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ISO 5208 and EN 12266-1 leakage rates in plain English — and how API 598 differs

What “Rate A”, “Rate B” or “Rate D” means on a valve order, why liquid and gas results are not interchangeable, and the one line your PO needs so nobody argues at the test bench.

Standards Comparisons2026-11-148 min read

ISO 5208EN 12266-1API 598leakage rate

Short answer: ISO 5208 and EN 12266-1 set seat tightness with lettered leakage rates: Rate A means no visually detectable leakage, and later letters permit small leakage that scales with valve size. API 598 instead sets allowable leakage by valve size and seat type. Always write the standard, edition and leakage rate into your PO.

By Kevin Lyu (吕中国), ValveWitness Inspection, Ningbo · Published 2026-11-14

This guide explains how the rates work. It does not reproduce the numeric leakage tables, which are copyrighted. Read exact leakage values, test pressures and durations from the licensed standard edition named in your PO.

What the lettered rates mean

ISO 5208:2015 defines ten lettered rates — A, AA, B, C, CC, D, E, EE, F and G. EN 12266-1 uses a family of lettered rates in its Annex A in the same way. The letter does not change the test; it changes how much leakage is accepted.

Rate familyPlain-English meaning
Rate ANo visually detectable leakage for the test duration. Typical expectation for soft-seated (resilient) valves.
Rates AA, B, C, CC, DSmall permitted leakage that scales with valve size (DN), stated per unit of time. Each step allows a little more than the one before.
Rates E, EE, F, GProgressively larger permitted leakage, used where the product standard or service allows it, for example some metal-seated designs.

Qualitative summary only. The permitted leakage for each rate, valve size and test fluid must be read from the licensed edition.

How API 598 does it differently

API 598 does not use letters. It gives a maximum allowable seat leakage that depends on valve size and on whether the seat is resilient or metal, with separate allowances for liquid and gas closure tests. So “API 598 seat test passed” and “ISO 5208 Rate B passed” are different statements — compare them only like with like.

ISO 5208 / EN 12266-1API 598
How seat leakage is setA lettered leakage rate chosen by the product standard or the orderAn allowance in the standard that depends on valve size and seat type (resilient or metal)
Soft-seated valvesUsually Rate ANo visible leakage is typically required
Metal-seated valvesA lettered rate that permits small leakage, per the product standard or POA small permitted leakage by size, stated separately for liquid and gas tests
What the PO must sayStandard, edition and the letter of the rateStandard, edition, seat type and any optional tests

Why “Rate A” is not the same on a water test and an air test

  • Different detection. On a liquid test you look for drops at the outlet; on a gas test you look for bubbles under water or in leak-detection fluid. Each method sees different things.
  • Different leak paths. Gas passes through paths that water does not, so a valve can be tight on water and leak on air.
  • Different closing force. High liquid pressure pushes the closure member onto the seat; low gas pressure barely helps. See our check valve case from the test bench.
  • The rate is tied to the test. A result is only meaningful together with the fluid, pressure and duration stated.

Soft-seated vs metal-seated expectations

  • Soft (resilient) seats — PTFE, RPTFE, elastomer: buyers usually expect no visible leakage (Rate A), and standards generally require it.
  • Metal seats — hard-faced gate, globe and some ball valves: small permitted leakage is normal and the rate must be stated. A metal-seated valve ordered as “Rate A” is a specific requirement the factory must agree to before production.
  • Check valves — the seat test direction and fluid matter as much as the rate.

How to write the leakage rate into a PO

Example wording (adapt rates to your product standard):

  • “Seat test to ISO 5208:2015, leakage rate A, liquid and gas, for all soft-seated ball valves.”
  • “Seat test to EN 12266-1:2012, leakage rate per the product standard; supplier to state the rate on the quotation and the test record.”
  • “API 598:2023, high-pressure closure test and low-pressure air closure test required; witness on a sample per ITP.”

One line that avoids most arguments

  • Standard + edition + leakage rate + test fluid(s), per valve type.
  • Who witnesses, how many valves, and that testing is a hold point.
  • That the test record states the observed leakage, not just PASS.

Compare the standards themselves in API 598 vs EN 12266-1 vs ISO 5208.

Questions buyers ask

What does Leakage Rate A mean?

No visually detectable leakage for the duration of the test under the stated test conditions — no drops for a liquid test, no bubbles for a gas test. It is a visual criterion, not a measured zero.

Is ISO 5208 Rate A the same as API 598 zero leakage?

Not automatically. Test conditions, durations and the way leakage is observed differ between the documents. Treat them as different requirements unless your PO links them in writing.

Which leakage rate should I specify?

Use the rate your valve product standard sets, or your project specification. If neither sets one, agree a rate with the supplier per valve type and seat design before the PO — and write it in.

Sources

We report facts from the samples inspected. Standards named in these guides are public standards we work to, not certifications or accreditations we hold. We do not issue product certificates or accreditation marks. Test pressures, durations, leakage rates and acceptance numbers must be read from the licensed edition named in your purchase order; we do not reproduce copyrighted tables from the standards.

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