These are used in similar applications as the N14210W- Watts 009 but offer superior control and maximum relief due to a patented "air-in/water-out" principle during emergency conditions of combined back siphoning and back pressure with both check valves fouled. The 3/4" - 2" are portable units (max 175 psi) that are furnished with ball type test cocks and quarter turn, full port bronze ball valves. The 2-1/2" - 4" are much larger units with flanged gate valves.