How are welds tested using vacuum box test ?

Vacuum box testing or Bubble leak testing is one of the NDT (Non destructive testing) methods used to detect leaks in welds and confirm the weld quality. This inspection process is recommended by various Classification Societies in the shipping industry and other international standards.

In this article we describe the procedure and requirements for undertaking the leak testing using the vacuum box technique for detection of leaks in a pressure boundary that cannot be directly pressurized such as welds, weld seams and other components in welded steel. The procedure has been explained in three simple steps. Information has been used from sources like Classification Society Rules, websites of vacuum box manufactures and books.

Equipment and Materials used to undertake the test :

A assembled Vacuum testing apparatus using a electric vacuum pump is shown in the pic.

Vacuum Box

Vacuum box is a portable box having a viewing window large enough to view the complete test area and to allow sufficient light to enter the box for proper examination. The box used, shall be capable of producing and holding a pressure differential of 5 to 12 psi. Various types of standard boxes readily available in the market are shown in the pics below.

Vacuum Source

The required vacuum can be developed in the box by any convenient method (e.g. air ejector, or vacuum pump). The gauge shall register a partial vacuum of at least 2 psi (94.1 in. Hg) below atmospheric pressure or the partial vacuum required by the referencing Code Section.

Bubble Solution 
  • The bubble forming solution shall produce a film that does not break away from the area to be tested and the bubbles formed shall not break rapidly due to air drying or low surface tension. The number of bubbles contained in the solution should be minnimized to reduce the problem of discriminating between existing bubbles and those caused by leakage.
  • Soaps or detergents designed specifically for cleaning shall not be used for the bubble forming solution.
  • The bubble forming solution shall be compatible with the temperature conditions of the test.
Pressure Gauge

The pressure gauge shall have a range of 0 psi to 15 psi, or equivalent pressure units such as 0 in. Hg to 30 in. Hg. Gauges shall be periodically calibrated and when there is reason to believe the readings are inaccurate.

Procedure :

Before undertaking the test the following points are to be ensured :

Surface Preparation :

Prior to performing the test, the surface where the vacuum box is to be placed, should be cleaned of all slag, scale, grease, paint or other material which would otherwise interfere with the examination or the interpretation of results.

Surface Temperature :

The temperature of the test surface shall not be below 5 0 C nor above 52 0 C throughout out the examination. Local heating or cooling is permitted provided temperatures remain in the 5 0 C to 52 0 C during the test.

Lighting :

Lighting shall be sufficient to ensure the detection of leaks and the formation of bubbles as leakage gas passes through the solution. Artificial illumination such as a flashlight or other auxiliary lighting may be used.

Step -1 Application of Solution

Apply the bubble forming solution to the test surface by flowing, spraying or brushing forming a continuous film, relatively free of bubbles, before placement of the vacuum box. The solution shall not be applied more than one (1) minute prior to the test.

Step- 2 Vacuum Box Placement

Place the vacuum box over the solution coated portion of the test surface and evacuate it to the required partial vacuum (differential pressure) of five (5) psi minimum.

Step -3 Vacuum Retention & Leak Detection

Observe the solution leakage (bubble formation) for a minimum of 15 seconds at the required partial vacuum. Mirrors may be used to improve the angle of vision and aids such as a magnifying lens may be used to assist in the evaluation.

Presence of continuous bubble growth on the surface of the material or weld seam indicates leakage through an orifice passage(s) in the areas under test.
Area being tested can be acceptable when no continuous bubble formation is observed. Any leak observed is considered as unacceptable and the location of leak shall be duly marked and recorded for repairs. 


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