O-Ring Fitting and Seal Types: ORB, Flange, ORFS, and Material Uses

O-rings are used in many hydraulic fitting and seal designs, but the material, geometry, and application are not the same in every connection. A seal that works in one fitting family may fail quickly in another.

The correct choice depends on connection style, pressure, temperature, fluid, shelf life, and whether the seal is static, dynamic, face-sealing, or used with a flared tube connection.

O-Ring Boss Seals

O-ring boss connections join fluid-carrying pipes, hoses, or tubing through a straight-thread male part and a female port. The threads provide the mechanical engagement, while the O-ring seals in the chamfered area of the port.

ORB seal sizes such as AS568-901 through 932 are commonly associated with straight-thread tube fitting boss seals. ORB connections are common in hydraulics, compressed air systems, and vacuum equipment.

Flange O-Ring Seals

Flange seals create a front-face static seal in flange connections. They can be used for internal or external pressure applications and are available in multiple materials.

Split-flange assemblies typically include a permanent flanged head, an O-ring, two clamp halves, and four bolts. The O-ring sits in a groove and seals against the flat port face as the bolts clamp the assembly together.

Flare-O and 37 Degree Flare Seals

Flare-O seals are usually made from Buna-N or Viton. They are used with SAE 37 degree flared tube fittings to improve sealing at the flare connection when the fitting style is appropriate.

These seals are often considered for 37 degree flare systems, including aerospace, marine, or older applications that have not moved to flareless or face-seal tube fitting designs.

Common O-Ring Materials

Buna-N, also called nitrile, is a common general-purpose material for quick couplings and many oil-related applications. It resists many solvents, oils, water, crude oil, gasoline, propane, and petroleum fluids, with a common temperature range around -40 F to 250 F depending on formulation.

Peroxide-cured EPDM is used where chemical resistance is important, including medical-grade polypropylene couplings, water and wastewater systems, automotive manufacturing, and some household or industrial chemical-service applications.

Viton-type fluorocarbon O-rings are chosen for resistance to high temperature, ozone, oxygen, mineral oil, synthetic hydraulic fluids, fuels, aromatics, and many solvents. Other fluorocarbon compounds may improve resistance to acids, fuels, water, or steam.

High-Temperature and Specialty Materials

Silicone O-rings are often used in dry heat applications and can be suitable for food, medical, electronics, automotive, and hardware uses where the material limitations fit the service.

FFKM materials such as Kalrez-style O-rings are used where very high chemical resistance, high temperature resistance, and long service life are needed in demanding environments. They are specialty choices, not general-purpose replacements.

Bonded Seals

Bonded seals, sometimes called Dowty-type seals or bonded washers, combine a metal outer ring with an inner rubber elastomer. They are frequently used as bolt seals in hydraulic equipment.

The same rubber-to-metal or rubber-to-plastic bonding concept can also support custom shapes when a normal O-ring groove is not the right seal design.

ORFS Face Seals

O-ring face seal fittings use a replaceable elastomeric O-ring held in a groove on the fitting face. SAE J1452 ORFS connections are used to reduce leakage in high-pressure hydraulic systems.

When the connection is tightened, the O-ring compresses against a flat face sleeve brazed to the tube end. The cavity supports the O-ring and helps keep the tube from twisting during assembly.

Conical Seals for Flared Fittings

Conical seals for 37 degree flare fittings may be made from aluminum, copper, nickel, or stainless steel. They are used where small nicks or defects in the fitting face can prevent a reliable metal-to-metal seal.

These soft annealed seals act like a crush washer on the male flare, helping create a leak-resistant connection while reducing the chance of further damage to the tube or fitting.

What to Stock and Confirm

Useful replacement groups include Buna O-rings, Viton O-rings, flange O-rings, metric bonded seals, British bonded seals, Flare-O seals, face seal O-rings, and conical seals.

Before replacing an O-ring, confirm the fitting family, seal material, size standard, temperature, pressure, fluid, and whether the seal is reused, damaged, or past its shelf-life limit.