Insulation and coating are two different problems. The coating is overcome with the right measurement mode; the insulation has to be opened, because ultrasound does not cross the air gap.
- ✓ Echo-Echo ThruPaint™: the coating stays out of the measurement
- ✓ CMX1-DL: measures metal and coating separately
- ✓ Permanent inspection windows on insulated lines
- ✓ Focused attention on corrosion under insulation (CUI)
- ✓ Probes and couplants for high temperature
- ✓ A-Scan to recognise a distorted reading
On plant piping the two words are often used together, but for whoever has to measure they mean opposite things. The coating — paint, epoxy, anti-corrosion protection — is in direct contact with the metal: ultrasound crosses it, and the only problem is that in Pulse-Echo it would be counted as thickness. The insulation, on the other hand, is a separate shell with air inside: ultrasound does not pass, and no measurement mode can make up for it.
For the coating the solution is the Echo-Echo mode, which measures between two echoes inside the metal: the surface layer drops out of the count automatically and nothing has to be removed. The CMX1-DL goes further and returns both values at once, metal and coating, useful when the thickness of the protection is itself a figure to be checked.
Need help choosing the right product?
For the insulation the only way is to open it at the measuring point. Since redoing the opening at every campaign is costly, established practice is to fit permanent inspection windows at the coded points: they are opened, the measurement is taken, they are closed again. It is also the chance to look at the condition of the surface, because wet insulation is the cause of corrosion under insulation.
Coating and insulation compared
| Situation | Does ultrasound pass? | How to proceed |
|---|---|---|
| Paint or epoxy | Yes, it is in contact with the metal | Echo-Echo mode: the layer drops out of the count |
| Thick adherent coating | Yes, with adequate gain | Echo-Echo, or CMX1-DL to read the coating as well |
| Insulation with an air gap | No, the air breaks the path | Opening of the measuring point, preferably with a permanent window |
Corrosion under insulation
CUI is one of the most insidious mechanisms in process plants, precisely because the insulation makes it invisible.
- Water enters through joints and penetrations and stays trapped against the metal
- It attacks in a localised way, not uniformly: a measurement at a nearby point may see nothing
- The critical areas are the low points, the supports and the breaks in the jacketing
- Scan mode over the opened area is needed, not a single reading in the middle
When the insulation is opened it is worth taking advantage: the visible surface says a great deal about the state of the protection, and the point must be measured over an area, not at a centre.
Recommended instruments
| Instrument | Why in this application |
|---|---|
| Dakota CMX1-DL | Measures metal and coating together, at the same moment |
| Dakota CMX2-DL | Six modes and A-Scan to validate the reading under a coating |
| Dakota ZX6-DL | Echo-Echo ThruPaint with data logger: line surveys on painted surfaces |
| Couplants for high temperature | Hot lines: the limit is the couplant, not the instrument |
Frequently asked questions
No. Contact ultrasound needs physical continuity between probe and metal, and the air gap of the insulation breaks the path. No measurement mode can make up for it: the point must be opened, preferably with a permanent inspection window to be reused at every campaign.
Yes, provided the instrument has the Echo-Echo mode. The paint is in contact with the metal and ultrasound crosses it; the only problem is that in Pulse-Echo it would be counted as thickness. By measuring between two echoes inside the metal, the surface layer drops out of the result automatically.
The limit depends on the thickness and type of coating, on the probe frequency and on the available gain: a very thick or highly attenuating coating weakens the echoes to the point of making the measurement unstable. The A-Scan is the practical way of checking whether the echoes being used are the right ones.
Back to Oil & Gas and Petrochemical. For an assessment of your case, write to us stating the type of coating, thickness and line temperature.
Further information and contacts
Systems division and sensor integration partner : www.roder.it
More information about RODER SRL : about us
Contact for general information : info@roder.it
Contact for information about these products : info@roder.it
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