Why do seals fail when the compatibility chart says compatible?

This answer covers elastomer seals such as O-rings, gaskets and lip seals in contact with process chemicals, where a published chart rated the fluid and polymer pair compatible. Mechanical causes appear only so you can rule them out. It does not replace qualification testing, such as ISO 23936-2:2011 for elastomers in oil and gas production equipment.

What does a seal failure despite a compatible chart rating look like?

A seal failure despite a compatible chart rating looks like chemical attack: the seal swells, shrinks, hardens, softens, cracks, blisters or leaks, although the chart showed the fluid and polymer pair as satisfactory. All Seals describes chemical degradation as blisters, cracks, voids or discoloration, and adds that "In some cases, the degradation is observable only by measurement of physical properties" (All Seals, n.d.). Apple Rubber describes excessive swell as a marked increase in seal dimensions that reduces physical properties and can leave the seal mis-sized for its gland, with dynamic seals especially prone because friction accelerates failure (Apple Rubber, n.d.).

Shrinkage is the less obvious sign. The Parker O-Ring Handbook calls shrinkage far more critical than swell and states that more than 3 or 4 % shrinkage can be serious for dynamic seals (Parker Hannifin, 2018). A seal that starts leaking after a shutdown, when the fluid drains and the seal dries out, may be showing extracted plasticizer rather than wear.

The pattern that points to a chart mismatch is timing and place. The failure appears after a change in process temperature, product concentration, cleaning chemical or seal supplier, or it appears only on the hot side of the same line. Write down the date the failure started and every change made before it; the diagnostic table below works from that record.

What conditions does a seal compatibility chart rating assume?

A seal compatibility chart rating assumes one pure fluid, one specific compound, one test temperature and one immersion time, usually judged by volume swell and property change after immersion. Marco Rubber states that most charts provide basic o-ring chemical resistance information "based on isolated material testing in optimal conditions at room temperatures and pressure" (Marco Rubber, 2025). The same page says ratings are usually based on swell percentage, with green ratings usually below 10 or 15 % volume swell and yellow ratings at 10 to 25 % swell that may suit a static application (Marco Rubber, 2025).

The test methods behind such data are ASTM D471-16a(2021) and ISO 1817:2026, which measure rubber properties before and after immersion in test liquids (ISO, 2026). ASTM D471 states that the method "may not give any direct correlation with actual part performance, since service conditions vary too widely" and that it yields comparative data (ASTM D471-16a(2021), section 4.3).

The Parker O-Ring Handbook rates each pair from 1, satisfactory, to 4, unsatisfactory, with 2 "usually OK for static seal", and names a recommended compound for each fluid (Parker Hannifin, 2018). Parker adds that a static seal rated 2 should in all cases be tested, and that the presence of a particular fluid may modify the published temperature limits of a compound (Parker Hannifin, 2018). A chart rating therefore describes a tested compound under tested conditions, not every seal made from that polymer family.

How do concentration, temperature and contact time change a seal compatibility rating?

Concentration, temperature and contact time change a seal compatibility rating because each one can move service outside the condition the rating was measured at. In rough order, the most likely causes when a chart-rated seal fails are service temperature above the chart condition, a concentration or mixture the chart did not list, contact time longer than the immersion test, a different compound in the same polymer family, and a mechanical cause that looks like chemical attack. The order reflects common practice, not a published survey.

Temperature above the chart condition

The Parker fluid compatibility table lists separate rows for "Sulfuric Acid, Concentrated Room Temp" and "Sulfuric Acid, Concentrated to 158°F" (70 °C), so a room-temperature rating does not cover the hot row (Parker Hannifin, 2018).

Concentration or mixture

The same Parker table lists "Nitric Acid (0 - 50%)" and "Nitric Acid (50 - 100%)" as separate entries (Parker Hannifin, 2018). A blend, a cleaning chemical or a contaminant is a different fluid from any single entry, and Parker recommends, where two or more fluids are sealed, a compound rated 1 for all of them where possible.

Contact time

Parker notes that the usual 70-hour ASTM immersion test can indicate swelling where a long-term test shows shrinkage, so short-term swell may be an interim condition (Parker Hannifin, 2018).

Compound grade

Nitrile sealing compounds vary from 18 % to 50 % acrylonitrile content (Parker Hannifin, 2018), so two NBR seals can behave differently in the same fluid.

Mechanical cause

Compression set, extrusion and abrasion can look like chemical damage; All Seals lists excessive compression and excessive temperature among the contributing factors for compression set (All Seals, n.d.).

How can you tell the causes of a chart-rated seal failure apart?

You can tell the causes of a chart-rated seal failure apart by setting each chart test condition against the matching service condition and checking which one differs, then confirming with the failed seal itself. Measure the failed seal's hardness and dimensions against a new seal from the same source before you look at the chart, because a seal with no swell, shrinkage or hardness change points away from chemistry altogether. Work from the dated change record, so each check compares the condition at the time of failure with the chart. The table below decides which gap between chart and service caused the failure.

The row that most often settles the question is the first: if the seal runs hotter than the chart's test condition, the rating does not apply until it is tested at service temperature. When two rows both differ, change one condition at a time in a test rather than replacing seals by trial, and keep the failed seal until the cause is settled.

How do you keep chart-rated seals from failing again?

You keep chart-rated seals from failing again by fixing the service conditions for each fluid and seal combination, recording every chart value with its source and test conditions, and judging each combination compatible, conditionally compatible or not compatible:

1. Fix the service conditions: the fluid name, its concentration range from the safety data sheet (SDS), the normal and maximum temperature in °C, the contact time (continuous, intermittent or cleaning cycle), and every other fluid the seal touches.

2. Record each chart value as a typical value. Note the chart's source, the compound it names, and its test temperature and time, or "conditions not stated".

3. Compare the two records line by line. A service condition outside the chart condition is a gap, even at a rating of 1.

4. Judge it. Compatible: the chart covers every service condition. Conditionally compatible: a rating of 2, or a gap, with the condition named, such as static only. Not compatible: a rating of 3 or 4, or failure in test.

5. Close each conditional gap with an immersion test to ASTM D471-16a(2021) or ISO 1817:2026 at service temperature and concentration, on the compound you will buy, for a time that reflects service exposure. Handle the test fluid as its SDS and your site's chemical handling rules direct.

6. Specify the compound, not only the polymer family, and recheck the judgment when a fluid, temperature or seal source changes.

The record then shows which judgments rest on chart data and which on a test.

When should you ask an elastomer specialist about a seal compatibility chart?

You should ask an elastomer specialist about a seal compatibility chart when the service fluid is not on the chart, when the chart rates the pair 2 or 3, when service temperature or concentration exceeds every listed row, or when a failure repeats after a like-for-like replacement. The Parker O-Ring Handbook treats a fluid that is foreign to the user's experience and not listed in the table as "major pioneering", where the most likely compound should be put on simulated service test (Parker Hannifin, 2018). In equipment for oil and gas production, ISO 23936-2:2011, last reviewed and confirmed in 2022, describes the requirements and procedures for qualification of elastomeric material (ISO, 2022), and a chart rating does not stand in for that qualification.

Bring the specialist the failed seal and a new one, the SDS of each fluid that touched it, the temperature record and the dates of every change. A failed seal that carried a hazardous fluid travels cleaned or packed as that fluid's SDS directs. With the record, a seal maker can test the actual compound under your conditions instead of quoting the chart again.

Lubinpla's compatibility evaluation fixes concentration, temperature and contact time for each combination, marks public chart values as typical values with their source and test conditions, and judges each combination compatible, conditionally compatible or not compatible.