3-way valves have grown in popularity over the past 10-15 years, as they have begun to find their way into relief systems. Today, codes allow “relief trees” to be used, so Pressure Safety Valves can be changed out without emptying and degassing the vessel. The 3-way valve is the means by which the PSV is isolated from the process. This comes with some risks, as some folks are never trained on the proper operation of the 3-way valve, and unfortunately, there seems to be a lack of conformance in the design. Some manufacturers offer a really simple legend on their valve handle, but others do not. A recent incident involved one of those who did not, and the worker was exposed to an HHC/EHS during a Hydrostatic RV change-out. Here is a pic of the legend on one brand of 3-way:
As we can see, in that position, the flow is to the left. When we move the handle to the left and its parrell with the pipe, the flow will be to the right. That simple light flow legend makes all the difference! Its the little things that can make a big difference in process safety.