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Compression and recovery

The optimum gasket
The optimum gasket would be highly compressible, in order to be able to adapt
to all  ange surface unevenness when it is tted. It would at the same time recover
100%. The concepts of compression and recovery are explained in detail below.
Compression
Compression plays a major role in practical application. This property determines
the adaptability of the gasket to the sealing surface. The more a material can be compressed, the greater its ability to adapt to faults in the sealing surface like scratches or grooves. Figure 1 is a diagram showing the surface structure of a  ange. Surface leakage – leakage between the gasket and the sealing surface – can be reduced signi cantly thanks to the adaptability of the material.
Recovery
Theoretically speaking, a gasket recovers when the distance between the sealing surfaces in the application is increased by certain peripheral conditions (e.g. internal pressure). In view of this, strong recovery would be a priority.
Test procedure
Several di erent approaches can be adopted to determine the compression of a gasket
ASTM F 36 J
There are di erent versions of the ASTM F36 test for di erent materials. Procedure J is outlined in detail here. In this method, three gasket thicknesses are measured with de ned loads using a tool with a
O of  1 “. Compression is de ned as the deformation of the gasket under the main load compared with the original thickness. Recovery is de ned as the change in thickness on removal of the load in relation
to the change in thickness between the preload and the main load. Figure 2 shows the three relevant gasket thicknesses during the ASTM 36 test.
The recovery  gure must not under any circumstances be considered in isolation; there is a direct connection between it and the prior compression.
Model calculation:
Original thickness P: 2.00 mm
Thickness under the main load
M: 1.8 mm   compression: 10 %
Thickness after load removal
R: 1.9 mm   recovery: 50 %
DIN 28090-2
DIN 28090-2 also describes a method for determination of gasket deformation, both at room temperature (cold compressibility) and at higher temperatures (hot creep). A standard sealing ring (55 x 75 mm) is tested in a hydraulic press in this context Recovery levels are also determined.
Cold compressibility, i.e. compression at room temperature, is de ned as the change in the thickness of the gasket under preload and main load in relation to the original thickness. Cold recovery is needed as the change in the thickness of the gasket between main load and preload in relation to the original thickness of the gasket. Hot creep and hot recovery are determined by similar means at a speci c test temperature per material (graphite 300°C,  bre material 200°C, PTFE 150°C).
All the results are expressed in a percentage of the original thickness h

ASTM 36 J DIN 28090-2 Compression graph

+ Simple testing equipment + The specimen tested is a practical sealing ring + The specimen tested is a practical sealing ring

- The gasket thickness tested must be at least 1.6 mm + Easily interpreted results + Test can be carried out at any temperatures

- Behaviour at only one de ned surface pressure level is tested + Test is carried out at higher temperatures too + A complete range of surface pressure levels is tested

- The test is only carried out at room temperature - Complex testing equipment - Complex testing equipment is required

- Recovery results can only be interpreted together with compression levels - Behaviour at only one de ned surface pressure level is tested
- Only a small area of the material is tested

Material inhomogeneity is not covered

Compression graph
The compression graph is speci ed in a standard, e.g. DIN EN 13555-2014, and has the purpose of determining the deformation properties of a gasket material. The force applied to the gasket
in a press is increased as de ned and the respective thickness of the gasket is measured. The measurements can be taken at room temperature or at higher temperatures. The graph reveals the
deformation in relation to the surface pressure.
The advantages of the compression graph are that a standard sealing ring is tested and that not just one surface pressure level is reviewed; the entire range from 0 MPa to about 250 MPa is covered instead
(depending on the capacity of the testing equipment).
Comparison of the methods
The advantages and disadvantages of the individual methods are compared in Table 1.
The ASTM test carried out on the test equipment
The following graph – Figure 3 – visualises The following graph – Figure 3 – visualises ASTM test procedure was carried out on two di erent materials on the test equipment: very adaptable gasket material(dashed black line) and relatively hard gasket material containing elastomer(uninterrupted green line). Most of the recovery with both materials only occurred at surface pressure levels lower than 5 MPa.
Summary
Both the standardised tests to determine compression based on ASTM F 36 and DIN 28090-2 produce results about the deformation of a gasket material at de ned surface pressure levels in each case. Both
processes are suitable ways to obtain an initial impression about material properties and to enable materials to be compared with each other.
The compression graph, on the other hand, provides a more in-depth insight into material compression, because it covers the entire range of surface pressure levels. In view of the many di erent requirements,
it is not possible to  nd an optimum gasket for all applications. The right gasket material must be selected according to the medium, the temperature, the pressure level, the structure of the sealing surface and many other parameters. Gasket manufacturers generally make the necessary information
about the di erent materials available.

By ESA member A. Will      

The European Sealing Association (ESA) has produced this article as a guide towards Best Available Techniques for sealing systems and devices. These articles are published on a regular basis, as part of their commitment to users, contractors and OEM’s, to help to  nd the best solutions for sealing challenges and to achieve maximum, safe performance during the lifetime of the seal. The ESA is the voice of the  uid sealing industry in Europe, collaborating closely with the Fluid Sealing Association
(FSA) of the USA. Together, they form the key global source of technical knowledge and guidance on sealing technology, which is the basis for these articles. For more information, please visit www.europeansealing.com