When foam shows at the sight glass or the level gauge of the vacuum chamber, the instinctive move is to open the tank door and clean it out. That step comes last in the handling order, not first. Opening the tank while the working vacuum of -0.08 ~ -0.0999 MPa is still on the vessel pulls gas and droplets out through the pressure difference, and at the same time carries outside air and oil mist back into the system. Foam is a symptom; stabilising the system first and looking for the source afterwards is the correct order.
The first step: stop the oil inlet, not the tank door
The first source of foam is usually the continuous supply. Closing the oil inlet valve cuts off the source of foam; once the oil inlet speed comes down, the load on the vacuum side comes down with it. This sets the starting point of the handling: what the operator can do sitting at the control cabinet deserves priority over running to the tank to open it.
At the same time, confirm whether the automatic de-foaming triggered by the infrared level and foam detection has actually acted. A rising foam level triggers the de-foaming action, and if the de-foaming side did not execute properly the foam continues to advance into the vacuum chamber. What has to be checked then is whether the action occurred and whether the foam was drawn away, not whether the vacuum level can be raised.
Next: reduce the speed, hold the temperature, watch the vacuum gauge
There is a fixed ratio between the oil inlet speed and the degassing capacity. If the oil inlet speed exceeds the current degassing capacity, foam is produced without end. After reducing the speed, watch the vacuum gauge: if the vacuum level still holds near the working vacuum range of -0.08 ~ -0.0999 MPa, the vacuum side is still stable and the foam is mainly still upstream of the vacuum chamber; if the reading becomes markedly weaker or fails to reach the working range for a long time, the foam has already entered the vacuum chamber, or the oil inlet speed itself is beyond the degassing capacity of this machine at that moment.
The temperature control has to be checked at the same time. The temperature control range is 45–80 °C, with the upper limit pulled down to 70 °C when the water content of the oil is high. When the water content is high, the higher the temperature the faster the water vaporises and the more foam is produced. Reading the temperature and the oil inlet speed together is often more effective than adjusting either one on its own.
Three actions to avoid
After foam has overflowed, the three actions below are the ones most likely to make the situation harder to handle; what they have in common is that each of them makes the work that follows more difficult.
| Action to avoid | If you do this | What to do first |
|---|---|---|
| Opening the tank while the negative pressure is still on the vessel | The pressure difference carries gas and liquid out of the tank, and outside air is drawn into the system | Stop the oil inlet, then stop the pump and the heating, and wait until the pressure is back close to atmospheric before venting and opening the tank |
| Blowing the foam apart with compressed air | Air is driven into the oil and creates a new foam carrying entrained air | Let the automatic de-foaming and the vacuum side work first |
| Increasing the oil inlet speed while the foam is still active | The foam production rises together with the oil throughput, and the overflow becomes worse | Run at reduced speed and confirm that the degassing capacity keeps up |
- After foam overflows, close the oil inlet valve first, and only then consider any dismantling or assembly work.
- Do not open any tank accessory while the system is under pressure or under negative pressure.
- Confirm that the automatic de-foaming has actually acted; do not look only at the command status of the de-foaming valve.
- When foam pours out in quantity, do not blow it away with compressed air; reduce the speed first and let the vacuum side stabilise.
Once it is stable: how to trace the cause back
With the handling order carried out, the machine is back under control, and the next step is to record the cause of this occurrence; otherwise the same foam will come back again. Three sets of records have to be kept: the oil inlet speed, the oil temperature and the vacuum gauge reading at the moment the foam appeared; the times of the infrared foam detection alarm and of the automatic de-foaming action; and the water content test result for this batch of oil, measured to GB/T 7600.
With these three sets of data the direction can be separated. A high water content, with foam that clearly increases as the temperature rises, points to water vapour over-saturation and has to be handled in the temperature control settings; a normal water content with foam that keeps recurring calls for the return oil line to be checked first, its sealing and the position of the suction inlet, to see whether air is being drawn in; where the de-foaming action has run but the foam cannot be held down, confirm that the detection point is installed before the vacuum chamber.
When foam overflows, do not open the tank yet: cut off the source and then look at the data. Opening the tank is the closing step, not an investigating step.
