Leak Testing (LT)

Leak Testing (LT) verifies the pressure boundary integrity of vessels, piping, and tanks by detecting and quantifying fluid leakage using methods such as pressure decay, bubble emission, mass spectrometer helium tracing, or ultrasonic leak detection.

Definition

Leak Testing (LT) is the NDT method used to confirm that a closed system holds pressure or vacuum without unacceptable leakage. Techniques range from simple soap-bubble visual inspection to mass spectrometer helium leak detection capable of resolving leaks below 10⁻¹⁰ Pa·m³/s.

Technical Context

Common methods include: pressure-decay (monitor pressure drop over time), bubble emission, helium mass-spectrometer (sniffer or hood), halogen detection, and ultrasonic leak detection. The method chosen depends on required sensitivity, system geometry, and gas compatibility.

When It Is Used

  • Final acceptance testing of pressure vessels and piping
  • Vacuum-system qualification in semiconductor and aerospace
  • In-service tank-bottom leak monitoring

Related Standards

ASME Section V Article 10, ASME B31.3 Chapter VI, ASTM E432, ISO 20485, API 510, API 653.

How it works

A pressure differential is established across a boundary and the passage of a tracer — air, a bubble-forming solution, halogen, helium — is detected on the low-pressure side. Sensitivity spans many orders of magnitude depending on the technique chosen.

What it finds

Through-wall breaches, whatever their cause. It answers the question of containment directly, where every other method answers it by inference from wall condition.

What it will not find

Anything that is not yet through-wall. A crack ninety percent through the thickness leaks nothing and passes a leak test, which is why leak testing complements rather than replaces volumetric and surface examination.

How it is actually done

Technique selection follows the sensitivity required: bubble testing for gross leakage, pressure change for closed systems, halogen or helium mass spectrometry where small rates matter. System cleanliness, temperature stability and hold time govern whether a pressure-change result means anything at all.

Governing codes and standards

ASME Section V Article 10 covers leak testing and its appendices define the individual techniques; ASME Section VIII references it for pressure boundary acceptance.

Where it goes wrong

Reading a pressure drop as a leak without correcting for temperature. A modest ambient swing moves system pressure enough to imitate a leak, or to mask one, on any test whose hold time is short relative to the thermal time constant.

Frequently asked questions

Is a hydrostatic test a leak test?

It demonstrates strength and gross tightness, but it is not a sensitive leak test. Water is a poor tracer at small leak rates and the test proves the boundary held at pressure — a different and coarser question than whether it leaks.

Where Leak Testing fits in an inspection programme

A term is only useful when it connects to a decision. Leak Testing appears in written procedures, in technique sheets, and in the records an owner or accreditation body reviews afterwards — which means the way it is defined in your documentation has to match the way it is applied on site. Where the two drift apart, audits find it. Atlantis writes and reviews procedures against the governing codes, trains inspection personnel to apply them, and builds the record-keeping that makes the evidence retrievable years later. Procedure development and code consulting · NDT training and certification · Ask us about your programme.

Related terms

  • Acoustic Emission Testing (AE) — Acoustic Emission (AE) is a passive NDT method that detects transient elastic stress waves emitted by active discontinuities such as growing cracks, plastic deformation, or leaks, using sensors mounted on the component during loading.
  • Pressure Vessel — A pressure vessel is a closed container designed to hold gases or liquids at a pressure substantially different from ambient, typically constructed per ASME Section VIII and inspected in service per API 510.
  • Storage Tank — A storage tank is a low-pressure, large-volume welded container for the storage of liquids — typically atmospheric (API 650), low pressure (API 620), or for water (AWWA D100) — inspected in service per API 653.

More method terms

Ultrasonic Testing · Radiographic Testing · Magnetic Particle Testing · Penetrant Testing · Eddy Current Testing · Visual Testing · Thermography / Infrared Testing · Microwave Testing · Phased Array Ultrasonic Testing · Time-of-Flight Diffraction

Where this comes up in practice

Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.

Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.

Leak testing techniques separate by sensitivity, not by preference. Bubble and vacuum-box methods resolve gross leakage; pressure change suits closed systems of known volume; halogen and helium mass spectrometry reach the fine end. ASME Section V, Article 10 and its mandatory appendices define each technique, and the required sensitivity is set by the referencing code, not by the technician.

Selection runs backwards from the leak rate that matters to the owner. A US pressure vessel built to ASME Section VIII, Division 1 gets a hydrostatic or pneumatic pressure test as a matter of construction; a sensitive leak test is an addition, invoked when the user's design specification, lethal-service rules or the purchase order names one. Refinery and petrochemical operators carry a second driver: EPA Method 21 fugitive-emission monitoring under 40 CFR Part 60 sets its own detection threshold on flanges, valves and pump seals, and that requirement is regulatory rather than code-driven. Three variables decide whether any result means anything — hold time relative to the system's thermal time constant, temperature correction, and cleanliness of the surfaces a bubble or tracer must reach. A pressure-change test read without temperature compensation reports weather. Every technique needs its sensitivity demonstrated on a calibrated leak before the test, not asserted from the equipment specification.

Source: ASME BPVC Section V, Article 10 — Leak Testing (2025 Edition), with its mandatory appendices covering bubble, pressure change, halogen diode, helium mass spectrometer and ultrasonic techniques; US EPA Method 21, 40 CFR Part 60

Technically reviewed by Anoop Rayavarapu — ASNT NDT Level III (UT, RT, MT, PT, VT, ET) · API 653 · ISO 9001:2015 Lead Auditor
Leak test techniques ranked by the leak rate each can resolve
TechniqueDetection mechanismApproximate sensitivity (Pa·m³/s)What the test needsWhere it is used
Bubble — direct pressureSolution applied to the surface forms bubbles at the leak site10⁻⁴Access to the pressurised side, clean dry surface, adequate lightWeld and flange checks on fabricated assemblies
Bubble — vacuum boxBox evacuated over the weld, soap film draws bubbles inward10⁻⁴Flat accessible weld, sound box seal, solution that survives the dwellTank floor lap welds and roof seams
Pressure change (decay or rise)Pressure drop across a timed hold, corrected for temperature10⁻³ and coarser, degrading as system volume risesIsolatable volume, thermal equilibrium, calibrated gauge, long holdClosed piping systems and vessels
Halogen diode probeHeated element responds to a halogenated tracer gas10⁻⁶Halogen tracer charge, ventilated area, probe calibration checkRefrigeration and process systems
Helium mass spectrometer — detector probeProbe traverses the outside of a helium-charged system10⁻⁸Helium charge, calibrated leak standard, controlled traverse speedLocating individual leaks on assembled equipment
Helium mass spectrometer — tracer probeHelium sprayed on the outside of an evacuated system10⁻⁹System under vacuum, controlled spray rate, spectrometer response timeVacuum systems and vessels under evacuation
Helium mass spectrometer — hoodWhole item enclosed in a helium atmosphere10⁻¹⁰ and finerEnclosure, evacuated item, helium fill, background controlTotal leak rate acceptance on sealed components
Ultrasonic leak detectorTurbulent gas escaping through the breach emits airborne ultrasoundGross leakage only — no quantified rateLine of sight to the leak, low ambient ultrasonic noiseIn-service surveys of valves, steam traps and compressed air systems
Sensitivities are order-of-magnitude capability, not acceptance criteria; 1 std cm³/s ≈ 0.1 Pa·m³/s. Every technique must have its sensitivity demonstrated on a calibrated leak standard before the test, and the required leak rate comes from the referencing code or the user's design specification.

Does ASME Section VIII require a leak test before a vessel enters service?

Section VIII, Division 1 requires a hydrostatic or pneumatic pressure test at completion — a strength and gross-tightness demonstration. A sensitive leak test to Section V, Article 10 is separate and applies when the user's design specification calls for it, when lethal-service rules govern, or when the purchase order names a technique and a leak rate. Absent one of those, no leak test is mandated.

How do I choose between helium mass spectrometry and pressure decay?

System volume decides it. Pressure decay measures the whole boundary at once and its resolution degrades as volume rises, so it suits small isolatable systems held long enough for pressure to settle. Helium mass spectrometry locates the leak and reaches roughly six orders of magnitude finer, at the cost of tracer gas, evacuation or hooding, and a much longer setup.

Why does a pressure-decay test fail on a hot day?

Gas pressure tracks absolute temperature. A few degrees of ambient swing across a test volume moves indicated pressure enough to imitate a leak, or to cancel a real one. The fix is measuring gas temperature at multiple points, correcting the reading to a reference temperature, and holding long enough that the system reaches thermal equilibrium before the timed period starts.

Can leak testing substitute for ultrasonic wall-thickness inspection?

No. Leak testing answers containment and detects only a breach that already passes through the wall. Corrosion that has consumed 80 percent of the thickness leaks nothing and passes. The two are sequenced: volumetric and surface examination find degradation while there is remaining life to manage, and leak testing confirms the boundary is intact once repairs are complete.

What qualification does a leak-testing technician need in the United States?

For code work, LT Level II certified by the employer against its written practice, which is written to ASNT SNT-TC-1A. The written practice sets training hours, experience and examination content technique by technique, because bubble testing and helium mass spectrometry are separate skills. Contracts that reference ISO 9712 instead require third-party certification, which an employer certificate does not satisfy.

Which leak test applies to an in-service storage tank bottom?

Vacuum box testing over the bottom lap and butt welds is the established technique: a transparent box with a soap film is placed over the weld and evacuated, and bubbles mark through-welds. API 653 inspection also uses tracer-gas monitoring beneath the floor and product detection in the containment, because a tank bottom leaks downward where no observer stands.