Static & Flange O-Rings
Face seal solutions for pipe flanges, valve bonnets, pump housings and equipment covers — designed for zero-leak static sealing.
Static flange sealing is the most common O-ring application in industrial equipment. A face seal (also called a flange seal or static gland) compresses the O-ring between two flat mating surfaces, creating a leak-tight joint without relative motion. Unlike dynamic seals, static face seals do not experience wear from sliding friction — their primary enemies are over-compression, chemical attack, thermal cycling and inadequate bolt preload. This guide covers the complete static flange sealing workflow: selecting the correct O-ring size and material, designing the groove for proper compression and fill rate, specifying surface finish and bolt torque, and preventing the most common failure modes including compression set, extrusion and thermal degradation. Use the linked calculators and standards pages to verify your specific design before cutting metal. For step-by-step material selection, see our [O-Ring Selection Checklist](/blog/o-ring-selection-checklist/). For installation best practices, refer to the [Installation & Storage Guide](/guides/o-ring-installation-storage/).
Application Requirements
Recommended Materials
NBR
Hydraulic manifolds, fuel flanges, general industrial pipe joints in petroleum-based fluids.
Temp: -40°C to +120°C
Default choice for oil and fuel service. Specify 70 Shore A for general use, 80–90 Shore A for high-pressure with backup rings.
FKM
High-temperature exhaust flanges, chemical process joints, aggressive fuel systems.
Temp: -20°C to +200°C
Specify high-fluorine grades for continuous operation above 150°C. Use PTFE backup rings above 100 bar.
EPDM
Water treatment flanges, steam valve bonnets, outdoor HVAC equipment.
Temp: -50°C to +150°C
Excellent for hot water and steam. Not compatible with petroleum oils or fuels.
PTFE
Chemical process flanges, semiconductor equipment, cryogenic static joints.
Temp: -200°C to +260°C
Universal chemical resistance. No elastic recovery — requires 20–25% compression and well-machined grooves.
FFKM
Aerospace fuel flanges, pharmaceutical reactors, oilfield wellhead seals.
Temp: -20°C to +320°C
Highest performance for extreme chemical and temperature combinations. 50–100× cost of NBR.
Design Tips
- 1.Design groove depth for 15–25% compression. Too little compression (< 12%) allows leakage under low pressure or thermal cycling. Too much compression (> 30%) accelerates compression set and increases bolt torque requirement.
- 2.Groove width should be 1.15–1.25 × O-ring cross section for face seals. This provides adequate gland fill (65–85%) while leaving room for thermal expansion and fluid swell.
- 3.Corner radius minimum 0.10 mm (0.004 in) on all groove edges. Sharp edges act as stress concentrators and can cut the O-ring during assembly or under pressure cycling.
- 4.Surface finish Ra 0.8–3.2 µm (32–125 µin) is typical for static face seals. Smoother is acceptable but unnecessary; rougher surfaces increase leak risk and accelerate compression set by creating stress peaks.
- 5.Bolt preload must be sufficient to maintain compression across the full temperature range. Thermal expansion of the flange material can reduce clamping force at high temperature — design bolt torque for the maximum operating temperature, not ambient.
- 6.Use a backup ring when pressure exceeds 150 bar or when the extrusion gap exceeds 0.15 mm. PTFE anti-extrusion rings are standard; harder elastomer backup rings are acceptable for moderate pressure.
- 7.For vacuum face seals, increase compression to 20–30% and specify a harder compound (80 Shore A minimum) to prevent gas permeation through the elastomer bulk.
Common Sizes
| Size | Typical Use |
|---|---|
| AS568-210 to -222 series | Common pipe flange sizes from 1/2 inch to 2 inch nominal bore. NBR and FKM are standard stock. |
| AS568-325 to -338 series | Large-diameter flange seals for pump housings, valve bonnets and equipment covers. EPDM for water, FKM for chemicals. |
| AS568-110 to -125 series | Small instrument and sensor face seals. VMQ for medical/food, NBR for industrial instrumentation. |
| ISO 3601 Group B and C | Metric equipment flanges. 2.65 mm and 3.55 mm CS are the most common for European machinery. |
Frequently Asked Questions
What is the difference between a face seal and a radial seal?
A face seal compresses the O-ring between two flat mating surfaces perpendicular to the axis of the bolt load. A radial seal compresses the O-ring between an inner and outer cylindrical surface (bore and shaft or piston and cylinder). Face seals are always static; radial seals can be static or dynamic. Face seals tolerate wider surface finish ranges and higher compression rates than dynamic radial seals.
Why does my flange O-ring leak after thermal cycling?
Thermal cycling causes the flange and O-ring to expand and contract at different rates. If the bolt preload was designed for ambient temperature, the clamping force may drop at high temperature as the flange expands more than the bolt. The O-ring then loses compression and leaks during cool-down. Design bolt torque for the maximum operating temperature and use Belleville washers or spring washers to maintain preload across the temperature range.
How tight should bolts be on a flange with an O-ring?
Bolt torque should be calculated to achieve uniform compression across the entire O-ring contact area. For standard face seals, 15–25% O-ring compression typically requires 20–40 Nm per M8 bolt or 40–80 Nm per M12 bolt, depending on flange size and material. Tighten in a star pattern (cross-sequence) to ensure even compression and prevent flange distortion. Always verify with a torque wrench — uneven tightening creates high spots that over-compress the O-ring and low spots that leak.
Can I reuse an O-ring in a flange joint after disassembly?
No. Once compressed, an O-ring takes a permanent compression set and will not return to its original cross section. Reusing a compressed seal results in reduced sealing force and premature leakage. Always install a new O-ring and lightly lubricate it with a compatible assembly grease before reassembly.
What causes extrusion in a static flange seal?
Extrusion occurs when pressure forces the O-ring material into the clearance gap between the flange mating surfaces. In static face seals, this is caused by: excessive pressure without a backup ring, insufficient bolt preload allowing flange separation under pressure, or an extrusion gap larger than recommended for the O-ring cross section. For pressures above 150 bar, use a PTFE backup ring or increase hardness to 80–90 Shore A.
When should I use a square ring instead of an O-ring for a face seal?
Square rings (also called square-cut rings or lathe-cut rings) provide a larger contact area and better stability in high-pressure or large-diameter face seals. They resist rolling during installation and maintain position better under cyclic pressure. However, square rings require more precise groove machining and are less universally available than O-rings. For most standard face seals, O-rings are the preferred choice due to lower cost and wider availability.
Do I need a metal gasket or can an O-ring handle high-pressure flanges?
O-rings can handle pressures up to 200 bar in well-designed face seals with backup rings and proper bolt preload. Above 200 bar, or in applications with significant pressure pulsation, metal gaskets or metal-to-metal seals may be more reliable. The decision depends on pressure, temperature cycling, flange stiffness, and allowable leak rate. For critical high-pressure applications, consult our engineering team for a joint design review.
What lubricant should I use when installing a flange O-ring?
Use a thin film of lubricant compatible with both the O-ring material and the system fluid. For NBR in hydraulic oil, use petroleum-based assembly grease. For FKM in chemicals, use fluorinated grease or dry PTFE powder. For EPDM in water systems, use water or glycerin. Avoid silicone grease on NBR or EPDM if the system fluid is incompatible. Lubrication reduces abrasion during assembly and provides initial sealing until system pressure is applied.
What O-ring size fits SAE Code 61 and Code 62 flange grooves?
SAE J518 (ISO 6162) four-bolt flanges use standardized groove dimensions by flange size — not arbitrary AS568 dash numbers. Code 61 (3,000 psi class) and Code 62 (6,000 psi class) each specify groove width, depth, and recommended cross-section for the port size (e.g. 1/2 in through 2 in). Match the O-ring to the groove width and depth in the standard table, then verify 15–25% compression with the groove land height. For ORFS face seals per SAE J1453, the O-ring sits in a machined face groove — use the ORFS size chart for the fitting thread size, not the flange bolt pattern alone.
What is an SAE flange O-ring face seal (ORFS)?
SAE O-ring face seal (ORFS, SAE J1453) places the seal in a flat annular groove on the fitting face. The O-ring is compressed when the flat faces mate; threads do not cut the seal during assembly. Torque the fitting to specification after seating the O-ring with a blunt plastic tool and confirming no twist. ORFS is common in high-pressure hydraulic lines where thread damage to the seal must be avoided.
How do I choose between AS568 dash sizes and metric flange groove sizes?
AS568 dash numbers describe nominal ID and cross-section for general-purpose O-rings. Flange and port grooves in SAE, DIN, and ISO standards often specify exact groove width and depth that may not match a single dash size. Measure the groove (width, depth, land height) and select an O-ring whose cross-section fills 65–85% of the groove at ambient temperature with 15–25% compression. When the groove is standard per SAE J518 or your equipment drawing, use the published groove O-ring size for that port rather than guessing from dash number alone.
Need Static Seal Design Support?
Our engineers review flange groove designs, recommend materials and provide failure analysis at no cost. Submit your drawing or operating conditions for a technical review within 24–48 hours.