Maintenance engineers and technicians are typically charged with responsibility of developing and implementing maintenance programs, making continuous productivity improvements, and solving daily equipment challenges.
A subject within the maintenance team that often surfaces is strategy; strategies for ensuring productivity remains at its highest levels and for methods to return systems to a productive state should a failure occur. Although many basic strategies exist, the following four maintenance strategies are those that are most often used: corrective, preventive, predictive and proactive.
The strategies allow facilities to set up systems to deal with both planned and unplanned outages. No one strategy is the be-all, end-all solution to keep production always running at the highest efficiency. Most often, a combination of these strategies will be implemented and used for various reasons, all with the goal of keeping efficiencies at the highest possible levels.
One component that determines efficiencies of production is mean time to repair (MTTR). MTTR is key as it determines how quickly a system can be returned to a productive state following an outage, planned or unplanned. MTTR, by definition, is the average time that a device will take to recover from failure.
Systems in the outage stage vary from self-resetting fuses (where the MTTR would be very short, probably seconds), up to complete system repair and replacement. MTTR is defined as follows:
MTTR = RT/N, where:
MTTR = Mean time to repair
RT = Total repair time
N = Number of failures.
An effective device that will quickly allow a system to return to a productive state from failure is a split seal. Since split seals are easy and fast to install, they help get production back up and working towards the primary goals of a business – shareholder wealth maximization and long-term viability.
In power transmission systems, the most time-consuming steps to return a system to a productive state include the disassembly and reassembly of bearing equipment. Split seals eliminate these time-consuming steps as the seal can quickly be replaced. Available split seal technologies are numerous, so a basic understanding will be required prior to the implementation of split seals.
Three basic groups of split seal technology currently exist: split radial lip seals, split face seals and split bearing isolators. Within split radial lip seals, two levels of technology exist: high-performance and general service radial lip seals.
Split radial lip seals
Split high-performance seals (Fig. 1) are used for the most demanding applications to ensure production continues running. High-performance seals include a few key technology differentiators to provide immediate benefits to businesses: specialized elastomer and moulded-in lip loading springs. The combination of these technologies provides the benefits of high productivity uptime and extremely fast MTTR.
The specialized elastomer was engineered to provide, namely, very high abrasion resistance. The abrasion resistance will allow a seal to remain in the application, in service, for a longer period of time. Generally speaking, the specialized elastomer offers abrasion improvements over traditional elastomers of 65% to 90% (Fig. 2).
The longer service life equates to fewer seal replacements and minimizes the need to implement the corrective maintenance strategy or firefighting, should a seal begin leaking.
Moulded-in lip loading springs used in high-performance split seals drive MTTR. Since the spring is physically retained in the seal itself, you no longer need to make the spring connection and install the spring into the seal. The spring technology offers rapid installation, without extra hassle, which will ensure the system returns to an operational state very quickly.
Some applications simply do not require high-performance seals and general service split seals will be better suited. Such applications could be simple gearboxes in a workshop, low-speed motors in ambient environments and similar pieces of equipment.
General service seals (Fig. 3) offer many of the same user benefits that high-performance split seals offer but do so in a different manner. Whereas the high-performance seals use specialty elastomers with high abrasion resistance, general service seals use standard elastomers that are often times suitable for many traditional applications.
General service seals are designed to use garter spring technology to provide appropriate lip loading. While the garter spring requires manual connection, the seal itself is simple to install due to the garter spring pocket design. The pocket design allows the spring to be snapped into place within the seal. Once the spring is snapped into place, risk of the spring coming out is significantly reduced, providing uptime reliability.
For both the high-performance and general service split seals, a common application is steel mill continuous casters. The high-performance seals would typically be used in caster applications that have limited cooling or when the seals are exposed to excessive debris. The general service seals would be used in more advanced casters that offer better roll cooling and debris protection. For both scenarios, having the ability to quickly change a caster seal is invaluable in the event of a breakdown. The caster can quickly be back up and running, since it is not necessary to pull the entire roll assembly out and send it to the roll shop for maintenance.
Split face seals
Split face seal technologies are very different from traditional radial lip oil seal technology. Face seals (Fig. 4) interact with the equipment in different ways and provide for a very different type of seal. First off, whereas an oil seal will provide ingress or egress protection by creating a dynamic seal at the rotating shaft, a face seal provides ingress protection by creating a dynamic seal at the housing contact point.
Split face seals are typically installed, in a static position, on a rotating shaft. The interface between the face seal and the shaft is purely static: no dynamic movement between the two. The dynamic sealing occurs at the seal-to- housing interface. In more simple terms, as the seal rotates with the shaft, the contact point between the seal and the static housing creates the dynamic seal, providing ingress protection.
Face seals are critical for applications exposed to coolant spray, debris and similar contamination. A common application for face seals includes backup and work rolls. The face seal is installed on the roll and provides sealing against the chock. The seal provides ingress protection for the internal seals and bearings, keeping slag and similar debris out.
Since backup and work roll applications are so aggressive, having the ability to change the seal quickly is paramount. Split face seals allow rapid seal change-out without the need to disassemble the entire system, thus providing rapid MTTR. The split face seal is simply installed around the roll and positioned against the chock.
Bearing isolator technology is a significant departure from radial lip seals. How a bearing isolator works, the materials of construction, the installation of the seal and the interaction between the seal and the equipment are vastly different.
Bearing isolators have historically been solid seals consisting of a rotor, stator and static o-ring seals at the shaft and housing interface. The installation of a solid bearing isolator can be very daunting in some applications as you would be required to disassemble the equipment, slide the bearing isolator down the sometimes very long shaft and then press it into the housing. Split bearing isolators (Fig. 5) solve this problem by allowing the seal to be installed at the equipment without the need to disassemble s
ystems Two dowel pins ensure proper alignment while two screws fully unitize the bearing isolator. Once installed, the split bearing isolator provides both ingress and egress protection similar to a traditional bearing isolator.
Bearing isolators represent an evolution in technology from radial lip seals. Bearing isolators significantly increase the operational life of the sealing system from a few hundred hours to thousands of hours.
Bearing isolators are most frequently applied to gearboxes and electric motors found in power transmission systems. An example of such an application is run-out tables found in most steel production facilities. Dozens of electric motors are arranged in the run-out table to move the steel along the process line. If a solid seal is used, the electric motor would need to be removed from the system for repair or replacement. With the split bearing isolator, you can leave the motor installed on the table and simply replace the seal, meaning there is no more need for costly downtime and hours of work.
Regardless if you plan to use split high-performance seals, split general service seals or split bearing isolators, the fact remains that you will clearly make significant improvements to your MTTR by using split-seal technology. With split seals the time, pain and expense of costly equipment disassembly, repair, reassembly and production downtime are vastly reduced.
The improvements to MTTR will have an immediate effect on the business and will ensure shareholder wealth maximization continues to occur and grow while providing for long-term business viability. MRO
Earl J. Rogalski is with Garlock Sealing Technology. This report is based on a presentation made at the Iron & Steel Technology Conference in May 2010.
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