On a line in service, corrosion works from the inside. Ultrasonic thickness measurement is the only way to know how much wall is left without stopping the line and without removing the coating.
- ✓ Measurement with the line in service, from outside
- ✓ Echo-Echo mode to measure through the coating
- ✓ Scan mode to find the thinnest point in the area
- ✓ Readings stored on coded CMLs for the trend
- ✓ A-Scan to validate the reading on an irregular back wall
- ✓ Reference standard API 570
Internal corrosion in a line gives no external sign until it is too late. The fluid attacks the wall on the side nobody sees, and the external coating — which is there to protect against atmospheric corrosion — also hides what is happening underneath. A stretch that looks perfect on visual inspection may have lost half of its design wall.
Ultrasonic measurement reverses the situation: resting the probe on the outside reads the actual remaining thickness, with the line running. The delicate point is the measurement mode. In Pulse-Echo the paint or coating layer is counted in and the value comes out higher than the real one: it would give a reassuring figure exactly where the risk is greatest. The Echo-Echo ThruPaint™ mode measures between two echoes inside the metal and removes the problem at its root.
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The second aspect is that a single reading is of no use. What matters is the corrosion rate, that is how much the wall has thinned between one inspection and the next, and from there the remaining life of the stretch. To obtain it, the readings must always be taken at the same coded points, stored and comparable.
How the campaign is set up
- The CMLs (Condition Monitoring Locations) are defined on the basis of risk: elbows, reducers, low points, downstream of valves
- On each CML a cluster of points is measured, not just one, using scan mode to hold the minimum
- The readings are stored in grid files associated with the point code
- They are downloaded to DakMaster™ and compared with the previous campaign
- From the delta the rate and the remaining life are derived according to the criteria of the standard
A current calibration of the instrument is what makes the data defensible in an audit.
Recommended instruments
| Instrument | Why in this application |
|---|---|
| Dakota CMX2-DL | Six measurement modes and A-Scan: the reference choice for API 570 campaigns |
| Dakota CMX10-DL | Colour A-Scan and prove-up: when a doubtful indication has to be qualified |
| Dakota MX1-DL | Extended grid files for networks with many CMLs to follow over time |
| Dakota ZX6-DL | Echo-Echo ThruPaint with data logger, IP65: line surveys on painted surfaces |
Frequently asked questions
Yes: the measurement is made from outside by resting the probe on the wall, without any intervention on the process. The practical limit is the surface temperature, which determines the choice of probe and couplant and requires correction of the sound velocity.
A single reading per point is not representative, because internal corrosion is irregular. Current practice calls for a cluster of measurements around the coded point, with scan mode active to hold the lowest value encountered. The exact number is set by the applicable inspection procedure.
The probe has to rest on the metal, so the insulation must be opened at the measuring point: ultrasound does not cross the air gap. This is why measuring points on insulated lines are fitted with permanent inspection windows, which are reopened at each campaign without redoing the work.
Back to Oil & Gas and Petrochemical. To set up a campaign on your lines, write to us stating diameters, material, coating and operating 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
The information contained on this web page is provided for information purposes only. Although it has been compiled with the utmost care, it does not constitute a contractual offer or a binding commitment to supply. It may contain transcription, translation or typographical errors. For precise and up-to-date information, please contact our company directly.
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