Air pressure-decay testing of tanks and vessels

LEAKLAB · TESTING AND ENGINEERING

A defined cavity is pressurised with clean, dry air, isolated from the supply and monitored after thermal stabilisation.

Test boundary and preparation

LeakLab develops and performs pressure-decay programmes for tanks and vessels whose documentation permits pneumatic testing with air. The test can cover the body, welds, hatches, seals and connections included within the agreed boundary. Each compartment is tested separately. Adjacent compartments are empty and unpressurised; common manifolds are isolated so cross-flow cannot mask a defect.

The customer supplies the passport, compartment and valve schematic, allowable pressures and repair history. The vessel is emptied, cleaned and degassed. Safe condition, gas analysis and work permission are established before air is introduced; fuel residues and hazardous vapours must be excluded.

Measurement arrangement

The loading train comprises a compressor, receiver, filtration and moisture separation, pressure regulator, restrictor, non-return valve and a reliable isolation valve. A verified high-accuracy MIDA digital gauge communicates directly with the vessel on the object side of isolation. The relief device remains connected to the object, with no intervening isolating valve; a separate valve provides controlled depressurisation. Exhausts are directed to a safe area.

The rig is checked with its outlet blanked before a series. Rig leakage cannot simply be subtracted from the vessel result without justification. Gas and ambient temperatures are recorded; barometric pressure is also needed when using a gauge-pressure instrument.

Proposed measurement sequence

Pressure is increased gradually to the object-specific value, then the supply is isolated. The proposed initial stabilisation pause is at least 15 minutes and is extended until thermal drift is acceptable. It is separate from the measurement hold.

The source programme proposes a 30-minute hold, automatic recording every 10 seconds and operator records every minute. These are proposed programme settings, not universal requirements. Pressure, acceptable drop, duration and recording interval must provide sufficient sensitivity for the actual volume and measurement uncertainty. If pressure is adjusted, stabilisation starts again.

Temperature correction and decision

For constant volume and representative gas temperature: p_abs = p_gauge + p_atm, and Δp_corrected = p_abs,1 − p_abs,2·T₁/T₂. Temperatures are in kelvins and pressures use consistent units. Atmospheric pressure is not added to an absolute-pressure reading. Volume changes and temperature gradients require assessment.

Conformity can be confirmed when corrected pressure drop plus expanded uncertainty remains within the agreed limit. A result whose uncertainty interval crosses the limit remains indeterminate. Cooling after pressurisation, deformation, outgassing and connection leakage can affect the result; stable displayed digits alone do not prove leak tightness.

Abnormal deformation, dangerous leakage, protection operation, loss of pressure control or measurement failure stops loading and requires controlled depressurisation. Connections are not tightened under pressure.

Preparing and verifying the test

1. Review the drawing, materials, internal volumes, test boundary and required result.
2. Agree the document edition, allowable pressure difference, vacuum and temperature, acceptance criterion and required system sensitivity.
3. Clean and dry the object, prepare fixtures and exclude leakage from the test connections.
4. Check instrument metrological status and the working characteristics of the complete arrangement; use an appropriate reference response for the selected method.
5. Perform the agreed scan or measurement cycle and record the actual conditions. Confirm suspected leaks by repeat tracer-gas exposure where applicable.
6. Check system stability after the test and report the result against the agreed criterion.

What the customer receives

The agreed scope can include a test programme, operating or inspection instructions, measured results, a map of located leaks and a technical conclusion. The report identifies the object and tested regions, applicable programme, instruments and metrological status, pressure and temperature, response checks, observations and limitations.

Define your task

Send a drawing or sketch, material, dimensions and volume, working pressure, allowable vacuum and temperature, leak-tightness requirement, number of items, site and schedule. These inputs determine the method, fixture, equipment and reporting scope.