Air tightness testing (gas leak testing) of low-temperature distillation columns is a critical quality control step that verifies the integrity of welded joints, flanges, gaskets, and seals before cryogenic service in air separation, LNG, and olefin plants. This article explains the purpose, applicable standards, testing samples, testing items, common methods such as pressure decay, bubble testing, and helium mass spectrometry, along with typical acceptance criteria and practical procedures for engineers and plant operators.
Technical Overview
A low-temperature distillation column is a cryogenic separation vessel that operates at temperatures ranging from approximately -40°C down to -196°C, depending on the process. Typical services include the upper and lower columns of air separation units, demethanizers, deethanizers, ethylene fractionators, and natural gas liquefaction columns. At cryogenic temperatures, differential thermal contraction between the shell, internals, piping, bolting, and gaskets can open leak paths that were tight at ambient temperature. Even a small leak can cause loss of valuable product, ice formation, energy waste, oxygen enrichment hazards, or the release of flammable gases.
An air tightness test (also called a gas tightness test or leak test) is therefore performed to confirm that the column and its associated joints can hold the specified pressure without unacceptable leakage. The test is normally carried out after the strength (hydrostatic or pneumatic) test has been passed and before the system is dried, purged, insulated with perlite or placed under vacuum jacketing, and cooled down. Key reference standards include ASME Boiler and Pressure Vessel Code Section V Article 10 for leak testing methods, EN 1779 for the selection of leak testing methods and techniques, EN 1593 and ASTM E515 for bubble emission testing, EN 13185 and ASTM E1603 for helium leak testing, ASTM E432 for the selection of leak test methods, ISO 20486 for the calibration of helium leak detectors, and GB 50236 and GB 50184 for the construction and acceptance of industrial metallic piping systems.
Because moisture in the test gas would freeze and block instrumentation or damage seals during subsequent cooldown, the test medium must be dry, oil-free nitrogen or dry compressed air with a sufficiently low dew point. Safety relief must be provided at all times, and pressurization must be controlled to avoid overpressure of components that are isolated during the test.
Testing Samples
Cryogenic Distillation Columns
The primary test object is the column itself, including the upper and lower columns of air separation units, crude argon columns, and cold-section fractionators in ethylene and gas processing plants. Both new fabrication acceptance and post-maintenance revalidation are covered.
Column Internals and Attached Heat Transfer Equipment
Condenser-reboilers, subcoolers, integrated plate-fin heat exchangers, and trays or packing supports are tested as part of the column system or individually when specified, since internal cross-leakage between process streams is not acceptable in cryogenic service.
Cold Box Piping and Interconnecting Lines
All cryogenic piping, manifolds, and process lines running to and from the column within the cold box or insulation enclosure are included in the test boundary, together with expansion joints and instrument connections.
Joints, Seals, and Accessories
Shell and nozzle welds, flanged connections, manway covers, threaded bosses, gasketed joints, valve seats and packing, safety device seats, and instrument tap points are examined individually for local leakage.
Vacuum-Insulated Assemblies
For columns or transfer lines with vacuum jackets, the annular space leak rate and the inner vessel tightness are verified separately using helium techniques or vacuum hold testing.
Testing Items
The scope of an air tightness test program for a low-temperature distillation column typically includes the following items:
- Overall system leak rate determined by a temperature-compensated pressure decay test.
- Local leak detection at shell weld seams, nozzle welds, and repair welds.
- Tightness of flanged joints, manways, and gasketed closures.
- Valve seat tightness, gland or packing leakage, and safety device seat tightness.
- Instrument connection and thermowell boss tightness.
- Internal cross-tightness between shell sides or passages in condenser-reboilers and plate-fin exchangers.
- Integrity of vacuum jacket and annular insulation spaces where applicable.
- Pressure hold performance over a specified duration to verify system-wide retention capability.
- Documentation of test pressure, medium, hold time, ambient conditions, and observed leak rate for the acceptance report.
Testing Methods
Pressure Decay Method
The system is pressurized with dry nitrogen or dry air to the specified test pressure, usually equal to the design pressure or 1.05 to 1.15 times the design pressure as defined by the project specification or applicable code. After a temperature stabilization period, the system is isolated and gauge or absolute pressure is recorded at fixed intervals over the hold time. Because measured pressure changes with ambient and gas temperature, the recorded values must be corrected using the absolute pressure and absolute temperature relationship before calculating the leak rate. The test is accepted when the corrected pressure drop is below the allowable limit.
Bubble and Soap Solution Testing
While the system is pressurized, a leak detection solution is applied to accessible joints and surfaces. The formation of continuous bubbles indicates a leak path. This direct pressure technique, covered by EN 1593 and ASTM E515, is simple and effective for flanges, welds, and threaded connections. For joints accessible from only one side, a vacuum box technique may be used. The surface must be clean and dry so that small bubbles are not masked, and sufficient observation time must be allowed for slow leaks.
Helium Mass Spectrometry Leak Detection
For stringent leak rate requirements, such as hydrogen or high-purity gas service, helium testing is used. The system is charged with helium or a helium-nitrogen mixture and a sniffer probe is moved systematically over welds and joints to detect escaping helium, achieving sensitivities in the range of 10⁻⁷ mbar·L/s or better. For individual components, vacuum chamber testing provides even higher sensitivity. Equipment calibration follows ISO 20486 and relevant ASTM practices. In cryogenic projects, a helium test is often specified before cooldown to establish a leak rate baseline, since leaks may grow at operating temperature due to thermal contraction.
Typical Test Procedure
- Confirm that strength testing, cleaning, and drying are complete and that all instruments are calibrated.
- Define the test boundary, isolate or blind components not rated for the test pressure, and provide relief protection.
- Introduce dry nitrogen or dry air in staged increments, holding at intermediate pressures to check for abnormalities.
- Reach the specified test pressure and allow the gas temperature to stabilize, typically for at least one hour.
- Isolate the system and record pressure, ambient temperature, and gas temperature at regular intervals throughout the hold period.
- Perform local detection using soap solution or a helium sniffer probe at all joints within the test boundary.
- Calculate the temperature-corrected leak rate and compare it with the acceptance criterion.
- Depressurize in a controlled manner, restore the system, and issue a signed test report with all recorded data.
Typical Acceptance Criteria
- Corrected pressure drop within the hold period shall not exceed the limit stated in the design document, commonly on the order of one percent of the test pressure or a value agreed in the test plan.
- No continuous bubble formation is permitted at any joint during bubble testing.
- For helium sniffer testing, the local leak rate shall not exceed the specified threshold, typically 10⁻⁶ mbar·L/s per joint or stricter for high-purity and hydrogen service.
- No visible leakage, audible leakage, or icing-indicating leakage is permitted during subsequent cooldown verification.
Applications
Air tightness testing of low-temperature distillation columns is applied across many cryogenic process industries and project phases:
- Pre-commissioning of air separation units producing oxygen, nitrogen, and argon, including upper columns, lower columns, and crude argon columns.
- Construction acceptance of ethylene plant cold boxes containing demethanizers, deethanizers, depropanizers, and ethylene fractionators.
- Natural gas liquefaction trains and LNG receiving terminals, where boil-off gas control depends on tight cryogenic equipment.
- Hydrogen and helium liquefaction systems requiring very low leak rates due to small molecule size and high flammability.
- Nitrogen generators and cryogenic gas purification units in chemical, electronics, and pharmaceutical facilities.
- Turnaround and post-repair revalidation, including verification after gasket replacement, weld repairs, or internal modifications.
- Periodic integrity assessments and regulatory re-inspection of aging cryogenic columns performed by accredited third-party laboratories or plant inspection teams.
FAQ
Why is air tightness testing so important for low-temperature distillation columns?
At cryogenic operating temperatures, materials contract at different rates, which can open leak paths in welds, flanges, and gaskets that appeared tight at ambient temperature. Detecting and eliminating leaks before cooldown prevents loss of product, icing damage, energy inefficiency, and serious safety hazards such as oxygen enrichment or flammable gas release.
What gas should be used for the test?
Dry, oil-free nitrogen is preferred because it is inert and eliminates combustion risks. Dry compressed air may be used when specified, but the dew point must be low enough, typically below -40°C, to prevent moisture freezing in valves, instrument lines, and seals during subsequent cooldown. If hydrocarbon residues may be present, inert gas is mandatory.
At what pressure is the air tightness test performed?
The test pressure is defined by the project specification or applicable code and is typically equal to the design pressure or 1.05 to 1.15 times the design pressure. It must never exceed the maximum allowable pressure of the weakest component within the isolated test boundary, and calibrated pressure gauges with appropriate ranges are required.
How is temperature variation handled during a pressure decay test?
Pressure readings must be corrected using the relationship between absolute pressure and absolute temperature. Ambient temperature, gas temperature, and barometric pressure are recorded throughout the hold period so that true mass loss from leakage can be distinguished from pressure changes caused by thermal effects.
How does a leak test differ from a hydrostatic strength test?
A hydrostatic or pneumatic strength test verifies the structural integrity of the pressure boundary against yielding or rupture at an elevated test pressure. An air tightness test, by contrast, verifies freedom from small leaks at or near operating pressure. The strength test is performed first, and the leak test follows, often using gas instead of water to avoid wetting lines that must remain dry for cryogenic service.
Can a leak that only appears at cryogenic temperature be detected in advance?
Ambient-temperature gas testing detects the majority of leak paths, but it cannot fully reproduce thermal contraction effects. This is why high-specification projects add helium leak detection at ambient temperature, and why joints are rechecked during initial cooldown when icing or condensation plumes reveal cold-gas leaks that require corrective action before full operation.
How long does the test take?
For a complete column system, including boundary isolation, staged pressurization, stabilization, hold, and local detection, one to several days may be required depending on system volume and the number of joints. The pressure hold period itself commonly lasts from one to twenty-four hours as defined by the test plan.
Who can perform and certify the test?
Testing may be executed by the construction contractor, the plant operator, or accredited third-party laboratories under an agreed inspection and test plan. Regardless of the executing party, calibrated instruments, qualified personnel, and full documentation in accordance with the referenced standards and the applicable pressure equipment regulations are mandatory for acceptance.