Vacuum Box Testing

Vacuum box testing for storage tank floors, shell-to-bottom welds and one-side-access seams across the UAE — bubble emission to ASME V Article 10, ASTM E515 and API 650, carried out by ISO 9712 qualified technicians under ISO 17020 accreditation. Last reviewed: August 2026.

Most leak testing depends on pressurising one side of the boundary. Tank floors, roof plates and many weld seams cannot be pressurised at all — there is no closed volume to fill. Vacuum box testing solves that by reversing the differential: a sealed box is placed over the wetted weld and evacuated, so atmospheric pressure on the far side drives air through any through-wall defect and forms bubbles in the solution.

How the test is performed

• The weld and adjacent surface are cleaned of scale, slag, oil and coating that would trap solution or obscure bubbles
• A leak indicating solution is applied to the test area, thick enough to form and hold a bubble but not so thick that it masks a small leak
• The box, fitted with a transparent window and a compliant gasket, is placed over the area and evacuated to the partial vacuum specified in the procedure — typically 20–35 kPa below atmospheric pressure
• The area is observed for the full hold period through the window, with the gauge reading verified as stable
• Boxes are overlapped along the weld by at least 50 mm so no length is left unexamined between placements
• Indications are marked on the plate for repair and the area is retested after rework

Vacuum level and hold time

Both are set by the governing code or procedure rather than by convenience. A partial vacuum below the specified level will not draw air through a tight defect, and an observation period shortened to keep pace with production is the most common reason a leak is missed — small leaks take time to form a visible bubble. Gauge readings and hold times are recorded for each placement, and the gauge is verified against a calibrated reference.

Where it applies

• Storage tank floorsBottom plate lap welds and butt welds, the classic application, where the underside is inaccessible and a leak means product to ground. Required under API 650 for new construction and API 653 for in-service inspection.
• Shell-to-bottom weldsThe annular weld at the tank periphery, tested during construction and at out-of-service inspection.
• Roof plates and seamsFixed and floating roof welds where vapour tightness matters.
• Repair weldsVerification after plate replacement or weld repair, before the tank is returned to service.
• Barges, decks and bulkheadsMarine structures where one side is inaccessible or cannot be flooded.
• Linings and membranesSeams in secondary containment and lined surfaces.

Standards applied

• ASME Boiler and Pressure Vessel Code, Section V, Article 10, including the appendix covering the vacuum box technique
• ASTM E515 — leaks using bubble emission techniques
• API 650 — welded tanks for oil storage, construction testing
• API 653 — tank inspection, repair, alteration and reconstruction, in-service work
• Client and project specifications where these set tighter requirements

Advantages and limits

The method is inherently safe — no pressurisation, no stored energy, no radiation source — and it locates the leak precisely rather than only confirming one exists. It needs no power at the test location beyond the vacuum pump, and results are immediate and visible.

Its limit is sensitivity. Bubble methods detect leaks orders of magnitude larger than tracer gas techniques can resolve, so where the acceptance criterion is a small quantified leak rate, helium leak testing is the appropriate method. The box also requires a reasonably flat surface for the gasket to seal, which restricts it at nozzle penetrations, corners and heavily distorted plate — those areas need an alternative technique and the report identifies them.

Personnel and accreditation

Testing is carried out by technicians qualified to ISO 9712 and ASNT SNT-TC-1A Level II. ITMAD is accredited to ISO/IEC 17020 as an inspection body, is an ADNOC approved inspection contractor, and is listed on the Abu Dhabi City Municipality register of Approved Third Party Inspection Bodies. Certificates are issued in the format required by the governing authority, including Dubai Municipality and the Dubai free zone authorities. Testing is performed at tank farms, terminals, fabrication yards and marine facilities across Dubai, Abu Dhabi, Sharjah and the northern Emirates.

What you receive

• A report identifying the item, weld seams tested and total length examined
• Vacuum level achieved and hold time applied at each placement
• Indications located and marked, with photographic evidence
• Areas that could not be tested by this method and why
• Retest results following repair
• Gauge calibration traceability and technician qualification