Why bubbles, sound and pressure decay cannot be converted directly into API 598 or ISO 5208 leakage classes

Engineering guide

Browse the BTR VSI knowledge hub for related valve maintenance guidance.

Start with our passing valves guide for the fault context.

The standards answer different questions

API 598 provides valve inspection and pressure-test acceptance criteria. ISO 5208 provides leakage rate classes under its specified valve test framework.

These are not two names for one conversion table. A result described against one framework should not be relabelled as a result from the other.

API 6D addresses pipeline valve manufacture and acceptance at a broader level. It also does not turn an informal field observation into a leakage class.

Why a bubble count is not a direct class

A bubble observation depends on the collection method, test fluid and applied pressure. Tube size, immersion arrangement and observation time can change what the operator sees.

Bubble size is not uniform. A count alone does not provide a traceable leak volume unless the approved method defines that relationship.

Report the actual arrangement, pressure direction, duration and observed count. Do not call the result an API 598 acceptance outcome or ISO 5208 class without a test that meets that framework.

Our field seat-leak testing guide compares bubble, bleed and pressure methods.

Why sound cannot define a leakage class

Audible hiss or acoustic instrument response can locate and rank possible leakage. It is screening evidence, not a direct leakage rate class.

Sound changes with differential pressure, fluid properties, valve geometry, background noise and sensor position. Two valves with equal flow may produce different signals.

State the instrument, sensor location, operating condition and comparison basis. Use sound to guide confirmation, not to assign a standards label.

Why pressure decay needs its own calculation

Pressure decay can support a defined field test when the isolated volume is known. Temperature, trapped fluid, connected branches and instrument accuracy can still affect the result.

A falling pressure shows system behaviour during that test. It does not by itself identify which seat leaked or convert into a standard class.

A stable pressure also has limits. The test may lack enough duration, differential pressure or instrument resolution to show a small leak.

Record start and end pressure, temperature, volume basis, duration, valve position and all boundary conditions. Then compare the result only with the approved field acceptance basis.

Use honest field language

1. Observation. State exactly what was seen, heard or measured.
2. Conditions. Record medium, pressure direction, temperature, duration and valve position.
3. Method. Name the equipment, collection arrangement and instrument resolution.
4. Interpretation. Write that the result indicates or suggests leakage where appropriate.
5. Confirmation. Name the approved test needed for acceptance or classification.

A useful report might say that downstream pressure rose during a defined isolation check. It should not say the valve is “ISO class” based on that observation.

See how to read a valve survey report and why full travel does not prove tight shut-off.

Choose the next test from the decision

Testing should answer an operating decision, such as isolation readiness, maintenance priority or return to service. The required confidence changes with that decision.

Where formal acceptance against API 598 or a leakage rate class under ISO 5208 is required, use the applicable controlled test and documentation. Do not reconstruct that claim from incomplete field notes.

For installed valves, an approved field method may be the correct endpoint. Our field survey service records conditions and evidence without upgrading screening signs into standards claims.