By Dan Herring

A frequently asked question is, “How can I keep my vacuum furnace performing like it was when it was brand new?” This goes hand in hand with the question “How should you operate and maintain your vacuum furnace to maximize your investment and produce repeatable high-quality results?” The next few articles will provide tips for doing just this.

We plan to cover the following subjects:

Tip #1:     Maintaining Your Vacuum Pumps
Tip #2:     Selecting the Right Vacuum Level for the Job
Tip #3:     Avoiding Eutectic Melting
Tip #4:     Maintaining Your “O” Rings Seals
Tip #5:     Avoiding Diffusion Bonding
Tip #6:     Controlling Partial Pressure Additions
Tip #7:     Properly Supporting Your Work (Grids, Baskets & Fixtures)
Tip #8:     Finding & Fixing Vacuum Leaks
Tip #9:     Selecting the Right Furnace for the Job
Tip #10:   Repairing and/or Replacing Your Furnace Insulation
Tip #11:   Maintaining Your Cooling Water System
Tip #12:   Calibrating Your Instrumentation
Tip #13:   Understanding Your Quench Parameters
Tip #14:   Control Process & Equipment Variability
Tip #15:   Doing the Little Things
Tip #16:   Understanding Ramp and Soak Rates
Tip #17:   Outgassing & Its Consequences
Tip #18:   Keeping Critical Spares on Hand
Tip #19:   Watching Your Amperage
Tip #20:   Ways to be Proactive Not Reactive
Tip #21:   Cleaning the Vacuum Furnace
Tip #22:   Repairing or Replacing Heating Elements
Tip #23:   Performing Temperature Uniformity Surveys
Tip #24:   Selecting the Best Backfill Gas
Tip #25:   Steps to Save Energy & Reduce Environmental Impact

Figure 1 – Typical Industrial Mechanical Vacuum Pump and Blower Combination (Photograph Courtesy of Oerlikon Leybold Vacuum USA Inc.)

We begin with a simple task that is often overlooked or taken for granted

Tip #1: Maintaining Your Vacuum Pumps

The heart of any vacuum furnace is its pumping system. Our goal is be to keep the vacuum pumping system operating at peak efficiency. Here’s a look at how we can do this.

First, to facilitate maintenance activities with any component on a vacuum system, but most especially the pumps, it is helpful to have a running hour meters installed to document the number of operating hours on the pump between service activities.

Mechanical Pumps

The mechanical (i.e. wet or oil-sealed) pump (Fig. 1) is the most common type found on most vacuum furnaces and is generally considered the workhorse of the industry. Oil-sealed pumps offer efficient, cost effective operation and very reliable performance. However, they require a great deal of maintenance in order to continue to maintain optimum performance. This includes daily maintenance (e.g., oil level checks, ballasting), routine maintenance (e.g., oil changes) and annual maintenance (e.g., replacement of poppet valves).

Common problems [1] with mechanical pumps also require routine maintenance and inspection and include:

• Oil Contamination
• Sludge buildup
• Loose or slipping belts
• Improper oil level (too low or too high)
• Stuck discharge valve
• Clogged oil lines or valves
• Damaged discharge valve
• Ingestion of foreign contaminants (metal fins, metal chips, etc.)
• Excessive vibration (pipe connection or floor mounting)
• Exhaust filters (more than 12 months in age)
• Oil temperature not being regulated between 60ºC – 70ºC (140ºF -160ºF)

Of the various mechanical pump problems that can arise, contamination of the oil in the mechanical pump is the most common. Vapors present in the gas being pumped may condense and mix with the oil. Moisture (water vapor) is especially problematic and if not removed will flash to vapor and tie up a large portion of the pump’s gas load capacity thus creating a significant loss in pumping efficiency (resulting in either extremely long pumpdown times or failure to achieve a low vacuum level, or both).

In order to rid the oil of water and other liquid condensates, a gas ballast is used; a gas ballast may be used in conjunction with correctly regulating the operating temperature of the oil with a water control valve assembly; a ballast valve on the pump can be opened (manually or automatically) to admit air, nitrogen, or argon into the pump, disrupting it’s operating efficiency, the result of which is a reduction in the compression necessary to exhaust the gases and, correspondingly, a decrease in the amount of vapor that condenses.  The use of a gas ballast increases the amount of oil carried out in the exhaust. The gas ballast valve is very effective in removing water vapor but actually is very ineffective in cleaning dirty oil or fixing oil that has cracked (fractionated) due to mixing with other downstream by-products.

In addition, the oil may break down chemically forming a sludge that causes numerous (short and long term) problems with pump operation especially as it relates to severe wear on internal components, often to the point where rebuilding is not possible. Disassembly and cleaning of the pump is the only solution to this problem.

Figure 2 [1] – Anatomy of a Mechanical Pump

A common question asked is “How often should the oil be changed?” The most common indicators are that

• The oil looks cloudy or milky despite gas ballasting;
• The pump routinely sees a large moisture load or is used on a “wet” system.
• After a furnace bake-out (burnout) cycle has been run if you suspect that a great deal of contaminants have been pulled out of the chamber and into the pump;
• If the pump will not pull down to its rated micron reading with the inlet valve shut, which is typically 0.026 – 0.067 mbar (20 – 50 microns). Note: this test is to be done using an electronic vacuum gage.

Mechanical pump oil must be changed on a routine basis (typically every 300 hours or about one month). The initial charge of oil in the pump when it is brand new should be changed after 100 hours of operation. Checking the amount of fluid in the pump reservoir during normal operation is strongly recommended. It is possible, due to improper operation, to have the pump oil backstream into the vessel in considerable quantities.

Here are some other common maintenance activities [2]:

1. Check the oil level daily. There is an oil sight glass on the side of the pump where you can check the oil level. In many pumps the oil level is designed to be at the midpoint of the sight glass (check your owners manual to be sure). If the oil level is low, add oil but be careful not to overfill the pump.
2. Use the proper oil. In replacing pump oil, be careful to use the type of oil recommended for the pump and be equally careful to apply precisely the right amount of oil. Either too much or too little oil in the pump reservoir will give rise to serious difficulties.
3. Change the filters. Change the filters ever time you change the oil in the vacuum pump, including cleaning or replacing the filter in the oil mist eliminator.
4. Lubricate bearings. The bearings in the pump should be lubricated every 3000 hours, or annual, whichever comes first.
5. Inspect the vanes. The vanes (a.k.a. beater bars) in the pump should be inspected every 3000 hours, or annual, whichever comes first for cracks or worn areas. If these are found, replace as a set.
6. Check the rotor rotation. Check the motion of the rotary to be sure it turns freely and make sure there is no dirt or debris in the rotor area.
7. Check the motor rotation. Be sure the motor is moving in the correct direction (often this is marked on the motor by a rotational arrow). This can be done by quickly starting and stopping the motor.

Figure 3 [3] – Dry Pump Internal Screw Assembly

Obviously, if you hear any abnormal noises in the pump, shut it down immediately!

Dry pumps (Fig. 3) are an alternative to wet pumps especially in applications where the gas load entering the vacuum system contains vapors or particles that tend to condense or deposit in the pumping system (e.g., sintering, brazing, carburizing). These pumps can be cleaned using a high flow purge and solvent flush between periods when the pump must be partially or fully stripped down for a more thorough cleaning.

Here are the common maintenance areas on dry pumps:

1. Change the filters. Change the filters ever time you change the oil in the vacuum pump, including cleaning or replacing the filter in the oil mist eliminator.
2. Inspect the screws. The screws in the pump should be inspected every 3000 hours, or annual, whichever comes first and cleaned thoroughly. Check for cracks or worn areas. If these are found, replace as a set.
3. Lubricate bearings. The bearings in the pump should be lubricated every 3000 hours, or annual, whichever comes first.
4. Check the motor rotation. Be sure the motor is moving in the correct direction (often this is marked on the motor by a rotational arrow). This can be done by quickly starting and stopping the motor.

Daniel H. Herring / Tel: (630) 834-3017) /E-mail: dherring@heat-treat-doctor.com

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.

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