Valve trays are essentially modified sieve trays. Like sieve trays, holes are punched in the tray floor. However, these holes are much larger than those in sieve trays. Each of these holes is fitted with a device called a “valve.” Vapor flowing up through the tower contacts the liquid by passing through valves in the tray floor (Figure 6-1 c). Valves can be fixed or moving. Fixed valves are permanently open and operate as deflector plates for the vapor coming up through the holes in the tray floor. For
moving valves, vapor passing through the tray floor lifts the valves and contacts the liquid. Moving valves come in a variety of designs, depending on the manufacturer and the application. At low vapor rates, valves will close, helping to keep liquid from falling through the holes in the deck. At sufficiently low vapor rates, a valve tray will begin to weep. That is, some liquid will leak through the valves rather than flowing to the tray downcomers. At very low vapor rates, it is possible that all the liquid will fall through the valves and no liquid will reach the downcomers. This severe weeping is known as “dumping.” At this point, the efficiency of the tray is nearly zero.