Used oil purifiers are beginning to appear on purchasing lists, usually for reasons of cost. Oil treatment equipment can indeed be bought second hand: the frame, the tank and the pipework are durable metal structures, and what actually ages are the sealing elements, the pump oil, the electrical components and the filter elements. The problem is that buying second hand saves the purchase price while handing over the cost of making a judgement; if the basis for that judgement is not clear, this account is easily calculated the wrong way round.
The first look is at identity, not performance
The first thing to confirm about a second hand machine is what it is. The position of the nameplate and of the equipment number carries the basic information of the machine, including the model, the processing flow rate, the rated power and the manufacturing information. A missing or covered nameplate means that every later part purchase, spare part matching and maintenance record has no anchor, and it is hard to imagine how a machine whose identity is not even clear could be maintained reliably.
The accompanying documents matter just as much. The factory test records and the certificate of conformity, the list of spare parts supplied with the machine, the manual and the operating instructions tell you what condition the machine was in when it left the factory, what was supplied with it, and what performance was promised at the time. A second hand machine with a complete set of documents at least has a traceable starting point.
The second look is at what has been changed
The part of a second hand machine most open to alteration is the filtration stage. The filter element is the easiest component to remove and also the easiest place for someone to fit a cheaper part. Once a machine has been fitted with elements of a non-original specification, its nominal accuracy no longer holds. There is a simple way to check this: compare the specification of the elements on the machine against the model of the machine. The common coarse elements are 150-250-50 and 110-250-50, the second stage element is 110-400-20, and the fine elements come as 110-600-5 and 150-500-5. If the specifications do not match, that is a sign that the machine has been tampered with.
Two more places are often changed: the pipework and the joints. Joints of different specifications do not match either in pressure or in flow rate, and once they are fitted they either leak or cause a large pressure loss. The electrical part has to be looked at as well. After a second hand machine has been dismantled and reassembled a number of times, whether the wiring in the cabinet has been tidied up again and whether there are any signs of burning are the first things to judge the electrical condition by.
| Inspection point | What to look at | What it tells you |
|---|---|---|
| Elements and filter housing | Whether the specification matches the model | A non-original specification means the nominal accuracy no longer holds |
| Vacuum pump | The condition of the pump oil and traces of a change | Deteriorated pump oil makes the pumping speed drop noticeably |
| Sealing elements | The shape and the leakage of the static and the dynamic seals | Ageing rubber parts are the most common source of faults on a second hand machine |
| Wiring in the cabinet | Signs of burning and the wire numbers | It reflects how the machine has been used |
The third pass is re-establishing the baseline
Once the machine has been bought, the performance baseline has to be re-established instead of taking the description of the seller on trust. The baseline includes the sound and the vibration during a no-load trial run, the flow rate and the pressure during loaded running, the way the vacuum level is established, the action of the heating and the temperature control, and whether each of the protective functions is effective. These are the same items as in the factory test of a new machine; the only difference is that the initial condition of a second hand machine needs more time to observe.
- No-load first and loaded afterwards; note the abnormal points during the no-load stage and do not put oil in too early.
- Treat all sealing elements as items for replacement; the cost is far below the loss of oil caused by leakage.
- Treat the filter elements as new items and establish the initial differential pressure baseline first.
- Verify the electrical safety items one by one: the protective earthing, the phase loss and phase sequence protection, and the overpressure alarm and shutdown all have to be confirmed as effective.
The condition under which a second hand machine really is worth it is that the price leaves room for both the time taken to re-establish the baseline and the cost of the replaced parts. If the money saved on a second hand machine amounts to exactly the cost of one full overhaul, then the point of the transaction has to be calculated again. The discount, the production downtime and the spare parts lead time have to be looked at together, and the third of these is the most critical: a machine with no source of spare parts turns the money saved into downtime loss at the end.
A second hand machine can be bought, but what is bought is a machine that has to be verified again, and the price has to take the cost of that verification into account.
