TWI Knowledge Summary

Visual inspection

• Direct visual inspection
• Remote visual inspection

Direct visual inspection

What will it find?

A visual examination of operational plant can be used to check for obvious problem areas, such as leaks, excess vibration or misalignment. On exposed metal surfaces it can also be used to check for corrosion. Specific guidance on the inspection of pressure vessels for corrosion and other flaws is given in API Recommended Practice 572, and on the inspection of piping, valves and fittings associated with pressure vessels in API Recommended Practice 574.

Plant coated with internal or external coverings, such as insulation, refractory protective linings and corrosion resistant linings, may be inspected visually if access permits. In such cases, a visual examination can be used to check that the protective layer has not separated and that it is free of breaks, holes and blisters.

In some extreme cases, a visual examination may also be able to detect cracking, particularly if the cracking is surface-breaking and extensive.

Equipment

There are various aids to visual inspection. As well as magnifying glasses, there are devices for measuring pit depths, weld leg length and surface profiles.

Where is it used?

Visual examination is applicable to most surfaces, but is most effective where the surfaces have been cleaned prior to examination, i.e. any scale, loose paint has been removed by wire brushing etc. However, care must be taken to avoid peening.

Remote visual inspection

What will it find?

An enhanced visual inspection using boroscopes or endoscopes can be used to check the internal surfaces of pipes or tubes for corrosion and/or erosion damage. On pipe containing a protective inner lining, boroscopes may be used to check that the lining is free of breaks, holes and blisters. Also, if the inner surface of the pipe or tube is sufficiently clean, boroscopes may be able to detect surface-breaking cracks, provided that they are sufficiently large.

Equipment

Where access is restricted, and it is not possible to examine the subject area with the naked eye, boroscopes or endoscopes may be used. In simple terms, these are telescopes, which can be inserted into pipes, tubing or access holes in machinery. Most modern endoscopes use optical fibres to illuminate the subject area and to relay the image back to a closed-circuit television camera. Optical fibres enable images to be retrieved over several metres and permit access to relatively complex geometries. The inspection can also be recorded on video. Equipment costs range from a few hundreds of pounds for a manually operated endoscope to several thousand for a sophisticated video recording system.

Where is it used?

Boroscopes and endoscopes are normally used to examine areas that are inaccessible to the naked eye. Perhaps, their most common application area is the examination of small diameter tube or pipe.

Further information

TWI offers training courses on visual inspection.

TWI Industrial Members and JoinIT North East users have access to the NDT Selector Toolkit

Additional information about visual inspection can be found in the items detailed below:

Visual inspection of arc welds - a guide to best practice

FAQ: What should I check during the visual inspection of clinched joints?

FAQ: What should be checked during simple visual inspection of self-piercing riveted joints?

You can use the Weldasearch literature database to supplement what you find in JoinIT.