return valve . </P> <P> Multiple check valves can be connected in series . For example, a double check valve is often used as a backflow prevention device to keep potentially contaminated water from siphoning back into municipal water supply lines . There are also double ball check valves in which there are two ball / seat combinations sequentially in the same body to ensure positive leak - tight shutoff when blocking reverse flow; and piston check valves, wafer check valves, and ball - and - cone check valves . </P> <P> Check valves are often used with some types of pumps . Piston - driven and diaphragm pumps such as metering pumps and pumps for chromatography commonly use inlet and outlet ball check valves . These valves often look like small cylinders attached to the pump head on the inlet and outlet lines . Many similar pump - like mechanisms for moving volumes of fluids around use check valves such as ball check valves . The feed pumps or injectors which supply water to steam boilers are fitted with check valves to prevent back - flow . </P> <P> Check valves are also used in the pumps that supply water to water slides . The water to the slide flows through a pipe which doubles as the tower holding the steps to the slide . When the facility with the slide closes for the night, the check valve stops the flow of water through the pipe; when the facility reopens for the next day, the valve is opened and the flow restarts, making the slide ready for use again . </P>

Where would a single check valve be used