Visual Examination (VT)

By admin

December 29, 2023

1. General

VT is the most extensively used NDE method for welds. It includes either direct or indirect observation of the exposed surfaces of the weld and base metal. Direct VT is conducted when access is sufficient to place the eye within 6 in. through 24 in. (150 mm through 600 mm) of the surface to be examined and at an angle not less than 30° to the surface, as illustrated in Figure 1. Mirrors may be used to improve the angle of vision.

Remote VT may be substituted for direct examination. Remote examination may use aids such as telescopes, borescopes, fiberscopes, cameras, or other suitable instruments, provided they have a resolution at least equivalent to that which is attained by direct visual examination. In either case, the illumination should be sufficient to allow resolution of fine detail. These illumination requirements are to be addressed in a written
procedure.
ASME Section V, Article 9, lists requirements for VT. Codes and specifications may list compliance with these requirements as mandatory. Some requirements listed in this article include the following:

a) a written, qualified procedure;

b) the minimum amount of information that is to be included in the written procedure;

c) demonstration of the adequacy of the inspection procedure;
d) inspection personnel must be certified by their employer for the specific method;
e) personnel are required to document annual completion of a J-1 Jaeger-type eye vision test (with or without correction);
f) direct VT requires access to permit the eye to be within 6 in. through 24 in. (150 mm through 600 mm) of the surface, at an angle not less than 30°;
g) the minimum required illumination of the part under examination;
h) indirect VT permits the use of remote visual examination and devices be employed;
i) evaluation of indications in terms of the acceptance standards of the referencing code;
j) a form adequately documenting the results per the relevant code section.

2. Visual Inspection Tools

2.1 General
To visually inspect and evaluate welds, adequate illumination and good eyesight are the basic requirements.
In addition, a basic set of optical aids and measuring tools, specifically designed for weld inspection, can assist the inspector. In the following subsections are some commonly used tools and methods for VT of welds.

2.2 Optical Aids
Optical aids used in visual inspection include the following:
a) Lighting: Inspection surface illumination is of extreme importance. Adequate illumination levels should be
established in order to ensure an effective visual inspection. Standards such as ASME Section V, Article 9,
specify minimum lighting levels of 100 foot-candles (1000 lux) at the examination surface. This is not
always easy to achieve, so inspectors have to be aware of the need to measure lighting conditions using
a light meter.
b) Mirrors: Valuable to the inspector, allowing them to look inside piping, threaded and bored holes, and
castings, and around corners if necessary.
c) Magnifiers: Helpful in bringing out small details and imperfections.
d) Borescopes and fiberscopes: Widely used for examining tubes, deep holes, long bores, and pipe bends
that have internal surfaces not accessible to direct viewing.

2.3 Mechanical Aids
Mechanical aids used in visual inspection include the following:
a) Steel ruler: Available in a wide selection of sizes and graduations to suit the needs of the inspection
(considered a nonprecision measuring instrument).
b) Vernier scale: a precision instrument, capable of measuring with a precision factor of 0.0001 in. The
Vernier system is used on various precision measuring instruments, such as calipers, micrometers, height
and depth gauges, the gear tooth, and protractors.

c) Combination square set: Consisting of a blade and a set of three heads: square, center, and protractor,
used universally in mechanical work for assembly and layout examination.
d) Mechanical thickness gauge: Commonly called a “feeler” gauge, and used to measure the clearance
between objects.
e) Levels: Tools designed to prove whether a plane or surface is truly horizontal or vertical.

2.4 Weld Examination Devices
Typical inspection tools for weld inspection include the following:
a) Inspector’s kit (see Figure 2): Contains some of the basic tools needed to perform an adequate visual
examination of a weld during all stages of welding. It includes everything from a lighted magnifier to a
Vernier caliper.
b) Bridge cam gauge (see Figure 3): Can be used to determine the weld preparation angle prior to welding.
This tool can also be used to measure excess weld metal (reinforcement), depth of undercut or pitting,
fillet weld throat size, or weld leg length and misalignment (high–low).
c) Fillet weld gauge: Provides a quick and precise means of measuring the more commonly used fillet weld
sizes. The types of fillet weld gauges include:
1) Adjustable fillet weld gauge (see Figure 4) measures weld sizes for fit-ups with 45° members and
welds with unequal weld leg lengths;
2) Skew-T fillet weld gauge (see Figure 5) measures the angle of the vertical member;
3) Weld fillet gauge (see Figure 6) is a quick go/no-go gauge used to measure the fillet weld leg size.
Gauges normally come in sets with weld leg sizes from 3 mm (1/8 in.) to 25 mm (1 in.). Figure 7
shows a weld fillet gauge being used to determine if the crown has acceptable concavity or convexity.
d) Weld size gauge (see Figure 8): Measures the size of fillet welds, the actual throat size of convex and
concave fillet welds, the reinforcement of butt welds, and root openings.
e) Hi-lo welding gauge (see Figure 9): Measures internal misalignment after fit-up, pipe wall thickness after alignment, length between scribe lines, root opening, 371/2° bevel, fillet weld leg size, and reinforcement on butt welds. The hi-lo gauge provides the ability to ensure proper alignment of the pieces to be welded. It also measures internal mismatch, weld crown height, and root weld spacing.
f) Digital or infrared pyrometer or temperature sensitive crayons: Measures preheat and interpass
temperatures.