7 Tips for Choosing Sealing Rings

A sealing ring may be small, but its failure can stop a pump, contaminate a process, or drain compressed air from a production line. The U.S. Department of Energy’s Improving Compressed Air System Performance: A Sourcebook for Industry reports that leaks can waste 20–30% of compressor output in poorly maintained systems. That figure covers many leak sources, not Sealing Rings alone. Still, it shows why seal selection and routine inspection deserve attention.

Choosing Sealing Rings means matching the ring to its real operating conditions: fluid, pressure, temperature, movement, and groove dimensions. A ring that fits a bench test may swell in oil or harden near a hot motor. Parker’s O-Ring Handbook offers detailed guidance on elastomer compatibility and seal design, while ISO 3601 addresses O-ring dimensions and tolerances. These references help narrow choices, but they cannot replace checking the equipment manufacturer’s specifications or testing the actual material in service.

Small details matter. A scratched shaft, an oversized groove, or a dry installation can shorten seal life. Even experienced technicians can miss one. The seven tips ahead focus on practical checks that reduce avoidable failures, from material selection to installation and maintenance. No checklist guarantees a leak-free system; real conditions can be messier than a catalog suggests. That is worth remembering.

Table of Contents [Hide]

1 Define Service Conditions: Pressure, Temperature, Motion, and Fluid

2 Specify Ring Dimensions and Tolerances Using ISO 3601

3 Match Elastomer Compounds to Fluids and ASTM D2000 Codes

4 Select Hardness: 70 Shore A Is a Common O-Ring Grade

5 Check Compression Set Using ASTM D395 and Assess Extrusion Gaps

Define Service Conditions: Pressure, Temperature, Motion, and Fluid

Define the real service envelope before choosing a sealing ring. Record normal and peak pressure, startup spikes, temperature range, motion type, speed, and the exact fluid mixture. A system running at 6 MPa steadily may still see much higher short pressure peaks. Those peaks can push an undersized or poorly supported ring into a gap, causing extrusion and early failure. Check the equipment during startup and shutdown, not just at steady operation.

Temperature changes both the elastomer and the fluid. NIST Chemistry WebBook, Standard Reference Database 69, reports water viscosity of about 1.00 mPa·s at 20°C and 0.355 mPa·s at 80°C. That change can affect lubrication and leakage around a moving seal.

Reciprocating rods, rotating shafts, and static joints therefore need different design checks. Match the ring compound to the actual fluid, including concentration, cleaning agents, and additives; a generic fluid name may hide important differences.

ISO 3601 provides dimensional requirements for O-rings, but correct dimensions alone do not confirm chemical or thermal suitability. It is easy to focus on the nameplate rating and miss brief temperature spikes or dry movement.

That detail deserves a second look.

Specify Ring Dimensions and Tolerances Using ISO 3601

ISO 3601 helps make O-ring dimensions and tolerances clear across drawings and suppliers. State the nominal inside diameter and cross-section, then identify the applicable ISO 3601 part and tolerance class. Use consistent units. Small differences matter when a ring sits in a narrow groove or stretches over a shaft.

Check the standard’s scope. ISO 3601-1 covers dimensions and tolerances; ISO 3601-2 addresses housing dimensions, while ISO 3601-3 covers quality acceptance criteria. These details do not replace seal design checks. Confirm the groove, squeeze, temperature, fluid, and pressure against the application. A ring that matches a table can still fail in a real assembly.

Be specific. A useful drawing calls out the size, tolerance class, material, and any required inspection criteria. Avoid relying on a rounded caliper reading from a compressed sample; measure carefully and use suitable equipment. I have seen drawings that looked complete but left the class unstated. That gap is easy to miss. Verify the current standard edition and review the finished specification with the seal supplier before release.

Match Elastomer Compounds to Fluids and ASTM D2000 Codes

7 Tips for Choosing Sealing Rings
Match Elastomer Compounds to Fluids and ASTM D2000 Codes

Fluid compatibility is not a material-name shortcut. Nitrile often suits mineral oils; EPDM commonly serves hot water and glycol; fluorocarbon compounds tolerate many oils and fuels. But additives, temperature, and exposure time can change performance. Check the actual fluid and operating conditions.

ASTM D2000 helps compare compound specifications, not predict every seal’s life. Its type letter indicates heat-resistance classification, while its class letter indicates oil-resistance classification. The callout also includes hardness and tensile-strength requirements, with suffixes for additional tests. ASTM D2000 specifies properties after defined conditioning; some heat-aging requirements use 70-hour exposures. Read the complete code, not just the first letters.

A practical check: place a sample ring beside the real fluid, then record its mass and dimensions before and after exposure. Swelling, softening, or a tacky surface are warning signs. Industry compatibility charts are useful screening tools, but they cannot capture every blend or temperature cycle. That gap matters. Confirm the compound with the seal supplier’s test data, and verify performance in the assembly. A code can look exact and still leave room for a poor fit.

7 Tips for Choosing Sealing Rings - Match Elastomer Compounds to Fluids and ASTM D2000 Codes
Tip	Fluid or Service	Elastomer to Consider	Typical Temperature Guidance	Compatibility and Design Checks	ASTM D2000 Check
1. Start with the actual fluid.	Petroleum oils, lubricants, and many hydrocarbon fuels	Nitrile rubber (NBR); hydrogenated nitrile rubber (HNBR) for some higher-temperature or more demanding service	NBR is commonly used around −30 to 100 °C; the usable range varies by compound and exposure.	Check the exact fluid formulation, additives, and expected exposure time. NBR generally has limited resistance to ozone and weathering; do not assume all fuels or oils behave alike.	Specify the complete compound callout, hardness, and any required suffix tests. Confirm that the selected grade has fluid-immersion data for the actual fluid.
2. Choose EPDM for suitable water-based service.	Water, many dilute aqueous solutions, and some glycol-based fluids	Ethylene propylene diene rubber (EPDM)	Many EPDM compounds serve approximately −40 to 150 °C; limits depend on the compound, fluid, and pressure.	EPDM is generally a poor choice for petroleum oils, fuels, and many hydrocarbon-based fluids. Check concentration, temperature, and whether steam or hot-water exposure is continuous or intermittent.	Review the full ASTM D2000 callout and the compound’s test results. The polymer family alone does not specify heat resistance, hardness, or additional requirements.
3. Consider FKM for many hot oil and fuel applications.	Many petroleum oils, fuels, and hydrocarbon chemicals	Fluorocarbon rubber (FKM)	Many FKM compounds are used around −20 to 200 °C, with the exact range depending on formulation and exposure.	Compatibility varies across FKM types and fluid blends. Check cold-start requirements and verify suitability for hot water, steam, and specific polar chemicals rather than relying on the generic material name.	Match the complete D2000 callout to the required temperature, hardness, tensile properties, and suffix requirements; verify the grade against the target fluid.
4. Use silicone when temperature flexibility is important.	Air, many dry environments, and selected static sealing applications	Silicone rubber (VMQ)	Many silicone compounds cover approximately −55 to 200 °C; some formulations differ substantially.	Silicone can offer a broad temperature range but often has lower tear and abrasion resistance than other common elastomers. It is generally not the first choice for many fuels, oils, or high-wear dynamic seals.	Confirm the exact D2000 properties and any special tests needed for the application. Check mechanical requirements as well as temperature capability.
5. Look at HNBR for combined heat and mechanical demands.	Selected oils, fuels, and applications involving elevated temperature or wear	Hydrogenated nitrile rubber (HNBR)	Many compounds are used around −30 to 150 °C; formulation and operating conditions determine the actual limit.	HNBR may provide improved heat, ozone, and wear performance over conventional NBR in appropriate grades. Confirm compatibility with the specific fluid, additives, and any sour-gas or chemical exposure.	Request the compound’s full specification and supporting test data. Check that any required aging, compression-set, or low-temperature properties are covered.
6. Check weathering and ozone exposure.	Outdoor equipment, ozone exposure, and weather-exposed static seals	EPDM for compatible fluids; FKM or another qualified compound where fluid compatibility governs	Temperature limits depend on the chosen compound and the combined effects of weather, fluid, and mechanical load.	Do not select by weather resistance alone. EPDM resists weathering well but is generally unsuitable for petroleum oils; verify the complete service environment.	Check applicable D2000 suffix requirements and test data for aging or other specified properties. Ensure the selected callout applies to the exact compound being supplied.
7. Validate the complete operating envelope.	Any application involving pressure, motion, temperature cycling, or changing fluids	Select from NBR, HNBR, EPDM, FKM, silicone, or another qualified family based on test data	Use the compound-specific limits, not a generic polymer range; account for temperature peaks, cold starts, and cycling.	Consider pressure, squeeze, gland design, extrusion gaps, motion, chemical concentration, and exposure duration. Test the finished seal when failure risk or fluid uncertainty is high.	ASTM D2000 is a material specification system, not a universal fluid-compatibility chart. Verify the complete callout, current specification requirements, and compound-specific documentation.

Selection note: Temperature ranges shown are general guidance, not guaranteed operating limits. Compatibility depends on the specific compound, fluid formulation, concentration, pressure, exposure time, and seal design. Confirm selections with compound-specific data and application testing where needed.

Select Hardness: 70 Shore A Is a Common O-Ring Grade

A 70 Shore A O-ring is a common starting point for many sealing jobs. The Shore A scale measures how strongly rubber resists indentation. At 70A, an O-ring is firm enough for many static seals while retaining some flexibility. It can be easier to install than a much harder ring. Still, the number alone does not predict sealing performance.

Check the actual operating conditions. A seal in a low-pressure water fitting may behave differently from one exposed to heat, oil, or repeated movement. Pressure, temperature, fluid compatibility, groove dimensions, and extrusion gaps all matter. For example, a 70A ring squeezed into a narrow groove may deform or wear faster than expected. A softer grade might seal better in one design, but it could also extrude under pressure. Details count.

Treat 70A as a useful reference, not a guarantee. That distinction matters. Review the material data and test the ring under realistic conditions, including assembly and temperature changes. A neat specification can still hide a poor fit. I would be cautious about choosing hardness from a catalog chart alone; the right grade depends on the complete seal design. A small trial can reveal issues that calculations miss.

Check Compression Set Using ASTM D395 and Assess Extrusion Gaps

When comparing sealing rings, check compression set rather than relying on a material label alone. ASTM D395 measures how much a test specimen stays deformed after compression under specified conditions. Methods, temperature, duration, and recovery time matter, so compare results only when the test conditions match. Small details count.

A low compression-set result can indicate better recovery, but it does not guarantee a long service life. The test may not reproduce your seal’s pressure cycles, fluid exposure, or temperature changes. I would treat it as one useful data point, not a verdict. A little caution helps.

Then inspect the extrusion gap: the clearance between mating surfaces where pressure could force the ring into a narrow opening. Check the largest likely gap, including manufacturing tolerances, wear, and thermal movement. A feeler gauge can help during inspection, but drawings and tolerance calculations are often needed for hidden or variable clearances. Consider peak pressure, not just normal operation. Higher hardness or a support ring may reduce extrusion risk, but either choice needs checking against the application. Real assemblies are rarely perfect; measure what you can, and question assumptions that cannot be verified.