How to Detect Corrosion Under Insulation (CUI)

The inspection of plant and pipework for CUI can be extremely expensive if all

insulation material has to be removed. 

Often windows are cut into the insulation for localised inspection. However, where selected areas of plant are exposed, confidence in the effectiveness of the examination depends on the ability of the inspector to identify the critical sections of pipework or plant.

In an attempt to improve the selection process, a number of non-destructive testing (NDT) techniques can be employed to detect corrosion under insulation or to identify areas that may be susceptible to corrosion without the need to remove insulation. Each technique has advantages and disadvantages and they have different capabilities. The inspector should understand the capability and limitations of any technique applied. 

The results of the NDT examination can then help to target problem areas where further examination may be necessary. However for critical plant/pipework where no leaks are acceptable full insulation removal may be required to secure proper inspection.

Examples of some NDT techniques available for detecting corrosion under insulation include:

1. Pulsed Eddy Current

The decay of an eddy current pulse is monitored within a ferritic pipe or vessel and the signal is used to calculate the remaining wall thickness beneath a coil unit. This technique can indicate areas of localised corrosion averaged over the area of the sensor. There are limitations on the thickness and type of insulation through which the eddy currents can penetrate.

2. Guided Wave Ultrasonics

This remote screening technique can be used to look for degradation of internal or external pipe surfaces. It can be used where access is restricted and is suitable for long pipe runs. It can detect losses of cross-sectional area of 10% and upwards and is useful to identify areas for more detailed examination.

3. Flash radiography*

This is an established technique which produces high energy X-rays in very short pulses of about 50 nano-seconds duration. When used for detecting CUI, a tangential exposure is made using very fast film. A variation of the system was developed using an unshielded hand-held 'gun' that allowed unacceptable levels of radiation exposure to operators. HSE radiation specialists have considered that this adaptation of the technique should not be used.

4. Real-time imaging radiography*

Developments in real-time radiography have been rapid in recent years and mobile systems are now available to carry out on-site monitoring with a direct visual display of the image. A specially built, shielded hand-held device is also available with this system that, it is claimed, gives negligible exposure to the operator.

Examples of some NDT techniques available for detecting moisture under insulation include:

1. Thermal imaging (thermography)

Detects temperature variations and has been widely used to detect breakdowns of thermal insulation of cryogenic storage vessels and thermal linings of furnaces etc. Areas of damaged and waterlogged insulation can be detected using this technique. Thermography does not give a definitive indication of corrosion, but highlights areas where corrosion may develop in the future. The possibility of intermittent wetting and drying-out of insulation, especially on hot plant and pipework, leading to a misleading result needs to be kept in mind.

2. Neutron Backscatter*

Neutron backscatter devices (hydrodetectors) can be used to locate areas of wet insulation on vessels and pipework, which are potential CUI sites. The system comprises a neutron source and detector assembly on the end of a telescopic pole allowing access to hard to reach areas. Typical screening rates are around 300m of insulated pipework per day.

*Activities subject to the Ionising Radiations Regulations 1999.

For more information about CUI, contact us as www.leopad.com

source:http://www.hse.gov.uk/foi/internalops/hid_circs/technical_general/spc_tech_gen_18.htm#detection