Selecting and designing seals for pressure systems

Pressure systems are among the most sensitive technical applications in mechanical engineering and in industry. Even the smallest leaks lead to pressure losses, energy losses or failures of entire systems. The selection and design of the right seal directly influence operational safety, maintenance effort and service life.

In practice, the actual challenge arises from the interaction of several factors. Pressure, temperature and medium act simultaneously on the seal and change its behaviour during operation. In addition, movement, vibration and installation conditions have an effect. A well-founded design takes these interactions into account from the outset and ensures stable processes.

Understanding requirements in pressure systems

Seals work in the interaction of pressure, temperature and medium. These three factors largely determine the behaviour.

As pressure increases, the mechanical load increases. Materials require dimensional stability and a low tendency to extrude. Temperature influences elasticity and ageing. The medium determines chemical resistance, swelling and hardening.

Materials for pressure seals at a glance

NBR is suitable for oil-based applications in hydraulic systems and in mechanical engineering.

EPDM covers water, steam and many chemical applications and remains stable at changing temperatures.

FKM meets requirements at high temperatures and with aggressive media and offers long service lives.

PTFE is suitable for extreme chemical and thermal loads as well as for very high pressures.

Typical design values at a glance

Material	Temperature range	Pressure range	Special features
NBR	-30 to +100 °C	up to approx. 200 bar	oil-resistant
EPDM	-40 to +130 °C	up to approx. 150 bar	water- and steam-resistant
FKM	-20 to +200 °C	up to approx. 250 bar	chemical-resistant
PTFE	-200 to +260 °C	up to >500 bar	extremely resistant

Comparison of seal types

Seal type	Area of application	Advantages	Limitations
Flat gasket	Static connections	simple installation	uniform compression required
O-ring	Static and dynamic	versatile	dependent on the installation space
Profile seal	Complex applications	high load capacity	more complex design

Installation space and design

The installation space determines the function. O-ring grooves require a defined width, depth and edge radii.

Guide values for O-ring grooves:

• Groove width: O-ring cross-section + 10 to 30 %
• Groove depth: depending on the desired compression
• Edge radius: prevents damage during installation

The gap dimension is one of the most critical parameters. As pressure increases, the permissible gap dimension decreases.

Example guide values:

Pressure range	Max. gap dimension without back-up ring
up to 100 bar	approx. 0.2 mm
up to 250 bar	approx. 0.1 mm
>250 bar	use of back-up ring

Tolerance chains influence the function considerably. Manufacturing and installation must be considered together.

Calculation and compression

Compression is achieved via defined squeeze.

• static: approx. 20 to 30 %
• dynamic: approx. 10 to 20 %

Too little squeeze leads to leakage. Too much squeeze increases friction and wear.

Simplified relationship:

• higher pressure → higher risk of extrusion
• smaller gaps or back-up ring reduce risk

Static vs. dynamic

Static seals work without relative movement between the components. The sealing effect is created primarily by uniform compression and a clean adaptation to the sealing surface. What is decisive here is a correctly designed compression, a suitable surface quality and stable installation conditions. In static applications, the focus is on long-term tightness and low material fatigue.

Dynamic seals work under movement, for example on pistons, shafts or rotating components. Here, additional loads arise from friction, temperature development and wear. The seal must not only seal, but at the same time accommodate movements and remain permanently stable.

Friction plays a central role in dynamic applications. It leads to heating and directly influences service life. Materials therefore require good sliding properties and high abrasion resistance. PTFE offers very low friction values, while elastomers are more dependent on lubrication.

Lubrication also influences the function considerably. Suitable lubrication reduces friction, lowers the temperature and extends the service life. At the same time, it has a protective effect against wear and material fatigue.

The design of dynamic seals therefore requires a significantly more precise coordination of material, geometry and operating conditions than in static applications.

Decision logic for selection

The selection follows a clear structure.

• Medium determines the material
• Pressure determines the design
• Temperature determines the material class

This logic creates clarity in the selection and reduces wrong decisions already in the early planning phase.

Sealing system instead of a single seal

In many applications, a single seal is not enough. Instead, a sealing system is used.

A typical system consists of:

• sealing element
• back-up ring for protection against extrusion
• adapted geometry in the installation space

This combination significantly increases operational safety and reduces the risk of failures.

Temperature behaviour and expansion

Materials change their behaviour as temperature increases.

• thermal expansion changes the compression
• elasticity decreases at high temperatures
• at low temperatures, stiffness increases

These effects must be taken into account in the design, as they have a direct influence on the sealing effect.

Comparison of high pressure vs. low pressure

Factor	Low pressure	High pressure
Material	standard elastomers	FKM, PTFE
Design	simple geometry	reduced gaps
Protection	rarely required	back-up rings necessary

Typical mistakes in selection

• wrong material for the medium
• insufficient temperature design
• missing protection against extrusion
• wrong squeeze
• imprecise tolerances

Typical damage patterns

• cracking due to overload
• extrusion due to high pressure
• swelling due to chemical influences
• hardening due to ageing

Fault analysis in practice

Leaks often arise from too little compression or insufficient squeeze in the installation space. Adjusting the groove geometry or the squeeze improves the sealing effect.

Extrusion occurs when the gap dimension is too large. Material is pressed into the gap. Back-up rings and reduced gaps solve the problem.

Material failure manifests itself through cracks, deformation or hardening. The cause usually lies in the material selection. A suitable material increases the service life.

Installation errors arise from damaged seals or incorrect installation positions. Clean installation processes ensure the function.

Service life and maintenance

The service life depends on pressure, temperature, medium and movement.

High temperatures accelerate ageing. Chemical media influence the material structure.

Dynamic applications increase wear through friction. Suitable materials and lubrication extend the service life.

Practical values:

• static seals: several years of operation
• dynamic seals: depending on cycles and load

Regular maintenance includes visual inspection, replacement intervals and condition assessment.

Norms and standards

Standards create a reliable basis for the selection, design and interchangeability of seals. For O-rings, DIN ISO 3601 applies, which defines dimensions, tolerances and quality requirements and thus enables consistent design of grooves and seals. Materials are assessed on the basis of standardised test methods, for example with regard to temperature and media resistance, pressure behaviour and ageing. These standardised tests facilitate comparability between materials and manufacturers and reduce the risk of wrong decisions. A design in accordance with standards leads to reproducible results, simplifies procurement and increases operational safety throughout the system.

Industry requirements at a glance

Mechanical engineering places high demands on flexibility and service life. Frequently changing loads require robust materials.

Hydraulic systems work with high pressures and oil-based media. Seals require high extrusion resistance and media resistance.

The chemical industry places extreme demands on resistance. PTFE and FKM are frequently used here.

The food industry requires additional demands on hygiene and material compatibility.

Practical examples from industry and mechanical engineering

• Hydraulic system with 250 bar, oil, 80 °C → FKM O-ring with back-up ring
• Water system with 10 bar, 60 °C → EPDM flat gasket
• Chemical plant with 150 bar → PTFE seal
• Mechanical engineering application with 50 bar → NBR O-ring

Cost vs. service life

The choice of seal influences not only the function but also the overall costs.

• inexpensive materials often lead to shorter service lives
• high-quality materials reduce maintenance effort
• longer service life lowers downtime costs

The most economically viable solution results from the combination of material costs and service life.

Consequences of the wrong seal selection

An unsuitable seal leads to direct and indirect problems.

• pressure loss and leakage
• unplanned downtimes
• increased maintenance costs
• safety risks in the system

These effects show the importance of a clean design.

Checklist before ordering

• Check medium, temperature and pressure
• Define installation space and tolerances
• Select a suitable material
• Determine the suitable seal type
• Allow for safety reserves
• Check requirements for service life and maintenance

Recommended action

A robust sealing solution results from a structured approach. The starting point is the analysis of the operating conditions. From this follows the selection of a suitable material.

The next step is the selection of the seal type as well as the geometric design. Installation space, squeeze and gap dimension must be precisely coordinated.

Additional design measures increase safety in demanding applications. These include back-up rings or special sealing profiles.

The result is a solution that functions reliably, reduces failures and lowers costs in the long term.

Frequently asked questions

Which seal is suitable for high pressure?

For high pressures, O-rings with back-up rings as well as PTFE seals are particularly suitable. Which solution fits additionally depends on the medium, the temperature and the existing gap dimension.

Which seal withstands 200 bar?

For applications around 200 bar, FKM seals or PTFE solutions are frequently used. What is decisive is the overall design of material, geometry, compression and protection against extrusion.

When are back-up rings required?

Back-up rings are used when high pressures act on the seal and there is a risk of material being pressed into the sealing gap. They increase dimensional stability and improve operational safety.

How can a wrongly designed seal be recognised?

Typical signs are leaks, extrusion, cracking, swelling or premature hardening. Unusually rapid wear or frequent failures also indicate an unsuitable design.

Which standard applies to O-rings?

DIN ISO 3601 is relevant for O-rings. It regulates dimensions, tolerances and quality requirements and thus creates a reliable basis for selection and interchangeability.

What role does the medium play in seal selection?

The medium directly influences the chemical resistance of the material. Oils, water, steam or aggressive chemicals act differently on elastomers and plastics and thus determine the service life of the seal.

What is particularly important with dynamic seals?

With dynamic seals, friction, temperature development and wear are the main focus. Material, lubrication and geometry must be coordinated so that the seal can accommodate movement and seal permanently.

How much may a seal be squeezed?

The permissible squeeze depends on the seal type and application. In static applications, typical values are often around 20 to 30 per cent, in dynamic applications rather lower.

What are the consequences of an unsuitable seal?

An unsuitable seal leads to pressure loss, leakage, increased maintenance effort, unplanned downtimes and, in the worst case, safety risks throughout the system.

Why is the gap dimension so important in pressure systems?

The gap dimension influences whether the seal can deform into the gap under pressure. The higher the pressure, the smaller the gap must be designed or the more likely additional back-up rings become necessary.

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