Vacuum Box Leakage Test(VBT) ：
a. Purpose:
It is used on weld and its heat-affected zone to detect through wall leakage caused by defects such as cracks, porosity, and non-fusion.
b. Principle:
A foamable solution is sprayed or brushed on the surface of the testing area, then a vacuum box with a transparent viewing window placed on the surface, and vacuum suction at least 0.55 bar (8 psi) to creating a differential pressure across the weld causing the formation of bubbles as leakage gas passes through the solution.
The system should be equipped with a calibrated pressure gauge to verify the specified pressure difference and to detect vacuum seals or other types of leaks in the box. Observe whether there are air bubbles appearing in the test area to confirm the evidence of through wall leakage.
c. Application:
1). It is used to detect butt weld or lap weld in the bottom plate of the tank and hull shell plate with a plane or radius of curvature greater than 8 meters.
2). Any plane surface (metallic or non-metallic) which may applied a pressure gradient, such as waterproofing sheets and membranes.
