In refrigerant vessels and piping constructed of carbon steel, problems such as external corrosion, dents, bulges and punctures provide visual indicators of potential failures. Left unchecked, these areas can become the source of excessive maintenance or downtime. Implementing a regular inspection program can help plants identify and correct piping problems before they affect production.

Physical Impact Damage to Industrial Refrigeration Piping

One of the more common causes of piping problems is a physical impact. Refrigeration system piping, evaporators and vessels can fall victim to the errant driving habits of fork-truck operators, overambitious cleaning crews or abuse as a ladder or climbing aid. These encounters can result in dented, punctured or severed pipes, vessels or related infrastructure such as the insulation system. Optimally, when damage occurs, it should be documented immediately so that the extent of the damage can be assessed and followup repairs made. Because immediate documentation does not always happen, visual inspections should include scanning the refrigeration equipment for dents or other indications of physical impact damage (deep scratches, flattened surfaces).

Most dents or bulges in uninsulated vessels and piping that are large enough to be of concern can be found during a visual inspection of the vessel or pipe surface. A powerful light directed from multiple angles toward the area being inspected will aid the inspector in identifying problem areas.

For insulated piping and vessels, look for signs of damage to the aluminum or PVC jacket. The damage might not extend all the way to the pipe or vessel surface, but even minor damage can compromise the integrity of the insulation system, including the vapor barrier. The puncture wound shown in figure 1 did not extend all the way to the pipe surface, but it provided easy access for rain and other forms of moisture incursion, which put the underlying piping at risk for accelerated external corrosion.

External Corrosion on Uninsulated Industrial Refrigeration Pipe

Piping that is on the high-pressure side of the system (i.e., condenser piping, high-pressure liquid) is not commonly insulated. The piping should be protected by an external layer of paint, galvanizing or epoxy coating to guard against external oxidation. If the piping or vessel’s protective layer is compromised and the corrosion process begins, visually observing the corrosion is easy -- assuming that the pipe or vessel is located in an accessible area.

In your piping inspection process, be certain to include piping that is located on the roof, suspended from the ceiling and in other potentially difficult-to-access locations by the best means available. During your inspection, look for signs of mild corrosion or degradation of the protective coating (peeling, cracking or flaking). Examine flanges, welds and piping supports for signs of deterioration. Inspect under tags or anything else covering the surface of the pipe or vessel. If corrosion is found, IIAR Bulletin 110 recommends checking the material thickness using ultrasonic gauges, or other methods if the metal loss is estimated to be 10 percent or more of the original thickness.

Because water flows “downhill,” places to be particularly aware of corrosion would be the bottom surface of a horizontal run of pipe; the bottom elbow of a vertical run of pipe; or any point where rain water can collect before it drips off the pipe. Also, pay extra attention in areas of heavy traffic where accidental impacts occur easily.

Breaches in the Outer Jacket of Industrial Refrigeration Piping

Exterior piping traditionally is covered with an aluminum jacket, while piping and vessels inside the facility are protected by a PVC jacket or other similar material. Both types of jackets are designed to protect an applied vapor barrier and underlying insulation from physical impact (such as hail, boots and fork trucks) and directly applied water (such as rain or overspray during a cleaning shift). Unfortunately, all jacketing materials degrade over time or can be subject to damage. If significant damage is found to the external insulation, the problem area should be noted for a more detailed followup inspection that might involve removing a section of the insulation and vapor retarder for examination and repair.

PVC jackets can degrade and crack due to exposure to sunlight and other chemicals. Look for signs of jacket degradation and be aware that the underlying insulation system may be at risk.

Aluminum jackets also can degrade by corrosion and biological damage. On aluminum jackets, corrosion appears as small blisters that eventually fall away, leaving small holes such as those seen in figure 2. While the process does not present an immediate danger to the safety of the piping or vessel, decomposition of the aluminum jacket provides the outside elements with direct access to the vapor barrier.

Biological growth can be an indicator of problems such as degraded insulation or breached jacket/vapor barrier. Figure 3 shows an example of moss growing on the underside of a horizontal pipe. In this example, the moss took advantage of an overlapping seam between two pieces of jacket to live off of moisture and dirt that collected there. If the insulation has degraded sufficiently so that water is continuously condensing moisture from the air, an adequate water supply will be available for moss to grow. Moss and small plants also have been observed growing out of moist insulation when the outside jacket and vapor barrier have been compromised.

Degraded or Saturated Insulation

The persistent presence of ice or moisture on the outside of vessel or piping-run jacketing is a good indication that the underlying insulation is degraded or saturated (especially if that moisture is present on warm and dry summer days). As insulation becomes saturated, the thermal conductivity of the material degrades and it is subject to increased physical damage (ice damming). The reduced thermal conductivity causes the surface temperature of the insulation and exterior jacket to decrease to the point where atmospheric moisture will condense and freeze readily on the jacket surface.

While moisture or ice on the outer surface indicates a problem in that area of the vessel or pipe, it might not be the penetration point where moisture entered the insulation system. Often, moisture enters the insulation system at a physically higher point and flows downhill to the problem area. The entry point also should be sought during an inspection. If an odd collection of moisture or ice exists on the surface of the insulation, think about what it is indicating and where the problem might have originated. Start at the point of moisture and work uphill until the insulation-system breach can be found. Plan to replace all wet and deteriorated insulation.

Piping with deteriorated or saturated insulation often will have ice or moisture on the jacket surface. Pay particular attention to piping that fluctuates significantly in temperature (such as ammonia defrost condensate piping or transfer system piping), which is prone to problems because of the thermal stresses on the pipe and insulation system. This piping might show signs of condensate/frost only when the evaporator is in defrost mode.

Wet suction return piping might have ice only on the bottom quarter of the pipe, where the liquid ammonia tends to collect, because the heat transfer from the ammonia vapor might not be able to cool the rest (i.e., top) of the pipe below the dewpoint.

Additionally, elbows or dead-legs at the bottom of a vertical run of piping will collect moisture that could enter much higher in the pipe run. The ice often will build up and split open the jacket, making identification easy, but the original damage still must be found and repaired. Also look for dips in a horizontal run of pipe if it was not installed properly.

The inspection also should focus on any piping with insulation that could be easily damaged by impact because of its location in or around heavy-traffic areas. Look for any penetration or break in the insulation system on vessels or piping, which include the following:
  • Pipe hangers, wall penetrations and other piping supports. Look for dripping from iced-over pipe hangers, and be certain to inspect the insulation under the pipe hanger for damage due to vibration or moisture.
  • Insulation jacketing seams located on the top of horizontal piping or any improperly lapped or sealed insulation jacketing.
  • Valves, valve trains, fittings and other irregular shapes that are difficult to fit with vapor retarders. These often are covered in ice, but the moisture can work its way into the face of the insulation system on the adjoining pipe.
  • Insulation system penetrations for instrumentation, pump-out connections, etc.
  • Interface between vessels and attached piping or vessel supports.
  • The bottom of vessels, even on the supports. Moisture will enter anywhere on the vessel or the attached piping and flow to the bottom of the vessel, where it will degrade the insulation. Ice or moisture will be seen on the outside jacket around the bottom of the vessel.
  • Sections of the insulation system such as removable plugs for thickness monitoring that might be poorly constructed or maintained.

Regardless of the refrigerant (e.g., ammonia, R22, CO2, R134a, or any other fluid), there are numerous opportunities for failures to occur. Some precursors to failure are internal and are difficult to detect reliably without either advanced nondestructive inspection technologies or physically opening the component for inspection. However, more commonly, mechanisms leading to failures of piping and vessels are external events that usually leave telltale visual signs of their existence. Dents, deformation, ice and condensation on the insulation jackets all provide visual indications of potential problems at the underlying pipe or vessel’s external surface. Fortunately, many of these defects can be found through a competent and thorough inspection program before failure occurs.

Sidebar:
5 Common Inspection Questions & Answers

How often should I inspect my system?
One of the first places to look for an answer to this question is the mechanical integrity section of your plant’s process safety management (PSM) program documentation. Many PSM programs base their mechanical integrity programs on IIAR Bulletins 109 and 110, which state that all vessels, piping, supports and other associated components should be inspected every 12 months. If your plant references these IIAR bulletins as a basis for the mechanical integrity program, are these provisions currently being followed? Piping that is subject to a repetitive freeze/thaw cycle - defrost ammonia condensate piping, transfer station piping, hot gas defrost run-outs - should be inspected more frequently than every 12 months.

Which piping do I need to inspect?
The simple answer is you must inspect all piping. IIAR Bulletins 109 and 110 do not exempt a given type of pipe from inspection; however, they do differentiate inspection requirements for insulated and uninsulated pipe. Some might argue that “always frozen” piping or vessels do not need inspection because ice does not corrode carbon steel. While it is true that the corrosion process is significantly slowed by the continuous presence of ice, can you guarantee that any given piece of equipment is “always frozen”?

How can I inspect all this piping? I have miles and miles of piping!
It does seem like a daunting task, but none of the provisions require all vessels and piping to be examined in one continuous inspection. Because most plants have a limited number of vessels, they typically are inspected at a single time. To handle the piping inspection, consider breaking the inspection task up into smaller portions and doing a little bit at a time. Just be sure that no portion of your piping system goes longer than 12 months without an inspection. Here are some ideas for how to complete your pipe inspection:
  • Break the job up into smaller segments and inspect a different portion of the piping system each month.
  • Include the piping inspection as part of inspection/maintenance of valves and other components. As the team moves from valve station to valve station, inspect the pipe in between.
  • Make the inspection part of on-the-job training for new hires. Send a senior member along to train on the first few, and then let the new guy inspect the rest with a senior member reviewing and confirming the inspection process and findings.

Be sure to document the findings of these inspections. Records of what is inspected and when are essential to avoid omitting vessels, piping or other components. Also, remember that without documentation, the inspection never occurred as far as your PSM program is concerned.

What do I need for documentation?
Section 1910.119(j)(4)(iv) of the PSM Standard states: “The employer shall document each inspection and test that has been performed on process equipment. The documentation shall identify the date of the inspection or test, the name of the person who performed the inspection or test, the serial number or other identifier of the equipment on which the inspection or test was performed, a description of the inspection or test performed, and the results of the inspection or test.”

Simply put, any piece of inspection data collected should be documented and stored. A good starting point for a documentation template is the Ammonia Refrigeration Safety Inspection Checklist, which can be found in the back of IIAR Bulletin 109. These sheets contain many of the items that should be examined during an annual visual inspection. Modify the sheets to include additional inspection information that you feel is important to the safety of your system.

Who can perform the inspection?
The choice of an inspector to conduct the annual inspection is up to the owner/operator of the system. Most choose a team of on-site personnel to conduct the inspection. If at all possible, at least one of these participants should not be involved in the day-to-day operation and maintenance of the system. This allows a new set of eyes to carefully examine all parts of the system for any mechanical integrity deficiencies. Choices might include the plant’s PSM coordinator, a maintenance technician from another department, the plant engineer, or perhaps a corporate engineer.

What equipment do I need?
The ASME Boiler & Pressure Vessel Code: Section V, “Nondestructive Examination,” Article 9 outlines the requirements that can be used in selecting and developing visual inspection procedures. Proper lighting is essential. A minimum of 100 foot-candles (1,000 lux), which is the equivalent of a 150-W incandescent bulb held 3' from the surface, is recommended. When possible, surrounding surfaces (walls, floors, ceilings, backdrops, etc.) should be light colors that reflect light well.

A clean surface also makes inspection easier. A built-up, opaque dirt layer, whether it is rust or other debris, obscures the inspector’s vision. When possible, clean the test surface prior to inspection. The visual inspection process can be enhanced without the application of a probing medium by using magnifying lenses and video technology. Both require sufficient lighting but also offer the added ability for magnification on the test surface.

Links