A mechanical seal is a critical component whose useful life significantly impacts the overall reliability and robustness of pump or centrifugal compressor. Unscheduled or premature failure of the seal leads to increased maintenance and increased overall equipment costs to the user. The overall performance of a mechanical seal often is most affected by the performance of the faces and the intervening lubricating film. This primary element of a mechanical seal represents a classic problem in the science of tribology, the study of friction, wear and lubrication.
The demands placed on seal faces require careful attention to the issues of wear resistance, chemical compatibility, mechanical and thermal properties—all of which are determined by the end use application and overall seal design. When a mechanical seal is used in a pump, the liquid in the pump is used to cool and lubricate the seal faces. The seal faces are in sliding contact and prevent the liquid in the pump from reaching atmosphere. This contact also generates frictional heat that must be removed from the seal faces. Failure to remove this unwanted heat often results in the boiling of the lubricating liquid film at the seal faces, usually leaving a deleterious residue and causing premature failure of elastomeric components in the seals (static-secondary seals). Both of these undesirable effects of elevated temperatures lead to premature seal failure.
The ability to maintain coolant on the seal face is even more critical when compounded by the demands associated with the pumping of abrasive media or a pump’s ability to withstand intermittent and unscheduled coolant loss. The ability to run two hard-faced seal materials such as SiC against each other often is desirable but not practical due to the premature failures that result from elevated temperatures caused by friction at the seal interface.
In the case of a centrifugal gas compressor, a noncontacting mechanical seal is used to contain the gas within the machine. A lift mechanism, such as spiral grooves, is included in the design of the seal. During operation the seal faces do not make contact except at startup and shutdown. The intense frictional heat occurring at this time must be controlled or face damage can occur.
Based on the benefits nature’s hardest substance would appear to offer for this application, the idea of using the diamond as a wear resistant face material in seals is not new. Diamond also possesses many other attractive properties, including extremely high thermal conductivity and chemical resistance. Unfortunately, previous attempts at integrating diamond into seal faces failed due to difficulties in ensuring that the diamond face presented the necessary surface finish required for such a demanding tribological application. Following extensive research and development and improvements in equipment and processes, those problems appear to have been solved.
Today, a new form of diamond with ultrananocrystalline grains has entered the industrial arena. Invented at Argonne National Laboratory and commercialized for seals by John Crane, Inc. and Advanced Diamond Technologies, Inc., UNCD®, as it is commercially known, provides the surface roughness typical of normal, unprocessed seal. UNCD has been dynamically tested and shown to signifi- cantly reduce the frictional heat and increase the life of the seal faces in accelerated wear. The work highlighted in this article was completed, in part, by funding from the National Science Foundation and the Department of Energy.
One of the major obstacles in providing a diamond-treated surface for a mechanical seal is maintaining the surface flatness and roughness necessary to achieve sealing. Early work in diamond surfacing placed extreme demands on finishing and polishing the diamond to meet the required metrology and geometric specifications of a seal face. Surfaces were rough and had a high degree of waviness. Additional lapping of the diamond surface to achieve sealing could not be done cost-effectively due to the hardness of diamond. Consequently, many researchers abandoned the idea of using diamond as a surface for seals.
The development of ultrananocrystalline diamond (UNCD), though, generated renewed interest in diamondtreated seal faces. The process demonstrated that the base material could be treated with diamond without changing its original flatness. This was a major breakthrough in the manufacturing technology for diamond-structured surfaces. At last, diamond could be applied to a seal face without any further work to achieve the desired flatness for sealing fluids. Moreover, UNCD, unlike other diamond films, has nanometer-scale roughness that allows as-deposited UNCD to have sufficient smoothness so that it doesn’t degrade a soft counterface. In other words, UNCD works in both hard on hard and hard on soft sealing applications.
Still, there was an additional obstacle to overcome. Work to this point was done to transfer this laboratory-scale process to meet the demand of seal production. New equipment and processes had to be designed to handle a larger volume of seals at one time. Once this was done, the new equipment and processes had to be validated. Tests were run on production parts and compared to those run on the smaller scale equipment. Continuous testing confirmed that the production parts met the early results for friction and wear testing of parts manufactured on the smaller scale equipment.
Friction plays an important part in the success or failure of a set of seal faces. Not all materials make good seal faces. Some materials have properties that hold heat in the seal face, while others wear too much. Applying diamond to a seal face reduces both friction and wear. One of the best substrate materials for diamond is silicon carbide. Silicon carbide and diamond have very similar material properties. Results of friction testing for UNCD on a SiC face are as follows:
The results for carbon versus UNCD on silicon carbide were expected. This is a normal value for friction in seal design work. The results for silicon carbide versus UNCD on silicon carbide were very good. When silicon carbide runs against itself without any diamond treatment, the coefficient of friction is greater than 0.1. For those applications requiring hard-on-hard seal faces, the application of a diamond-treated seal face is a major improvement. Several groups of seal faces have been tested, resulting in the same friction values.
An important step in qualifying materials for a mechanical seal is dynamic testing in hot water. This test involves running a 1.375” diameter seal in 250 F water at 100 psig and 3450 rpm. At these conditions, the pressure-velocity value for the seal is 170,000 psi x ft/min. This test in hot water is very demanding of the seal. Each test run consists of running a group of four pumps, three fi tted with seals treated with diamond and one without a treated surface. The results of hot-water testing were outstanding for the diamond-treated seals. Very minimal or no measureable wear occurred over the 100-hour test. The untreated silicon carbide seals in each case failed due to heavy wear across the entire seal face. The conditions for the untreated and treated seals are shown in Figs. 1, 2, 3 and 4. In each test, the seals were run against carbon.
These results (as depicted in Figs. 1, 2, 3 and 4) were typical for each test run. The UNCD-treated surfaces were in excellent condition with very little carbon wear. The untreated seal had high wear for both the carbon and silicon carbide seal faces.
An interesting measurement taken after testing was surface flatness. The diamond-treated seals had no change in surface flatness during testing. The untreated silicon carbide had an increase of 240 microinches. This is an indication that the untreated surface was running hotter than the diamond-treated surface. Continued increases in flatness or surface waviness will lead to unwanted seal leakage and wear.
The development of ultrananocrystalline diamond (UNCD) and the improvements in equipment and processes have resulted in an excellent material for seal faces. When applied to a base material such as silicon carbide that has been lapped flat, no further processing is required to achieve a working seal face. Results in friction testing also have been excellent. UNCD shows promise when run against carbon or silicon carbide. Tests in hot water demonstrate no visible wear occurring during the 100-hour tests. Untreated silicon carbide failed to pass the hot-water test. Tests have shown that diamondtreated seal faces will improve seal performance. MT