Important Tips for Improving Vacuum Performance and Operation - Tip #1 Maintaining Your Vacuum Pumps

August 6, 2019 by VAC AERO International

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 I operate and maintain my vacuum furnace to maximize my investment and produce repeatable high-quality results?” In this article will provide important tips for doing just this.

Tip #1: Maintaining Your Vacuum Pumps

The heart of any vacuum furnace is its pumping system. Our goal is 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 meter installed to document the number of operating hours on the pump between service activities.

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

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 pump-down 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 lot 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 every 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 every 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.

Blowers

Vacuum blowers (a.k.a. booster pumps) typically need little day-to-day maintenance, which normally consists of simply monitoring the oil level in the pump. The blower (Fig. 4) is used in conjunction with the mechanical pump to improve pump-down rates and ultimate vacuum levels.

Table 1 [4] – Oil Viscosity Selection Chart

Contained within the pump housing (Fig. 5) are pistons and timing gears in a gear case fitted with troughs, which control the amount of oil, supplied to the gear teeth. A typical procedure is to fill the gear case with the appropriate grade of oil until it overflows from the oil level/filler hole. Allow the excess oil to drain before inserting the oil level filler plug. This must be done when the blower is not in operation. Overfilling must be avoided, as this will cause the gears to run hot, resulting in damage. The oil level should be checked weekly or every 100 working hours. Drain and refill the gear case to the correct level every six to twelve months depending upon the condition of the oil. When there are considerable seasonal temperature variations, it may be necessary to change the grade of oil at certain times during the year.

The oil used should be suitable (i.e. have the proper viscosity) for the minimum ambient temperature (cold starting) and for the highest oil temperature reached on maximum load (Table 1). The recommended oil for most booster pumps is straight mineral oil, which may contain anti-wear, anti-rust, anti-foam, and anti-oxidant additives. The oil should not contain either an extreme pressure additive or additives that emulsify. The oil should have a viscosity of not more than 2,500 centistokes at the minimum ambient temperature (cold starting) and not less than 30 centistokes for the highest oil temperature reached on maximum load. When there are considerable seasonal variations in ambient temperatures, summer and winter grades of oil will be required.

Typical maintenance activities for blowers include the following:

1. Timing gears. Wear should be negligible over a period of years of normal service. Gear teeth are provided with the correct amount of backlash, and a reasonable amount of tooth wear can normally be accommodated without permitting contact between lobes of the two impellers. A high oil level in the gearbox will cause churning and excessive oil heating, indicated by unusually high temperature in the bottom of the sump. If the operation is continued under this condition, gears will heat and the teeth can be affected by rapid wear, which will lead to impeller lobe contact or unit seizure.
2. Shaft bearings. These bearings are critical in the service life of the booster. Gradual wear may allow a shaft position to change slightly until rubbing develops between impeller and cylinder or head plate. This will cause spot heating detected by feeling these surfaces. Sudden bearing failure is usually more serious. Since the impeller shaft assembly is no longer supported and properly located, extensive general damage to casing and gears is likely to occur immediately after the bearing fails.
3. Driveshaft seal assembly. This assembly consists of two individual lip-type seals submerged in oil, and located in the drive end cover, should be considered as expendable. It should be replaced as an assembly whenever oil leakage through the inboard member becomes excessive as indicated by a rise in oil level at the sight gauge. Leakage through the outboard member is not likely to be excessive unless that seal or its shaft bearing surface is damaged. Shaft smoothness and freedom from scratches have a considerable effect on the performance of this type of seal. Operating problems may also result from causes other than parts damage. Since clearances are only a few thousandths of an inch, interferences and rubs can be caused by shifts in the mounting or changes in pipe supports.
4. Casing issues. Foreign materials entering the casing will also cause trouble. If this type of problem is experienced, and the casing is found to be clean internally, try removing strains. Disconnect piping and loosen mounting bolts, and reset the leveling and drive alignment. After tightening the mounting, make sure all piping meets unit connections accurately and squarely before reconnecting.

Figure 4 [5] – Typical Roots Blower.

Figure 5 [7] – Internal Blower Components.

Diffusion Pumps

In day to day operation, most diffusion pumps (Fig. 6) require little maintenance, only monitoring of the proper air/water cooling (e.g. flow rate, temperature, pressure), electrical power and that the pump has the proper type and quantity of fluid.

Maintaining a day-to-day log of pump and system performance (including pump-down times and ultimate vacuum level achieved) is the best indicator of the condition of the pump and provides a record of any variations that might require corrective action. In the event of a problem, confirm electrical power output, cooling water temperatures (in/out) and oil level & quality.

Common problems [1] with diffusion pumps include:

• Power failures
• Excessively high foreline pressures
• Backstreaming
• Process byproducts clogging oil returns in boiler plate
• Defective heaters and/or broken wiring on the boiler
• Water inlet temperature above 45oC (115oF)
• Water exit temperature above 65oC (150oF)
• Mixtures of hydrogen-based oils with silicone family oils
• High leak rates on the system when being pumped on
• Water-cooled copper lines full of mineral (calcium) deposits negating proper heat transfer

Figure 6 [9] – Diffusion Pump Operation.

Figure 7 [9] – Anatomy of a Diffusion Pump.

Of the various diffusion pump problems, exposure of the hot pump oil to the atmosphere or interruption/loss of the coolant flow is of the most concern. Accidentally introducing air when the diffusion pump is at too high a temperature almost inevitably leads to a pump malfunction or failure and oftentimes require expensive and lengthy repairs (most often at the manufacturer). Severe cracking (breakdown) of the oil and oxidation will occur, depending on the type of oil. These lead to excessive backpressure and the products of the oil breakdown will deposit on the jet structure blocking the openings or deposit in the area of the oil heater, burning it out. Overheating due to inadequate coolant flow also decomposed the oil and can cause excessive backstreaming into the vacuum furnace chamber. Depending on the actual amount of air in the hot pump, coupled with what previously deposited materials in the base of the pump, the oil may expand excessively in vapor form with a significant pressure buildup.

When servicing a diffusion pump (Fig. 7) you want to disassemble the pump so as not to damage any of the internal components, inspect the pump for damage, clean everything thoroughly and reassemble the pump properly. Common industry practice is to use acetone and then an isopropyl alcohol rinse to clean internal pump components (e.g., pump body, jet assembly). With respect to the diffusion pump fluid, remove oxidized deposits, check fluid level and color, replace fluid and have the old fluid analyzed to determine its condition (when compared to new fluid) so as to help ascertain the frequency at which it should be replaced. Reassemble the pump making sure to install new “O” rings, check for proper jet alignment, add the correct amount of new diffusion pump fluid and inspect/verify heater wiring.

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