TOFD for In-Service Inspection: Crack Sizing Under API Codes

Short answer: In service, TOFD (time-of-flight diffraction) is used mainly to measure how deep a crack goes through the wall. It times the weak signals diffracted from the crack's upper and lower tips, so height comes from arrival times rather than echo amplitude. API 510 and API 570 do not mandate TOFD. They require sizing by qualified angle-beam examiners when crack data is needed, and they send crack-like flaws to an engineer for assessment.

This guide is for inspection engineers, API 510 and API 570 inspectors, and integrity leads in the US and Canada who need crack data from operating vessels and piping. It explains how TOFD sizes flaws, where it fits under the in-service codes, its dead zones, why it is usually paired with phased array, and what the data package should hold. API 510 is cited from the eleventh edition (2022) and API 570 from the fourth edition (2016); API 570 is now in its fifth edition (2024). Confirm any clause against your licensed copy. Acceptance, repair and assessment decisions belong to the owner's inspector and engineer.

How TOFD measures crack height

Direct answer: Two angled compression-wave probes face each other across the weld, one sending and one receiving. The receiver picks up a lateral wave along the surface, a back-wall echo, and diffracted signals from the top and bottom tips of any flaw in between. Crack height is the difference in depth between the two tip signals, worked out from their travel times.

Conventional pulse-echo UT sizes flaws largely from how much sound reflects back, and reflected amplitude depends heavily on flaw orientation and surface roughness. TOFD works differently. The edges of a crack act as small sources of diffracted sound that spread in many directions. The receiver records them whatever the flaw's orientation, and because the probe spacing and sound velocity are known, each arrival time maps to a depth. The amplitude matters for detection, but the size comes from timing.

A TOFD scan is usually shown as a greyscale image, with time (depth) on one axis and probe position on the other. A clean weld shows two continuous bands: the lateral wave at the top and the back wall at the bottom. A buried crack shows as an upper and a lower tip signal between them, typically with opposite phase. A crack breaking the inside surface disturbs the back-wall signal and shows a single upper tip. A crack breaking the scanning surface interrupts the lateral wave.

Scans are either non-parallel, with the probes moving along the weld so the scan covers its length, or parallel, with the probes moving across the weld to refine the position and height of a known indication. Most in-service work starts with non-parallel scans over the weld length and adds parallel scans at indications that need sizing. For the method basics, our TOFD explainer covers equipment and set-up.

What the API in-service codes say about crack examination

Direct answer: API 510 and API 570 name TOFD as one of the available techniques but leave the choice to the inspection plan. They set the qualification for examiners who detect and size flaws from outside. They also require crack-like flaws to be assessed by an engineer or corrosion specialist under API 579-1/ASME FFS-1.

The API 510 eleventh edition mentions TOFD in two places. For vessels in cyclic service, such as coke drums, mole sieves and pressure swing adsorbers, it lists external angle-beam UT, wet fluorescent magnetic particle testing and TOFD among the NDE options for fatigue cracking. In its list of examination techniques, it names advanced ultrasonic methods, giving TOFD and PAUT as examples, for detecting high-temperature hydrogen attack as described in API RP 941. Our HTHA guide covers that application.

Both codes set a qualification rule for angle-beam work. The owner must specify industry-qualified UT angle-beam examiners when it needs ID-surface-breaking flaws detected from the OD, or flaws detected, characterised or sized through the wall. API 570 defines such an examiner as someone holding an API angle-beam qualification (it gives API QUTE and QUSE detection and sizing tests as examples) or an equivalent the owner approves. API 510 also applies the rule where manual UT is used on welds in place of a pressure test. Collecting data for fitness-for-service evaluations is one of the examples the codes give.

When crack-like flaws are found in service, both codes ask for further examination to find their size and whether they come from fabrication or from a service mechanism. API 510 says crack-like flaws and environmental cracking shall be assessed by the inspector together with an engineer or corrosion specialist, referring to API 579-1/ASME FFS-1. API 570 assigns that assessment to an engineer and/or corrosion specialist. The NDE supplies the dimensions. It does not decide whether the component can stay in service.

For examination technique, the usual reference is ASME BPVC Section V, Article 4, which covers ultrasonic weld examination and includes a mandatory appendix for TOFD. ASTM E2373 gives a standard practice for writing TOFD procedures, and ISO 10863 covers TOFD for welds in international work. In-service procedures normally borrow from these, adapted for the access and condition of the equipment.

What in-service TOFD is used for

Direct answer: The main in-service uses are sizing known or suspected cracks, monitoring crack growth between inspections, scanning welds for service cracking where access is from outside, and measuring weld-root erosion or corrosion without removing the weld cap.

• Through-wall sizing of indications. When PAUT, shear-wave UT, magnetic particle or a leak flags a crack, TOFD is often added to measure its height and remaining ligament, which are the numbers an engineering assessment needs.
• Monitoring known flaws. An encoded TOFD record taken at a fixed datum can be repeated at the next outage, or on-stream, and compared to see whether the tip positions have moved.
• Service cracking in welds. Fatigue cracking in cyclic vessels, and environmental cracking such as wet H2S cracking (SOHIC and SSC near welds), amine or caustic cracking. TOFD is one tool. Surface methods such as WFMT are often used alongside it where internal access allows. Our NDE method selection guide sets out the options by damage mechanism.
• Weld root erosion and corrosion. With a probe on each side of the weld cap, TOFD can measure loss at the root without grinding the cap. North Sea non-intrusive inspection guidance (DNV-RP-G103, and the HOIS recommended practice on weld corrosion, as summarised by equipment makers) treats TOFD as a leading method for this task.
• HTHA screening support. As part of an advanced-UT approach under API RP 941, TOFD can help characterise suspect zones, alongside other techniques the owner's specialists select.

Construction-stage TOFD, such as UT in place of radiography on new welds, runs under the construction code and is a different job with different acceptance criteria. In-service work does not use workmanship acceptance criteria. It produces sizes that the engineer assesses.

Dead zones, limits and why TOFD is paired with PAUT

Direct answer: TOFD has a near-surface dead zone hidden by the lateral wave and a smaller one near the back wall. It gives less precise lateral position and length than height. It needs access on both sides of the weld, and it can be confused by grain noise and geometry. PAUT covers most of these gaps, so in-service scopes commonly combine the two.

The pairing is practical. PAUT sectorial and linear scans detect and locate, and they reach the near-surface zone and the root from one side. TOFD then gives an independent height measurement that relies little on amplitude. When both techniques agree on a flaw, the engineer has more confidence in the size. When they disagree, the difference shows where more work is needed. Our PAUT vs TOFD comparison covers the trade-offs in more detail.

A worked example, described qualitatively

Direct answer: A typical in-service job moves from detection, to sizing, to assessment and repeat monitoring. The NDE team produces locations and sizes. The owner's inspector and engineer decide what to do.

A carbon steel amine regenerator reflux drum is due for its inspection. The damage review flags possible amine cracking near welds that were not post-weld heat treated. The plan calls for an external PAUT scan of the long and circumferential seams and the nozzle welds, with TOFD added on the main seams. Wet fluorescent magnetic particle testing is reserved for the inside surface at the next entry.

The PAUT scan finds a small cluster of ID-connected indications near the toe of one longitudinal seam. TOFD on that seam shows a single upper-tip signal and a break in the back-wall signal, consistent with a crack breaking the inside surface. The analyst measures the tip depth and the remaining ligament from the TOFD data. Parallel scans and the PAUT data confirm the position relative to the weld centreline. The PAUT data, adjusted with the parallel scans, give the length.

The report goes to the owner's API 510 inspector with the encoded files, the procedure and the examiners' qualifications. Under the code, the inspector and an engineer or corrosion specialist assess the crack-like flaw. That might mean a fitness-for-service assessment the owner commissions, a repair, a process change or a shorter interval. If the drum stays in service, the same TOFD set-up at the same datum becomes the monitoring baseline. A repeat scan can then show whether the tips have moved. Atlantis's role in a job like this ends at producing the data.

Accuracy, qualification and procedure

Direct answer: TOFD sizing is only as good as the procedure, the set-up and the analyst. In-service procedures should be demonstrated on representative blocks with known flaws. Examiners who size flaws for API 510 or API 570 work should hold the qualification the owner specifies under the code.

Sizing reliability depends on probe frequency and angle, probe spacing, the depth of the flaw relative to that spacing, sound velocity in the actual material, surface condition and the clarity of the tip signals. Branched cracks, such as stress corrosion cracks, give several tips and can be harder to read than a single fatigue crack. Good practice is to:

• Write a technique sheet for each thickness and geometry, under a procedure approved by the NDE provider's Level III.
• Demonstrate the set-up on a block of similar material and thickness, with notches or flaws at known depths, including near the surfaces.
• Record the velocity used and how it was checked, especially on hot surfaces.
• Use encoders and keep raw data, so another analyst can review the result.
• Report the uncertainty the procedure supports, and do not state sizes to a precision the technique cannot give.

For personnel, ASNT SNT-TC-1A or CP-189 certification in UT under the employer's written practice is the usual base. TOFD-specific training and practical experience sit on top of that. Where the code calls for industry-qualified angle-beam examiners, the owner specifies the API qualification or an approved equivalent. See TOFD vs phased array for weld inspection for a broader look at technique choice.

What the data package should contain

Direct answer: The engineer needs each flaw's location, height, length, depth to the upper tip, remaining ligament and whether it breaks a surface, with the uncertainty and the coverage limits. The raw files and procedure details should come with it.

• Equipment, weld and drawing reference, with datum and scan direction marked so the scan can be repeated.
• Procedure and technique sheet reference, including probes, angles, spacing, frequency and calibration.
• Scan plan showing which welds and lengths were covered, and where access or geometry stopped coverage.
• For each indication: position along and across the weld, depth to the upper tip, through-wall height, remaining ligament, length, and whether it breaks the ID or OD.
• Characterisation notes, for example planar or volumetric, single or branched, and likely fabrication or service origin where the data supports it.
• Supporting PAUT or other data used to place or confirm each indication.
• Examiner qualifications and any owner-specified industry qualification.
• Raw encoded files in a format the owner can archive.

Common mistakes with in-service TOFD

Direct answer: The usual errors are relying on TOFD alone near surfaces, overstating length, skipping procedure demonstration, losing the datum for monitoring, and treating sizes as an accept or reject decision.

• Ignoring the dead zones. A clean TOFD scan does not rule out shallow cracks at the scanning surface or tiny root flaws. Add PAUT or surface methods.
• Using length from a single non-parallel scan. Diffraction arcs exaggerate length unless they are corrected.
• Applying construction acceptance criteria to service cracks. In-service flaws are assessed by an engineer under API 579-1/ASME FFS-1, not accepted or rejected against a fabrication table.
• No demonstration block. An undemonstrated set-up on a heavy or coarse-grained weld gives sizes nobody can defend.
• No repeatable datum. Growth monitoring needs the same start point, direction and set-up each time.
• Wrong examiner qualification. If the owner's programme calls for industry-qualified angle-beam examiners, a general UT certification alone may not meet it.

Jurisdiction, FFS and Canada notes

Direct answer: Crack sizing feeds decisions that regulators care about. In the US that means OSHA PSM for covered processes and state pressure-vessel programmes. In Canada it means provincial regulators under CSA B51. Fitness-for-service assessment is an engineering service the owner commissions. It is separate from the NDE.

Under OSHA PSM (29 CFR 1910.119(j)), equipment deficiencies outside acceptable limits must be corrected before further use, or in a safe and timely way with measures to keep operation safe. An in-service crack is exactly that kind of deficiency until the owner's engineer shows otherwise. In states that regulate pressure vessels, repairs may also need an authorised repair organisation and inspector involvement under the National Board Inspection Code. In Canada, provincial authorities such as ABSA in Alberta and TSSA in Ontario oversee pressure equipment, and repairs or continued operation with known flaws may need the regulator's acceptance. Confirm the rules with your jurisdiction. For how the API 510 inspector and the construction-code inspector differ, see ASME authorized inspector vs API inspector.

How Atlantis supports this

Atlantis NDT performs encoded TOFD and PAUT examinations on in-service vessel and piping welds, under procedures approved by an ASNT Level III and demonstrated on representative blocks. We deliver flaw locations, heights, lengths and remaining ligaments, with the raw data, to the owner's API 510 or API 570 inspector and engineers. They stay responsible for assessment, repair decisions and intervals. Atlantis supplies sizing data only and does not offer fitness-for-service or RBI assessments. See TOFD testing and phased array inspection services. Request a TOFD/PAUT scope and quote. We respond within 24 hours.

Frequently asked questions

What is TOFD used for in in-service inspection?

Mainly measuring the through-wall height of cracks and the remaining ligament, monitoring known flaws for growth, scanning welds for service cracking from outside, and measuring weld-root erosion without removing the cap.

How does TOFD size cracks?

It times signals diffracted from the upper and lower tips of the crack. Since the probe spacing and sound velocity are known, each arrival time converts to a depth, and the height is the difference between them.

Is TOFD more accurate than PAUT for crack height?

TOFD height sizing relies on tip timing rather than amplitude, which is why it is widely used for through-wall sizing. PAUT is better for detection, placing the flaw and near-surface coverage. Most in-service scopes use both and compare the results.

Does API 510 require TOFD?

No. API 510 names TOFD as an option for fatigue cracking in cyclic service and, with PAUT, for HTHA under API RP 941. The inspection plan chooses methods able to detect the expected damage.

What is the TOFD dead zone?

Near the scanning surface, the lateral wave can hide shallow flaws. A smaller zone near the back wall can hide very shallow root flaws. Its size depends on probe spacing, frequency and thickness, so the technique sheet should state it and other methods should cover it.

Who can perform TOFD for API 570 crack sizing?

UT personnel certified under the employer's written practice and trained in TOFD. Where the owner needs flaws detected and sized from outside, API 570 requires industry-qualified angle-beam examiners, such as holders of API QUTE or QUSE or an approved equivalent.

Can TOFD be done on hot equipment?

Yes, within the limits of high-temperature wedges and couplant, with the velocity checked at temperature. Very hot surfaces limit scan time and data quality, and the procedure should be demonstrated for the temperature.

What happens after TOFD finds a crack in service?

The owner's inspector and an engineer or corrosion specialist assess it, usually referring to API 579-1/ASME FFS-1. The outcome may be repair, continued service with monitoring, or a change to the interval. The NDE provider supplies the sizes.

Can TOFD detect weld root erosion without removing the cap?

Yes. With a probe on each side of the weld, TOFD can measure root loss under an intact cap, which is why non-intrusive inspection guidance uses it for this task.

Talk to an NDE Level III about sizing a known indication or plan TOFD and PAUT for your next outage.