https://www.nachi.org/gallery/piping/pressure-vacuum-breaker-backflow-preventer
and
https://www.nachi.org/gallery/piping/double-check-valve-assembly
and
https://www.nachi.org/gallery/piping/atmospheric-vacuum-breaker-backflow-preventer
and
https://www.nachi.org/gallery/piping/reduced-pressure-zone-backflow-preventer