There is one class of fault that is particularly well hidden: the machine runs normally all round, the flowmeter reads normally, the outlet pressure is normal, the oil at the outlet looks clean, and yet the oil never reaches the cleanliness it should. The reason is that a valve has not closed fully: the internal leakage has formed a bypass path, and the oil takes that path around the filtration section and goes straight to the oil outlet.
How internal leakage pulls down the filtration result
The normal sequence is oil inlet, heating, filtration, vacuum degassing, oil outlet. If any one of the diverter valves or the bypass valves has not closed completely, two parallel paths exist: one through the filtration section and one around it. Because the resistance of the filtration section is far higher than that of the bypass path, most of the oil in fact takes the path of lower resistance.
This produces a misleading result: the throughput does not fall, it may even be higher, because the oil that goes through the bypass is counted in the flow rate as well. The flowmeter therefore shows a normal reading, and seeing oil discharge steadily on site leads to the conclusion that the machine is working well. In reality the filtration accuracy has already been lost.
The pressure behaviour is more insidious still: when the filtration section has not built up sufficient differential pressure, the differential pressure reading across the filter element is below the design value. If this differential pressure is not recorded as a baseline in normal operation, the low reading also looks like a normal value.
How to confirm internal leakage rather than something else
Use four steps of cross-verification; no single one of them on its own gives a conclusion.
- First, look at the differential pressure: record the current inlet and outlet differential pressure of the filtration section and compare it with the baseline kept from this machine's normal operation. If it is clearly low, suspect a bypass.
- Second, take comparative samples: sample the inlet oil and the outlet oil at the same time and test them for cleanliness, then compare against the NAS 1638 grading. If there is almost no difference in particle level between the inlet and the outlet, the oil has in effect not been filtered.
- Third, isolate section by section: close and isolate each path in turn as described in the machine manual and watch how the flow and the differential pressure change. If the flow clearly drops when one path is closed, the flow was going through that bypass.
- Fourth, run a comparison test: use a known oil containing particles as the reference. If the machine has almost no particle separation capacity, internal leakage is confirmed.
Why it is hardest to find during acceptance
Three reasons stack up. First, acceptance is normally carried out with oil that is already clean, so oil that bypasses the filtration section and oil that passes through it cannot be told apart; only a comparison with oil containing particles brings the fault out. Second, instruments such as the flowmeter and the outlet pressure gauge reflect the overall state of the system, and the oil in the bypass path carries pressure in the same way, so the instrument readings are normal. Third, site practice is to judge by the appearance of the outlet oil, while filtration accuracy cannot be seen; the appearance only shows that the large particles have been removed.
To make the acceptance stage meaningful, the key is to add one step that is also part of the routine: record the differential pressure of the filtration section as a baseline and compare against it at intervals.
| Symptom | What it points to | How to confirm | Action |
|---|---|---|---|
| Normal flow, outlet cleanliness fails the requirement over a long period | A bypass path exists | Take comparative inlet and outlet samples and test cleanliness | Stop the machine and check every diverter and bypass valve |
| The differential pressure of the filtration section is clearly below the baseline | The filtration section is not carrying the full flow | Compare against the differential pressure baseline of normal operation | As above, with the valve disc and the valve seat as the first items to check |
| The flow clearly drops after one path is closed | The flow was going through that path | Isolate path by path and watch the flow and the differential pressure | Confirm that the valve on that path is in the correct working position |
| A valve that has not been moved for a long time will not open and close properly | The valve stem is seized or corroded | Move the valve by hand and check whether the travel is smooth | Stop the machine, service the valve stem and the valve disc, and add lubricant |
Maintenance and prevention
Once internal leakage is confirmed, stop the machine, release the pressure, disconnect the power and remove the valve in question to check how the valve disc seats against the valve seat. Light wear can be corrected by lapping the sealing face and refitting the pair; a badly scored sealing face or a deformed valve disc means replacement. After refitting, re-test the differential pressure of the filtration section and make one comparative cleanliness test between the inlet and the outlet to confirm that the bypass has been eliminated.
Three measures carry the prevention in normal operation: work the valves by hand at regular intervals and record in the maintenance log that this has been done; fit pressure gauges on the inlet and outlet pipe sections so that the differential pressure can be read at any time; write the differential pressure baseline into the maintenance records and compare against it at intervals instead of judging by impression.
A normal flow rate does not mean normal filtration. A valve that leaks internally takes away accuracy, not output, and that is why it is harder to find than a machine that has stopped.