ISO 5208 Rate A: What a Valve Seat-Leakage Class Does and Does Not Promise

A seat-leakage class is a number multiplied by the valve size, measured with one test fluid, discharged to atmosphere. ISO 5208:2015 Rate D allows 0.1 mm³/s per DN of liquid leakage through a closed valve; at DN100 that is 10 mm³/s, and the gas figure for the same letter is 300 times larger.

That structure is why "Rate A" and "zero leakage" are not the same sentence, and why a rate quoted without the test fluid tells a buyer nothing.

This article sets out what the rate table measures, why the same letter carries two numbers, where the Chinese grading sits against the international one, and what a leakage rate does not say.

Read it and you can ask a supplier for a seat-leakage statement that can be checked against a document.

What does a leakage rate actually measure?

It measures leakage through the closed valve — past the seat — while the test fluid is discharged to atmosphere, at the closure test pressure the standard fixes.

ISO 5208:2015 states its own scope as the examinations and tests a manufacturer performs to establish the integrity of the pressure boundary of an industrial metallic valve and to verify the degree of closure tightness and the structural adequacy of the closure mechanism. It is written to be applied together with the product standard for the valve, and where the product standard differs, the product standard applies. The standard also states it is not applicable to the safety aspects of pressure testing.

Two consequences follow, and both are routinely missed.

The shell test and the closure test are different tests. The shell test asks whether the pressure boundary holds. The closure test asks how much passes the seat. A valve that passes the first can fail the second.

The applicable rate is not chosen by the buyer's habit. The standard's own note sets the order: the rate is the one identified in the valve product standard, or a more stringent one identified in the purchase order. Where neither says anything, there is nothing to enforce.

Why does the same letter carry two numbers?

Because the table gives a liquid column and a gas column, and gas is allowed far more.

ISO 5208:2015, Table 4 — maximum allowable closure test leakage rate, as reproduced across independent public reproductions of the table:

Rate D, the figure most often quoted as the general-purpose metal-seated benchmark, differs by a factor of 300 between the two columns. Rate C differs by a factor of 100. A specification that names "Rate D" without naming the test fluid has specified two different valves at once.

Why does the allowance grow with the valve?

Because every rate except Rate A is multiplied by DN.

At DN100, Rate D permits 10 mm³/s of liquid and 3 000 mm³/s of gas. At DN600, the same letter permits six times as much. The standard is not saying a large valve is worse; it is saying that a fixed absolute figure would be unachievable at large sizes, so the allowance scales with the sealing line length.

That scaling has a practical consequence for buyers comparing quotations: two suppliers can both be compliant with Rate D and offer valves whose actual leakage differs by an order of magnitude, because the size differs. Compare the rate and the DN together, or the number means nothing.

Who has put what on the record?

GB/T 13927 carries the Chinese closure-tightness grading, GB/T 26480 the inspection-and-testing route, and ISO 5208:2015 the table and its correspondences. The Chinese national grading runs alongside the international one, and the two are not always the same vintage.

The practical point for a buyer of Chinese valves: a works certificate written to GB/T 13927 and one written to ISO 5208 are working from the same family of grades, but the edition matters. GB/T 13927-2008 was aligned to the 2008 vintage of the international text; it has since been replaced. Ask which edition the certificate cites.

What does a rate not promise?

It promises no zero, nothing about the stem or the body joints, nothing about the valve in the crate, and nothing about the fluid the valve will actually see.

Rate A is an observation, not a zero. It is defined as no visually detectable leakage for the duration of the test. It depends on how the test is observed and for how long. It is not a measured quantity that can be carried into another standard.

It is not a fugitive emissions result. Seat leakage passes through the closed valve. Fugitive emissions are leakage to atmosphere past the stem and body joints. They are different leak paths, different standards, and different class letters — see this site's ISO 15848 article.

It is not a production release. The rate is measured on a test. Whether the valve in the crate matches the tested configuration is a production acceptance question.

It says nothing about the test fluid the valve will actually see. A rate earned on water is not a statement about gas service, which is exactly what the two columns of the table are there to show.

When does this go to a human?

Five boundaries for seat-leakage questions: any duty below −101 °C (−150 °F), where automatic coverage ends unconditionally; any specification that names a leakage rate without naming the test fluid; any requirement for a shut-off class the valve type cannot support at that size and temperature; any duty at DN1200 (48 inches) or above, or above Class 2500 or 420 bar (6 090 psi); and any duty above 425 °C (797 °F), where the body material group moves to chromium-molybdenum steels.

• Any duty below −101 °C (−150 °F), where this site's automatic coverage ends unconditionally
• Any specification that names a leakage rate and does not name the test fluid, the standard and the edition
• Any requirement for a shut-off class that the valve type cannot support at the size and temperature asked for
• Any duty at DN1200 or above, at a class above 2500, or above 420 bar
• Any duty above 425 °C, and any duty above 538 °C (1 000 °F) without exception

What people assume, and what the standard says

Common questions about valve seat leakage

Should I specify Rate A?

Ask for the rate the seat can actually hold. A resilient-seated valve can be asked for Rate A; a metal-seated one is normally asked for Rate D, and asking for Rate A on a metal seat is asking for a figure the construction will not deliver.

My supplier quotes "Class VI". Where does that sit?

That is the control-valve seat leakage classification, not the ISO 5208 rating. The two use different scales and different test methods. Ask which scale the certificate is written to before comparing figures.

Does a gas test always give a worse result?

The gas column allows far more — 300 times more at Rate D — which is why a gas test is the harder test to pass at the same letter. That is also why the test fluid has to be named.

Is a leakage rate the same as a shut-off class?

No. The rate is what the standard permits. The shut-off class is what the process requires. A specification is only complete when it states the second and the acceptance route for the first.

What this article does not cover

Note that this article does not select a seat material, does not determine whether a valve complies with a standard or regulation, does not calculate emissions or leak rates for a duty, and does not provide certification advice. It sets out what the ISO 5208 leakage rates measure, why the same letter carries two numbers, and where the Chinese grading sits.

Where the next constraint sits

A seat-leakage rate is measured at a temperature, and the seat is the part that decides whether the rate survives the duty. Where the seat material, rather than the certificate, sets the limit — and why a metal seat buys temperature at the cost of tightness — is set out in Soft-Seated vs Metal-Seated Valves: Where the Temperature Line Is.

The rate also has to be read against the machine: which of the three isolation types can hold which seat construction, at what size and class. Ball vs Gate vs Globe Valves: The Three-Way Comparison sets the three side by side, and API 607 and ISO 10497: What a Fire-Safe Valve Certificate Covers takes the next step — what happens to that same soft seat when the temperature stops being a process variable and becomes a fire.

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