Some Straight Talk on Vacuum Pump/Blower Rebuilds
When choosing a rebuilder, you must ask yourself (and the prospective rebuilder) a few questions:
- Do they have the facilities and resources to do the job with no outsourcing?
- Do they have the deep engineering expertise to analyze and problem-solve during the rebuilding process?
- Do they have factory-trained and certified technicians who know the difference between meeting and exceeding OE specifications?
- And most important, do they have the commitment to quality to put the pump through the toughest testing regime in the business before it is shipped?
Critical Dimensions and ClearancesWhen a worn pump or blower arrives at the plant, it is logged in and then fully disassembled by factory-certified technicians. All parts are carefully examined for damage and measured to determine critical clearances. Part dimensions are recorded and compared to the original specifications. All deficient parts are noted so they can be replaced or remanufactured in-house. This is unlike some rebuilders who replace just gaskets, bearings and valves.
MHV’s engineering staff will advise you how to avoid problems in the future. Advice may include recommending oil filtration where corrosive vacuum processes are involved or heavy-duty valve springs that resist corrosion, which can cause premature pump failure. If foreign matter (like broken glass) damages the pump, the addition of an inlet trap may be recommended.
Parts RemanufacturingEccentrics for rotary-piston mechanical pumps can be sleeved to restore sizes and finishes on bearing surfaces (Fig. 1). Sleeves are made from pearlitic continuous-cast gray iron, which has the same wear resistance as the OEM part. It is necessary to finish grind bearing surfaces after sleeving. There is an oil groove under the sleeve for lubrication. The small ring shown is for repairing wear at the end of the eccentric, around the hole (where the main shaft goes through the eccentric).
Piston-slides can be sleeved to restore diameter and finish on the bearing surfaces (Fig. 2). The sleeves are shrink-fit, and no setscrews are used. MHV has remanufactured hundreds of piston-slides using this method, and they have accumulated tens of thousands of hours of reliable operation. The sleeves are bored after installation for proper size and surface finish.
Also, piston-slide tangs can be ground to restore finish. The tang is the flat portion protruding from the cylindrical portion of the piston-slide. When the tang is ground, the mating hinge bars must be replaced with thicker ones to make up for the metal removed from the tang.
When the hinge-bar bore in a rotary-piston pump becomes scored, technicians line-bore the block to restore the proper surface finish (63 rms or better). Taper is held to 0.001 inch or less. MHV utilizes special fixtures that eliminate chatter and keep the bore in the factory location. New hinge bars (Stokes) or slide-pins (Kinney) are supplied to fit the new larger bore.
Diffusion PumpsVacuum diffusion pumps are still plentiful because they have several advantages. They are reliable, simple in design, run without noise or vibration, and are relatively inexpensive to operate and maintain. In fact, diffusion pumping can be the most economical means of creating high-vacuum environments. These pumps also tolerate operating conditions such as foreign particles and reactive gases that might damage other types of vacuum pumps.
Breakdowns can occur when there is an exposure to oxygen and overheating to a level that will degrade or decompose the oil. This can be a control failure or backing-pump breakdown. Failure of the cooling coils surrounding the diffusion pump can also cause overheating, and they must usually be replaced. This generally causes the internal parts to become covered in a difficult-to-remove layer of decomposed tar or gunk. Also, low oil level can elevate the boiler temperature to the point that the boiler plate is dished, or made concave, by the vacuum.
A vacuum diffusion pump is basically a stainless steel shell containing vertically stacked cone-shaped jets (Fig. 3). Typically, there are three or four jet assemblies of diminishing sizes, with the largest at the base. At the bottom of the chamber is a pool of a specialized type of oil having a low vapor pressure. The oil is heated to boiling by an electric heater at the base of the pump. The vaporized oil at a pressure of 1-3 torr moves upward and flows through the jets at sonic velocity. It is the collision of this vapor with the residual gas molecules “diffusing” into the pump inlet that gives this type of pump its name. The vapor is then condensed by water tracing on the shell of the pump and flows back to the boiler for reheating. Meanwhile, the residual gas molecules are forced downward and eventually end up at the foreline connection of the pump. Usually, the foreline must be held to less than 400 millitorr to prevent it from influencing the vacuum level on the top jet.
Roots-Type Blowers or Booster PumpsThese consist of two double-lobed, figure-eight impellers that run in an oblong cylinder with timing gears between the drive and driven impellers that maintain the proper clearances. Small clearances are maintained between the impeller and cylinder and impeller to endplates. As the impellers rotate, gas is compressed and ejected into the outlet, or foreline. At inlet pressures below about 5 torr, the compression ratio (or staging ratio) is usually set in the range of 4:1 to 10:1 by sizing the downstream pump at ¼ to 1/10th the inlet speed of the booster. Rebuilding consists of disassembly, inspection and measurement, caustic cleaning, remanufacturing of parts as needed, and then careful reassembly with new premium gaskets and seals.
Leak Testing and Final OperationAfter reassembly, all rebuilt pumps are helium leak tested for 24 hours to assure they are vacuum tight. MHV typically warrants rebuilt pumps for periods that equal or exceed that of the original manufacturer, although they can last a lot longer with proper maintenance. Oil changes on mechanical pumps are vital, especially when corrosive vapors are present. IH
For more information: Visit our website: www.expectmore.tv or call MHV at 877-787-9880 to learn more and view the plant-tour video. Not sure what’s wrong with your pump? Get our diagnostic kit that contains practical “How-to” information free at www.expectmore.tv
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: rotary-piston pump, mechanical pump, vacuum diffusion pump, backing pump, foreline, impellers