A vacuum oil purifier that has been in use for three or five years produces oil that is not as clean as that from a new machine, and this kind of feedback is very common. The mistake most easily made on site is to change the filter element straight away and still find the result non-conforming; or the other way round, to strip the pump for maintenance while the element stays blocked. The real situation is usually that several parts are ageing at the same time, and the question is which of them to deal with first.
Sorting the order follows two lines: one uses the result to set the direction, the other uses the checks to set the sequence. The result line follows the trend of the four indicators: if the cleanliness deteriorates, suspect the filtration section first; if the gas content deteriorates, suspect the vacuum system first; if the water content deteriorates, check the sealing and the condensation first; and a deterioration in the breakdown voltage is usually the result of the water content and the particles acting together. The check line separates the reversible from the irreversible: cleaning, resetting and re-tightening are reversible, while ageing of the seals, wear of the pump and saturation of the filter media are irreversible.
What each of the four indicators points to
| Indicator that deteriorates first | Section to suspect first | First check to carry out | Which category it belongs to |
|---|---|---|---|
| Cleanliness | Saturation of the filter media, an end cover not pressed tight, an element fitted the wrong way round | Strip and inspect the element, and look at the state of the media and the seals | Saturation can be replaced with a new element; the end cover and the reversed fitting can be put right |
| Gas content | Sealing of the vacuum system, residual liquid in the low points of the pipework, degradation of the pump oil | Stop the pump and hold the pressure to see how fast it falls | Cleaning allows recovery; ageing of the sealing parts requires replacement |
| Water content | Tank sealing, the condensation section, insufficient heating | Check that the temperature control has reached its setpoint and that the condensation water is being drained | Scale can be cleaned off; ageing of the seals is irreversible |
| Breakdown voltage | The combined effect of the water content and the particles | First re-test the water content and the cleanliness separately | Usually reversible; bring the two into line first |
Separating these four classes of problem also has a practical benefit: each class has its own basis for judgement, and they do not interfere with one another. Saturation of the filter media is judged from the differential pressure curve and the appearance of the element; ageing of the seals from the rate at which the pressure falls during a pressure-hold test; the deviation of the temperature control from the difference between the displayed temperature and the measured temperature; and wear of the pump from the running current. All four can be measured once without stripping the machine, and once they have been measured it can be decided where to strip, which saves a great deal of pointless dismantling and reassembly.
The order of the checks: start with what does not need the machine opened
- Look at the gauges first. When the readings of the pressure gauge and the vacuum gauge cannot be trusted, every later judgement rests on wrong numbers; if the readings differ greatly before and after the gauge is changed, the problem is in the gauge and not in the machine.
- Then check the temperature. If the displayed temperature differs a lot from the hand-held measurement, drift of the probe distorts the temperature control judgement; scale on the heat transfer surface reducing the heating efficiency is also possible.
- Then look at the current. A running current clearly higher than in the period when the machine was new often points to increased resistance in the pump and the bearings; this is an item for maintenance rather than one that consumables can solve.
- Only then open up the filtration section. An element fitted the wrong way round, an end cover not pressed tight and a seal squeezed off centre can all be seen directly once the section is opened.
- The vacuum system is dealt with last. After stopping the pump, look at how fast the pressure falls: a fast fall points to a sealing problem, a slow fall to the pumping speed of the pump set, and the two lead in completely different directions.
The irreversible part has to be accounted for separately
Cleaning, resetting and re-tightening can be done as many times as necessary and cost nothing; ageing of the seals, wear of the pump and saturation of the filter media cannot be repaired indefinitely, and every replacement is a cost that also means a shutdown. On an old machine, therefore, all of the reversible part is dealt with first, and only then is it assessed how much of the irreversible part is left; what remains is what actually has to be paid for, and this is the practical basis for deciding whether a machine should be repaired or replaced.
The part that has become dirty can be cleaned, the part that has become old can only be replaced; first establish which of the two it is, and only then decide whether money is to be spent.
