F390 (3-inch) and F387 (4-inch) air set by-pass regulators The by-pass regulating valves are normally mounted in pump bypass return lines. They are usually installed as secondary pressure controls. However, where maximum low flow stability is required, they may be utilized as primary pressure controls when used in conjunction with an inline air set demand regulator similar to the 4-inch model F370. The air set demand regulator valve will provide positive required deadman closure and surge control features not contained in the by-pass regulator. The by-pass regulators will maintain a constant pressure at a downstream sensing point, such as a venturi, as a function of the supplied reference pressure. An optional feature allows the valve to also control maximum inlet pressure providing pump outlet pressure protection. Normally these units are capable of acting as secondary controls to larger sized primary or deadman controls. It is our opinion that the by-pass valve should normally be used as the primary control.