Process side stress corrosion cracking (SCC) was observed on a carbon steel branched, transgranular cracks were ultimately attributed to CO and C02 in These findings prompted a company wide inspection program at similar units. heat exchanger tube. The process environment. SCC has been found in the analogous equipment at all plant locations which have performed internal inspections. At a few of these locations, SCC has occured at gas compositions leaner in CO than previously reported in the literature. Replacements have been made with type 304 L SS which is not susceptible to CO-C02 SCC.
In late 1993, branched transgranular cracks were observed propagating from the process side of a carbon steel heat exchanger tube. Initial inquiries failed to identify a known problem with process side SCC. Over the next six months, cracking of carbon steel piping was reported at two other company plants in the same service.
FAILURE ANALYSIS
The heat exchanger described above was an overhead condenser for a light hydrocarbon separation tower. Propylene in the overhead stream is condensed by water at about 400 psig (28 bar) and 120 F (50 C). Noncondensibles such as ethane and C02 are vented to a fuel gas system. The branched transgranular cracking looked like stress corrosion cracking (SCC). See Figure 1. However, no SCC mechanism could be readily identified. Cyanides can cause transgranular SCC of carbon steel, but none were thought to be present. Neither were other species such as hydroxides, carbonates, ammonia or nitrates which can cause intergranular SCC of carbon steel. A gas analysis from the plant indicated CO in significant quantities as well as C02. A partial gas analysis of the reflux drum vapors is listed under Plant 1 in Table 1.
