We continue our discussion of ways to maintain your vacuum pumps by considering what needs to be done with blowers and diffusion pump.
Tip #1: Maintaining Your Vacuum Pumps (continued)
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 pumpdown rates and ultimate vacuum levels.
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 a 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:
- 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 operation is continued under this condition, gears will heat and the teeth can be affected with rapid wear, which will lead to impeller lobe contact or unit seizure.
- 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.
- Drive shaft 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 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.
- Casing issues. Foreign materials entering the casing will also cause trouble. If this type 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  – Typical Roots Blower
Figure 5  – Internal Blower Components
In day to day operation, most diffusion pumps (Fig. 6) require little maintenance, only monitoring of the proper air/water cooling (e.g. flowrate, 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 pumpdown 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 event of problem, confirm electrical power output, cooling water temperatures (in/out) and oil level & quality.
Common problems  with diffusion pumps include:
- Power failures
- Excessively high foreline pressures
- 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  – Diffusion Pump Operation
Figure 7  – 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 often times 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 back pressure 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
4. Blower Maintenance Manual DRI-114-0502, Dresser Roots
5. Dresser Roots (www.dresserroots.com)
6. RGS HVB High Vacuum Booster Installation, Operation & Maintenance Brochure ILRB-3008 rev 0510, Dresser Roots.
7. Know How, Vacuum Generation, Pfeiffer Vacuum (www.pfeiffer-vacuum.com).
8. McCarthy, David, Diffusion Pumps for Vacuum Furnace Applications, Agilent Technologies Inc., 2009.
9. Vacuum Furnace Operation and Maintenance, Ipsen-U, 2009.
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.