VISUAL INSPECTION OF CRITICAL PIPING -ABC OF THERMAL POWER STATION

Visual inspection

Visual inspection is most widely used NDE tool. It is simple widely used and can be

carried out quickly and cheaply. A specimen can be tested with different NDE process

but before carrying out any examination visual inspection to be carried out. A visual

inspection can reveal many things such as quality of weld, presence or absence of

cracks, surface porosity, misalignment etc. Basic principle of carrying out visual

inspection is to illuminate the weld area sufficiently to observe the defect. Visual

inspection can be carried out with many observation tools like mirror, telescope,

periscope and boroscope. 

Present day Boroscopic inspection is required to be carried out at different places

of thermal power station. A boroscope inspection is carried out in different headers,

tubes etc wherever easy access of visual inspection is not there. A modern day

Boroscopic instrument consist of light fibre optic cable, lenses etc to get the exact

visual of the weld area.

Different types of gauges are also available as aid for visual inspection. Theses

are Taper gauges, Hi-Lo gauges, Filler gauges and multipurpose gauges.

Before welding a few points needs the attention of site engineers.

1. Review of drawings and specification.

2. Check the qualification of personnel and procedures.

3. Check the alignment /fit up of weld joints.

4. Follow WPS and Field Quality Plan

5. Surface cleanness of the weld.

Welding Procedure (WPS)

-        It should ensure correct welding procedure is employed.

-        Welding procedure should be well documented considering all aspect of welding.

-        Prior to welding base material identification including cleanness to be checked.

-        For joint fit up one should check groove angle, Root Opening, Backing, etc.

During Welding following aspect needs to be checked.

Quality of Root bead to be checked.

Joint Root preparation before the welding second side.

Preheat and inter pass temperature.

Sequence of weld Pass.

Inter Pass Cleaning

After Welding Following points need to be checked. 

Final weld appearance, Final weld Size.

Weld length, Dimensional accuracy, amount of distortion,

Post heating and Post weld heat treatment, Checking Discontinuity.

Following Discontinuities are generally present in the weld.

Porosity, Incomplete /lack of fusion, Incomplete Joint penetration, Undercut,

Overlap, Cracks, Slag inclusion, Excess Reinforcement etc.

While code permits limited number of such discontinuities. But discontinuities listed

Below are never allowed.

Cracks and incomplete fusion.

Undercut, overlap and improper contour results in stress raiser

One of the important aspects is illumination during visual inspection. Some codes specify

16 lux for general inspection and 54 lux for discontinuities.

Welding training is going on both at class room and at site with welding gauge by welding

experts from NDE examination.

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