Management and Identification of Dead Legs

Hydrocarbon processing facilities encounter numerous damage mechanisms threatening the asset integrity. However, many of those damage mechanisms become even more detrimental when the flow is stagnant and the process parameters are unlike the ones in the normal operating circuits. For instance, in piping with hydrofluoric acid service, segments with stagnant flow are more prone to damage due to higher corrosion rates compared to segments where the flow is normal. Similar observations are also evident in process piping with varying types of service, as numerous incidents have occurred where unexpectedly higher corrosion rates surfaced in areas where the process flow is still. Dead legs are areas of a piping system that rarely see flow, yet are still exposed to process, even if not explicitly cut off. These areas pose a substantial threat to piping integrity because they are especially prone to contamination and corrosion (Examples shown in Figures 1 and 2).

Ideally speaking, during the design stage of a plant, all potentially unavoidable dead legs, like a bypass to a control valve, should be identified and efforts should be made to minimize or eliminate their existence where possible. However, that is often not realistic. Most operating facilities have piping circuits containing one or multiple dead legs. These dead legs are either operational or related to the plant construction, and are required to be identified on drawings and tagged at the site as well. Designers, however, are still responsible for eliminating or minimizing unnecessary dead legs at the design stage. It is not a difficult task to replace the piping length of instrument connections with forged fittings which would practically eliminate the dead legs. Long weld-neck flanges (forged) can be used for drain and vent connections, which are potential dead legs on equipment, in order to avoid any possible thinning of those forged components over the years with stagnant flow inside. Similarly, it is also practical to have forged fittings wherever a vent or a drain connection has to be provided on static equipment. Likewise, dummy legs can also be eliminated by providing structural supports wherever required. There are cases where it is simply not possible to eliminate a dead leg, however. Examples of these include bypass lines, pump and compressor recirculation lines, level bridles, relief valve inlet and outlet piping, sample points, etc. Broad categories of dead legs are described in Table 1.