High-pressure seals

High-pressure seals are used wherever media must be permanently and reliably sealed under extreme pressure, high temperatures and heavy mechanical loads. In industrial applications, these loads often act simultaneously and over long periods. Even minor deviations in material, design or installation have a direct effect on the sealing function. High-pressure seals are therefore among the safety-relevant components in plant and mechanical engineering.

This technical article examines in detail which requirements high-pressure seals must fulfil, which materials and designs have proven themselves and how an application-appropriate selection is made for high thermal and mechanical stresses.

Requirements for high-pressure seals under extreme conditions

High-pressure seals must perform considerably more than standard seals. They are permanently exposed to high forces and changing operating conditions. Typical requirements are:

• very high system pressures and surface pressures
• high and frequently fluctuating operating temperatures
• thermal expansion of adjacent components
• mechanical vibrations and load cycles
• chemical resistance to media and process substances

Combinations of temperature and pressure changes are particularly critical. These lead to settling phenomena, material fatigue and stress redistribution, which a high-pressure seal must reliably compensate for.

Thermal loads in detail

High temperatures influence the sealing behaviour of high-pressure seals in several respects. As temperature rises, the mechanical properties of almost all materials change. Strength, elasticity and resilience decrease, while at the same time thermal expansion of the seal, flange and adjacent components occurs. These effects lead to stress redistribution in the sealing system, which must be compensated for over the entire operating period.

In addition, high temperatures accelerate ageing processes in the material. In fibre-based seals, binders can break down, causing the internal structure to lose stability. Elastomers tend to become brittle or increase in hardness under continuous thermal load, which reduces their ability to adapt to the sealing surfaces. Metallic materials, in turn, are subject to oxidation processes and creep deformation at high temperatures, which influence contact pressure in the long term.

Applications with frequent temperature changes are particularly demanding. During heating and cooling, cyclic expansion and contraction movements occur, which promote settling phenomena and change the preload of the bolted connections. High-pressure seals for high thermal stresses must permanently compensate for these effects and reliably maintain their sealing effect even at temperature peaks. Graphite-based and metallic high-pressure seals have proven particularly effective in this environment thanks to their high temperature stability and favourable adaptation behaviour.

Mechanical stresses and pressure load

Alongside temperature, mechanical pressure is a central load variable. High pressures generate extreme surface pressures on the seal. In addition, the following act:

• bolt preload forces
• dynamic pressure surges
• vibrations and oscillations
• relative movements between components

A suitable high-pressure seal must remain dimensionally stable and must not extrude or deform plastically under pressure. The right combination of material strength and adaptability is decisive here.

Typical areas of application for high-pressure seals

High-pressure seals for high thermal and mechanical stresses are found in almost all key industrial sectors:

• chemical and process industry
• power plant and energy technology
• oil and gas production as well as refinery technology
• mechanical and plant engineering
• hydraulic and high-pressure systems

In these applications, leaks frequently lead to production downtime, safety risks or environmental pollution. The requirements for seal quality and design are correspondingly high.

Materials for high-pressure seals at a glance

The choice of material decisively determines the performance of a high-pressure seal. Proven materials are:

• Graphite seals: very high temperature resistance, good adaptation to flange surfaces, suitable for high pressures
• Metallic high-pressure seals: extreme pressure and temperature resistance, often designed as ring-type joint or lens seals
• Fibre-based seals: highly compressed materials with temperature- and pressure-stable binders
• Elastomer-based high-pressure seals: for dynamic applications with pressure peaks and moderate temperatures

In many cases, multi-layer or combined sealing systems are used in order to combine the respective advantages of the materials.

Designs and seal geometries

The design and geometry of a high-pressure seal are always precisely matched to the respective application. The aim of the structural design is to use the existing bolt forces and system pressures in such a way that a permanently stable surface pressure is created. Metallic high-pressure seals often work with very small, clearly defined contact surfaces that deliberately generate high pressures and thereby ensure reliable sealing even at extreme pressures. A controlled plastic deformation is intended here, through which the seal adapts optimally to the mating surfaces.

Softer high-pressure seals, by contrast, have larger contact zones and a greater material thickness in order to compensate for settling phenomena and to offset surface irregularities of the flanges. The inner and outer contours are designed so that yielding or extrusion of the material under pressure is avoided. The right geometry ensures that the sealing effect is maintained over the entire service life, even under changing thermal and mechanical loads.

Influence of the flange and mating surfaces

The quality of the flange and mating surfaces has a direct influence on the performance of high-pressure seals. Even optimally designed seals can only fulfil their function if the adjacent components provide the required conditions. A defined surface roughness ensures that the seal can adapt in a controlled manner without being damaged or subjected to uneven loading.

Flat and stress-free flange surfaces are equally decisive. Distortion, edges or local irregularities lead to uneven surface pressure and promote leaks. At high pressures, the stiffness of the flanges also plays an important role, since components that are too flexible deform under load and reduce the preload of the seal. High-pressure applications therefore require a coordinated consideration of the seal and the mating surfaces as a functional unit.

Installation and operational safety

Installation is a central factor for the performance of high-pressure seals. Important aspects are:

• even tightening of the bolts in a defined pattern
• correct calculation of the bolt preload
• clean, grease-free sealing surfaces
• observation of settling behaviour after commissioning

Many seal failures can be traced back to installation errors or incorrect preload forces.

Selecting the right high-pressure seal

The selection of a high-pressure seal for high thermal and mechanical stresses is always application-related. Relevant criteria are:

• maximum operating pressure and temperature range
• temperature and pressure changes during operation
• type of medium and chemical load
• static or dynamic application
• flange type and installation situation

Individually manufactured high-pressure seals and precisely fitting cut pieces increase operational safety and reduce downtime.

Economic aspects and service life

Alongside the technical requirements, economic aspects also play an important role for high-pressure seals. High-quality materials and a precise design do cause higher procurement costs, but have a positive effect on the overall operating costs of a plant. A durable high-pressure seal reduces maintenance effort, extends service intervals and decreases unplanned downtime.

The longer service life lowers the risk of consequential damage to adjacent components and loss of media. In safety-critical applications, costs due to production downtime or repairs can also be avoided. Considered over the long term, the increased operational safety clearly compensates for the investment in a high-quality high-pressure seal.

Summary and classification for demanding high-pressure applications

High-pressure seals for high thermal and mechanical stresses perform a central function in the safe operation of industrial plants. They must permanently withstand extreme pressures, high temperatures and changing mechanical loads while reliably maintaining their sealing effect. What is always decisive is the interaction of a suitable material, a precisely designed geometry, matching flange and mating surfaces and professional installation.

Standard solutions quickly reach their limits under these conditions. Only high-pressure seals selected specifically for the application enable stable surface pressure, compensate for settling phenomena and offset thermal expansion in a controlled manner. As a result, service lives can be extended, maintenance intervals optimised and unplanned plant downtime reduced. For operators, this means greater operational safety, plannable processes and, in the long term, more economical plant operation.

FAQ on high-pressure seals

Which temperatures can be achieved with high-pressure seals?

The permissible temperature range depends on the material and extends from moderate temperatures to well above 500 °C for graphite- or metal-based high-pressure seals.

Which pressures can high-pressure seals withstand?

Metallic high-pressure seals are designed for very high pressures in the three-digit bar range and are used specifically for extreme high-pressure applications.

Are high-pressure seals suitable for dynamic applications?

Certain designs and materials are also suitable for applications with pressure and load changes, provided that the geometry and preload are matched to them.

Why do leaks occur despite high-quality high-pressure seals?

Leaks often arise from unsuitable flange surfaces, insufficient bolt preload or deviations during installation.

What role does surface roughness play in high-pressure seals?

Surface roughness influences the even surface pressure and thus the adaptability of the seal. Deviating roughness values have a direct effect on the sealing effect.

When are individually manufactured high-pressure seals worthwhile?

Individual high-pressure seals offer advantages at extreme pressures, high temperatures or special installation situations in which standard solutions can barely achieve the required service life and operational safety.

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