Every plant has thousands of thickness readings and a spreadsheet going back twenty years. The trouble is that a reading tells you about the metal under the probe and nothing whatsoever about the metal beside it.
A spot reading is a probe on the outside of the wall timing an echo off the inside. It gives a number that is accurate to a few hundredths of a millimetre, at that point, and it is the foundation of nearly every integrity programme in the world.
The number is not the problem. The problem is what happens between the numbers.
Take an ordinary condition monitoring location: a four-point grid on a 6-inch line, readings taken every outage. The four points cover about 300 mm² of a pipe surface that runs to hundreds of thousands of square millimetres per metre of length.
You are inspecting well under one per cent of the surface and inferring the rest.
That inference is sound if — and only if — the damage is uniform. For genuine general corrosion, where the whole wetted surface wastes at roughly the same rate, any point genuinely does represent its neighbours, and spot readings are not just adequate but ideal. Cheap, fast, repeatable, trendable.
For anything localised the inference collapses. A groove at the six o'clock position from water dropout, a patch under a failed coating, a wormhole from microbiological attack, erosion downstream of a bend: none of these are obliged to sit under your grid, and the grid was placed where the scaffolding was convenient.
| Damage | Shape | Will a grid find it? |
|---|---|---|
| General corrosion, sulphidation, oxidation | Uniform | Yes — this is what spot readings are for |
| Flow-accelerated corrosion | Local patch, orientation-specific | Only by luck — it sits where the flow puts it |
| Corrosion under insulation | Local patch at water ingress | Only by luck |
| Under-deposit and MIC | Discrete pits, often deep | No |
| Erosion at bends and tees | Local, downstream of the disturbance | Only if you deliberately put points there |
| Acid dewpoint corrosion | Local, at the condensing zone | Only by luck |
Read that column again. Most of the mechanisms that actually cause loss of containment in process plant are in the rows a grid does not reliably find.
Mapping is the same ultrasonic measurement taken continuously over an area rather than at points, usually with an encoded scanner so the position of every reading is known. The output is a thickness image rather than a list.
Three things follow from that, and only the first is obvious.
Grids find the minimum of the points you took. Mapping finds the minimum of the area. On localised damage those are routinely different by several millimetres.
This is the part that gets overlooked. A Part 5 assessment of a local thin area needs the critical thickness profile — how the remaining wall varies along and around the flaw. You cannot construct a critical thickness profile from four spot readings. If a Level 1 or Level 2 local metal loss assessment is in your future, the mapping is not optional; it is the input data.
An encoded scan can be repeated at the next outage over the same footprint and subtracted. That gives a corrosion rate for an area, which is a far better number than the difference between two spot readings taken by two technicians three years apart.
Two readings a few years apart, each accurate to ±0.1 mm, differencing to a loss of 0.3 mm, gives a rate with an uncertainty of well over half its own value — before you account for the probe not landing on precisely the same spot. Short intervals and small losses produce corrosion rates that are mostly measurement noise, and remaining life calculated from them inherits all of it.
This is why a suspiciously high or low rate should prompt a re-measure before it prompts a decision.
You cannot map everything; there is not enough outage. The rule that survives contact with a real budget:
If a single point in a grid reads far below the rest, the instinct is to re-shoot it and, when it repeats, to log it as the minimum and move on. That point is telling you the grid landed on the edge of localised damage. The real minimum is somewhere nearby and nobody has measured it. Scan around it before you calculate anything.
The quick check takes a thickness, a diameter and a pressure and gives you the minimum required thickness, the MAWP and the remaining life — on screen, free.
Open the quick checkWho writes this. A mechanical engineer with twelve years in oil and gas — in-line inspection, fired heater and furnace inspection, and pipeline integrity. What is here comes from the published standards and from what those years in the field actually looked like. It is not written by an API-certified inspector.
This is not an assessment. Nothing on this site may be used to justify a decision about real equipment. Assessing plant requires the current editions of the applicable codes, data from a licensed source, and a competent engineer who signs for the answer. · Integrity Field Guide