Your questions answered: Tips on avoiding leakage, downtime and equipment failure
CFE Media & Technology presented a webinar on “How the right seal face technology overcomes process challenges and extends equipment life,” sponsored by John Crane.
During industrial processing operations, every mechanical seal application experiences its own unique set of challenges, including:
- Inadequate seal face lubrication
- High heat generation
- Dry-running conditions.
These issues can cause leakage, unplanned equipment downtime and even catastrophic equipment failure, negatively impacting operations and profitability.
Mean time between repair (MTBR) intervals for mechanical shaft seals can occur frequently in these harsh conditions, which means maintenance costs go up and equipment run-time goes down.
During the course of the webinar, the presenters, Brian Kalfrin, PE, regional engineering manager; and Jack Bagain, PE, senior staff engineer, both of John Crane, answered a wide range of questions from attendees.
Question: What is the LaserFace pressure limit?
Answer: LaserFace technology can be applied to many existing seal designs. The grooving has no effect on pressure capability of the seal. Therefore, the pressure limit is the same as the seal it is applied to.
Q: Is the LaserFace technology uni-directional?
A: LaserFace is not dependent on shaft direction of rotation it is bi-directional.
Q: To maintain adequate seal pot pressure, how can we calculate seal chamber pressure?
A: Seal chamber pressure varies by the equipment being sealed in terms of configuration, clearances, orientation, etc. To determine seal chamber pressure (pressure acting on the mechanical seal), proceed as follows:
- Obtain calculated seal chamber pressure value from the equipment OEM – they know the internals of the machine and can provide these details.
- Measure the seal chamber pressure with a local pressure gauge – this requires access to the seal cavity through some means.
- Estimate a value based on known equipment variables – this is more applicable in the case of centrifugal pumps and compressors and relies on calculations based on known quantities such as pressure balancing devices, differential pressure, etc.
It is best to determine the seal cavity pressure initially, then adjust your seal pot pressure ─ in the case of a pressurized system ─ at the greater of either 10% or 30 PSI above the maximum pressure in the seal chamber.
Q: In high-temperature applications, such as a hot oil pump, do diamond treated seal faces require pre-heat?
A: There are no pre-heat requirements for diamond-treated seal faces. Preheat requirements would be more applicable to the equipment and mechanical seal in terms of ensuring even heating for thermal expansion, growth, and so forth. The actual temperature limitations of a diamond-treated seal would be set by the limitations of the seal configuration itself, considering secondary sealing elements, and other seal-related variables – the addition of diamond treatment to the seal faces do not impart a temperature limitation to the seal design.
Q: For general purpose pumps with tandem seals, what is John Crane’s experience or opinion regarding dry versus wet secondary seal reliability?
A: There are pros and cons associated with Arrangement 2 seals using either wet or dry containment seals. The advantages of the dry configurations, especially with non-contacting designs, are significant in considering both the initial cost of the support system, reduced maintenance, and longevity of the containment seal. In a contacting dry seal, the longevity proves less of an advantage, but the simplified support system is still considered a significant advantage. All options are recognized by API 682 as viable options – the point being that in all cases the containment seal is meant to only contain more elevated levels of inner seal leakage, not operate as a complete back-up. The linked tutorial covers several aspects of Arrangement 2 containment seal strategies in more detail.
Q: I see how dry is preferred with respect to installation cost & operation (smaller support system), but I believe wet is older & more established technology. Is the laser face technology well established for dry secondary seals?
A: Dry containment options have been in use for quite some time. LaserFace technology is more suited for wetted seal applications and narrow vapor pressure margin services. Dry containment seals, especially of the non-contacting variety, use a uni-directional or bi-directional feature (spiral grooves) to generate hydrodynamic lift when the seal is in operation.
Q: What do you do for adhesive fluids?
A: Dependent on the application, but typically optimized face hydraulic balance and material considerations are key in terms of mitigating leakage and preventing face damage or hang-up. In many instances, dual pressurized seals are required to protect critical components of the seal from the process fluid. We suggest reviewing the application with your local John Crane representative to ensure the best approach for the specific application.
Q: What is the difference in cost between diamond face and other technologies?
A: The face technologies discussed have similar costs in terms of operation for application. Keep in mind the technologies are based on seal face application and those faces are subsets of a mechanical seal cartridge or assembly. The cost of the entire mechanical seal should be considered in terms of size, speed, pressure and certification or testing requirements as greater drivers compared to simply the application of grooves or treatments to one component. The JC Diamond treatment discussed in the training does not require application on both seal faces for proper operation – it can be applied to one face and run against a traditional hard counter face. This will reduce overall seal assembly cost when compared against other available diamond technology currently available.
Q: What is the minimum velocity for Y Technology for proper function?
A: Minimum velocity for Y-groove is less critical than with traditional hydrodynamic grooves in dry gas seal applications. The Y-groove technology is intended for low DP oil sealing – provided the oil is present, then the faces will receive lubrication by the grooves, even more so at lower speed. At higher shaft speeds is where zero face contact and fluid film stiffness is more critical as the low differential pressure of the oil is often not enough to overcome centrifugal effects. At high speeds, the Y-groove technology provides a benefit in terms of ensuring face lubrication and providing film stiffness to mitigate damage from face contact at these high velocities.
Q: What is a common failure mode with diamond face treatments?
A: “Failure” would be attributed to extended dry run periods, where the underlying substrate would typically experience damage and cracking before the surface treatment degradation. While the amount is greatly reduced, all diamond treatments would wear in the absence of fluid film support. During these instances of dry running, the wear profiles of the faces will change and over time substrate breakdown could occur. This is primarily why intermittent dry run periods, validated by actual testing simulating ‘real-life’ conditions should be adhered to when considering application of the technology. One common concern is also the fear of delamination of the diamond treatment from the substrate – therefore the JC Diamond technology discussed in the training has been tested to ASTM standards to validate adhesion strength of the treatment under extreme levels. This may not hold true for all other available diamond technology in the industry currently.