Profile Radiography for Piping Wall Loss and CUI

Short answer: Profile (tangential) radiography aims the beam so it skims the pipe wall. The detector then shows the wall as a band whose width you can measure, so remaining thickness and corrosion shape can be read without stripping insulation. It is a standard way to find CUI and internal wall loss on small and medium piping, fittings and deadlegs. It is less precise than UT, needs radiation controls, and works best on smaller diameters.

This guide is for piping inspectors, integrity engineers and maintenance planners in the US and Canada who are deciding when to use profile RT, how to scope it, and what to do with the results. It covers how the technique works, how it differs from double-wall radiography, the standard that describes it (ISO 20769), its limits, radiation-safety planning, and where API 570 and API 510 point to it. Code references are given in our own words. API 570 is cited from its fourth edition (2016), and its fifth edition (2024) is current. API 510 is cited from its eleventh edition (2022). Confirm details against your licensed copies. The owner's API inspector evaluates the results and decides what happens next.

How profile radiography works

Direct answer: The source sits to one side of the pipe and the detector on the other, with the beam tangent to the wall. Where the beam grazes the wall, it passes through a long chord of steel. The wall shows up as a light band at the edge of the pipe image, and the inner edge of that band traces the bore. Measuring the band, with a correction for magnification, gives the remaining wall.

A single exposure shows two wall profiles, one on each side of the pipe image. Because the beam passes the wall on edge, the image shows the wall in section, much like a cut-away. Internal pitting appears as notches in the inner edge of the band. External CUI appears as loss on the outer edge, often with scale or corrosion product lifting off the surface. General erosion shows as a thinner band. Insulation and jacketing absorb little radiation compared with steel, so the pipe can be imaged through them.

Since each shot sees only the two tangent points, a full picture of a fitting or a short run needs several shots at different angles around the pipe, for example at 0° and 90°. Pipes run horizontally, so the bottom of the line, where water and deposits collect, is a usual target. The planner chooses angles to cover the features most likely to corrode.

Profile RT can use film, computed radiography (CR) with phosphor plates, or digital detector arrays (DDA). Digital methods allow on-screen measurement, contrast adjustment and faster turnaround, and they often allow a lower dose or shorter exposure. That matters on live plants where exclusion zones disrupt work. Our digital radiography vs film comparison covers that choice.

Tangential vs double-wall radiography

Direct answer: Tangential radiography measures wall thickness at the tangent points. Double-wall radiography shoots through both walls and shows corrosion as changes in image density across the pipe's face. The two are often combined: tangential for measured wall at the edges, double-wall to find pits across the area between them.

ISO 20769, published in two parts, sets out both methods for radiographic inspection of corrosion and deposits in pipes by X- and gamma rays. Part 1 covers the tangential technique and Part 2 covers double-wall radiography. According to publisher summaries, Part 1 treats "pipe" broadly, including tubes, penstocks, boiler drums and pressure vessels, insulated or not. It covers internal or external corrosion and erosion, using film, CR or DDA. It is about service damage such as pitting, general corrosion and erosion, not weld fabrication flaws. It describes basic and improved technique classes, with the improved class for the harder job of measuring local pitting. ISO lists a revision of Part 1 under development, so check the edition your procedure cites.

Real-time radiography and other digital screening systems work on the same principles. They are used to scan long insulated runs quickly and find areas worth a measured shot.

Where API 570 and API 510 point to profile radiography

Direct answer: Both codes name profile radiography as a way to find minimum thickness and localised loss, and as an option that avoids removing insulation. API 570 says RT or insulation removal with visual is normally required at damaged or suspect CUI locations. It also calls for profile RT on small-bore deadlegs and favours RT for injection points and mixing points.

In the API 570 fourth edition:

• Thickness monitoring. Minimum thickness at a CML can be located by ultrasonic scanning or profile radiography.
• CUI. The CUI clause names real-time radiography, to see scale behind insulation, and profile radiography, pulsed eddy current and guided wave as techniques that help locate damage. The extent-of-inspection clause says RT, or insulation removal with visual, is normally required at damaged or suspect locations. Our guide on how API 570 and API 510 treat CUI explains the class-based follow-up.
• Injection and mixing points. Radiography and UT scanning or close-grid UT are the preferred methods. See our injection point guide.
• Deadlegs. Inspections should include profile radiography on small-diameter deadlegs such as vents and drains, with scanning UT or RT on larger ones. See the deadleg inspection guide.
• Welds. If a profile shot of an in-service weld shows a crack-like indication, further examination with weld-quality RT and/or UT should be used to size it.

The API 510 eleventh edition uses the same idea for vessels. Its definition of scanning NDE gives profile radiography of nozzles as an example. It says spot UT or profile RT may be used for thickness as long as it gives a minimum, and it prefers scanning UT or profile techniques where corrosion is local or wall is near the required thickness. It notes that radiographic profile techniques, which need no insulation removal, can be an alternative to UT at insulated CMLs. API 510 also warns that profile or density radiography can reveal what looks like a weld flaw, which then needs further examination.

Practical limits: diameter, wall, insulation and product

Direct answer: Profile RT works best on small and medium bore piping. As diameter and wall grow, the steel chord at the tangent point gets longer, so more source energy and longer exposures are needed, and image quality falls. Insulation adds distance and magnification. Product in the line matters more for double-wall shots than for tangential ones.

• Penetrated thickness. At the tangent point the beam crosses a chord of steel much longer than the wall thickness, and the chord grows with diameter and wall. That sets the practical ceiling for each source and energy. Large, thick lines are usually better served by UT at windows, PEC screening or guided wave.
• Magnification and calibration. The pipe wall sits at some distance from the detector, and insulation adds to it, so the image is enlarged. Procedures correct for this with geometry or by placing a reference object of known size in the plane of the wall. Without the correction, the measured thickness is wrong.
• Unsharpness. Source size, distances and detector type all blur the edges of the band. ISO 20769 sets technique classes and image-quality checks for this reason. Precision is lower than a good UT reading on a clean surface.
• Product and scale. Liquid in the line and heavy deposits add attenuation, especially for double-wall shots. Deposits also show on the image, which can be useful because the technique can identify fouling as well as corrosion.
• Geometry. Elbows, tees, valves and supports complicate the tangent. Shot plans for fittings take more exposures and more interpretation.
• Insulation type. Most insulation and aluminium or steel jacketing are easy to image through. Dense or wet insulation adds some attenuation, and metal jacketing seams can create lines to interpret.

Source choice follows the job. X-ray tubes and lower-energy isotopes such as selenium-75 suit thinner sections. Iridium-192 covers a wide middle range, and cobalt-60 is used for thick sections. Each needs its own exclusion-zone planning. The radiographic testing service page covers equipment and coverage.

Radiation safety and licensing

Direct answer: Field radiography with isotopes is licensed in the US by the NRC or by the radiation control programme of an NRC Agreement State. X-ray equipment is registered with the state. In Canada, gamma exposure devices are licensed by the CNSC and operators must be certified. On a live plant the exclusion zone is often the biggest scheduling issue.

In the US, industrial radiography with byproduct-material sources is governed by 10 CFR Part 34 in NRC states and by equivalent Agreement State rules elsewhere. Those rules cover radiographer certification, the requirement for a second qualified person at temporary job sites, surveys, posting, and controls on locking and storing sources. X-ray machines are regulated by the state radiation control programme. Confirm the current licensing authority with the state where the work happens, because Agreement State status and rules vary.

In Canada, the Canadian Nuclear Safety Commission licenses nuclear substances and exposure devices, and it certifies exposure device operators. X-ray equipment falls under provincial or federal occupational health rules, depending on the workplace.

For planning, the practical points are:

• Exclusion zones on live units need agreement with operations, permits and barrier control. Night shifts or unit-by-unit sequencing are common.
• Digital detectors and collimators can reduce exposure times and zone sizes compared with film.
• Gamma sources cannot be switched off, so storage, transport and emergency arrangements are part of the job plan.
• Work near other contractors needs coordination so no one strays into a controlled area.

Scoping a profile RT campaign: a worked example

Direct answer: A good campaign starts from the inspection plan's list of susceptible locations, picks shot angles to suit each feature, uses screening to rank long runs, and feeds measured wall back into the CML records for the inspector to evaluate.

A Gulf Coast unit has a few hundred insulated small-bore and medium-bore carbon steel lines in the CUI temperature range. The owner's API 570 inspector has finished the external visual and marked locations with damaged jacketing, low points, support points and bottoms of vertical runs. The plan also lists small-bore vents and drains on deadlegs, and an injection point downstream of a wash-water connection.

The NDE planner groups locations by line size and access and selects digital radiography to keep exclusion zones short. For each straight-run location, the plan calls for two tangential shots at right angles, with the bottom of the pipe in one of them. Elbows and tees get extra shots. The injection point gets a set of tangential and double-wall shots covering the zone the plan defines. Reference objects go in the plane of the wall on every shot.

The results show most locations with intact profiles, a few with surface scale under the jacketing but little measured loss, and two low points with clear external wall loss. One deadleg drain shows internal pitting. The planner reports measured minimum wall at each location, with images, and flags the two low points and the drain as priorities. The inspector orders insulation windows and UT at the two low points to confirm the readings. Under API 570, the CUI findings widen the follow-up on those circuits. The engineer works out corrosion rates and remaining life, and the inspector sets next steps. The NDE contractor's job ends with the data.

Profile RT vs other CUI and wall-loss methods

Direct answer: Profile RT is the most direct way to see wall loss through insulation on smaller piping. UT gives better precision where the surface can be reached. PEC and guided wave screen larger areas and longer runs. A practical programme uses each where it is strongest.

For a broader look at CUI screening without removal, see CUI detection without removing insulation.

What the inspector should receive

Direct answer: Each location should come with the images, measured minimum wall and where it was measured, the shot geometry and calibration method, interpretation notes, and the limits of coverage. That lets the inspector enter the readings against CMLs and decide on follow-up.

• Line, circuit and CML or location reference matched to the owner's isometrics.
• Shot plan: source type, energy or activity, distances, angles and detector type.
• Magnification correction method and the reference object used.
• Measured remaining wall at each tangent point, with the minimum flagged, and the nominal or previous value where available.
• Interpretation notes: internal or external loss, pitting or general loss, scale or deposits, and any crack-like indication that needs weld-quality examination.
• Image quality checks and the technique class used, where the procedure follows ISO 20769.
• Personnel certifications under the employer's written practice and the radiography licence the work was done under.
• Digital image files in a format the owner can archive and compare at the next inspection.

Under OSHA PSM (29 CFR 1910.119(j)), inspection records for covered processes must be documented. Keeping the images, not just the readings, lets the next campaign compare like with like.

Common mistakes

Direct answer: The usual errors are skipping magnification correction, using profile readings as precise UT values close to minimum thickness, shooting only one angle, ignoring diameter limits, and failing to plan exclusion zones with operations.

• No magnification correction. Uncorrected band widths overstate the wall.
• Treating profile wall as UT-grade data near the minimum. Where remaining life is short, confirm with UT at a window.
• One shot per location. Corrosion at the bottom of a line can be missed by a single side-on shot.
• Pushing beyond the technique's range. Large or thick lines give poor images. Use other methods there.
• Ignoring deposits. Scale and fouling on the image are information for the process team, not just noise.
• Late radiation planning. Exclusion zones that clash with operations or other trades cause most schedule losses.

How Atlantis supports this

Atlantis NDT performs profile (tangential) and double-wall radiography of piping with film, CR and digital detectors. Radiography crews are licensed where the work takes place, and procedures are approved by an ASNT Level III. We deliver images, measured wall and interpretation notes to the owner's API 570 or API 510 inspector, who evaluates the findings and decides follow-up, corrosion rates and intervals. See radiographic testing, CUI inspection services and piping circuit and CML inspection. Request a profile RT scope and quote. We respond within 24 hours.

Frequently asked questions

What is profile radiography of piping?

It is a radiographic technique where the beam is tangent to the pipe wall, so the image shows the wall in section. The band width, corrected for magnification, gives the remaining wall thickness and shows the shape of any corrosion.

Is tangential radiography the same as profile radiography?

Yes. Both names describe the same technique. ISO 20769-1 calls it tangential radiographic inspection.

Can profile radiography detect CUI without removing insulation?

Yes, on most small and medium piping. Insulation absorbs little radiation, so the wall profile and external corrosion can be seen through it. Confirm critical readings by UT at a window.

How accurate is profile radiography for wall thickness?

It depends on the geometry, source, detector, unsharpness and magnification correction. It is generally less precise than UT on an exposed surface, which is why codes treat it as a scanning or profile method and why critical values are often confirmed by UT.

What pipe sizes suit profile RT?

It works best on small and medium diameters with moderate wall. As diameter and wall grow, the steel chord at the tangent point lengthens, needing higher energy and giving poorer images. The procedure qualification sets the practical range.

What is the difference between tangential and double-wall radiography?

Tangential measures the wall at the edges of the pipe image. Double-wall shoots through both walls and shows pits across the face as density changes. They are often used together.

Does API 570 require radiography for CUI?

The fourth edition says RT, or insulation removal with visual inspection, is normally required at damaged or suspect CUI locations, and other NDE may be used where it applies. Confirm the wording in the fifth edition.

Do you need a licence for profile radiography?

Yes. Gamma radiography needs an NRC or Agreement State licence in the US and a CNSC licence in Canada, with certified operators. X-ray equipment is registered under state or provincial rules.

Can profile radiography be done on live lines?

Yes. It is commonly done on-stream, subject to exclusion zones agreed with operations. Product in the line affects double-wall shots more than tangential ones.

Which standard covers profile radiography of pipes?

ISO 20769-1 covers tangential and ISO 20769-2 covers double-wall radiographic inspection of corrosion and deposits in pipes. Owners may also write their own procedures under ASME Section V principles.

Talk to an NDE Level III about a profile RT campaign or send your CUI location list for a quote.