See the related feature article, "Pinpointing Piping Problems," to learn how implementing a regular inspection program can help plants identify and correct piping problems before failures occur.
In a liquid state, water serves as an electrolyte that enables the carbon steel materials used in the ammonia refrigeration industry to corrode. In a solid state (ice), water is a much less effective electrolyte, thereby substantially reducing or eliminating the potential for enabling corrosion of the underlying pipe or vessel. If a component is consistently below the freezing point of water, the moisture will form frost or ice, leading to significantly lower corrosion risks. But what constitutes a “constantly” frozen pipe, vessel or other component?

Many times, large sections of piping and certain vessels are ignored in mechanical integrity inspections because they are assumed to be “constantly” frozen by the plant owners/operators. In actuality, some components cyclically freeze and thaw the infiltrating moisture. The formation of ice under the insulation tends to degrade the insulation quickly. When the pipe warms up, the water then is free to act as an electrolyte and begin to oxidize the underlying steel surface. Piping that undergoes the repetition of this freeze/thaw cycle - for example, defrost ammonia condensate piping, transfer station piping, hot gas defrost run-outs - should be inspected with greater frequency.

Some components only have a portion of their surface that is constantly frozen. A good example is surge drums for flooded evaporators and transfer vessels (figure 1). The bottom half of this formerly insulated, vertically oriented transfer vessel (Section D) always holds liquid ammonia at temperatures below freezing, which keeps the bottom portion constantly frozen. When the vessel goes into transfer mode, the upper portion of the vessel warms up, allowing any trapped moisture to melt. Again, this creates an environment conducive for external surface corrosion. In the figure, corrosion was seen in section A and uniform corrosion in section B, but section D still had some of the original paint in place.

Horizontally run, wet-suction-return piping with poor or degraded insulation is another common location for corrosion under insulation (figure 2). In this case, the bottom portion of the pipe is full of liquid ammonia and maintains itself below freezing. The upper portion has only vapor, which cannot maintain the pipe surface below freezing on a warm, sunny day with degraded insulation. The corrosion occurs on the top and sides of the pipe where the infiltrated moisture is still in liquid form. On the outside, the bottom of the jacket is nearly always wet or covered with frost/ice. The top might be damp but will dry off as the day warms up.

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