- 13 December 2006 -

Elastomer seals taking the pressure

"When the going gets tough, the tough get going" is as true for seals as it is for motivational training. Seals are increasingly being used in extreme processing environments, and pumps are no exception. Paul Haworth, technical manager at Precision Polymer Engineering looks at how recent elastomer seal developments are coming to the aid of the pump engineer.

Elastomers have a unique property that sets them apart from all other types of pump sealing material - their elasticity. When a deforming force is applied, elastomers will retract to their original shape once the force is removed. It is this ability for the elastomer to return to its original, undeformed shape that gives it sealing force. The way in which a seal performs is an important factor in pump efficiency.

There are two important factors when specifying a seal - design and material selection. An excellent seal material that is fully compatible with the application conditions can still fail if it is incorrectly designed. Equally, an excellent design can be compromised by selecting the wrong material. Installing the wrong type of seal by accident can damage the pump and lead to premature seal failure. On the other hand, if the seal is compatible with the process, correctly designed and installed, seal failure can be an early indication of mechanical problems in the pump. If the pump's bearings are damaged, for example, the rotating shaft will tend to 'flex,' which can increase wear on the seal and lead to early seal failure. Not catching the early warning signs of these kinds of failures can be expensive, as one oil company discovered when the gas compressor on its production platform failed. With no spare seals on board, production had to be halted for a day, which cost more than £1 million in lost revenue.

The use of analytical techniques such as TGA (thermo gravimetric analysis), DSC (differential scanning calorimeter), and FTIR (Fourier transform infra-red) can give valuable information about seal wear and an insight into why a seal has failed. Moreover the results of the analyses will indicate whether or not the seal material is suitable for the pumping application and, where necessary, provide an indication of how it may be fine-tuned to obtain the best possible performance.

Recent elastomer developments

Recent developments in elastomer seals for pumps have tended to focus on extreme pumping environments of decompression, pressure, and temperature, where the inflexibility of rigid sealing elements reduces plant productivity.

In most cases, pump seals are not exposed to particularly hazardous conditions. For example, in pneumatic pumps/compressors the seals are subjected to a mix of air, water, fine oil mist, and moderate heat, all well within the capability of mid-range performance elastomers. In hydraulic pump seals, the operating environment again is not overly aggressive and consists of moderate heat, oil, and, in some cases, mild acidity. However, operating pressures are significantly higher. As a consequence, seals are usually made of either low-cost NBR (nitrile) or the higher performing fluoroelastomer (FKM).

There are some important exceptions, however. For pump applications such as laboratory test rigs or gas compressors, special elastomers are required. In such cases, extremely high pressures (in excess of 300 Bar) are experienced and de-pressurisation rates are spectacularly swift, from 300 Bar to atmospheric within one second. This exerts tremendous forces on the seal and it can become saturated, through permeation, by the pressurised media. This phenomenon is called explosive decompression.

Explosive decompression (ED) is a failure mechanism of elastomer seals that is due to the rapid depressurisation of a gaseous media. When elastomer seals are exposed to high pressure gas for a prolonged period the gas will permeate into the seal material. When the external gas pressure is reduced, the gas dissolved within the seal material expands. As the gas expands, it may permeate out of the seal material. However, if the rate of decompression and the resultant expansion is high, gas trapped within the seal can cause fissuring and result in seal failure. Elastomers have differing resistance to this mechanism depending upon a number of physical properties, including permeability, tensile strength, elongation at break, and maximum void size. Many variables can influence the possibility and degree of explosive decompression failure, including seal cross-section, temperature, seal compression, and groove-fill. Special elastomer compounds are now available that use reinforcing fillers to reduce the incidence of explosive decompression damage. hese materials must be fully tested for explosive decompression resistance using dedicated test equipment able to simulate high pressures at varying temperatures, with rapid decompression rates, while being immersed in a variety of gases.

One such material is AFLAS (tetrafluoroethylene/propylene (TFE/P) elastomer). It can be compounded to be resistant to explosive decompression under conditions specified by National Association of Corrosion Engineers. (NACE TMO192-98 "Evaluating Elastomeric Materials in Carbon Dioxide Decompression Environments") Yet, until recently, AFLAS was unsuitable for applications where the elastomer was exposed to temperatures above 200°C. By customising the polymer formulation, explosive decompression-resistant AFLAS seals are now available with temperature resistance up to 250°C and, for short periods, 290°C. Of course, these AFLAS seals still retain the excellent steam and chemical/sour gas resistance that are characteristic of this material.

Taking the pressure

High pressure pumping applications present unique challenges for elastomeric seals in both design and elastomer compound type. Elastomers are essentially high viscosity liquids, and as such will flow under extreme pressures. In these applications it is possible for the seal material to extrude into the clearance gap, it may be necessary to use a back-up ring. The back-up rings are either flat or contoured rigid rings that are inserted on the low pressure side of the seal gland to prevent extrusion. If the rings are used, additional gland width is required. As with the elastomer itself consider the chemical and thermal compatibility of the ring as well as the ease of installation carefully when making your choice. For instance, the AFLAS elastomer can also be combined with other materials such as metal substrates or reinforcing fabrics for high-pressure applications. The modified AFLAS has successfully operated up to 30,000 psi in conjunction with a PEEK (polyetheretherketone) anti-extrusion ring.

Conversely, low pressure applications are equally challenging because the performance of a vacuum chamber can be greatly influenced by the materials of construction, and, most importantly, by the seals.

All materials will permeate gas under extreme vacuum conditions. (At ultra high vacuum of 10-9 Torr, hydrogen molecules can permeate through the steel walls of the chamber.) Elastomeric seals will not be suitable for vacuum lower than approximately 10-7 Torr because of their inherent gas permeability. Some elastomers are better than others, but eventually all types will permeate gas. Low molecular weight elements within the elastomer, stemming from the polymer itself or process aids compounded into the elastomer, can create a phenomenon known as outgasing. This can influence the stability of the vacuum over time, or the time it takes to achieve full vacuum pump down. Similarly, any entrapped air or gas from the extrusion and moulding processes of the seal can outgas with the same effects.

Nano-engineered elastomers

New sealing materials are now becoming available that offer exceptionally low permeability designed to meet the needs of vacuum processes and environmental emissions regulations. Values in helium leak-testing have indicated that nano-filled compounds are up to ten times more effective than standard FKM materials. These nano-filled materials also offer other benefits. The first of a new type of nano-engineered fluoroelastomer uses fully fluorinated nano-fillers bound within the structure of the elastomer, enabling the delivery of increased chemical resistance and exceptionally low levels of permeability. The polymer's structure significantly reduces the gas permeability of the elastomer compared with standard fluoroelastomers and perfluoroelastomers, leading to reduced swelling from exposure to solvents. Moreover, the absence of metallic or carbon-based fillers produces an exceptionally pure elastomer that is less prone to attack by chemicals.

New developments in elastomer seals offer pump engineers a practical solution to meeting their sealing needs. All that remains is for them to take the initiative and work more closely with seal developers and manufacturers to fine-tune the performance of the seal, and in so doing, increase the competitiveness of their pumps.

*AFLAS is a registered trade mark of Asahi Glass

To receive news and features like this direct to your inbox sign-up for the World Pumps enewsletter.

Simply register your details to receive a fortnightly roundup of the latest news from the pump and sealing industries direct to your inbox.

If you would like to advertise in the World Pumps enewsletter please contact our sales team.

If you have some news for the pump industry or would like to comment on any of the articles on this site, contact our editorial team.

You can also access the full list of contacts here.