Storage tank airtightness testing is a mandatory quality control and safety procedure that verifies the leak-tightness of tank bodies, weld seams, roofs, bottoms, and connections before commissioning and throughout the service life of the equipment. Performed in accordance with recognized standards such as API 650, API 620, EN 14015, GB 50128, and GB 50341, these tests protect stored products, personnel, and the environment from the consequences of leakage. This guide explains the applicable sample types, testing items, test methods, acceptance criteria, and frequently asked questions related to storage tank gas-tightness verification.
Technical Overview
Storage tank airtightness testing, also referred to as gas-tightness or leak-tightness testing, is a non-destructive examination performed to confirm that a storage tank and all of its welded joints, sealed connections, and closure devices can contain the stored medium without measurable leakage. For tanks holding flammable, toxic, or volatile substances, even small leaks can lead to product loss, fire and explosion hazards, environmental contamination, and regulatory penalties, making airtightness verification a critical acceptance activity for new construction, repair, alteration, and periodic in-service inspection.
The test is typically conducted after completion of all welding, non-destructive testing, and heat treatment (where applicable), and after the structural strength test has been passed. Depending on the tank design pressure, geometry, and service, tightness verification may be carried out using compressed air or nitrogen pressurization, vacuum box examination, hydrostatic filling, or tracer gas detection. Governing specifications include API 650 for welded atmospheric oil storage tanks, API 620 for large welded low-pressure tanks, API 653 for in-service inspection and repair, EN 14015 for site-built vertical cylindrical flat-bottomed tanks, and the Chinese national standards GB 50128 and GB 50341 covering construction, design, and acceptance of vertical cylindrical welded steel tanks, with GB 150 applied to pressure-vessel-type storage equipment.
Accredited third-party laboratories and inspection bodies commonly perform or witness these tests, issuing documented evidence of test pressure, hold duration, leak indication, and acceptance conclusions that form part of the tank's permanent quality file.
Testing Samples
Atmospheric Vertical Welded Steel Tanks
Flat-bottomed, vertical cylindrical welded steel tanks operating at or near atmospheric pressure, widely used for crude oil, refined products, edible oils, water, and aqueous chemicals. Both fixed-cone-roof and fixed-dome-roof configurations are covered, with tightness verification focused on shell, roof, and bottom plate weld seams.
Low-Pressure and Spherical Storage Tanks
Welded tanks designed for low internal pressure, including spheres, spheroids, and cylindrical vessels for liquefied gases such as LPG, LNG service components, and refrigerated storage. These items are tested in accordance with API 620, GB 150, or equivalent pressure equipment codes, where gas-tightness testing at design pressure follows the strength test.
Floating Roof and Internal Floating Roof Tanks
Tanks equipped with external floating roofs or internal floating covers, where the rim seal systems, roof penetrations, and drain assemblies must be verified for vapor tightness to control emissions and product evaporation losses.
Underground and Horizontal Storage Tanks
Buried double-wall tanks, horizontal cylindrical tanks at fuel dispensing facilities, and shop-fabricated tanks requiring tightness confirmation of the primary containment, interstitial space, and manway or fitting connections before backfilling or placement into service.
Tank Components and Accessories
Individual items that determine overall leak-tightness, including manholes and manways, nozzles and reinforcement pads, flanged joints and gaskets, bolted covers, emergency vents, blanketing connections, water draw-off sumps, and roof seal assemblies.
Testing Items
Depending on the tank type and applicable code, the airtightness examination program typically includes the following items:
- Shell-to-bottom and bottom plate weld seam tightness
- Shell course circumferential and longitudinal weld seams
- Roof plate weld seams and shell-to-roof attachment joints
- Nozzle, manway, and reinforcement pad welds, including tell-tale hole pressurization checks
- Flanged bolted joints, gasketed covers, and mechanical seals
- Floating roof rim seals, roof drains, and penetrations
- Vent devices, pressure and vacuum relief fittings, and inert gas blanketing connections
- Overall tank pressure decay rate under sustained pressurization
- Interstitial monitoring spaces of double-wall tank systems
Testing Methods
Pneumatic Tightness Test
The tank is pressurized with clean, dry compressed air or nitrogen to the specified test pressure, which for atmospheric and low-pressure tanks is commonly in the range of 1.05 to 1.15 times the design pressure as directed by the applicable code, while pressure-vessel-type equipment is generally tightness tested at design pressure after passing the strength test. Pressurization is performed in gradual stages with calibrated gauges mounted at the highest point and at grade level. Once the pressure stabilizes, a soap or surfactant solution is applied to all weld seams and joints under inspection, and the surface is observed for bubble formation, which indicates a leak path.
Vacuum Box Testing
Used primarily for tank bottom plate welds and other welds that cannot be conveniently pressurized from the reverse side. A soap film is applied to the weld, a transparent vacuum box is placed over it, and a specified partial vacuum, typically not less than 53 kPa (approximately 400 mmHg) in accordance with GB 50128 or as required by API 650, is drawn. Any through-wall defect draws air through the soap film and produces visible bubbles inside the box.
Hydrostatic Water Fill Test
The tank is filled with water in controlled increments to the design liquid level and held, typically for a minimum of 24 hours, to verify bottom and shell tightness while simultaneously monitoring foundation settlement. Weld seams are examined visually for seepage, sweating, or drips. This method also serves as the strength test for atmospheric tanks and validates floating roof travel and seal performance.
Tracer Gas Leak Detection
For high-sensitivity requirements, the tank or component is charged with a tracer gas such as helium, ammonia, or a halogenated compound, and leaks are located using a mass spectrometer sniffer probe, indicating paper or solution, or an electronic detector. Helium detection performed in accordance with ASTM E499 achieves sensitivity down to the 10 to the minus 9 mbar-liter-per-second range, making it suitable for stringent services where even minute leakage is unacceptable.
Pressure Decay Monitoring
The tank is brought to the specified test pressure, isolated from the pressure source, and monitored over a defined hold period using calibrated instruments. Recorded pressure changes are corrected for ambient and media temperature variation using the absolute pressure-temperature relationship, and the corrected decay rate is compared against the allowable limit defined by the engineering specification or code.
All pressurized testing must follow strict safety controls, including staged pressurization, verified relief devices, exclusion of personnel from the danger zone during pressurization, and prohibition of any weld repair or striking of the equipment while under pressure.
Applications
Storage tank airtightness testing is applied across the full life cycle of containment equipment, from fabrication acceptance through decades of operation. Typical application scenarios include:
- Pre-commissioning acceptance of new welded storage tanks at refineries, tank farms, and distribution terminals
- Tightness verification of low-pressure spheres and vessels for LPG, ammonia, and other liquefied gas service
- Emission control and evaporation loss management for floating roof tanks storing volatile organic liquids
- Regulatory compliance testing of underground and aboveground fuel storage tanks at service stations and industrial sites
- Post-repair and post-alteration verification following welding, seal replacement, or component changeout
- Periodic in-service integrity inspection in accordance with API 653 or equivalent national requirements
- Storage of water, wastewater, food-grade liquids, and chemicals where contamination ingress must be prevented
FAQ
What is the difference between a strength test and an airtightness test?
A strength test, usually hydraulic, demonstrates that the tank structure can safely withstand a pressure above design level without permanent deformation or rupture. An airtightness test is performed at or near design pressure to detect leakage paths through welds, gaskets, and seals. For pressure equipment, the tightness test is normally conducted only after the strength test has been passed.
What test medium and pressure are used?
Clean, dry compressed air or nitrogen is the standard medium; nitrogen is preferred when the residual contents or the tank material are sensitive to oxygen or moisture. Test pressure is specified by the applicable code and design documents, commonly equal to design pressure for vessel-type equipment and up to about 1.15 times design pressure for pneumatic strength and tightness combinations. Pressurization above atmospheric tank ratings must never exceed the value authorized by the design specification.
How are leaks located during the test?
Localized leaks are located by applying a soap solution to joints under pressure and observing for bubble growth, or by vacuum box examination where a partial vacuum is drawn over soap-filmed welds. For very small or inaccessible leaks, tracer gas methods such as helium sniffer detection provide quantitative leak-rate measurement with far higher sensitivity.
How long must the test pressure be held?
Localized soap-film or vacuum box examinations typically require a stable observation window of at least 10 to 15 minutes per area. Overall pressure decay tests are held for periods defined by the code or specification, often several hours and up to 24 hours for full-tank verification, with continuous or periodic recording of pressure and temperature.
What are the acceptance criteria?
The standard acceptance requirement is the absence of visible bubbles, seepage, or tracer gas response at any examined weld or joint. For decay testing, the temperature-corrected pressure drop over the hold period must not exceed the allowable percentage stated in the specification. Gauges and detectors must be within calibration, and all observed defects must be repaired and the affected areas retested until acceptance is achieved.
How often should in-service tanks be retested?
Reinspection intervals depend on service severity, corrosion rates, and jurisdictional rules; risk-based inspection programs under API 653 and comparable national standards are used to establish the schedule. Tightness verification is always required after any repair involving welding on the pressure boundary, replacement of seals or gaskets on critical nozzles, or change of stored medium.