Vacuum Maintenance (part 1)
We live in the real world, and things break down or otherwise drift from factory settings. Patchwork repairs (that all too often become permanent) and temporary fixes are poor substitutes for properly planned and well-executed maintenance activities. But how do we accomplish this? Let’s learn more.
Planned Preventive Maintenance
Having a good preventive-maintenance (PM) program for your vacuum equipment ensures years of reliable operation. Most furnace manufacturers and third-party suppliers can provide training on their systems, offer troubleshooting advice and help design PM programs, which always need to be customized to the individual company running the equipment. Often, however, help with process applications is outside the scope of their work. If this is needed, it should be negotiated before purchase.
Vacuum furnaces are not unique. As with any piece of manufacturing equipment, proper maintenance at regular intervals is essential to extend service life and provide (relatively) trouble-free operation (Fig. 1). The operating and maintenance manuals supplied with most furnaces provide detailed information on maintenance and troubleshooting. While these manuals should be read and understood before commencing furnace operation, this is seldom done. Instead, people refer to the manual only when a problem occurs. Reading the manual first, as a preemptive step, helps to more quickly maintain the furnace.
Typically, the mechanical components in a vacuum furnace require normal maintenance practices (e.g., lubrication, cleaning, proof of operation, etc.). However, where vacuum furnaces differ from other types of machine tools or heat-treatment equipment is that their successful operation depends heavily on maintaining a leak-free vacuum environment.
When performing any type of maintenance activity, the impact on the integrity of the vacuum system must be considered. It is not uncommon when an unacceptable leak rate is observed to troubleshoot the problem to the last place maintenance was performed. Leaks in areas such as seals, braze joints and the like, coupled with contamination of furnace internals, will adversely affect the ability to maintain proper operational vacuum levels and ultimately the quality of the end product produced. In addition, finding and correcting leaks is a highly time-consuming and painstaking process.
The location of tiny leaks often requires the use of helium leak detectors or even more sophisticated methods (e.g., residual-gas analysis). However, keeping track of where leaks have previously occurred and especially replacing temporary sealing compounds (e.g., Glyptol®) used as stopgap measures are a critical part of a good PM program and will minimize future problems.
Failure to execute a properly planned maintenance program is often a precursor to unanticipated equipment downtime, which effects production and can have significant consequences (one of which is unpredictable and high repair costs). By optimizing the performance and therefore the life of your vacuum furnace, the total cost of operation over time will be lower than taking a “wait until it breaks and fix it” approach.
To support internal plant maintenance, working with an OEM or third-party supplier committed to field service and having the availability of original replacement parts is critical.
At a minimum, a good PM program involves the proper care of the vacuum pumps on the system, replacement of O-rings (especially on doors and moving or rotating seals) every six to nine months, daily leak-up checks, daily inspection of exposed flange sealing surfaces and inspection/cleaning of the furnace hot zone.
In addition, regular inspection of the power feed-throughs (for arcing) and hot zone (for signs of deterioration due to attack by oxygen) helps prevent downtime due to leaks and other issues (Fig. 2). In addition, continuous monitoring of vacuum levels during processing can help identify potential problems before they develop into major repairs.
Remember that the final product quality is a function of many factors, including such items as uniform temperature distribution throughout the hot zone, proper gas circulation (partial pressure or quench gas), and the cycle times and temperatures chosen. In addition, anticipation of potential problems leading to extensive maintenance or downtime is beneficial.
For example, use of a sacrificial layer of insulation material in the bottom third of the furnace hot zone during brazing can minimize downtime, as can the proper selection of the form of the braze alloy (i.e., if brazing paste is used, its composition and outgassing characteristics must be fully understood).
All of these steps will lead to achieving the best performance and ultimately the highest quality standards while achieving the required productivity even for the most challenging of vacuum applications.
What To Do
A planned PM program involves the following: planning the activity, executing the plan, evaluating the results of the maintenance effort and revising the plan to make it better going forward (the PEER system). This includes the following activities:
- Identifying essential spare parts (via critical spares inventories)
- Monitoring of component usage times (via hour meters)
- Detailed record keeping
- Root-cause determination when problems do occur
- A complete explanation of repairs (why, what, where, when and how)
- Establishing mean time between failure (MTBF) of critical components
Success of any planned PM program involves:
- Understand the external constraints imposed on you.
- Equipment usage
- Understand the equipment being serviced.
- How should it operate?
- How is it working now?
- Tailor the plan to meet realistic expectations.
- Identify critical spares and have them in your stock or a consignment inventory at the supplier.
- Understand which spares must come from an OEM provider and which spares can be purchased from third-party suppliers, with the same quality assurance.
- Divide the furnace and work effort into manageable parts serviced by specific disciplines.
- Focus on those components or assemblies (internal or external) that are critical to the functionality of the operation.
- Do an exterior and interior review and observe how components interact.
- Put the repair information into usable (i.e., searchable) and retrievable form.
- Review needs with management.
- Get feedback through team meetings.
- Revise the plan as needed.
- Establish a mean time between failure for key components.
- Conduct cause-and-effect analyses.
- Determine the root cause of a failure (don’t just fix the obvious).
- Be disciplined.
- Realize the benefits by having a carefully structured, rigorously adhered-to program (this is not punishment but prevention).
- Do the job right (or not at all).
- Have the tools and supplies on hand to succeed.
We will conclude our discussion of vacuum furnace preventive maintenance in December’s issue.