Over the past ten years the operating procedures for oil treatment equipment have been written more and more finely: changing the filter element, changing the seals, reading the pressure gauge, each step has a fixed action. But what really decides how well the equipment is used is often not those actions themselves but the judgement that comes before them: is the phenomenon in front of me a fault of the equipment or of the oil. Get that step the wrong way round and every action that follows is done correctly in the wrong way.
Changing the element and judging the cause are two different things
Changing the element is work of action: read the manual and it can be done correctly. Judgement is work of analysis: the same phenomenon can have three or four different causes behind it, and the readings on site, the oil sample data and the equipment history have to be read against each other. The longer the equipment has been in service, the wider the gap between the two grows: with a new machine the manual is enough to avoid errors, while the modifications, the oil changes and the temporary treatments accumulated on an old machine are not in the manual, and the judgement becomes harder.
More practically, the people who can actually get readings on site are usually only one or two, while the equipment may be spread over several areas. That means the judgement has to be built on a few observable indicators rather than on a test system that needs specialist instruments. This is also why many sites end up with only two habits: reading the pressure gauge and reading the vacuum gauge.
What is scarcest now is telling the equipment side from the oil side
Take the case of a vacuum level that will not come up. The causes on the equipment side are a vacuum pump oil in poor condition, a leak at a pipe joint and foreign matter on a sealing face; the causes on the oil side are a high water and gas content in the oil itself and a large amount of dissolved gas in the system. The two lead in completely opposite directions: on the equipment side the machine has to be stopped and overhauled, on the oil side the degassing time has to be extended or the work done in batches. The common misjudgement on site is to dismantle and inspect the vacuum pipework again and again once the vacuum level will not come up, and find nothing.
| Phenomenon | The reading or action to look at first | Which side is more likely |
|---|---|---|
| The vacuum level stays at -0.08 MPa and will not come up | After shutdown, carry out a pressure holding test and watch the rate at which the pressure recovers | A fast recovery points to the equipment side, a slow one to the oil side |
| The water content will not come down to the target value | Observe the separation in the heating section and the vacuum section | Mostly the water is present in emulsified or dissolved form |
| The element replacement interval is clearly shorter | Check the reset of the bypass valve and the direction in which the element is fitted | Mostly a matter of the operation on the equipment side and of the pipework |
| The outlet pressure fluctuates widely | See whether the period of the fluctuation is related to the flow rate | Related to the flow rate points to the pump side, related to time points to the control side |
How this judgement is practised
The precondition for practising it is records, not memory. The point is not to learn the conclusions by heart but to write down the handling of every abnormality: what the phenomenon was, which readings were taken, which step was carried out and what the result was. After ten such entries many judgements become a conditional checklist, and a similar phenomenon no longer has to be worked out from the start; it can also be handed to a new member of staff.
- Carry out a pressure holding test at every shutdown and enter the recovery data in the ledger; this is the lowest-cost action for telling the equipment side from the oil side.
- Fix the position of the sampling point and the sampling method, so that samples taken at different times are comparable.
- When changing an element, record the differential pressure and the running time before and after the change, so that the machine builds up its own life baseline.
- Leave a conclusion behind every abnormal event, even when the conclusion is that the cause has not been identified; a blank costs the most time.
Why it decides how well the equipment is used
The specification of the equipment is fixed; the standard of the work on site varies. The same machine on a site where the judgement is accurate usually has a stable vacuum level and an element life close to expectation; on a site where the judgement is poor there is one round of reversals after another: the element is changed and blocks again within a few days, the vacuum level will not come up so the vacuum extraction time is increased, and the energy consumption goes up with it. The judgement of the engineers on the line is in effect a second layer of capacity on top of the nominal capacity of the machine.
The manual says what the machine can do; the site engineer decides how much of that it actually achieves on this production line.
The judgement has a further role that is easily overlooked: it decides whether the problem reported back to the manufacturer is a useful one. A problem that comes with a pressure holding curve, oil sample data and replacement records can be located directly by the manufacturer; a remark that the vacuum will not come up gets nowhere after several rounds of discussion. Turning the experience of the site into a record that can be reproduced is in itself one of the most valuable outputs of a technical engineer.