Pipe Corrosion (IIAR 6, Part 13)

In Part 12 of our series on ANSI/IIAR 6-2019 we examined the ITM requirements related to pressure vessel corrosion. While many of the same principles apply, it is fitting to explore unique considerations when inspecting ammonia pipe.

Table 11.1 requires non-insulated ammonia pipes to be inspected annually for pitting and surface damage (Inspection Item A). Similarly, insulated pipes must be inspected for “damage or moisture incursion in insulation” (Inspection Item B). These requirements essentially mimic the requirements for pressure vessels in Table 10.1.

Per §11.1.1, ” Where pitting, surface damage, general corrosion, or a combination thereof, is visually observed on a metal surface of the piping, deficient areas shall be further evaluated per Sections 11.1.1.1 – 11.1.1.3.” The three options available for further evaluation include:

1. §11.1.1.1 – Where corrosion is believed to have materially reduced the wall thickness, the pipe thickness must be measured using appropriate techniques (see Part 12 for a list of appropriate techniques);
2. §11.1.1.2 – Where corrosion has not materially reduced the wall thickness, the pipe must be cleaned and recoated (painted) to arrest further deterioration;
3. §11.1.1.3 – Where corrosion has reduced the remaining wall thickness beyond the owner’s acceptance criteria, the pipe must be evaluated to determine suitability for further operation.

With regard to Option 3 in the list above, the standards grants some freedom in developing an acceptance criteria, but the criteria must establish a replacement thickness that is not less than the calculated thickness for pressure containment. According to ASME B31.5-2016 §504.1.2 (Equation 3a), the minimum required thickness for ammonia refrigeration pipes is:

where:

t = minimum required thickness of pipe (in)

P = internal design pressure (psi)

D_o = outside diameter of pipe (in)

S = stress value for material (psi)

Y = coefficient (0.4 for steel pipes)

For those wanting to avoid complex equations, Table A.11.1.1.3.1 contains “alert thickness” and “replacement thickness” values for standard pipes sizes and schedules. While the content of the table is informative, it is likely to be widely adopted by the ammonia refrigeration industry.

Per §11.1.1.3.2, where the evaluation demonstrates that the pipe is not suitable for continued operation, “the owner shall immediately isolate the pipe from service and proceed with a plan for its replacement or decommissioning (for decommissioning, see ANSI/IIAR 8)“.

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This is Part 13 of a series on IIAR 6. You can access previous blogs in this series below:

• Part 1: It’s Finally Here!
• Part 2: IIAR 6 Overview
• Part 3: Purpose, Scope, and Applicability
• Part 4: Definitions
• Part 5: Program Administration
• Part 6: Inspection, Testing, and Maintenance
• Part 7: Task Frequency
• Part 8: Record Keeping
• Part 9: Qualified Inspectors
• Part 10: The Tables!
• Part 11: Testing Compressor Safety Devices
• Part 12: Vessel Corrosion