Vacuuming

The vacuum-bubble method of control allows to reveal through defects. When testing by this method, a vacuum chamber is installed on the side of the tested section of the welded joint, moistened with an indicator solution, and a vacuum is created in it. Due to the resulting pressure drop, atmospheric air penetrates through through defects, causing their bubble indication.

Equipment:

The control installation should include: a fore vacuum pump, a vacuum receiver, one or a set of vacuum chambers with three-way valves, vacuum gauges and lighting fittings, hoses. 

Requirements for the preparation of the control object: 

One-sided access to the object of control is sufficient for carrying out control.

The temperature of the welded joint or the base metal during the control should not exceed 40°C. It is not allowed to apply paint and varnish coatings before the inspection. Paint can make it difficult to identify defects. Scale must be removed from welds.

Observation of the surface of the tested section of the welded joint during vacuum bubble testing should be carried out through the transparent top of the vacuum chamber throughout the entire period of creating the established pressure difference plus 10 … 15 s.

Marking defective areas:

Defective places found during the inspection are marked next to the vacuum chamber and, after checking this area, are transferred to the welded joint.

If there are doubts about the reliable indication of defects in a given section of the welded joint or the main metal, a second inspection is carried out.

When checking the next section of the welded joint, the vacuum chamber is installed so that it is possible to check the joint that was under the sealing gasket of the chamber, as well as located near it.

The results of the control should be recorded in a log, map or diagram indicating: the type of welded joint, indices assigned to the joint, assembly, welded structure; length of checked joints, ambient air temperature and welded joints, indicator solution composition, type of installation for vacuum bubble testing, maximum pressure drop created during testing, type and location of through defects, results of checking defective places after their repair, names of the welder, flaw detector and other necessary data.

Most often used on the following objects:

To control the tightness of welded joints and structures, the following control methods are used:

• evacuation;
• sample “chalk – kerosene”;
• overpressure;
• hydraulic testing of the reservoir.

The following nodes are examined by the vacuum bubble method:

Seams of the bottom, seams of the lining with the bottom, seams of the bottom at a distance of 250 mm from the outer edge, the seam between the pipe and the wall, the seam of the wall with the bottom, the seams of the roof decking, roof shields, the seam of the branch pipe with the roof, the seams of the central part (floating roof (steel pontoon), seams of branch pipes with a roof (floating roof (steel pontoon).