Mechanical integrity and carbon steel refrigerant piping

ASHRAE Journal, Oct, 2007 by Daniel J. Dettmers, Douglas T. Reindl

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The goal of the U.S. Occupational Safety & Health Administration's process safety management (PSM) standard (29 CFR 1910.119, Process

Safety Management of Highly Hazardous Chemicals) is to protect people from large-scale catastrophic incidents that can occur in systems using hazardous or flammable chemicals. A portion of the PSM standard requires that end-users develop and implement mechanical integrity (MI) programs capable of managing the ongoing safe operation of systems that use hazardous chemicals, including the ammonia used in many large-scale industrial refrigeration systems.

The intent of the mechanical integrity provision in the PSM standard is to ensure the safety of plant personnel by preventing releases that result from equipment failure. This rule has produced significant changes in the industrial refrigeration community from reactive maintenance to proactive or predicted maintenance in an effort to meet the requirements of this standard.

The PSM standard has driven many industrial refrigeration end-users to develop and implement mechanical integrity programs that include regular equipment inspections and tests. An effective mechanical integrity program pays dividends in protecting people but can also deliver collateral benefits to the business including: decreased likelihood of unscheduled downtime, reduced insurance rates, and improved public image (by fewer chemical releases). Less downtime for the refrigeration system allows higher production rates while fewer uncontrolled refrigerant releases improves plant relations with both their communities and regulatory agencies.

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An important part of developing and implementing an effective mechanical integrity program is identifying those areas of industrial refrigeration systems that present risk of refrigerant releases. Historically, refrigerant piping has been the system component involved with larger scale releases making it a priority to address as part of a plant's mechanical integrity program. In comparison, compressors, refrigerant pumps, and vessels also require attention and preventive maintenance but these components tend to be centrally located making inspections and tests easier. The distributed nature of piping (including valves) as it runs throughout the facility makes inspection and testing to insure its mechanical integrity more challenging.

Because the first step in establishing an effective MI program is recognizing the common failure modes of carbon steel refrigerant piping, our emphasis here is on introducing basic mechanisms of mechanical integrity failures applicable to carbon steel piping (and vessels). We also provide some basic information on practices that can be taken to prevent loss of mechanical integrity of piping systems. Armed with this information, industry professionals can work with end-users to facilitate the implementation of more robust mechanical integrity programs.

While the focus of this article is on ammonia refrigeration systems, most of the failure modalities discussed also apply to refrigeration systems with carbon steel piping and vessels using other refrigerants. An MI program should be implemented whether the refrigerant is ammonia, carbon dioxide, R-22, R-507 or any other halocarbon refrigerant. Using other materials, such as stainless steel or copper, introduces a whole new set of failure mechanisms. For example, stainless steel is extremely susceptible to stress corrosion cracking in the presence of chlorides. Anyone dealing with copper should be aware of formicary (ant's nest) corrosion.

Pipe Wall Material Loss

By far, the most common failure mode for carbon steel piping is by wall material loss from external corrosion (rusting) due to prolonged exposure to water, and most often occurring under the piping insulation system. Corrosion is defined by the National Association of Corrosion Engineers (NACE) as "the deterioration of a material, usually a metal, by reaction with its environment." (1) The two most common forms of corrosion found in ammonia refrigeration systems include uniform corrosion and pitting corrosion. A third mechanism of wall material loss is attributable to "erosion." The first two are found on the external surface of the pipe, while the last is mostly internal.

Uniform corrosion is the gradual thinning of wall material due to the oxidation of material by progression of the corrosion process. Visual examination of the surface alone may not yield an adequate indication of the corrosion's severity since the entire surface is being uniformly removed. If uniform corrosion is suspected, a wall thickness measurement using an ultrasonic thickness gauge or similar device is recommended to quantify the material loss. Photo 1 shows uniform corrosion on the exterior section of carbon steel pipe.

Many factors can conspire to increase the rate of corrosion including oxygen (presence of dissolved oxygen in water accelerates corrosion), solutes (presence of acids accelerate corrosion), and temperature (corrosion rates double for every 18?F [10[degrees]C] rise in temperature). Unfortunately, several of these variables are largely uncontrolled in refrigeration applications.