What Is a Bonded Seal?
A bonded seal is a composite sealing component made of two parts – a rigid metal outer ring and an elastomeric sealing lip vulcanised permanently to the inner diameter of that ring. The metal ring provides structural support and defines the pressure capacity of the seal. The rubber lip deforms under compression to create a positive, leak-free contact against the face of a port, fitting or fastener head.
This combination of metal and elastomer is the reason the bonded seal has remained the preferred static sealing solution for threaded hydraulic connections for decades. The metal carries the mechanical load. The rubber creates and maintains the seal. Neither material could perform this function as effectively alone.
The term “Dowty seal” along with dowty ring, dowty washer, dowty bonded seal, dowty sealing washer and dowty bonded washer – all refer to the same product. The Dowty name originates from the Dowty Group, the British engineering company that first developed and commercialised this bonded washer design for aerospace and hydraulic service. Today, the bonded seal is an industry standard used across oil and gas, the valve industry, aerospace, chemical processing, heavy machinery and general manufacturing wherever threaded ports, banjo fittings, hydraulic adapters, bolt heads and pipe plugs require reliable static sealing.
Why choose bonded seals instead of flat washers
The limitation of a flat copper or aluminium crush washer is straightforward. It seals by plastic deformation – it is permanently compressed when the fastener is tightened and it cannot recover. In a system that sees thermal cycling, vibration or pressure fluctuation, the joint gradually loses clamp load. The washer creeps, the seating stress reduces and eventually the connection leaks.
In most industrial applications – hydraulic systems, oil and gas pipework, valve assemblies, compressor connections – thermal movement, vibration and pressure variation are routine operating conditions, not exceptions. A sealing solution that depends entirely on permanent deformation is therefore not appropriate for long-term reliability in these environments.
A bonded seal addresses this directly. The elastomeric lip, vulcanised to the inner bore of the metal ring, compresses elastically when the fastener is tightened. Unlike a crush washer, it stores energy during that compression. If the joint experiences thermal movement or vibration, the rubber lip uses that stored energy to maintain its contact force against the port face. The metal outer ring, meanwhile, bears the bolt load entirely independently of the sealing lip — it prevents over-compression, stops extrusion and maintains the dimensional geometry of the joint.
The practical outcome for procurement and engineering teams is fewer in-service leaks, reduced maintenance frequency, longer inspection intervals and a lower total cost per sealing point over the service life of the equipment. This is the reason bonded seals displaced flat washers in hydraulic and fluid power systems and why they remain the specified self-sealing washer solution in any application where static sealing reliability is a requirement.
Design Elements of a Bonded Seal
A bonded seal has two distinct design elements and each one plays a specific role in the performance of the finished component. For engineers specifying these parts and for procurement teams sourcing them, understanding these elements clarifies why manufacturing quality matters.
The Metal Outer Ring
The outer ring is the load-bearing element of the seal. Its inner diameter matches the nominal bore of the port or fitting being sealed, while its outer diameter and thickness determine the clamp load distribution across the port face. Dimensional accuracy is critical – incorrect seating geometry will cause the elastomeric lip to either under-seal or extrude.
The ring material is selected based on the corrosion and chemical resistance requirements of the service environment. Stainless steel covers general industrial, hydraulic and offshore service. Inconel is specified for elevated temperature applications involving oxidising or acidic conditions. Hastelloy is used in the most aggressive chemical environments – concentrated acids, chloride-bearing fluids and highly corrosive process streams.
The Elastomeric Sealing Lip
The rubber inner lip is vulcanised to the metal ring under heat and pressure, forming a chemical bond between the elastomeric compound and the pre-treated metal surface. It is not adhesively bonded after the fact the vulcanisation process integrates the two materials at a molecular level. The lip profile is typically trapezoidal in cross-section, which concentrates contact stress at the sealing interface and provides inherent resistance to extrusion under pressure. The elastomeric compound is selected based on the process fluid, the operating temperature and any chemical compatibility requirements.
Bonded Seals Material Specifications
Material selection is the most important decision in bonded seal specification. The wrong elastomer in contact with the process fluid will swell, harden, crack or chemically degrade – all of which result in loss of sealing force and eventual leakage. The wrong metal ring material in a corrosive environment will corrode, losing its structural integrity and dimensional accuracy. The guidance below covers the standard material options and the service conditions each is suited to.
Elastomeric Materials
The elastomeric material is selected primarily on the basis of chemical compatibility with the process fluid and the operating temperature range of the application.
| Trade Name | Material type / Grade | Temperature Range | Suitable For |
|---|---|---|---|
| Cerulean N | Nitrile Butadiene Rubber (NBR) | -55°C to +135°C | Mineral oil, petroleum-based hydraulic fluids, fuels, greases, pneumatic systems |
| Vertex H | Hydrogenated Nitrile Butadiene Rubber | -52°C to +160° | Sour gas (H₂S), steam injection, high-temperature hydraulic circuits, oilfield service |
| Cerulean EP | Ethylene Propylene Diene Monomer | -55°C to +150°C | Water, steam, glycol-based fluids, polar solvents, water treatment systems |
| Vertex FC | Fluoroelastomer (Viton) | -20°C to +230°C | Synthetic hydraulic fluids, phosphate esters, aromatic solvents, fuel blends, sour-gas well fluids, aggressive chemical media |
| Vertex A | Tetrafluoroethylene propylene (FEPM / Aflas®) | -3°C to +230°C | High-temperature steam, strong acids and bases, H₂S, amines, sour gas, oilfield chemicals where FKM is not suitable |
| Vertex F | Perfluoroelastomer (FFKM) | -40°C to +315°C | Highly aggressive chemicals, strong acids, strong bases, aggressive solvents, high-temperature critical sealing in chemical processing, semiconductor and oil and gas service |
Metal Ring Materials
The metal ring material is selected primarily based on corrosion resistance requirements in the service environment.
| Metal | Application |
|---|---|
| Stainless Steel 302 | Steel 302General industrial service, light corrosive environments |
| Stainless Steel 304 | Standard industrial and hydraulic service, food industry |
| Stainless Steel 309L | High-temperature oxidising environments, heat-resistant applications |
| Stainless Steel 316 | Marine, offshore, chemical processing and moderately aggressive media |
| Stainless Steel 316L | Low-carbon variant of SS316, preferred where post-weld corrosion resistance is required in chemical and offshore service |
| Inconel | Elevated temperature service involving oxidising or acidic conditions — high-temperature chemical processing, gas turbine systems and demanding oil and gas applications |
| Hastelloy | Highly aggressive chemical environments including concentrated acids, chloride-bearing fluids and corrosive process streams where stainless steel grades are insufficient |
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Partner with ISMAT to source bonded seals in NBR, FKM, HNBR, AFLAS and FFKM with metal rings in stainless steel, Inconel and Hastelloy – engineered for reliable static sealing in oil and gas, valve, aerospace, chemical and heavy industrial applications.
How a Bonded Seal Works
When a bonded seal is placed against a port face or under a fitting head and the fastener is tightened, the metal outer ring is clamped between the two mating surfaces. The ring distributes the clamp load evenly around the sealing circumference and prevents the connection from bottoming out on the thread. At the same time, the elastomeric lip on the inner bore is compressed against the port face. This controlled compression – typically 25 to 35 percent of the lip height – conforms the rubber to any minor surface irregularities on the port face and creates the primary seal contact.
The sealing mechanism has an additional characteristic that makes it particularly reliable under pressure. As system pressure acts on the inner face of the seal, it pushes the elastomeric lip harder against the port face, increasing contact stress as pressure rises. This self-energising effect means the seal becomes more effective under pressure rather than less. A correctly specified hydraulic bonded seal therefore performs reliably across the full operating range of a system – from zero pressure at start-up through to maximum rated working pressure.
In applications involving vibration, such as hydraulic power units, compressor manifolds, valve actuators and mobile equipment, the elastic recovery of the rubber lip maintains the seating load in the joint even as the fastener experiences dynamic loading. This is one of the properties that distinguishes a bonded seal from a simple metal washer and it explains the product’s long service record in vibration-prone industrial environments.
Bonded Seals Applications and Industries
The bonded seal is used wherever a threaded port, banjo fitting, hydraulic adapter, bolt head or pipe plug connection requires reliable static sealing in service. Its combination of high-pressure capability, vibration resistance, elastic recovery and broad material availability makes it the standard solution across a wide range of industrial sectors.
Oil and Gas
In upstream operations, hydraulic bonded sealing washers seal the threaded ports on wellhead equipment, choke and kill manifolds, hydraulic control circuits and BOP accumulator systems. Operating conditions in this sector – high pressure, sour-gas exposure, elevated temperature and safety-critical containment requirements – make material selection non-negotiable. HNBR and FKM are the standard elastomer specifications for these applications, supporting API and NORSOK compliance requirements. In downstream refinery and petrochemical service, bonded seal instrument connections, sample ports and process valve actuator connections in aggressive chemical environments.
Valve Industry
Bonded seals appear at every hydraulic actuation port, instrument connection and vent or drain plug on a valve assembly or hydraulic actuator package. The volume of sealing points on a typical valve manifold or actuator block is high and the consequence of a leak on a process valve in an operating plant is significant. Dowty seals are the standard static sealing component for these connections across the global valve industry and they are specified in both OEM production and MRO replacement contexts.
Aerospace
Aerospace hydraulic systems, fuel systems and landing gear assemblies use bonded seals at threaded port interfaces throughout the aircraft. Material selection is particularly critical in this sector because no single elastomer is compatible with every fluid on an aircraft. Fuel systems require FKM or Fluorosilicone for resistance to hydrocarbon-based jet fuels. Hydraulic systems using phosphate ester fluids such as Skydrol must use EPDM. This is a safety distinction – FKM will degrade rapidly on contact with Skydrol and EPDM will fail on contact with jet fuel. Specifying the wrong elastomer is not a procurement error; it is a safety risk.
Aerospace procurement carries requirements that go beyond material selection alone. Dimensional tolerances are tighter than in most other industries, full batch traceability back to the elastomeric cure date is a standard requirement and certification compliance – including AS9100 and AS4451 – must be confirmed and documented before supply. These requirements should be clearly stated at the enquiry stage.
Chemical Industry
Chemical processing plants use bonded seals on instrument connections, sample valves, vent and drain plugs and hydraulic control lines throughout the facility. The primary technical consideration in chemical service is fluid compatibility – the range of process media in a chemical plant is broad and the elastomeric compound must be specifically confirmed against the chemical data sheet for the process fluid in use. An incorrect elastomer in a chemical service can fail rapidly, with consequences that include fluid release and unplanned shutdown.
Heavy Industry and Manufacturing
Hydraulic systems in heavy industry – metal processing, press machinery, injection moulding, construction equipment, offshore cranes and machine tools – use bonded seals at virtually every port, adapter and fitting in the circuit. The primary requirement in this sector is dimensional consistency across production batches, availability in standard sizes and material quality sufficient to support long intervals between planned maintenance stops.
Why Choose ISMAT for Bonded Seals?
Not all bonded seals in the market are manufactured to the same standard. The difference between a precision-manufactured seal and a low-quality equivalent is not always visible from the outside – it shows up in service, in bond failures, dimensional inconsistency and premature leakage from joints that should have remained sealed for years.
Engineering Precision
Dimensional accuracy is maintained across every production batch, not just on a sample. This ensures the elastomeric lip compresses correctly when the fastener is torqued to specification – preventing both under-sealing and extrusion in the field.
Metal-to-Rubber Bond Integrity
Every compound and primer system combination is validated through adhesion testing, heat ageing and chemical immersion testing. A bond that fails in service produces immediate leakage – this is an area where ISMAT’s in-house vulcanisation process adds direct, measurable value.
Elastomeric Compound Quality
All elastomeric compounds are qualified through testing ASTM D2000 or equivalent, confirming that hardness, compression set, tensile strength and chemical resistance meet specifications before the compound enters production.
Quality and Compliance
Material test reports, certificates of conformity and batch traceability are standard deliverables. ISMAT holds ISO 9001:2015 and IATF 16949 certifications and supports API 6A, NORSOK M-710, WRAS BS6920, FDA and NSF/ANSI 51 compliance requirements.
Customisation
Where standard catalogue sizes do not meet the requirements, ISMAT manufactures bonded seals to customer drawings – in non-standard sizes, custom material combinations and bespoke outer ring profiles. Bonded seal kits in specified sizes and elastomers are also available for maintenance and field service applications.
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For hydraulic and fluid system connections where leakage is not an option, choose bonded seals engineered for reliable static sealing under high pressure, thermal cycling and chemically aggressive service conditions.
Contact ISMAT for application-specific bonded seal solutions - in the right elastomer, the right metal and the right size - to meet your exact pressure, temperature and media requirements, from standard BSP and Metric sizes to fully custom specifications.
FAQs
There is no functional difference. The term “Dowty seal,” “Dowty washer” or “Dowty ring” refers to the same product as a bonded seal or hydraulic sealing washer. The Dowty name comes from the Dowty Group, the company that originally developed and commercialised this design. All these terms describe a metal outer ring with a vulcanised elastomeric inner lip used for static sealing at threaded ports and fittings.
The standard recommendation is to fit a new bonded seal each time the connection is remade. After the elastomeric lip has been compressed once, it has conformed to the surface of the port face and its elastic recovery is partially consumed. The seal may still function reassembly but contact stress and sealing reliability will be lower than with a new component. In safety-critical applications like oil and gas, aerospace and chemical processing – reuse of bonded seals is not recommended and is frequently prohibited by maintenance procedures.
A bonded seal kit is a pre-selected assortment of bonded seals in a range of standard sizes, packaged for use by maintenance departments, service engineers and field repair teams. Kits are typically supplied in one or two elastomeric materials and cover the port sizes found in a specific type of equipment or system. They can be built to a custom specification if the standard assortment does not match the range of sizes in use.
An elastomeric compound that is not chemically compatible with the process of fluid will swell, harden, crack or degrade in service. The result is loss of sealing force, compression set and eventually leakage. In aggressive media, an incompatible elastomer can fail within hours of first contact with the fluid. Material selection should always be confirmed against the process fluid and operating temperature before finalising the specification.
Yes. Bonded seals are available with elastomeric compounds meeting WRAS BS6920 approval for potable water service, FDA-compliant formulations for food and pharmaceutical equipment, NSF/ANSI 51 for food equipment materials and API 6A-compatible compounds for oil and gas wellhead service. The compliance requirement should be stated at the enquiry stage so that the correct compound and documentation can be confirmed.