NDT methods: what each one sees, and what it cannot

The methods, and what each one is for

Ultrasonic thickness measurement

The workhorse. A probe on clean steel gives remaining thickness at that point. Spot readings are cheap and are the basis of corrosion-rate trending; scanning versions cover an area rather than a point and find localized loss that spot readings walk straight past.

Magnetic flux leakage floor scanning

The standard way to cover a tank floor. A scanner is walked over the plates and indicates areas of metal loss, top or bottom side, which are then confirmed by ultrasonic measurement. Coverage is a percentage, and that percentage belongs in the scope, not in a conversation.

Vacuum box testing

Applied to welds, typically on the floor: a box, a soap solution and a vacuum reveal through-leaks. It answers a different question from thickness, not how much metal is left, but whether it is currently tight.

Surface methods: magnetic particle and dye penetrant

For cracks and discontinuities that break the surface, particularly at welds and repairs. They see nothing buried in the plate, which is exactly the trade-off shown in the diagram below.

Radiography

For weld quality, typically on repairs and new work rather than as a routine condition tool. It brings its own access, exclusion and safety requirements.

In-service screening

Acoustic emission and robotic in-service tools screen a floor without emptying the tank. Treat their output as a prioritization signal, which tank to open first, rather than as an internal inspection performed remotely.

The three numbers to put in the scope

1. Floor scanning coverage, as a percentage of plate area, and how the critical zone near the shell is treated.
2. The number and pattern of shell thickness readings, per course, per orientation, and whether locations are marked for repeatability.
3. What triggers follow-up: when a scan indication becomes a confirmed measurement, and who decides.

Two bids that differ by a large amount usually differ on the first of these. Ask both bidders for the figure before assuming one of them is expensive.

Who holds the qualification

NDT is performed by qualified technicians working to written procedures, under a documented qualification scheme, and the report should say which. It is a fair question to ask at bid stage, and the answer is a differentiator between providers who have a quality system and providers who have equipment.

The interpretation of the resulting data, corrosion rates, remaining life, next interval, is separate work again, and it is the subject of integrity assessment.

Frequently asked