The rubber seat is the single most important component in a resilient-seated butterfly valve. It determines the valve's temperature range, chemical compatibility, sealing performance, and service life. Choosing between EPDM and NBR (Buna-N) is the most common seat material decision engineers face — and getting it wrong can mean seat swelling, cracking, or outright failure within months of installation. This guide provides the technical data and selection logic to make the right call every time.
Drinking water, treated water, raw surface water, cooling water, dilute acids and alkalis, ozone exposure, outdoor UV exposure, chloramine-treated water, steam condensate (low pressure)
Wastewater with oils or grease, petroleum products, hydrocarbon solvents, animal fats and vegetable oils, compressed air with oil carryover, fuel handling, hydraulic oil, any media containing mineral oil
EPDM is a synthetic rubber with a saturated polymer backbone — meaning the main carbon chain has no double bonds. This saturated structure is what gives EPDM its signature properties: excellent resistance to ozone, UV radiation, weathering, and polar solvents (water, alcohols, glycols). Because EPDM contains no unsaturation in the backbone, it does not degrade under ozone attack — a major advantage for outdoor installations and drinking water systems where ozone or chloramines are used for disinfection.
EPDM is classified by ethylene content (typically 45-75%). Higher ethylene content increases tensile strength and reduces permeability, while lower ethylene content improves low-temperature flexibility. Standard EPDM for butterfly valve seats is typically 55-65% ethylene, Shore A 65-75 hardness.
NBR is a copolymer of acrylonitrile (ACN) and butadiene. Unlike EPDM, the butadiene component contains carbon-carbon double bonds in the polymer backbone — this unsaturation is what gives NBR its excellent resistance to hydrocarbon oils, fuels, and non-polar solvents. The tradeoff is that those same double bonds are vulnerable to ozone and UV attack, making NBR unsuitable for outdoor exposure unless specially compounded with anti-ozonants.
NBR is classified by ACN content (18-50%). Higher ACN = better oil resistance but poorer low-temperature flexibility. Standard NBR for butterfly valve seats is typically 28-35% ACN (medium nitrile), Shore A 65-75 hardness.
This is the factor that most often determines seat selection. The table below provides specific resistance data for common industrial media encountered in butterfly valve service:
| Media | EPDM | NBR | Notes |
|---|---|---|---|
| Water (fresh, potable) | Excellent — NSF 61 certified | Good | Both suitable; EPDM preferred for drinking water |
| Seawater / Brackish Water | Excellent | Good | EPDM preferred unless oil contamination is present |
| Sodium Hypochlorite (bleach, ≤10%) | Good | Fair — swells significantly | NBR will absorb chlorine and degrade; EPDM is standard for chlorinated water |
| Chlorine Gas (dry) | Fair — surface hardening | Not recommended | Consult manufacturer; PTFE may be required for dry chlorine |
| Ozone (dissolved) | Excellent | Poor — cracks rapidly | EPDM's saturated backbone is inherently ozone-resistant |
| Chloramines (monochloramine, NH₂Cl) | Excellent | Poor | NBR degrades rapidly from amine attack on the nitrile group |
| Sulfuric Acid (≤30% at 25°C) | Good | Poor — acid attacks butadiene | EPDM resists dilute H₂SO₄; NBR is not suitable for acid service |
| Nitric Acid (≤10% at 25°C) | Good | Not recommended | Both degrade at higher concentrations; consider PTFE |
| Hydrochloric Acid (≤10%) | Good | Poor | EPDM acceptable for dilute HCl at ambient temperature |
| Sodium Hydroxide (≤20% at 25°C) | Good | Fair — swells | EPDM preferred for caustic service |
| Phosphoric Acid (≤30%) | Good | Poor | EPDM is standard for phosphoric acid at moderate concentrations |
| Mineral Oil / Hydraulic Oil | Not recommended — swells severely | Excellent | This is the defining difference: EPDM absorbs oil like a sponge |
| Diesel / Gasoline / Kerosene | Not recommended — swells 100%+ | Good to Excellent | NBR is the standard for fuel and petroleum handling |
| Aromatic Solvents (toluene, xylene) | Not recommended | Fair — high ACN (>40%) NBR improves resistance | Neither is ideal; FKM or PTFE is preferred for aromatics |
| Vegetable Oil / Animal Fat | Not recommended | Good | NBR is suitable for edible oil and food-grade fat processing |
| Alcohols (methanol, ethanol) | Good | Fair — slight swelling | EPDM preferred for ethanol and polar solvent service |
| Glycols (ethylene, propylene glycol) | Excellent | Good | EPDM is standard for HVAC chilled water and glycol loops |
| Compressed Air (oil-free) | Excellent | Good | EPDM preferred; no ozone degradation, no oil damage |
| Compressed Air (with oil carryover) | Not recommended | Excellent | Oil from compressor carryover will swell EPDM; use NBR |
| Steam Condensate | Fair — limited to low-pressure, intermittent | Poor — hydrolyzes at high temperature | Neither is rated for continuous steam; specify EPDM-HP or PTFE for steam service |
| Temperature Condition | EPDM | NBR |
|---|---|---|
| Minimum continuous service | -40°C (-40°F) — standard grade | -30°C (-22°F) — medium ACN; -10°C (14°F) — high ACN |
| Maximum continuous service (water) | +120°C (248°F) — standard; +150°C (302°F) — HT-EPDM peroxide cured | +90°C (194°F) — water; +100°C (212°F) — oil |
| Maximum intermittent (short excursion) | +130°C (266°F) — standard; +160°C (320°F) — HT-EPDM | +110°C (230°F) |
| Recommended max for butterfly valve seat | +80°C (176°F) — conservative rating accounting for compression set under load | +80°C (176°F) in water; +90°C (194°F) in clean oil |
| Glass transition temperature (Tg) | Approx. -55°C (-67°F) | -35°C to -15°C (-31°F to +5°F) — depends on ACN content |
| Steam resistance | Poor — hydrolyzes above 130°C; do not use for continuous steam | Very poor — hydrolyzes and hardens above 100°C |
| Property | EPDM (Standard) | NBR (Medium ACN) | Significance for Butterfly Valve Seats |
|---|---|---|---|
| Hardness (Shore A) | 65-75 | 65-75 | Seat hardness affects sealing pressure and torque. Softer = better seal but higher wear. Standard 70±5 Shore A balances both. |
| Tensile Strength | 10-15 MPa | 10-17 MPa | Higher tensile = better resistance to tearing at seat edge during installation |
| Elongation at Break | 300-500% | 300-500% | High elongation allows the seat to conform to disc edge irregularities for bubble-tight sealing |
| Compression Set (22h @ 70°C) | 15-25% | 15-25% | Lower is better. Compression set is the permanent deformation after long-term compression — the primary aging mechanism for butterfly valve seats |
| Compression Set (22h @ 100°C) | 25-40% | 35-50% | At elevated temperature, EPDM retains its resilience better than NBR |
| Tear Resistance (Die B) | 25-35 kN/m | 30-45 kN/m | NBR has better tear resistance — important for seats exposed to abrasive particles (grit, sand) |
| Abrasion Resistance (DIN 53516) | 120-180 mm³ loss | 80-120 mm³ loss | NBR is ~30-40% more abrasion-resistant than EPDM. Lower number = better abrasion resistance |
| Resilience (Rebound) | 40-50% | 25-35% | Higher resilience = better dynamic sealing under pressure fluctuations. EPDM rebounds faster |
| Gas Permeability (air, 25°C) | Low | Very low | NBR has slightly lower gas permeability — relevant for vacuum or gas service |
| Water Absorption (volume %) | <1% — essentially zero swelling in water | 1-3% in water; 3-5% in 50% water-oil emulsion | EPDM is dimensionally stable in water; NBR will absorb some water over time |
| Specific Gravity | 1.10-1.15 | 1.00-1.10 | NBR is slightly lighter |
| Application / Industry | Recommended Seat | Reasoning |
|---|---|---|
| Municipal drinking water distribution | EPDM | NSF 61 certified; excellent ozone and chloramine resistance; zero water absorption |
| Water treatment plant — raw water intake | EPDM | Raw surface water is EPDM-compatible unless petroleum contamination is present |
| Municipal wastewater — raw influent | NBR | Abrasion from grit + potential oil/grease in municipal sewage; NBR handles both |
| Wastewater — treated effluent | EPDM or NBR | If oil-free: EPDM. If trace hydrocarbons persist after treatment: NBR |
| Cooling water (power plant / HVAC) | EPDM | Clean treated water with biocides; EPDM resists chlorine and bromine treatment chemicals |
| Seawater intake / desalination | EPDM | Excellent seawater resistance; no swelling. NBR if oil contamination possible at port intake |
| Oil & Gas — produced water | NBR | Produced water always carries residual hydrocarbons; EPDM will swell and fail |
| Oil & Gas — crude oil transfer | NBR (or FKM) | High ACN NBR for crude oil; FKM for sour service (H₂S) or high-temperature crude |
| Chemical processing — acids/alkalis | EPDM | Dilute acids and caustic solutions; verify concentration and temperature against resistance chart |
| Chemical processing — solvents | NBR or PTFE | NBR for mild solvents; PTFE-lined valve for aggressive or aromatic solvents |
| Food & beverage — edible oil | NBR | NBR resists vegetable oil; FDA-compliant NBR compounds available for food contact |
| Food & beverage — water-based (brewery, dairy CIP) | EPDM | EPDM handles hot water, CIP chemicals (caustic, acid, hypochlorite), and steam flush at moderate temperature |
| Compressed air — instrument air (oil-free) | EPDM | Clean dry air; no oil content; EPDM's ozone resistance is a plus for compressor discharge |
| Compressed air — plant air (with oil carryover) | NBR | Oil mist from lubricated compressors will swell EPDM; NBR required |
| Fire protection — wet pipe sprinkler | EPDM | Stagnant water with potential corrosion inhibitors; EPDM is standard |
| Fire protection — foam concentrate | NBR | Firefighting foam contains hydrocarbon surfactants; EPDM degraded by foam agents |
| Irrigation and agriculture | EPDM | Clean water with occasional fertilizers (dilute); EPDM is cost-effective |
| Mining — slurry water | NBR | High abrasion from suspended solids + potential oil contamination from mining equipment |
This is the most common misapplication. When EPDM contacts mineral oil, diesel, or hydraulic fluid, the non-polar hydrocarbon molecules are absorbed into the EPDM polymer matrix. The rubber swells — sometimes increasing in volume by 100-200%. The swollen seat loses all mechanical strength, becomes soft and gummy, and cannot maintain a seal against the disc. In severe cases, the swollen seat extrudes into the flow path and is torn off by the disc during operation. Recovery is impossible: once oil-swollen, the EPDM seat must be replaced.
NBR's unsaturated butadiene backbone is vulnerable to ozone attack. When exposed to ozone (from outdoor air, corona discharge from electric actuators, or ozonated water treatment), the rubber develops characteristic perpendicular cracks on the tensioned surface — known as ozone cracking. These cracks propagate with each disc opening cycle until the seat splits completely. Chlorine and hypochlorite at concentrations above 10 ppm accelerate this attack by oxidizing the double bonds in the butadiene backbone.
Above 100°C in water, both EPDM and NBR undergo accelerated compression set — the rubber permanently deforms under the continuous load from the closed disc. The seat develops a groove matching the disc edge profile, and when the disc opens, the groove remains. When the valve closes again, the disc seats into the same groove but with reduced interference, leading to increasing leakage over time. This is a gradual failure mode (months to years) rather than a sudden one, and is the reason butterfly valve seat temperature ratings are conservative.
| Factor | EPDM | NBR |
|---|---|---|
| Raw material cost (relative) | 1.0 (baseline) | 1.2-1.4× |
| Finished seat cost premium | Baseline | +10-20% |
| Global availability | Excellent — the most widely used butterfly valve seat material | Excellent — second most common |
| Standard stock | Nearly all sizes in stock | Common sizes (DN50-DN600) typically stocked |
| Lead time for non-stock sizes | 2-4 weeks | 3-5 weeks |
| NSF 61 certified compounds | Widely available | Limited — fewer NBR compounds carry NSF 61 due to lower demand for NBR in potable water |
| FDA / food-grade compounds | Available | Available |
| WRAS (UK water) approved | Widely available | Available but less common |
For applications outside the EPDM/NBR envelope, the following seat materials are available on butterfly valves:
| Material | Temperature Range | Key Advantage Over EPDM/NBR | Typical Application |
|---|---|---|---|
| FKM | -20°C to +180°C | Superior chemical resistance — handles aromatics, concentrated acids, and high-temp hydrocarbons that damage both EPDM and NBR | Refinery, chemical plant, high-temperature oil |
| PTFE | -29°C to +180°C | Near-universal chemical resistance; zero swelling in any media | Aggressive chemicals, solvents, strong acids — see our full PTFE lined butterfly valve |
| Silicone | -50°C to +180°C | Extreme low-temperature flexibility; good for dry heat | Food industry dry applications, low-temp water; poor abrasion resistance |
| HT-EPDM (Peroxide Cured) | -40°C to +150°C | Extended high-temperature capability +30°C over standard sulfur-cured EPDM | District heating, low-pressure steam, hot water above 80°C |
| HNBR (Hydrogenated NBR) | -30°C to +150°C | Combines NBR's oil resistance with EPDM-like ozone/heat resistance | Oilfield water injection, high-temp oil + ozone exposure |
| Metal Seat (SS316 / Stellite) | -196°C to +600°C+ | No elastomer degradation; fire-safe design | High-temperature gases, cryogenic, fire-safe per API 607 — see our AWWA C504 range |
Yes, on all Laux Valve butterfly valves with a replaceable seat design. The seat is mechanically retained (typically by a bolted SS316 clamping ring) and can be removed and replaced in the field. When changing seat material (e.g., EPDM to NBR), the valve body bore and disc edge must be thoroughly cleaned to remove any residue of the old seat material and process contaminants. Seat replacement does not require removing the valve body from the pipeline on lug and double flange butterfly valves.
Field test: place a small drop of mineral oil (or clean engine oil) on a non-sealing surface of the seat. Wait 5 minutes. If the rubber has swelled or softened at the oil spot, it is EPDM. If it is unchanged, it is NBR. Laboratory confirmation via FTIR spectroscopy or thermogravimetric analysis (TGA) is recommended for critical applications.
If the process stream contains both water and trace oil — for example, a wastewater stream with occasional oil slugs, or a cooling water system with heat exchanger leaks — always select the seat material for the most aggressive component. In these cases, the oil will damage EPDM, so NBR is the safer choice. The NBR may have slightly higher water absorption, but it will not fail catastrophically in water. EPDM will fail catastrophically in oil.
Standard NBR is not NSF/ANSI 61 certified and is not recommended for potable water contact. NSF 61-certified NBR compounds exist but are less common and more expensive than NSF 61 EPDM. For drinking water applications, EPDM is the standard and preferred seat material. If a drinking water system also requires hydrocarbon resistance (extremely rare), contact the manufacturer for a certified material recommendation.
In clean treated water at ambient temperature, an EPDM seat typically lasts 15-25 years before compression set causes measurable leakage. NBR seats in non-aggressive service have a similar life. Factors that reduce seat life: elevated temperature (>60°C), abrasive particles, chlorine/oxidizer concentration >10 ppm, frequent cycling (>500 cycles/year), and high differential pressure at opening. A properly selected and installed seat should outlast the valve's coated components.
Compression set is the permanent deformation that remains after a rubber specimen has been compressed for a specified time at a specified temperature, then allowed to recover. For a butterfly valve seat, the disc compresses the rubber when the valve is closed. Over time, the rubber loses its ability to spring back fully — this is compression set. Eventually, the disc-to-seat interference is reduced to the point where the valve no longer seals. This is the normal end-of-life mechanism for resilient seated butterfly valves, not a manufacturing defect.
No. AFFF (Aqueous Film-Forming Foam) concentrate contains hydrocarbon surfactants and glycol ethers that will degrade EPDM. For foam concentrate valves, use NBR seats. For water-only fire protection lines (sprinkler supply, standpipe), EPDM is appropriate. Always verify the foam concentrate manufacturer's elastomer compatibility chart before selecting valve seat material.
