Mechanical seals are used as shaft seals in pumps, agitators, compressors and other rotary machines handling liquid and gaseous media. The process pressures which have to be contained by the seals range from vacuum to more than 400 bar. In some cases, the chemical and thermal stability required of the mechanical seal is very high. The design of the seal and the choice of material from which it is made play a crucial role in making sure that it operates reliably for a sustained length of time under the conditions for which it was designed and for which the material was chosen.
Of the key components of a pump, the mechanical seal has so far been the one with the shortest average service life. It therefore has a major influence on the MTBF / MTBR values of the system as a whole. The primary aim of all operators must be to improve these parameters significantly and minimize the process downtime costs which result from unplanned interruptions.
Why Choose a Diamond Coating?
Any system can only be as efficient as the lifespan of its weakest component. Investigations have shown that damage to the sliding faces in combination with inadequate lubrication (which results in dry-running) is still the main threat to the integrity of mechanical seals. The choice of suitable sliding materials is essentially determined by the solids content in the medium and its lubrication properties. With pure fluids, predominantly hard / soft sliding combinations are selected, with the soft component usually being an impregnated carbon graphite. The range of applications for soft carbon graphites is limited by the solid particles present in the medium, maximum permissible abrasive wear rates and, in some cases, inadmissible product contamination (pharmaceuticals).
The SiC/SiC material pairs which have been increasingly favored for seal faces during the last 10-15 years offer excellent chemical resistance and high resistance to abrasion as well as being virtually wear-free, but only if adequate lubrication is provided in the sealing gap. Any form of under-lubrication which causes dry-running results in massive temperature increases and damages the sliding faces and secondary seals (O-rings). In extreme cases, the time to total failure of the sliding faces may be just a matter of a few seconds. In practice, insufficient lubrication can occur due to the level of gas in the medium being unacceptably high, incorrect operation or even evaporation of the medium in the sealing gap.
This is precisely where EagleBurgmann’s diamond technology comes into its own! Outstanding abrasive resistance can be achieved by depositing a crystalline diamond coating onto the SiC sliding surfaces from an activated gaseous phase and this also considerably improves sealing efficiency when there is insufficient lubrication.
Diamonds Are Not All the Same!
Amorphous diamond-like carbon (DLC) coatings have been successfully used for wear protection in gas-lubricated mechanical seals for approx. 10 years. Until now, however, the coating adhesion limits and wear resistance of DLC coatings have often been exceeded in liquid-lubricated applications or under high-load conditions.
In contrast to this, the EagleBurgmann DiamondFace crystalline diamond layers, which have been specially developed for the specific applications in mechanical seals, have shown excellent chemical and thermal stability as well as unmatched hardness. In inert atmospheres, diamond is stable up to a temperature of approx. 1500°C, and in an oxidizing environment it begins to burn at approx. 600°C. Acids and lyes have no effect on diamond.
For the large-area, uniform coating of ceramic seal faces, an optimized HF-CVD (Hot Filament Chemical Vapor Deposition) process is used to meet these requirements. The surfaces are coated in a vacuum chamber using electrically heated tungsten wires. While this is happening, the topography of the surface and the orientation of the diamond crystals can be adjusted by selectively varying the process parameters.
Extensive general and application-specific tests have been performed in order to optimize the composite material, SiC diamond. The scope of these tests included examining different substrate variants, coating thicknesses, substrate pretreatment procedures and coating types. The most important objective during the optimization process was to reduce the friction coefficient and wear rate of the new tribological pairing. Further criteria for optimization, which are essential for predictable seal behavior, were the adhesive strength and evenness of the layers.
The result of the optimization – EagleBurgmann DiamondFace – is a high-efficiency composite material which is ideal for use in gas- and liquid-lubricated mechanical seals.
The friction factor found for dry running with an uncoated SiC/SiC standard pairing is around 0.65. After a short running-in period, silicon carbide with intercalated carbon graphite achieves a friction factor of approximately 0.2. After 5,000 seconds, however, the friction factor begins to rise in this case also. The diamond-coated sliding pair shows stable dry-running behavior at values of around 0.15.
Benefits for the Operator
By optimizing and classifying crystalline diamond coatings, it is possible to increase the efficiency of mechanical seal systems considerably when lubrication is inadequate or even when sealing against abrasive media. By reducing the friction factor, both during dry-running and under lubricated conditions, and by increasing the wear resistance, the reliability is considerably improved, particularly under difficult operating conditions.
In comparison to uncoated SiC/SiC pairings, which can only run for a few seconds when unlubricated, EagleBurgmann DiamondFace can run for up to several hours unlubricated, depending on the load. This has been confirmed by extensive field tests in various applications.
During normal operation, the amount of frictional heat produced is also reduced by the reduced sliding friction. The lower power consumption of sliding surfaces allows reduced cooling with double seal systems and higher temperature limits with low-boiling-point media. The universal chemical stability and insensitivity to solid-loaded media shown by the product enables it to be used in the chemical and pharmaceutical industry as standard.
Analyses have shown that the return on investment due to the significantly increased service life achieved by a diamond-coated mechanical seal can be obtained after just 1.5 years. And the average increase in the service life of diamond-coated mechanical seals has made it possible to combine maintenance with other engineering or plant components such as bearings or impellers.
Conclusion from Practice
Of course, apart from the field tests already mentioned, some seals are already in continuous use under harsh conditions. Particularly worthy of mention in this context is an application where seals are being used with extremely abrasive media in multi-phase pumps. These are used for the cost-effective exploitation of the enormous oil-sand fields in Canada. This project has been given a huge impetus by the significant rise in oil prices.
The new Steam Assisted Gravity Drainage process (SAGD), in which the oil components are recovered by steam, uses pumps which have to deliver an extremely abrasive mixture consisting of gas and solids. The single seal which was previously used failed regularly after only about 14 days. However, the same seal but with diamond-coated surfaces has improved the situation substantially by guaranteeing completely trouble-free operation for 12 months.
A further significant improvement in the service life of the seal has been achieved in a treatment unit for the grinding and wash emulsion used for the treatment of slip-ring seals. This unit is used in our production line and broke down regularly due to leaks after approximately 1,500-2,000 hours. The EagleBurgmann DiamondFace technology has been used here too. So far, the service life has been increased by a factor of 6, i.e. more than 11,500 operating hours without any measurable leakage. During an intermediate inspection, the surface of the slip-ring seals was found to be in perfect condition.
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For more information on EagleBurgmann DiamondFace, contact email@example.com.