A safe option

Three material types have established themselves on the market for asbestos-free gaskets: PTFE, elastomer-bonded fibre materials and flexible graphite. Unlike expanded graphite, PTFE gaskets have a tendency towards cold and warm flow, which can cause sealing problems in many types of media.

Elastomer-bonded fibre gaskets consist of glass, carbon or synthetic (e.g. aramide) fibres embedded in an elastomer binder. Typically, they can only be used for short periods at temperatures up to 200 °C. The temperature limit is determined by the binder.

At temperatures above approximately 150 °C, the material starts to decompose. After prolonged use, the resulting binder loss or „degassing“ gradually destroys the sealing properties. As the sealing material loses its flexibility, it becomes hard, brittle and porous. This results in a sharply increasing leakage rate and loss of creep strength under compressive stress.

The same process also takes place at temperatures below 150 °C, though not to the same extent. Hence, embrittlement at these lower temperatures does not become apparent until after prolonged use. The embrittlement of fibre gaskets is a physical process, which cannot be prevented by using the new graphite-enriched material recently launched on the market. Embrittlement is primarily due to the use of organic binders.

Sigraflex Hochdruck gaskets contain no binders of any kind. Depending on assembly conditions and the level of atmospheric oxygen present, they are suitable for use at temperatures up to approximately 450 °C. In steam applications, their maximum operating temperature is even higher.

The gaskets are made from thermally expanded and densified, natural, high-purity graphites. They are not subject to ageing. When used within the specified temperature range, their creep compression values under standard gasket pressures are negligible. They therefore differ from elastomer-bonded fibre or PTFE gaskets in that they need only a bare minimum of retightening and main-tenance.

The Sigraflex Hochdruck multilayer material comprises 0.5 mm thick layers of high-purity graphite foil and 0.05 mm thick stainless steel foils. The graphite and stainless steel foil layers are bonded together without using adhesives. This special bonding technique accounts for the unique properties of the Sigraflex Hochdruck sealing material. Thanks to this structure, the bonded sealing material can absorb very high gasket pressures beyond those which can normally be applied to conventional graphite gaskets. This ensures maximum safety, especially in tongue-and-groove applications and in cases of severe flange unevenness or under extreme operating conditions. Crucially, these properties are maintained even at high temperatures up to 450 °C owing to the absence of adhesives.

In contrast, the maximum permissible gasket pressures for the familiar bonded versions are relatively low. This is because the adhesive used may allow the graphite layers to become more easily detached from the stainless steel foils and slide out.

Sigraflex Hochdruck affords maximum safety during assembly and operation: with gaskets meeting DIN 2690, the permissible gasket pressure in raised-face DIN flanges cannot be exceeded. Depending on the operating and assembly conditions, the recommended maximum operating pressure for these gaskets is approximately 250 bar.

The purity or ash content of graphite has a direct influence on the leakage rate, corro-sion resistance and thermal stability of the sealing material.

The ash content of laminated Sigraflex graphite foils in Sigraflex Hochdruck material is in the high-purity range below 0.15% by weight. Sealability, corrosion resistance and thermal stability are also influenced by the type of contaminants. Hence, all Sigraflex Hochdruck sealing material is produced from special graphite grades.

Sigraflex Hochdruck is supplied with impregnated surfaces. The surface treatment provides the following special effects:

• reduced moisture absorbency and better impermeability as a result of sealing the graphite pores close to surface,

• improved scratch resistance and rigidity,

• reduced adhesion of the gasket to the flange sealing face during dismantling.

This special impregnation of the Sigraflex Hochdruck material ensures sealing values that meet German clean air regulations, class 1. Such values have previously been unattainable with graphite gaskets.

The stainless steel foil reinforcement allows the material to be easily cut on the customer’s site with commercially available cutting tools. Cost-intensive production stoppages can thus be minimized.

The impregnation presents no health hazards whatever.

Sigraflex Hochdruck has been certified as fire-safe in accordance with BS 6755-2. To be classed as fire-safe, fittings, pumps and valves must withstand short-term exposure to fire during operation. This means that the components must meet specified demands regarding external sealability under extreme conditions and thus ensure a high level of safety.

Sigraflex Hochdruck is particularly suitable for meeting such fire safety demands, e.g. resistance to high temperatures, high tensile and compressive strength and high recovery. The level of these properties is even higher than in spiral-wound and cam-profile gaskets.

The reason for the stringent demands relating to tensile strength and recovery is that the gasket pressure in the flange joint usually drops in the event of fire. The effect on the required sealability is then compensated by the high recovery of the Sigraflex Hochdruck sealing material.

If a pipe system is exposed to fire, the thermal stresses which occur will cause major flange plate distortions that produce higher compressive stresses on the flange gasket. The high compressive strength values required at such high temperatures can be obtained with pure metal gaskets on the one hand or adhesive-free, metal-reinforced high-quality Sigraflex Hochdruck gaskets on the other.

Sigraflex Hochdruck sheet material is avail-able in dimensions of 1500 mm x 1500mm. This allows relatively large gaskets to be produced in a single piece. The sheet specially developed for process equipment requirements is designated (Sigraflex Hochdruck AP (V30011Z3I).

SGL Technik GmbH

Fax: ++49/8271/83-2419

Further information cpp-202