Risk and Corrosion Control

Similarly there is a lot of knowledge in process operations. In the process industry, for example, units which start up and shut down frequently are more susceptible to stress corrosion cracking (SCC) than identical units in the same service which operate for long periods at a steady state. This is because start-up and shutdown produce thermal stresses and internal pressure surges which add a low-rate cyclic component to the static stress of normal operation [7]. 

In a single-train unit, obviously the greatest concern will be for capital intensive items such as major stills and heat exchangers and for items whose failure might affect production. Components within a given train which run hotter or that could experience more concentrated solutions than associated equipment, are worthy of extra attention. Reboilers, for example, frequently crack first when a 

There may be a multiplicity of welds in a piping system that may be susceptible to aggravated attack unless full penetration welds are achieved. In addition, cold-work due to bending operations, and especially that associated with flared pipe, provide areas highly susceptible to environmental cracking (Fig. 12.4). 

All weldments in a reactor contain high residual stress and, unless they are suitably stress-relieved, they are focal points for environmental cracking. However, the circumferential welds by which nozzles are attached to the vessels are particularly vulnerable. The radius of a dished head also contains residual stress from the forming operation, and the top head is often the first point of failure when volatile species (e.g., traces of hydrogen or ferric chloride) are involved (Fig. 12.5) [7]. External jackets can cause problems, unrelated to the process, that arise from the use of cooling water or steam in the jacket which causes external SCC of the vessel projjer. 

In the particular cases of columns, weldments, especially circumferential nozzle welds, are primary areas for corrosion, as are the radii of dished heads. Nozzles themselves, if in the horizontal plane, may accumulate and hold chlorides or other corrosive agents, making them particularly susceptible to SCC (Fig. 12.6) [7]. 

---