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Sparkling coating

Crystalline diamond increases service life of mechanical seals
Sparkling coating

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. By optimising 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.

Dr. Wolfgang Berger

Of a pump’s key components, 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 minimise the process downtime costs which result from unplanned interruptions.
Robust slide faces
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. For pure fluids, predominantly hard/soft sliding combinations are selected, with the soft component usually being impregnated carbon graphite. The range of applications for soft carbon graphites is limited by the solid particles present in the medium, the maximum permissible abrasive wear rates and, in some cases, inadmissible product contamination (pharmaceuticals).
The SiC/SiC material pairs which have been increasingly favoured for seal faces during the last ten to fifteen years offer excellent chemical resistance and high resis-tance to abrasion as well as being virtually wear-free, although 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 an unacceptably high level of gas in the medium, incorrect operation or even evaporation of the medium in the sealing gap.
This is precisely where Burgmann’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.
Not every diamond is the same
Amorphous diamond-like carbon (DLC) coatings have been successfully used for wear protection in gas-lubricated mechanical seals for approximately ten years. Until now, however, the 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 Burgmann Diamond Faces crystalline diamond layers, which were specifically developed for use 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 approximately 1500 °C and in an oxidising environment it begins to burn at around 600 °C. Acids and lyes have no effect on diamond.
An optimised HF-CVD (Hot Filament Chemical Vapour Deposition) process is employed to meet these requirements for the large-area, uniform coating of ceramic seal faces. 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 to optimise the composite material, SiC diamond. The scope of these tests has included examining different substrate variants, coating thicknesses, substrate pre-treatment procedures and coating types. The most important objective during the optimisation process was to reduce the friction coefficient and wear rate of the new tribological pairing. The adhesive strength and evenness of the layers were among the other optimisation criteria essential for predictable seal behaviour.
The result of the optimisation – Burgmann Diamond Faces – is a high-efficiency composite material which is ideal for use in gas and liquid-lubricated mechanical seals.
The friction factor measured 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 5000 seconds, however, this factor also begins to rise. The diamond-coated sliding pair exhibits stable dry-running behaviour at values of around 0.15.
Benefits for the operator
By optimising 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 increasing the wear resistance, reliability is substantially improved, particularly under difficult oper-ating conditions.
In comparison to uncoated SiC/SiC pairings, which can only run dry for a few seconds without lubrication, Diamond Faces achieve 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 likewise reduced due to 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 industries as standard. (Carbon graphite materials, which can lead to inadmissible product contamination under abrasion, can in many cases be replaced.)
Analyses have shown that a return on investment is possible within a mere eighteen months due to the significantly increased service life achieved by a diamond-coated mechanical seal. What’s more, the average increase in the service life of diamond-coated mechanical seals makes it possible to combine maintenance with that for other engineering or plant components such as bearings or impellers.
Lessons learned in practice
Apart from the field tests mentioned above, a large number of seals are already in continuous use under harsh conditions, of course. An application where seals are used with extremely abrasive media in Borne-mann multi-phase pumps is particularly worthy of mention in this context. These pumps assure the cost-effective development of Canadian oil-sand fields in a project that has been given a huge impetus by the significant rise in oil prices.
The Steam Assisted Gravity Drainage process (SAGD), in which the oil components are recovered by steam, makes use of 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 with diamond-coated surfaces has improved the situation substantially by guaranteeing completely trouble-free operation for a full four months.
A further significant improvement in the service life of the seal was achieved by a treatment unit for the grinding and wash emulsion used for slip-ring seals. This unit, manufactured by Atec, is installed in Burgmann’s own production line and broke down regularly due to leaks after approximately 1500 to 2000 hours. Under the supervision of Atec, Diamond Faces technology was also introduced here. So far, the service life has been increased sixfold, 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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