Type of CUI inspections: Pulsed Eddy-Current (PEC)

Assessing condition of pipework and pressure vessels under insulation can be advantageous to plant operators. The Pulse Eddy Current (PEC)  system was developed as a solution to the detection of corrosion under insulation (CUI) and validated by Shell Global Solutions.

Compared with conventional eddy-current testing, pulsed eddy-current inspection requires no direct contact with the object being tested. Measurements can be made through any material not conducting electricity including coatings, insulation materials, weather sheeting and even corrosion
products. It is a very useful characteristic that also enables high temperature non-destructive testing (NDT) inspections.

Inspection approach

The PEC instrument probe is placed against the metal weather sheeting (non-ferrous) of the insulated pipe or vessel. The geometry of the test object should be simple. A magnetic field is created by placing an electrical current in the transmitting coil of the probe. This field penetrates through the weather sheeting and magnetizes the pipe wall. The electrical current in the transmission coil is then
switched off, causing a sudden drop in the magnetic field.As a result of electromagnetic induction, eddy-currents will be generated in the pipe wall. The eddy-currents diffuse inwards and decrease in strength. The rate of decrease of the eddy currents is monitored by the PEC probe and is used to
determine the wall thickness.

Comparison with ultrasound wall thickness measurement

Both pulsed eddy-current and ultrasound wall thickness measurement have strong and weak points. The relevance of these strengths and weaknesses varies greatly from application to application.

Benefits at a glance

• ƒNo loss of production, as inspection can take place while the inspection object is in service.
• ƒReduced inspection costs, as insulation material does not need to be removed.
• Significantly lowered costs for underwater inspections.
• Speedy inspection, as surfaces do not require any preparation.
• Good reproducibility of PEC readings at the same locations.
• Plus minus 10% accuracy for corrosion detection under insulation and only plus minus 0.2% accuracy for corrosion monitoring.
• Inspections within a temperature range from -100°C to 550°C (-150°F to 1000°F).
• Inspection of objects with a wall thickness of 3 to 35 millimeter.
• Inspection of objects with a pipe diameter above 75 millimeter.

Areas for application

Pulsed eddy-current can be effectively applied for corrosion 

monitoring and detection on pipes and vessels made of 

carbon steel or low-alloy steel without making contact with 

the steel surface itself.

ƒInsulated and/or coated equipments

• ƒ Objects under high temperature conditions
• ƒ Heavy corroded equipments
• ƒ Offshore risers and caissons
• ƒ Objects behind concrete fireproofing
• ƒ Laminations
• ƒ Annular rings
• ƒ Bridges

Source: http://www.tuv.com/media/corporate/industrial_service/NDT_Pulsed_Eddy_Current_TUV_Rheinland.pdf