2021 RETA Breeze May-June

MYTHBUSTER

CORROSION RATE IS (NOT) A VIABLE MEANS TO DETERMINE LIFE OF A COMPONENT IN AMMONIA REFRIGERATION By Jim Kovarik, VP NDT Technology, Gamma Graphics Services (GGS) and LIXI, Inc.

As a rule, for industries that manage a piping condition monitoring system to support operations, corrosion rates can be used to estimate the remaining life of equipment such as piping, pressure vessels, reactors, distillation columns, etc. A corrosion rate determines the speed at which metal deteriorates in a specific environment.

Accurate, usable corrosion rates require the following characteristics to be present: 1. A systematic process in which conditions are controlled, repetitive and routine. 2. Corrosion or erosion occurs internally (on the inside wall) of the equipment. 3. The ability to measure process metrics that affect the reaction between the process chemical (including pressure, velocity, temperature, conductivity, diffusion, etc.) and the piping or other equipment.

For example, in the Oil and Gas industry, hydrocarbons pump through the pipes at a consistent, calculable temperature, pressure and velocity. The chemical itself at a given pressure and velocity wears the piping overtime, and the chemical is contained within the pipe. This systematic process is generally uninterrupted by environmental or other influences that may disrupt or change the process pattern. As a result, the oil and gas industry is capable of measuring the rate of internal corrosion on their piping and they know that internal erosion/corrosion factors will deteriorate the piping long before external factors will cause leaks.

In contrast, ammonia does not degrade metal. Ammonia is not a corrosive chemical to carbon steel or stainless-steel piping. Consequently, piping in closed- circuit ammonia refrigeration systems is generally not susceptible to internal corrosion. The damage mechanism that thins pipe wall in ammonia refrigeration systems occurs on the outside of piping. Corrosion Under Insulation (CUI) occurs when water or condensation breaches the vapor retarder and rests in the insulation against the surface of the pipe metal. Piping with moisture in the insulation becomes corroded and eats away the metal on the pipe surface, thinning the pipe wall.

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