Having a good preventive maintenance 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 preventive maintenance 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, so 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. 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 highly time-consuming and a 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 is a critical part of a good preventive maintenance program and will minimize future problems.
Failure to execute a properly planned maintenance program is often a precursor to unanticipated equipment downtime, effecting production and which can have significant consequences (one of which is unpredictable and high costs of repair). 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 preventive maintenance 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 6 – 9 months, daily leak-up checks, daily inspection of exposed flange sealing surfaces as well as 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 to prevent downtown due to leaks and other issues. In addition, continuous monitoring of vacuum levels during processing can help to 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. By way of example, use of a sacrificial layer of insulation material in the bottom 1/3 of the furnace hot zone during brazing can minimize downtime, as can the proper selection of the form of the braze alloy, that is, 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 you should expect from yourself
A planned preventative maintenance 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 spare 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 preventative maintenance program involves:
- 1. Understand the external constraints imposed on you with respect to such issues as:
a. Equipment usage
- Understand the equipment being serviced, in other words:
a. How should it operate?
b. How is it working now?
- Tailor the plan to meet realistic expectations including:
a. Identifying critical spares and have them in your stock or a consignment inventory at the supplier.
b. Understanding 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.
a. Focus on those components or assemblies (internal or external) that are critical to the functionality of the operation.
b. Do an exterior and interior review and observe how components interact.
- Put the repair information into usable (i.e. searchable) and retrievable form.
a. Review needs with management
b. Get feedback through team meetings
c. Revise the plan as needed
- Establish a mean time between failure for key components
a. Conduct cause and effect analyses
b. Determine the root cause of a failure (don’t just fix the obvious)
- Be disciplined
a. 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)
a. Have the tools and supplies on hand to succeed.
Establishing a PM Plan
Divide and conquer. Begin by understanding the heat treat process(es) you will be asking the unit to perform and compare these to the design ratings/limitations of the equipment, items such as:
- Temperature Rating
- Normal and maximum operating temperature
- Cyclic operating conditions
- Idling conditions
- Load size including volume or weight limitations
- Load distribution and the necessity for load ballast
- Maximum and minimum gross load weight as a function of temperature
- Atmosphere Requirements
- Type and function of gas(es) – partial pressure and backfill
- Gas flow rate, pressures, etc.
- Quench Requirements
- Type of quenchant
- The volume of quenchant – if a liquid (in relation to gross load weight)
- Quenchant temperature
- Flow characteristics of the quenchant around the part
- Special Requirements
- Baskets & fixturing
- Quench restrictions
- Access & site ports
- Water systems
- Design Specific Features
- Special features
- Support/ancillary items (heat exchangers, water circulating systems, etc.)
Next, take the time to divide the equipment into logical “sections” so that the maintenance on each of these areas focuses on those components or assemblies that are critical to their functionality (and ultimately that of the entire machine). Then walk around the exterior and inspect the interior. Note: confined entry training/permits may be required. Observe how all components interact. This takes a surprisingly short amount of time and yields a significant amount of information. Next, understand the “external” constraints being placed on the equipment (usage, budget, etc.). These factors are important in tailoring your plan to meet the expectation. Identify critical spare parts and have them in stock. Understand which spare parts must come from the OEM manufacturer and which ones can be purchased through alternative suppliers.
Steps you can take include:
- Intelligent equipment purchases – Taking the time to select the right equipment and features (standard and custom hardware and software) including consideration of future growth plans.
- Commitment to a robust maintenance program – Using a planned preventative maintenance approach rather than a haphazard “fix it when it breaks” approach.
- Having user-friendly HMI, instrumentation, and controls – Taking full advantage of today’s technology (including remote access capability, intelligent sensors and anticipatory controls (such as cloud-based reporting of abnormal operating conditions is almost mandatory today. This allows one to manage data through an intuitive, graphical interface so that changes to the system in real-time can be observed and corrective actions (including maintenance) initiated.
- Safety – Safety is mandatory and cannot be compromised at any time. Whether this is following a set of company rules (e.g., confined entry regulations) or plain common sense and respect for the equipment and processes being run. There is a myriad of safety issues that must be considered when maintaining vacuum furnaces to avoid physical injury, electrocution, asphyxiation, burns and other injuries to yourself or others. Code requirements, such as NFPA standards must be followed by the OEM and in addition, there are several unique considerations specific to vacuum furnace equipment. For example, on furnaces equipped with oil diffusion pumps, maintenance should only be attempted after the pump has been allowed to cool to ambient temperature. The diffusion pump works by boiling oil to form a vapor. Heated by coils in the base of the pump, oil temperatures reach 240ºC (464ºF) and higher. At these temperatures, vapors from hydrocarbon-based oils can react explosively with air. As a result, the fill port in the pump should never be opened while the pump is operating. Silicone-based pump oils eliminate this explosive risk.
Maintenance of furnace chamber internals should only be conducted using approved confined space entry and electrical lockout procedures. Residual quench gases remaining in the tank even after the door is opened can cause asphyxiation. Particular care should be taken entering furnace chambers after argon has been used as a quench gas. Argon is heavier than air and can remain in low lying areas for some time. It has no discernable odor, and there is usually no advanced warning before unconsciousness occurs. Vacuum conditions in a furnace tank are even more lethal. Lockout procedures to prevent furnace operation must be in place before entering any furnace chamber.
What you should expect of your furnace OEM or Third-Party Supplier
In this day and age, one should expect support from their supplier partners in the form of:
- Technical Support (24/7) – Internet and/or telephone support by personnel trained in supporting vacuum equipment. A prompt response and courteous reply followed by rapid resolution of the problem should be expected, especially if a critical issue is involved.
- Diagnostic help (remote dial-in) – The ability of the OEM to access the vacuum furnace to aid in the troubleshooting process (this often requires the installation of a dedicated telephone line). Today, a number of cloud-based reporting systems are available so that the system can be monitored for abnormal operating conditions in real-time with alerts sent to both users and the OEM. Predictive analytics help determine an optimal preventative maintenance schedule as tasks are carried out only when needed.
- Root cause analysis – Taking the time to determine the root cause of a failure and analyzing the various ways in which it can be corrected is essential to equipment uptime and ultimately high productivity. Temporary fixes to keep the equipment running in the short term may be necessary, but these should be instituted only with the understanding (and preset timetable) of shutdown for a permanent repair and/or replacement.
- Availability of spare parts – Anticipation, as well as the availability of critical spare parts, is an important aspect of a properly planned preventative maintenance program. This should include both “standard” spare parts as well as specially designed components that may require extended lead times. Evaluation of which spares to keep in stock or on consignment at the OEM or third-party supplier is as critical as any aspect of the preventative maintenance plan.
- Field support – Components fail and systems malfunction. This happens in the real world but dealing with an unanticipated breakdown of your vacuum furnace can bring operations to a standstill despite the best-laid plans. If the issue is significant enough to require service by an OEM or third-party provider, they must be available, have individuals with the right skill sets (mechanical/electrical/process) and be familiar with the style and type of vacuum furnace. This is essential to a quick resolution of the problem.
- Periodic Service Visits – Setting up quarterly service visits by the OEM or third-party maintenance provider serves as a wellness check and helps prevent more serious problems from occurring before they become critical and produce unanticipated downtime. with a suitable maintenance program. Regular inspection, lubrication, testing, repairs, and fine-tuning adjustments to ensure safe and efficient operations.
- Training – The OEM or third-party maintenance provider should be able to train your maintenance staff on routine service issues so that they can do the work themselves. In addition, they should be able to provide safety alerts and information on product upgrades, obsolescent and the like. Choosing a partner that will guarantee a level of professionalism and quality in the performance of their work is critical to success.
Figure 1 Key
1. “O” Ring seal care – after each run
2. Water system & hoses – visual inspection regularly
3. Cooling & Blower motor – inspect every 6 months
4. Heat exchanger – annual inspection
5. Vacuum gauge calibration – biannual
6. Pumping system – daily/weekly checks
7. Hot Zone inspection – daily checks
8. Hearth inspection – after each run
Figure 2 Key
1. Chamber door seal care – after each run
2. Water system and hoses – visual inspection daily
4. Hot-zone inspection – daily checks
5. Instrument calibration – per AMS 2750D
6. Hearth inspection – after each run
3. Pumping system – daily/weekly checks
Successful use of a vacuum furnace should be measured in terms of up-time productivity. Today, uptime should be in the range of 90% or greater and depends ultimately on a leak-free and well-functioning vacuum system. While there are numerous factors that influence uptime (e.g., planning, scheduling, identifying the root cause of a failure, having the tools and spare parts needed for the repair) fixing problems right the first time is critical to success.
1. Herring, Daniel H., Vacuum Heat Treatment, BNP Media, 2012.
2. Pritchard, Jeff, “Maintenance Procedures for Vacuum Furnaces”, Vac-Aero International (www.vacaero.com), 2008.
3. “Maintenance of Vacuum Furnaces”, Vac-Aero International, 2018
4. “Technical Support and Maintenance Services”, FurnaCare (www.furna.care)
5. “The Harold – Blogging for the Heat Treatment Industry”, Ipsen (www.ipsenharold.com)
6. “The Vacuum Furnace Blog – Maintenance”, TAV Vacuum Furnaces (www.tav-vacuumfurnaces.com/blog/category/2/en/maintenance)
Daniel H. Herring / Tel: (630) 834-3017) /E-mail: [email protected]
Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.