What can go wrong

Petrochemical is a broad term. Here we mean the plants that make basic chemicals from oil and gas feedstocks: steam crackers producing ethylene and propylene, aromatics units, and the polymer and derivative plants downstream of them. Refining is covered in our article on refineries, though many of the same damage mechanisms apply. The ones that most shape inspection are:

  • Furnace tube damage. Radiant tubes in cracking furnaces run very hot. Over time they suffer creep, slow stretching and bulging under stress at temperature, and carburization, in which carbon diffuses into the alloy and makes it brittle.
  • High-temperature hydrogen attack (HTHA) in steels exposed to hot, high-pressure hydrogen.
  • Corrosion under insulation (CUI) on the miles of insulated piping and vessels in any plant.
  • Fatigue of small-bore connections and branch fittings shaken by vibration from pumps, compressors and flow, and thermal fatigue where hot and cold streams mix.
  • Process-specific corrosion and cracking, and in cold services, embrittlement risks for materials that are not suited to low temperatures.

HTHA is a good example of why damage mechanisms matter. On April 2, 2010, a nearly 40-year-old heat exchanger at the Tesoro refinery in Anacortes, Washington, catastrophically failed during a maintenance operation, and seven employees were killed. The CSB found that the exchanger had been damaged by HTHA, and among its recommendations called for stronger industry guidance on HTHA in API RP 941 and for verifying actual operating conditions against the materials chosen. Although that was a refinery, the same mechanism applies to hydrogen service in any plant.

Where NDT is used

  • Furnace tubes are inspected during outages by specialized systems that crawl along or through the tubes, combining diameter measurement (to track creep growth), ultrasonic wall thickness and electromagnetic techniques that respond to carburization. The results feed a remaining-life assessment for each tube.
  • HTHA is looked for with advanced ultrasonic techniques, including phased array, TOFD and related methods, applied by specialists with procedures developed for the purpose. Early-stage HTHA is difficult to detect reliably, so inspection is paired with checks that operating conditions keep equipment within safe limits for its material.
  • CUI is screened with pulsed eddy current (PEC), guided wave testing (GWT), profile radiography and, where appropriate, infrared thermography to find wet insulation, followed by targeted insulation removal, visual inspection and UT at the suspect areas.
  • Welds and pressure equipment are examined with UT, phased array, magnetic particle and penetrant testing during turnarounds, and thickness is tracked at condition monitoring locations.
  • Acoustic emission (AE) monitoring can screen whole vessels and spheres for active flaws during a controlled pressure increase, directing other methods to specific areas.
  • Exchanger and cooler tubes are inspected with the techniques described in our guide to tubing analysis.

Turnarounds and on-stream inspection

Petrochemical units may run for years between turnarounds, and internal inspection of furnaces, reactors and columns can only happen when they are down. Turnaround scopes are planned long in advance from risk-based inspection results, previous findings and the plant's damage mechanism reviews. On-stream inspection, from the outside while the unit runs, carries the load in between: thickness monitoring, CUI screening, vibration surveys of small-bore piping and thermography of hot equipment.

Inspection findings often lead to a fitness-for-service assessment under API 579-1/ASME FFS-1, which decides whether damaged equipment can run to the next turnaround, needs a lower operating limit, or has to be repaired now. The quality of that decision depends directly on the accuracy of the NDT sizing behind it.

Regulations and standards

  • OSHA's Process Safety Management standard (29 CFR 1910.119) applies to most petrochemical processes, and its mechanical integrity element requires written procedures, trained personnel, inspection and testing to recognized and generally accepted good engineering practice (RAGAGEP), documentation of every inspection and correction of deficiencies. EPA's Risk Management Program (40 CFR Part 68) has a parallel requirement.
  • API documents are widely used as RAGAGEP: the inspection codes API 510 and 570, damage mechanisms in API RP 571, hydrogen service in API RP 941, risk-based inspection in API RP 580 and 581, integrity operating windows in API RP 584, and fitness-for-service in API 579-1/ASME FFS-1.
  • ASME construction codes (Section VIII for pressure vessels and B31.3 for process piping) govern new work and repairs, and ASME Section V covers how the NDT methods are applied.

What this means for owners

Petrochemical inspection succeeds or fails on the link between damage mechanism and method. Has every circuit had a damage mechanism review, and is the NDT technique chosen actually capable of detecting that damage? For specialized techniques such as HTHA and furnace tube inspection, has the vendor demonstrated its procedure on relevant samples, and are its people certified in that technique? Are CUI screening results followed up, not just filed? And are the results trended from turnaround to turnaround so remaining life estimates improve over time?

Frequently asked questions

What is corrosion under insulation and how do you detect it?

Corrosion under insulation (CUI) is corrosion of steel piping and vessels beneath thermal insulation, where water gets in and cannot dry out. Because it is hidden, plants screen for it with techniques that work through or around insulation, such as pulsed eddy current, guided wave testing, profile radiography and thermography to find wet insulation, then remove insulation at suspect areas for visual and ultrasonic inspection.

What is high-temperature hydrogen attack, and can NDT find it?

HTHA is damage to carbon and low-alloy steels in hot, high-pressure hydrogen service, where hydrogen reacts with carbon in the steel to form methane that creates fissures and weakens it. Advanced ultrasonic techniques applied by specialists can detect it, but early stages are hard to find reliably, so inspection is combined with keeping equipment within safe operating limits for its material, as described in API RP 941.

How are cracking furnace tubes inspected?

During outages, with specialized systems that measure tube diameter to track creep growth, ultrasonic wall thickness and electromagnetic responses that indicate carburization. The data feeds a remaining-life assessment that decides which tubes to replace.

How NDT LLC can help

NDT LLC helps petrochemical operators and inspection contractors with technique selection for specific damage mechanisms, NDT procedures and written practices, technician qualification, vendor audits before turnarounds and independent review of inspection data. Contact us to discuss your program.

This article is general information for a business audience, not engineering, legal or regulatory advice. Always work to the current edition of the codes, standards, regulations and manufacturer data that apply to your equipment and contracts.

NDT LLC is an independent consulting firm. It is not affiliated with, endorsed by or sponsored by the American Petroleum Institute (API), ASME, OSHA, EPA, the U.S. Chemical Safety and Hazard Investigation Board (CSB), or any other organization named in this article; names are used only to identify the organizations and documents referred to.