In a manufacturing plant, when a hydraulic system has a problem, few people go back to the cleanliness management; most suspect the pump or the valve first. From the point of view of contamination control, however, the contamination introduced during manufacture is far greater than that introduced during operation: new pipework still holds welding slag and rust, new seals shed fine debris, and cylinder bodies retain machining fluid and burrs. These are carried into the system by the hydraulic oil during commissioning, and once they reach a servo valve or a proportional valve the damage is irreversible.
Contamination enters in stages
Taken apart stage by stage, the path by which contamination enters is segmented, and each segment has its own sources and its own means of control. If they are discussed together as a single question of cleanliness, the measures will not land, because the actions that really bring the cleanliness of the system back to target take place during assembly rather than during operation. This is also why many sites put all their attention on changing the filter element: the direction was wrong from the start.
| Stage | Main contamination sources | Control actions |
|---|---|---|
| Oil storage and supply | Uncleaned storage tanks, residual liquid and sediment in drums | Check and sample before acceptance into the plant, confirm the cleanliness |
| Pipework assembly | Welding slag, rust, cutting debris | Flush before assembly, blank the pipe ends while routing |
| Seal installation | Debris shed by the seals, contamination brought in by assembly tools | Purpose-made installation tools, cleaned and inspected before fitting |
| Cylinder bodies and actuators | Machining residues, burrs, residual old oil | Cleaned separately before assembly, with a defined route for the residual liquid |
| Commissioning and flushing | Residues inside the system carried into sensitive components | Flush before the sensitive components are connected, to the scheduled plan |
A cleanliness target has to be stated as a definite class
A target such as "a bit cleaner" cannot be worked to. A workable target normally states the cleanliness class and the corresponding filtration accuracy. If the system target is set at NAS 1638 class 5, then the final filtration has to be at the 1 μm level rather than leaving it to the coarse stage to get lucky. Once the target is clearly stated the configuration has a basis, there is a reference for acceptance, and the two sides do not end up with different readings of the same number.
The cleanliness of the system itself also has to be distinguished from the filtration accuracy of the element. The first is the target, the second is the means. Fitting a 1 μm element does not mean the system is at NAS 1638 class 5; if contamination enters the system in quantity ahead of the filtration stage, the fine filtration that follows can only make good the damage. The main ground of contamination control is in the pipework assembly and the flushing, not in the accuracy figures of the elements. If that order of importance is inverted, no amount of investment has any effect.
The flushing schedule is the step most often skipped
That a new system must be flushed before it goes into service is agreed; the flushing is nevertheless often cut back. The usual reason is a tight schedule or the cost of the flushing oil. The practical result is that the residues in the system are pushed into the sensitive components without adequate flushing, the fault rate during commissioning rises, and a shutdown and inspection is needed in the end, which makes the schedule longer rather than shorter, and by then the extra time is production time. The cost has to be counted either way; it has only moved from the commissioning period to after the plant is running, and its nature has not changed: the time saved by shortening one process is eaten up by the shutdown that follows.
A workable approach is to put the flushing into the process document and specify the volume of flushing oil, the flushing time, the position of the drain point and the acceptance criteria. The acceptance criteria do not have to be complicated, but they need at least a clear signal, for example the cleanliness test result on the drained oil sample, or an observation of how the particles settle in the drained stream. Flushing without acceptance criteria is in essence only circulating the oil once.
The rhythm of sampling and oil changes
After the plant is running, contamination control is maintained by sampling. The sampling point has to be fixed, normally located after the return line filter element, at the dirtiest point in the system; the sample containers have to be clean, and samples from different batches must not be reused. On frequency, the usual arrangement is to increase the density during the initial period of operation and then work to a cycle, the length of which depends on the load and the rate at which contamination is generated. The sampling records have to be kept for the long term.
Oil changes and element replacements should likewise be triggered by the sampling result rather than by the calendar. A deterioration in the cleanliness class means that the particle accumulation has exceeded the buffer capacity; a change in the moisture or the acid value means that the medium is degrading. These two signals point to different actions: the first calls for an element change, the second for an oil change and an investigation of the source of the water ingress or of the overheating. Attributing every measure to an oil change is a common misjudgement on site; the judgement still rests on those few readings themselves.
- Write the cleanliness class into the equipment technical file and compare the tests at acceptance by the same method.
- Arrange a flush with defined acceptance criteria between the pipework assembly and the installation of the sensitive components.
- Fix the sampling point and the sampling method, so that samples taken at different times are comparable.
- Decide on the oil change from the oil analysis result; do not let a calendar cycle stand in for the judgement.
On a manufacturing site the defence against hydraulic oil contamination is mainly built into the two operations of assembly and flushing.