Pump engineers have a wide range of sealing options available that cater to diverse pump applications. In designing pumps and other associated equipment like compressors and valves, elastomers are one flexible sealing option. An elastomer's ability to return to its original, un-deformed shape after the removal of force (unlike rigid sealing materials like PTFE) helps the pump designer prevent leakage under demanding pressures and operating environments.
Pump users need maximum operating efficiency from their investment, and sealing solutions have to be designed to function between long service intervals. Users may also seek to run equipment for extended periods and often under greater working demands given current global economic pressures. These conditions often create increased chemical, thermal and pressure related stresses on the pump and seals, multiplying the risk of seal failure and pump leakage.
Under Pressure
In many applications, pump seals are not exposed to particularly hazardous conditions or operating pressures. For most pneumatic pumps and compressors, the mix of air, water, fine oil mist and moderate heat to which they are subjected can be typically handled by mid-range elastomers such as nitrile (NBR) and EPDM. For hydraulic pumps, however, operating pressures are significantly higher. Consequently, more appropriate materials would include HNBR or the higher performing fluoroelastomer (FKM), which exhibit high strength and good oil resistance.
High pressure pumping applications present unique challenges for elastomeric seals. Since all elastomers act similarly to high viscosity liquids, they can flow under extreme pressure conditions. In such cases, back-up rings are often used to prevent the seal material extruding into the clearance gaps, ensure seal performance and prevent seal leakage once pressure is relieved. As with the elastomer itself, the chemical and thermal compatibility of the back-up ring and its ease of installation need careful consideration.
Keeping It Tight
Low pressure vacuum applications are equally challenging as the performance of a vacuum system will be significantly degraded if air or other gases are allowed to enter. Different elastomer types and compounds within the same material family will exhibit a varying degree of gas permeability depending upon the gas and environmental conditions. Additionally, low molecular weight elements sometimes included in the elastomer formulation such as process aids, or even air or gas entrapped during the manufacturing of the seal, can create a phenomenon known as out-gassing. This will negatively impact the stability of the vacuum system and seal performance over time, and can increase the time taken to achieve a full pump down.
However, new sealing materials are increasingly available to help vacuum pump manufacturers enhance performance and prevent seal leakage and out-gassing. These materials offer exceptionally low permeability to meet the needs of vacuum processes and environmental emissions regulations. Values in helium leak-testing have indicated that specially developed self-reinforcing polymer compounds can be up to 10 times more effective in vacuum applications than standard FKM materials.
One new type of self-reinforcing compound uses a unique polymer structure that delivers increased chemical resistance and 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 a pure elastomer less prone to chemical attack.
The Heat Is On
Elastomer sealing in both high and low temperature environments poses particular difficulties. There is often a compromise between elastomers that offer high or low temperature capabilities and those that offer good chemical resistance.
As pump users increase the time a pump is running and reduce cool down periods to improve output during cost cutting campaigns, the risk of seal failure due to heat damage becomes a real danger. Pumps are designed with sufficient cooling in mind, but the heat-soak during shutdown can damage the seal or reduce the seal life. A crucial consideration is that the temperature of the process is not necessarily the highest temperature the seal will have to withstand. It could be higher or lower depending on the seal location and the effectiveness and use of the cooling system. It is critically important that the correct sealing material is selected initially.
For pump applications, the latest elastomer materials are capable of operating in temperatures between -100-deg C (-148-deg F) for silicone, up to +325-deg C (+617-deg F) for perfluoroelastomers (FFKM). One new FKM grade is a low compression set compound designed to operate in high temperature pump applications up to +200-degC (+392-deg F) and provides heat ageing properties combined with chemical resistance. With a compression set of just 5 percent*, this FKM grade retains its mechanical properties significantly longer than conventional FKM grades, allowing extended seal service life and increased efficiency.
Modified elastomer polymer architecture results in a low temperature fluoroelastomer (FKM) that extends the low temperature capability of FKM down to -40-deg C (-40-deg F) while still providing an upper operating temperature of +200-deg C (+392-deg F).
Both of these components present chemical engineers, pump manufactures and service organizations with the opportunity to increase service intervals, prevent leaks and generally increase pump efficiency. Moreover, the improved sealing efficiency provided by these elastomers will allow pumps to run quieter-an important consideration in enclosed environments. Both exhibit the same chemical and mechanical properties as conventional FKM grades and can be manufactured into almost any sized O-ring (standard and non-standard) as well as custom designed parts.
When Pressure Suddenly Changes
For seals that may encounter explosive decompression (ED) forces, a number of solutions are available: groove redesign, seal redesign, use of back-up rings or a specially developed ED resistant elastomer. Tetrafluroethylene/Propylene or TFE/P can be formulated to resist ED under industry standard conditions. However, until recently this material had low temperature capability-an important factor when operating temperatures can be high and sudden decompression can create low sub-zero temperatures.
These issues have now been overcome with customization of this polymer formulation, delivering seals that operate down to -25-deg C (-13-deg F) and up to +250-deg C (+482-deg F) (or +290-deg C (+554-deg C) for short periods) while still retaining the steam and chemical/sour gas resistance characteristic of this polymer type.
In the Event of Failure
Seal failure in pumps can be caused by many factors including accidental fitting of the wrong seal, damage through abrasion, chemical attack or heat ageing. When a seal is compatible with the process and is correctly designed and installed, seal failure can be an early indication of mechanical problems in the pump.
The use of polymer testing techniques, using various analytical equipment, can indicate why a seal has failed and ensure that the specified material is suitable. TGA (thermo gravimetric analysis) can be used to assess the elastomer's composition and thermal stability, DSC (differential scanning calorimeter) is used to evaluate any degradation within the polymer structure and FTI (fourier transform infrared) measures any changes within the elastomer's chemical structure. Test data obtained from these analysis techniques can then be used to fine-tune sealing materials to optimize performance.
Summary
Avoiding seal leakage or failure is a complex task for pump designers and one that must be taken seriously in difficult economic times when reducing downtime is critical. New developments in elastomer materials and seal design techniques mean that pump engineers can avoid seal leakage and ensure optimum performance in demanding operating environments involving high and low pressures and temperatures.
By working closely with material developers and seal manufacturers, pump manufacturers and users can improve the pump performance and contribute to increased operating efficiency during these challenging times.
Notes
* Compression set test ASTM D395, 24 hrs at +200-deg C (+392-deg F)