Visual Inspection : Testing Requirements and Application

Visual Inspection means the inspection of equipment and structures using a combination of human senses such as vision, hearing, touch and smell. Visual Inspection is sometimes carried out in conjunction with devices such as a low power magnifying glass, boroscopes, fiberscopes, digital video borescopes, camera systems and robotic crawler systems. Visual Inspection is the most basic of the different non-destructive testing control methods, but no less important for it.

From the very moment in which a component (valve, pump, pipe, vessel, etc.) is installed, control by visual inspection is implemented as the initial element of judgment for the acceptance of an individual part or an overall system prior to its entry into service, or as an initial control for the subsequent performance of tests verifying its operability.

In accordance with the requirements of the different standards applicable to the different types of installations, Tecnatom, S.A. provides top quality services for the performance of inspection technical procedures and direct visual inspections of ASME Code categories VT-1 and VT-2:

• Category VT-1. Visual inspection to determine the general conditions of a component, part or surface.
• Category VT-2. Visual inspection for the location of leaks in the pressure boundary of systems containing fluids. This category includes the in-service visual inspection of systems, functional tests, leak tests and hydrostatic tests.

Visual Inspection Testing Requirements

• The luminance at the surface shall be minimum 500 lx. If required to obtain a good contrast and relief effect between imperfections and background, an additional light source should be used.
• For performance of direct inspection, the access shall be sufficient to place the eye within 600 mm of the surface to be inspected and at an angle not less than approximately 30degree. If not otherwise agreed all weld connections in question should be 100% visually inspected.
• The testing volume shall as a minimum cover the zone which includes welds and parent metal for at least 20 mm on each side of the weld. In case of doubt, visual testing should be supplemented by other non-destructive testing methods for surface inspections.
• The weld surface shall be free of weld spatter, slag, scale, oil, grease, heavy and loose paint or other surface irregularities which might avoid imperfections from being obscured. It can be necessary to improve the surface conditions e.g. by abrasive paper or local grinding to permit accurate interpretation of indications.

Visual Inspection Applications

• Visual inspection VT-1 of welds, components, bolts, nuts and housings, required by the ASME Code to be performed every 10 years.
• Visual inspection VT-2 of different elements of systems and components (welds, piping sections, flanged unions, valves, etc.) as required by the ASME Code.
• Nuclear Class 1 Systems. This is performed under rated pressure and temperature conditions during plant start-up following each scheduled maintenance outage.
• Nuclear Class 2 and 3 Systems. Functional tests will be performed every 3 years on systems not in operation during normal operation of the plant, along with in-service inspection of systems that do operate during plant operation.

source : ndtindia.org and tecnatom.es