Current Trends in Maintaining Vacuum Systems
This article was originally published on September 8, 2014.
Return on investment (ROI) can be maximized by reducing unplanned downtime through predictive/preventive maintenance.
For some unknown reason, roughing pumps and blowers always seem to lock up, develop oil leaks or make weird noises on a Friday afternoon. As a result, needed production is lost or, in the worst case, a furnace load is scrapped.
With predictive maintenance, these types of outages will become rare. Predictive maintenance requires pulling out a functioning piece of equipment based on accumulated service hours and bearing vibration signatures. The key is to get the equipment off-line and repaired before catastrophic failure occurs. This is the cheapest alternative in the long run because the downtime for change-out is planned, and rebuild costs are lower since fewer hard replacement parts are usually needed. This is a very hard sell to management if the norm is to “run it ‘til it breaks.”
A two-pronged approach is needed for predictive maintenance:
• An in-depth vibration analysis on all rotating equipment
• The use of a custom-designed checklist for each furnace system
An “fft” Analysis
Baseline vibration levels need to be established on all roughing pumps and blowers. (Holding pumps are exempt. More about these later.) An outside contractor is usually brought on board to do this specialized testing. They will attach hard pick-up pucks on the drive and dead ends of the rotating equipment to locate accelerometer position during a vibration scan. In this manner the pick-up geometry remains fixed and not a function of who is running the scan. A fast fourier transform (fft) analysis of the accelerometer output will be plotted for each puck location.
Figure 1 shows a plot for a remanufactured Roots-type 5-HP blower running at 85 Hz (5100 rpm) under load. We have determined that this machine should have a primary unbalance velocity amplitude of less than 0.100 inch/second to ensure acceptable life of the rebuild.
The plot shows a 0.098 inch/second amplitude at 5,009 rpm due to primary imbalance of the motor rotor/impeller system. The 0.040 “second peak” at 9,931 rpm (second harmonic) is due to flow characteristics of the rotary lobe blower under load (i.e. two discharge peaks per revolution). Smaller peaks are due to bearing ball pass frequency on the inner and outer races. These amplitudes will increase as run time elapses on the equipment.
Figure 2 shows the drive end of a larger Roots-type 30-HP blower (Leybold RA9001) running at 1,800 rpm. Note a primary imbalance of 0.050 inch/second at 1,757 rpm and various smaller peaks extending out to 7,470 rpm due to ball/race vibrations. Figure 3 is the gear end of the same machine showing slightly lower primary imbalance.
The figures form a base plot of vibration and are sent along with the rebuild. The end customer would then run his own plots for baseline data. Quarterly or semiannual re-scans would be done to indicate impending bearing problems. The aim is to determine when the equipment should be taken out of service. In the case of the large blower presented, change-out might occur every 3-4 years under 24/7 operation (25,000-35,000 hours).
A time line needs to be established, and this can be done in several ways. A simple approach is to wire 120-VAC hour meters across selected motor starters. If a PLC is present with an HMI, a “maintenance screen” can be developed to show accumulated hours on each motor along with suggested maintenance intervals. Hour meters (480-VAC) are available to connect directly across the pump or blower motor, but they will need enclosures mounted near the motor with interconnect wiring.
These are the key to a successful maintenance program. Checklists need to be made for all subsystems on a typical vacuum furnace. These are filled out weekly during a scheduled walk-around. Pay attention to new noises, oil leaking from shaft seals, compressed air leaks, etc. Specific items to check and perform:
• Drive belts on pumps
• Oil levels on rough pumps, condition of oil (clear or milky)
• Oil levels on lip seal cavities on blowers if so equipped
• Oil levels on drive and gear ends on blowers
• Coupling condition on blowers if direct drive (we have seen couplings become completely disengaged)
• On large Roots 10-inch and larger gear machines, check seal pockets on drive and gear ends, drain if necessary and note.
• Drain oil reservoirs on rough pumps – check for water and adjust gas ballast valve(s) if needed to sweep out water contamination.
• Drain any oil carried over to oil mist separators if equipped.
• Exhaust back-pressure during roughing if so equipped with gauging
• Record oil temperature on rough pumps.
• Record oil delta P on rough-pump filters if so equipped.
• Note moist oil areas around mechanical shaft seals or gross oil leaking.
• Oil level in diffusion pump (note whether hot or cold reading)
• If diffusion pump has a foreline ejector (Varian models VHS 6-10, HS-16-20-35), feel temperature on the foreline elbow to ascertain that the ejector jet is functioning and properly aimed into the foreline.
• Listen for gear/bearing noise on blowers.
• “Oil slap” noise on rotary piston rough pumps when basing is OK. If there is “heavy pounding” that is not silenced with a little gas ballast, there are excess clearances/bad bearings in the pump.
• Oil level in air line lubricator
• Oil level in hydraulic power pack if so equipped
Do not neglect holding pumps! Small direct-drive pumps need to have a small amount of oil drained to make sure it has not polymerized. We have seen failures on Leybold D25B pumps that indicated good oil level, but the oil reservoir was completely full of tar-like oil.
Figure 4 shows a typical checklist form for a vacuum furnace. The “date” and “tech” line items must be filled out. Specific problems can be jotted down in the “notes” area. This form is set up for a furnace having a pair of MHV HS430 roughing pumps with on-board filtration. (This is a first attempt and will need some tuning up.) For the survey, we suggest two technicians to run the list. It should take 15-20 minutes per furnace. This assumes pump oil is handy.
Preventive Maintenance Guidelines
Maintenance consists of oil/filter changes, valve/valve spring replacement, etc. Stokes recommends an oil change every 300 operating hours because their pumps have no on-board oil-filtration system. This is only 12-13 days on a 24/7 system! If you adhere to this oil-change frequency, you’ll see long pump life and high oil costs. Oil changes should be dictated by the results of oil analysis, not operating hours. A vacuum oil-quench furnace running only a few days per week might need rough-pump oil changes every four months. We recommend using multi-viscosity engine oil with a detergent and rust/oxidation package in rotary-piston and vane-type roughing pumps. The higher base pressure of these oils is not a problem since most applications involve a rotary-lobe blower in front of the pump. This would make the base vacuum at the blower inlet only a few microns above that obtained with a vacuum-distilled pump oil with little or no additive package.
Clapper valve and spring changes might be required yearly on rotary piston pumps operating 24/7 year-in and year-out. It would make sense to convert these clapper-type valves to the poppet type to eliminate chances of valve spring ingestion.
Normal oil-change frequency on rotary-lobe blowers is once or twice per year, but we have seen applications with heavy refractory particle ingestion that would require weekly oil changes to get acceptable gear and bearing life!
The larger Roots high-vacuum blowers have four pressure-lubricated mechanical seals. Oil from normal seal leakage ends up in a seal cavity located at the bottom of the head plates. These pockets need to be drained to remove accumulated oil. If you need to drain about a quart of oil per week, the mechanical seals need replacing. If draining is not done, the oil level will increase and spill over through the impeller necks into the cylinder. This oil eventually ends up in the roughing pump.
Diffusion pumps should be pulled every 1-2 years, disassembled and the jets glass beaded. After pulling the jet assembly, lay the pump down on its side to allow easy access to the heaters and boiler plate. At this time the boiler plate can be inspected for excess curvature (we allow 3/8 inch maximum on the HS35 before a new boiler plate is recommended). We also recommend the use of OEM-style heaters because they conform better to a dished boiler plate than the cast-iron aftermarket replacements.
• #10MG-538 high-temperature wire with Panduit P10-10RHT6-D lugs are recommended for heater connections. Insulate the lug/wire junction with Scotch #27 glass tape.
• Check heater current in all three legs. Unbalanced currents mean an open heater. Replace heaters in complete sets.
• Missing clamp-plate studs on the boiler plate need to be attached by TIG welding to maintain adequate clamping force.
• The cooling lines should be pressure-flushed with a 50-50 solution of hot water and muriatic acid. Circulate this solution with an air operated plastic diaphragm pump and a 5-gallon bucket for an hour or two. Then flush with clear water.
• After reinstalling, evacuate and helium leak check the cold-cap bushings and tubing if equipped. Internal water leaks on the cold cap will ruin the base vacuum on the pump.
• Note: Leybold DIF-series pumps have cartridge heaters, which can be extremely difficult to remove.
Sensor Packages for Stokes 412-Style Pumps
We have provided a package powered by two motor legs that allows factory communication to the pump via Ethernet I/P. Figure 5 (intro. image) shows such a package. Figure 6 shows the Allen Bradley Micrologix 1100 PLC that will interface with Ethernet I/P. This allows monitoring of pump-oil temperature, level and line pressure. A prox switch senses flywheel rotation to warn of drive-belt failure. Pilot lights allow easy inspection of pump operation during a walk-through. IH