NDT in Chemical Plants: Mechanical Integrity and Inspection

What can go wrong

A chemical plant's damage mechanisms depend on its chemistry, which makes chemical production more varied than refining. Common threats include:

• General and localized corrosion by acids, caustics and process streams, often concentrated at welds, in dead legs, at injection points and where flow changes direction.
• Stress corrosion cracking (SCC): chloride cracking of austenitic stainless steels, and caustic and amine cracking of carbon steel, especially at welds that were not stress relieved.
• Corrosion under insulation (CUI) on insulated piping and vessels.
• Failure of non-metallic and lined equipment: fiberglass-reinforced plastic (FRP) tanks and piping that degrade chemically or crack, and linings in steel equipment that fail and let the process attack the shell.
• Erosion in slurry service and fatigue at vibrating connections.

Where NDT is used

• Ultrasonic thickness measurement at condition monitoring locations remains the backbone, with automated or phased array corrosion mapping where corrosion is patchy and single readings could miss the worst spot.
• Penetrant testing (PT) finds chloride stress corrosion cracking in stainless steel, and wet fluorescent magnetic particle testing finds caustic and amine cracking at carbon steel welds. Phased array UT can size cracks found this way for a fitness-for-service decision.
• Pulsed eddy current, guided wave testing and profile radiography screen insulated piping for CUI without stripping it all.
• Radiography shows wall loss, deposits and blockages in small-bore piping.
• Visual inspection is central for FRP equipment, looking for blistering, cracking, fiber exposure and chemical attack on the inner surface, supported by hardness checks and, for larger vessels, acoustic emission testing, which is well established for FRP.
• Holiday (spark) testing finds pinholes in linings and coatings before and after service.

Mechanical integrity under OSHA PSM

For processes that handle highly hazardous chemicals above threshold quantities, OSHA's Process Safety Management standard, 29 CFR 1910.119, makes mechanical integrity a legal requirement. Paragraph (j) applies to pressure vessels and storage tanks, piping systems including valves, relief and vent systems, emergency shutdown systems, controls, and pumps. In summary, it requires the employer to:

• have written procedures to maintain the ongoing integrity of that equipment;
• train the people who maintain it in the process, its hazards and their procedures;
• inspect and test the equipment, following recognized and generally accepted good engineering practices (RAGAGEP), at frequencies consistent with manufacturers' recommendations and good engineering practice, and more often if operating experience says so;
• document each inspection with the date, the name of the person who did it, the equipment identifier, a description of the inspection and its results;
• correct deficiencies that are outside acceptable limits before further use, or in a safe and timely way with measures in place to keep operating safely; and
• apply quality assurance to new equipment, installation, and maintenance materials and spare parts.

EPA's Risk Management Program rule, 40 CFR 68.73, contains a parallel mechanical integrity requirement for covered processes.

The standard does not name specific codes. In practice, RAGAGEP for chemical plant equipment usually means API 510 for pressure vessels, API 570 for piping and API 653 for tanks, plus ASME construction codes, manufacturers' recommendations, ASME RTP-1 for FRP vessels, and the plant's own documented practices where no published standard fits. Whatever the plant chooses, it should write down which practice applies to each equipment type and follow it.

Building an inspection program that holds up

An effective mechanical integrity program for a chemical plant usually rests on a short list of habits:

• An equipment list that covers everything in scope, with materials, design data and service.
• A damage mechanism review for each circuit or item, so inspection targets what can actually happen.
• Inspection plans with methods, locations and intervals justified by corrosion rates or risk, often using risk-based inspection.
• Qualified people: inspectors who understand the codes, and NDT technicians certified in the technique they use.
• Records that meet the PSM documentation requirement and are easy to trend.
• A deficiency process that makes sure findings are evaluated and closed, and management of change so new chemistry or conditions trigger a fresh look.

What this means for owners

Auditors and regulators look for evidence, not intentions. Can you show the RAGAGEP you chose for each equipment type? Do inspection records carry every item the standard lists? Are overdue inspections and open deficiencies visible to management? And do your NDT procedures and technician certifications match the technique actually used in the field?

Frequently asked questions

How NDT LLC can help

NDT LLC helps chemical manufacturers and their inspection contractors with mechanical integrity program gap assessments, NDT procedures and written practices aligned to the plant's RAGAGEP, technician qualification, and independent review of inspection data. Contact us to discuss your program. Related reading: NDT in petrochemical plants and API inspector certifications.