What can go wrong in a refinery

Refinery equipment fails in ways that depend on what it carries, how hot it runs and what it is made of. Inspection people call these damage mechanisms, and knowing which ones apply to each circuit is what tells them where to look and with which method. A few matter almost everywhere:

  • Sulfidation: high-temperature corrosion of carbon and low-alloy steel by sulfur compounds in crude oil. It can thin one component much faster than the pipe around it.
  • High-temperature hydrogen attack (HTHA): damage to steel in hot hydrogen service, such as hydrotreaters, that weakens it from within and is hard to detect.
  • Corrosion under insulation (CUI): water trapped under insulation corrodes the pipe or vessel out of sight.
  • Wet hydrogen sulfide cracking in sour water and gas services, naphthenic acid corrosion in some crude units, creep in fired heater tubes, and acid corrosion in alkylation units.

The consequences of missing one are well documented. On August 6, 2012, a carbon steel pipe in the crude unit at Chevron's refinery in Richmond, California, ruptured and released flammable hydrocarbon that ignited. The CSB found that the line had been thinned by sulfidation corrosion, that low-silicon steel components had corroded much faster than the rest of the line, and it recommended inspection of every component in susceptible circuits, or replacement with more resistant material. The lesson for inspection planning: a few thickness readings per circuit can miss the one component that fails.

Where NDT is used

  • Ultrasonic thickness measurement (UT) at condition monitoring locations is the backbone of piping and vessel programs. Repeat readings give a corrosion rate and a remaining life. Automated UT and phased array corrosion mapping cover larger areas where the damage is patchy.
  • Phased array UT and TOFD examine welds for cracking and are among the advanced ultrasonic techniques used to look for HTHA. Detecting HTHA reliably takes specialized procedures and people; it is not a routine thickness check.
  • Wet fluorescent magnetic particle testing (WFMT) finds fine surface cracking on the inside of vessels during turnarounds, for example in wet hydrogen sulfide service.
  • Radiography (RT), including profile radiography, shows wall loss and deposits in small-bore piping and can image piping through insulation.
  • Pulsed eddy current (PEC) and guided wave testing (GWT) screen insulated and hard-to-reach piping for CUI and wall loss, so insulation is stripped only where it needs to be.
  • Magnetic flux leakage (MFL) scans storage tank floors, and tube inspection techniques cover heat exchangers and air coolers (see our guide to tubing analysis).
  • Positive material identification (PMI) confirms alloy components are what they should be, and infrared thermography finds hot spots on fired heaters and refractory-lined equipment.

How inspection fits into refinery operations

Refinery inspection is organized by equipment type, and each has its own API code. API 510 covers in-service inspection, repair and alteration of pressure vessels, API 570 covers piping, and API 653 covers aboveground storage tanks. Each sets out how intervals are established from corrosion rates and remaining life, what records must be kept, and what an authorized inspector must approve.

Most refineries layer three practices on top. Risk-based inspection (RBI), described in API RP 580 and 581, ranks equipment by the likelihood and consequence of failure so inspection effort goes where risk is highest. Integrity operating windows (IOWs), described in API RP 584, set limits on temperature, pressure and chemistry so damage stays predictable, and alert the inspection group when a unit runs outside them. And when inspection does find damage, a fitness-for-service (FFS) assessment under API 579-1/ASME FFS-1 decides whether the equipment can keep running, for how long, and under what conditions.

Much of the internal inspection happens during turnarounds, when a unit is shut down, opened and cleaned. NDT demand peaks sharply: many crews, tight schedules, and a flood of data that has to be reviewed and acted on before the unit restarts. On-stream inspection between turnarounds, done from the outside while the unit runs, carries the rest.

Terminals and distribution sites downstream of the refinery are mostly tanks and piping, inspected to the same API documents. Breakout tanks on pipelines regulated under 49 CFR Part 195 must be inspected to API 653, which ties the tank world to pipeline rules as well.

Regulations and standards

  • OSHA Process Safety Management (29 CFR 1910.119), in its mechanical integrity paragraph (j), requires written procedures, training, inspection and testing of process equipment following recognized and generally accepted good engineering practices (RAGAGEP), at frequencies consistent with manufacturers' recommendations and good practice, with each inspection documented and deficiencies corrected. The API inspection codes are widely treated as RAGAGEP for refinery equipment.
  • EPA's Risk Management Program (40 CFR Part 68) contains a parallel mechanical integrity requirement.
  • API documents on damage mechanisms (RP 571), hydrogen service (RP 941), material verification (RP 578) and the inspection codes above set the technical content. ASME construction codes set how much examination new and repaired equipment needs, and ASME Section V sets out how the examination methods are applied.

What this means for owners

An inspection program is only as good as its link between damage mechanism, method and data. Does every circuit have its credible damage mechanisms identified, and is the NDT technique chosen able to find them? Are corrosion rates calculated from readings you trust, taken at the same locations each time? Are the technicians certified in the specific technique they are using, and do the procedures match the code and the owner's specification? When a turnaround ends, is every finding closed out before restart?

Frequently asked questions

What is risk-based inspection (RBI)?

RBI plans inspection by risk, the likelihood of a failure multiplied by its consequence, rather than by fixed calendar intervals. High-risk equipment is inspected more often or with more effective methods, and low-risk equipment less, so the same inspection budget buys more safety. API RP 580 sets out the principles and API RP 581 a quantitative method.

What is fitness-for-service?

A fitness-for-service assessment is an engineering evaluation of whether equipment with known damage, such as wall loss, pitting or a crack, can safely keep operating, and for how long. API 579-1/ASME FFS-1 organizes it in three levels of increasing detail: a conservative screening, a more detailed evaluation by an engineer, and an advanced analysis, often using stress modeling.

How often should refinery piping and vessels be inspected?

It depends on the corrosion rate, remaining life, risk and the applicable API code. API 510 and 570 set maximum intervals and allow risk-based intervals under defined conditions; the owner's program must justify its choice and adjust it when readings change.

How NDT LLC can help

NDT LLC helps refiners and their inspection contractors with NDT procedures and written practices, technique selection for specific damage mechanisms, independent review of inspection data, vendor audits before a turnaround, and audit readiness. Get in touch to discuss your inspection program. For the wider picture, see our overview of NDT in oil and gas.

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), PHMSA, or any other organization named in this article; names are used only to identify the organizations and documents referred to.