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Vacuum pump maintenance general troubleshooting

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In this section we propose general instructions on troubleshooting and pump maintenance. In addition, we discuss potential errors and their causes.

General operating information for vacuum pumps

The direction of motor rotation should be checked with the intake and outlet ports open prior to installing the pump. The drive shaft, seen from the motor end, should rotate counter-clockwise. Note the arrow on the motor or housing indicating the direction of rotation. If the motor turns in the wrong direction, this is rectified by interchanging two of the phases at the motor connection terminal block.

Causes of faults when the desired ultimate pressure is not achieved or is achieved too slowly

If the desired ultimate pressure is not reached at all in vacuum equipment or if it is reached only after an excessively long pumping period, then the following problems may be the reason:

the vacuum system may be leaky or dirty,
the pump may be dirty or damaged,
the vacuum gauge may be defective.

If the desired ultimate pressure is reached only after a very long pump-down time, then

the vacuum system may be dirty,
the pumping line may be restricted or too small in diameter,
the pump may be dirty or with too little capacity,
the pumping speed may be restricted due to other causes.

In order to locate the fault, you should proceed by separating the evacuated vessel from the pump system and ensure that the pump system alone will reach ultimate pressure. If this does not reach ultimate pressure, refer to the individual pump sections for the type of pump in use.

Next you need to check the vessel and pipework/flange connections for leaks and contamination using, for example, the pressure rise method. Flange seals are known to be places at which leaks can appear the most easily, resulting from slight scratches and mechanical damage which initially appears insignificant. If the vessel is free of defects in this regard, then the measurement system will be checked for cleanliness (see page on Maintaining Gauges) .

Contamination of vacuum vessels and eliminating contamination

A commonly used procedure to detect contamination is to compare the pressure indication of one and the same vacuum gauge with and without a cold trap inserted in the line: Filling the cold trap with liquid nitrogen will cause the pressure to drop abruptly, by one power of ten or more, if the container is contaminated since the vapors will freeze out in the trap.

Eliminating contamination at metallic equipment

Metal components will usually exhibit traces of contamination by oil and greases. If these cannot be readily removed by pumping down the vessel, then an appropriate organic solvent (denatured alcohol is unsuitable in all cases) will have to be used for cleaning. Maximum cleanliness can be achieved with vapor baths such as those commonly found in industry. To achieve extremely low pressure ranges (< 10^-7 mbar)the metal components will have to be baked out at temperatures of up to 200°C after cleaning. Seriously contaminated metal components will first have to be cleaned by cutting away or sandblasting the top surface. These methods risk that the surface area are treated will be increased through roughening and active centers may potentially be formed which would readily adsorb vapor molecules. Supplementary cleaning in the vapor bath is advisable. In some cases, electrolytic pickling of the surface may be beneficial. In the case of high vacuum components, it is necessary to ensure that the pickling does not turn into etching, which would seriously increase the surface area. Polishing surfaces which have been sandblasted is not necessary when working in the rough and medium vacuum ranges, since the surface plays only a subordinate role in these pressure regimes.

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