Watts 0061888 3/4 In Bronze Dual Check Valve Backflow Preventer with Intermediate Atmospheric Vent, Female Union Inlet and Outlet Connections

The Watts 9D, model 61888, is a dual check valve with an intermediate atmospheric vent. It guards against both backpressure and backsiphonage, and it is rated for non-health-hazard (pollution) applications only. Typical uses are boiler feed lines and similar connections where the substance in the line is a nuisance rather than a toxin. If the cross connection involves a health hazard, you need a higher class of assembly such as an RPZ, not this one.

Both ends are 3/4 inch NPT. The inlet is a union female threaded connection and the outlet is a union joint, so the valve can be pulled out of the line without cutting pipe or spinning the body. Standard 3/4 inch threaded nipples or male adapters mate directly; for copper you will need 3/4 inch male-adapter transitions.

The intermediate atmospheric vent sits between the two checks. If the upstream check fails and pressure reverses, the vent opens and dumps to atmosphere so contaminated water cannot be pushed back into the potable supply. It will discharge water when that happens, so mount it where a spill is acceptable or pipe the vent to a drain. Never plug the vent port.

Maximum working pressure is 175 psi, and the operating range runs from 33F (0.5C) up to 250F (121C). It is rated for continuous pressure service, so it can stay under line pressure indefinitely rather than being limited to intermittent use. Note that it is not built for hot and harsh water service, so recirculating hot supply or aggressive water chemistry is outside its intended duty.

A strainer is included on the 9D, so you do not need a separate inline strainer ahead of it to keep debris off the check discs. The checks use rubber discs and rubber seats, which are the parts grit would otherwise damage.