Writing and Qualifying a UT Procedure for Olefins and Derivatives Units

A UT Procedure Is a Contract with the Referencing Code

ASME Section V does not stand on its own. Article 1 requires written procedures and sets the general rules; Article 4 supplies the ultrasonic requirements and the table of essential and non essential variables. What Section V never does is tell you which welds to examine, how much of them, or what is acceptable. That comes from the referencing code: Section VIII for the vessel, B31.3 for the process piping, or the owner user's in service programme under API 510 and API 570. A procedure that cites Section V and nothing else has stated a method and omitted the contract.

Getting the referencing code right is not clerical. B31.3 acceptance for a given fluid service, Section VIII Division 1 workmanship acceptance, Division 2 with its different examination regime, and an in service assessment under API 579 give four different answers about the same indication in the same weld. Petrochemical sites run all four simultaneously: new construction on a project, tie ins under the piping code, and in service examination during the turnaround. One procedure cannot serve all of them honestly, and the compromise document that tries usually defaults to the most lenient criterion in the pile.

Edition control is the other half of the contract. The jurisdiction adopts a code edition, the client's specification may freeze a different one, and your procedure cites a third. In Texas the adopted edition and the contract normally agree; on export and Canadian work they frequently do not. Fix the edition and addenda explicitly in the procedure and record which version applied to each job, because the question always arrives years later, when the equipment file is opened after a failure.

Essential Variables: the Table That Decides When You Requalify

Article 4's variable table is the operating core of the document. Weld and base material types, geometry configuration, thickness range, surface condition, couplant, examination technique, beam angles, search unit frequency and element size, special wedges and shoes, the DAC or DGS reference level, scan directions, extent and overlap, the method of demonstrating coverage, automatic alarm and recording arrangements, and personnel requirements. Each entry is a promise about the conditions under which the procedure was shown to work, and each one is a boundary you can be held to.

The common failure is not ignorance of the table but range inflation. A procedure written to cover six to one hundred and fifty millimetres, all butt and nozzle configurations, three angles, two frequencies and both manual and encoded acquisition looks convenient until someone asks where the demonstration at one hundred and fifty millimetres is. The opposite failure is just as common: the technique sheet in the field names a seventy degree wedge that the procedure never listed. Both are recorded as findings, and the second one puts every report produced with that wedge into question.

Non essential variables can be changed by revision without requalification, which makes the classification decision worth taking seriously rather than copying from a template. Whether the demonstration is required at all comes from the referencing code and the owner's specification, not from Section V by itself. Write that trigger into the procedure explicitly, so the next revision does not have to relitigate whether a mock up is needed. The cheapest procedure is the one that states its own change rules.

Olefins Service: the Damage Mechanisms That Choose the Technique

Start from API 571 rather than from a probe catalogue. Caustic circuits in ethylene units crack from the inside surface, typically at weld heat affected zones where post weld heat treatment was omitted. Wet hydrogen sulphide services in amine, sour water and quench systems produce hydrogen induced cracking, stepwise cracking and stress oriented hydrogen induced cracking, much of it lying parallel to the surface at mid wall, which is close to the worst possible orientation for a beam aimed at volumetric weld flaws. NACE/AMPP SP0296 exists precisely because these mechanisms need a deliberate detection strategy rather than a general weld scan.

Inside surface cracking sets the angle. A forty five degree shear wave in the first leg approaches the inside surface at an unhelpful angle and leaves a near surface region where response is poor, so a procedure that offers only forty five degrees will systematically under report the very cracks the unit produces. Higher refracted angles, creeping wave or high angle refracted longitudinal arrangements, and phased array sectorial sweeps with an inside surface focus are the usual answers. Whichever is chosen, the reason belongs in the procedure, because that sentence is what a client's own Level III will look for.

High temperature hydrogen attack is the mechanism that most punishes routine thinking. It develops as methane filled fissures at grain boundaries in carbon and carbon half molybdenum steels above the API 941 Nelson curve thresholds, and it removes essentially no wall until it is far advanced. A thickness survey will call the equipment healthy right up to failure. Detection requires advanced ultrasonic approaches such as backscatter analysis, velocity ratio measurement, time of flight diffraction and high frequency phased array, each with real limitations that a serious procedure states rather than hides.

Cracking Furnaces, Cast Alloys and Where Ultrasound Stops

Radiant coils in an ethylene cracking furnace are centrifugally cast high chromium nickel alloys, and their coarse dendritic structure scatters and attenuates ultrasound badly. Creep cavitation in that material is not reliably detectable by conventional ultrasonics at the sizes that matter, and carburisation is a bulk metallurgical change rather than a reflector. Programmes therefore lean on eddy current and magnetic techniques that respond to the permeability change carburisation causes, diametral growth measurement, and metallographic replication, with ultrasonics restricted to specific geometries where it has been demonstrated to work.

This matters commercially, not just technically. A procedure that promises creep damage detection in cast coils cannot be qualified, and writing it exposes the contractor rather than protecting them. The right document states the material and product forms the technique is valid for and excludes the rest explicitly, so that the exclusion becomes a scope conversation before the turnaround instead of an argument after a tube ruptures. Owners respect a procedure that names its own limits; they lose confidence in one that appears to cover everything.

The same reasoning applies to austenitic and dissimilar metal welds in derivative units, to weld overlay and clad interfaces, and to the aluminium brazed cold box equipment in the cold section of an olefins plant. Each is a different acoustic problem, and each needs either a demonstrated technique or an explicit statement that this procedure does not apply. Silence is what gets read as coverage.

Detection and Sizing Are Different Procedures with Different Proof

A detection procedure asks a binary question: does the response from this region exceed the reference level. A sizing procedure asks for a number with an error band, usually flaw height through wall, and that number goes into an engineering calculation. Under API 579 a crack like flaw assessment is driven by depth; a two millimetre error in height moves a Level 2 assessment result far more than most engineers expect. Feeding an unquantified amplitude based estimate into that calculation produces an answer with false precision and no defensible basis.

Sizing therefore has to be qualified on its own terms. That means a blind trial on specimens containing flaws of the type you will size, at the depths and orientations you will encounter, with results compared against destructive sectioning or an accepted reference, and a stated accuracy expressed as an error band rather than a claim of competence. Tip diffraction, time of flight diffraction and encoded phased array can all support sizing when demonstrated; amplitude drop methods on service induced cracks generally cannot, whatever the instrument brochure suggests.

There is a documentation consequence as well. If sizing feeds a fitness for service assessment, the engineer performing that assessment is entitled to know the ultrasonic accuracy and to apply it in the calculation. That means the procedure must state the qualified accuracy, the report must carry it, and both must survive being read by someone who was not on site. Most contested fitness for service arguments in petrochemical plants come down to whether the flaw height was ever measured by a method qualified to measure it.

What the Qualification Demonstration Has to Show

A demonstration is an experiment with a predetermined pass condition, not a site visit. It should use mock ups representing the actual joint geometry, material, thickness extremes and surface condition, containing flaws of the type the mechanism produces, at the depths and orientations that matter. Machined notches are convenient and misleading: they are smooth, open and reflective, and they respond far more strongly than a tight service induced crack. A credible programme includes at least some real or realistically produced cracking, and states plainly where notches were used as a substitute.

The demonstration must also prove coverage rather than assert it. That means a scan plan showing where the beam actually goes at each qualified angle, including the fusion faces, the inside surface region and the volume claimed, with the skip geometry worked out for the thickness extremes rather than the nominal case. Where access limits coverage, the limitation belongs in the procedure and on the report. For encoded and automated systems, examination system qualification under Section V Article 14 or an owner's performance demonstration provides a structured route.

Finally, the demonstration should be run by the people who will do the work. A technique that works in the hands of the Level III who designed it, but not in the hands of the crew dispatched at two in the morning, has not been qualified in any useful sense. Recording who performed the demonstration, at what certification level and under which written practice, is what turns the exercise from a marketing artefact into evidence.

Gulf Coast and Alberta: One Procedure, Two Personnel Regimes

The Gulf Coast olefins and derivatives corridor, from the Houston Ship Channel through Chocolate Bayou, Freeport and across to Lake Charles, runs on employer certification. The written practice under SNT-TC-1A or CP-189 certifies the technician, the owner user procedures govern in service examination under API 510 and API 570, and OSHA process safety management requirements under 29 CFR 1910.119 turn documentation into a compliance issue as well as a quality one. The procedure and the personnel file are both employer controlled, which is efficient and puts every burden of proof on your own records.

Alberta works differently in a way that catches US contractors out. Pressure equipment there is administered provincially under the Safety Codes Act, with CSA B51 as the construction and registration standard, and non destructive testing on pressure equipment is normally performed by personnel certified centrally by Natural Resources Canada under the CAN/CGSB-48.9712 scheme. Your written practice does not certify anyone for that work. The ultrasonic procedure can transfer once it is aligned to the adopted code; the technician credentials cannot, and mobilising a US crew to the Industrial Heartland northeast of Edmonton or the ethylene complex near Red Deer on employer certificates alone is a scope failure discovered at the gate.

The practical answer is to write the procedure so the personnel clause references the applicable certification regime rather than assuming one. A single sentence stating that personnel shall be certified under the employer's written practice for US scopes, or hold the nationally issued certification required by the jurisdiction for Canadian scopes, saves an entire revision cycle and makes the document usable in both markets.

Findings That Recur When a UT Procedure Is Audited

The recurring set is short and predictable. The procedure cites a code edition that neither the contract nor the jurisdiction invokes. Acceptance criteria are lifted from a construction code and applied to in service degradation. Essential variable ranges do not include the wedge, angle or frequency named on the technique sheets in use. Transfer correction is required by the procedure and recorded nowhere. Calibration block traceability is missing, or the block material and heat treatment do not match the parts. And the procedure is approved by a Level III who is not the Level III named in the written practice.

Two findings do deeper damage. The first is the missing sizing qualification behind numbers that were fed into a fitness for service assessment, which puts an engineering decision rather than a report in question. The second is the calibration verification rule: procedures state that if a check fails beyond tolerance, everything examined since the last valid check must be re examined, yet almost no contractor can produce a record of that ever happening. An auditor reading a decade of files with no failed calibration check does not conclude that your equipment is exceptional.

Fixing these is mostly a matter of writing documents that can actually be complied with, then designing report forms that force the evidence to exist. If the procedure requires a number, the form should have a field for it, and a review step should reject the report when the field is empty. Controls that depend on memory are not controls.

How Atlantis Develops and Qualifies UT Procedures

Work starts from your equipment and your damage mechanisms, not from a template library. That means reading the corrosion study or the integrity operating windows if they exist, listing the mechanisms credible for the circuits in scope against API 571, and deciding technique from there: angle and frequency selection, calibration basis, whether encoding is needed, and whether the scope requires detection alone or detection plus qualified sizing. The output is a procedure with defensible essential variable ranges, technique sheets for the joints you actually have, and a written statement of what the procedure does not cover.

Where a demonstration is required, Atlantis specifies the mock ups and flaw types, defines the pass criteria in advance, runs or witnesses the demonstration with your personnel, and documents the result in a form that survives a client's technical review. Where sizing feeds an API 579 assessment, the accuracy is established by blind trial and carried through to the report template so the engineer receives the error band along with the number. Existing procedures are revised and extended rather than discarded, so your qualification history remains usable.

Atlantis is led by an ASNT NDT Level III in multiple methods who is also an API 653 authorised inspector, and it supplies inspection management and reporting software, digital twin asset visualisation, NDT training and independent report validation alongside procedure consulting. Positioning is affordable, accessible and fully customisable. To scope procedure development for an olefins or derivatives unit on the Gulf Coast or in Alberta, request a consultation and a written quote at info@atlantisndt.com.

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