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Roots pump maintenance and operation guidelines

General operating instructions, installation and commissioning

Roots pumps must be exactly level since the clearances inside the roots pump are very small. When attaching the pump, it is necessary to ensure that the pump is not under any tension or strain whatsoever. Any strains in the pump casing caused by the connection pipes should be avoided. Any strain to which the pump is subjected will endanger the pump in operation..

Roots pumps are positive displacement pumps without internal compression. This means that they usually cannot compress directly to the atmosphere, but always work in combination with a forevacuum pump. High gas loads will cause excessive load on the motor and temperature issues on the pump rotors themselves.

Leybold offers different types of Roots pumps that are protected from excessive loads in different ways. While classic Roots pumps are protected by a pressure switch, pumps operated with frequency converters or bypass lines can run permanently.

Classic roots pump may be switched on only after the forevacuum pump has evacuated the vacuum vessel down to the switch-on pressure. The pump system must be equipped with a pressure switch or a pressure sensor that is suitably evaluated in order to switch on the Roots pump within the permissible pressure range.

Permissible switch-on pressure PE will depend on the reduction ratios of the roots pump as against the forevacuum pump and is calculated by dividing the permissible pressure differential Δpmax by the compression ratio, reduced by the value of 1:

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If a combination of roots pump and forevacuum pump is to pump highly volatile substances such as liquids with a low boiling point, then it is advisable to use a roots pump which is equipped with an integral bypass line and a valve which will respond to a pre-set pressure. Example: Roots vacuum pumps RUVAC WHU / WSU / WAU.

Roots pumps from the RUVAC-WHU/WAU/WSU series, being equipped with bypass lines, can generally be switched on together with the forevacuum pump. The bypass protects these roots pumps against overloading.

Oil change and maintenance work

Under clean operating conditions, the oil in the roots pump will only degrade as a result of the natural wear in the bearings and in the gear box. We nevertheless recommend making the first oil change after about 500 hours in service, in order to remove any metal particles which might have been created by abrasion during the run-in period. Otherwise, it will be sufficient to change the oil every 3000 hours of operation. When extracting gases containing dust or where other contaminants are present, it will be necessary to change the oil more frequently. We recommend using our specially formulated LVO 100 oil.

Under “dirty” operating conditions it is possible for dust deposits to form a “crust” in the pump chamber. These contaminants will deposit in the pumping chamber and on the rotors. They may be removed, once the two connection pipes have been detached, either by blowing out the system with dry compressed air or by rinsing with a suitable cleaning agent, such as petroleum ether (naphtha).
The oil in the roots pump will then have to be changed. The rotor may be turned only by hand during cleaning since, due to the high speed when the motor is running, the deposits could damage the pump as they dislodge from the surfaces.
Grime which cannot be eliminated by washing can be removed mechanically using a wire brush, metallic scrubber or scraper.

Important note

The dislodged residues should not remain in the pump chamber. After cleaning is completed, check the pump for operability by slowly turning the rotors by hand. There should be no resistance to rotation. It is generally not necessary to dismantle the roots pump. If this should nevertheless be required due to heavy contamination, then it is highly advisable to have this done by the manufacturer.

Actions in case of operational disturbances

1. Pump becomes too warm:

The maximum operating temperature of the pump is 100°C - 115°C. Possible causes of the pump becoming too warm include:

  • Overloading: Excessive heat of compression due to an excessively high pressure ratio. Check the pressure value adjustments and the tightness of the vacuum chamber!
  • Incorrect clearances: The distances between the rotors and the housings have been narrowed due to dirt or mechanical strain.
  • Contaminated bearings: Excessive friction in the bearings
  • Incorrect oil level: If the oil level is too high, the gears will touch the oil, causing friction resistance. Where the oil level is too low the system will not be lubricated.
  • Incorrect oil type: An SAE 30 grade oil must be used for the pump.

2. Excessive power consumption:

All the factors which can lead to elevated temperatures can also cause excessive amounts of power to be drawn. The motor is defective if excessive power requirements do not coincide with a rise in the temperature at the pump.

3. Oil in the pump chamber:

Possible causes:

  • Oil level too high: Oil is subjected to excessive thermal loading. Oil foam is swept along.
  • Oil mixed with the product: degassing of the oil.
  • Pump leaking: Air ingress through the oil drain or filler screw will cause a large stream of air and leaking of oil into the pumping chamber.

4. Abnormal running noises:

Possible causes:

  • Dirt or residue at the rotors
  • Bearing or gearing damage
  • Rotors are touching the housing

In the case of bearing or gearing damage or where the rotor scrapes the housing, the pump should only be repaired by the manufacturer.

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