Using Visual Inspection of Piping and Vessels to Avoid Failure
February 7, 2008
Mechanical-integrity programs for refrigeration systems that include visual inspection provide many benefits for the end users, including improved reliability, according to a report in the Cold Front newsletter from Madison, Wis.-based Industrial Refrigeration Consortium.
Increasingly, end users of industrial refrigeration system are realizing that run-to-failure maintenance is not consistent with their operational needs and goals. Apart from the obvious safety risks to personnel, unplanned outages create process interruptions with the potential for significant direct and indirect impacts to the business. A good mechanical-integrity program that includes visual inspections can help avoid these problems.
When establishing a mechanical-integrity inspection program or training staff to do visual inspections, it is important to understand some of the typical failure mechanisms for refrigeration piping and vessels. The failure mechanisms discussed by IRC arise in industrial ammonia refrigeration systems, but it is worth noting that most of these failure methods are common to all refrigeration systems using carbon steel piping.
Visual inspection can be an effective tool against the three most common mechanisms that lead to the loss of mechanical integrity in piping and vessels: external corrosion, erosion and impact damage.
By far the most common failure mechanism for carbon steel piping and vessels in ammonia refrigeration systems is by external material loss from corrosion, which tends to form in numerous locations on metals at surface imperfections, inclusions or a lack of material homogeneity. The two most common forms of corrosion found in ammonia refrigeration systems are uniform external corrosion and external pitting corrosion.
Uniform external corrosion is the gradual thinning of large areas of a wall due to the removal of material by the corrosion process. Visual examination of uniformly corroded surfaces does not always give a good indication as to the severity of the problem. If significant corrosion is suspected, thickness measurement of the component using a nondestructive technique such as ultrasonics is recommended.
Corrosion under insulation (CUI) also can be tricky to identify using visual inspection. Often, a discolored insulation jacket may be the only sign that CUI may be active. Because it is difficult to detect, IRC recommends preventing it by using a good outer jacket, properly applied vapor retarder and the use of closed cell insulation material with low-moisture permeability.
One internally driven failure mode is erosion, which is the loss of material due to the repeated impingement of high momentum liquid droplets or solids on the internal surface. Although not possible to detect via visual inspection, a sound visual inspection program will identify areas at risk for erosion so they can be routinely inspected using proper non-destructive evaluations methods.
As a general rule, there is an increased risk of erosion in those portions of system with high concentrations of suspended particulate matter (weld slag, roof ballast, dirt or other debris), especially in piping systems that experience high velocities and changes in flow direction. Interior portions of a vessel can be at risk as well, if the entrained liquid droplets or solid particles impact the side of the vessel out of a misdirected or eroded inlet nozzle.
While the number and variation of failure methods for carbon steel refrigeration vessels and piping systems seems limitless, many can be found through a competent and thorough inspection program before failure occurs. Visual inspection is an important undertaking to monitor and locate external deficiencies that occur over time and allows the identification of telltale signs of problems before they lead to failure, according to the IRC.