Several detection channels are fitted in the control cabinet. They all appear to do nothing more than show the operator a number or issue a signal, yet the failure mode associated with each of them is different. Infrared level detection answers the question of how much oil is in the tank; foam detection answers the question of whether foam that should not be there has appeared in the tank. The two measure two different states inside the tank, with different physical quantities and different trigger conditions.
Level detection governs how much oil is in the tank
A level that is too high means that the oil inlet rate exceeds the amount the machine can handle at the set vacuum level. The excess oil accumulates in the tank, the gas space above the oil surface is compressed, and the degassing space shrinks accordingly, so gas that has not been fully released is pushed back by the next oil feed. The result is that the gas content cannot be brought down to the required level, the vacuum level is established slowly, and the flow reading does not agree with the actual amount treated.
A level that is too low means that the effective working volume has been emptied out, the proportion of gas in the tank rises, the volume of gas that the vacuum pump has to remove increases, the time for which the same batch of oil is repeatedly evacuated becomes longer, and the flow performance drops accordingly. In severe cases there are phenomena such as intermittent oil supply and the pump being evacuated before the pump. Continuing to feed oil in such a condition only amplifies the problem, and the evacuation load on the pump rises with it.
Foam detection governs whether foam that should not be there is present in the tank
Foam is a different matter. Air carried in with the oil, emulsified liquid, or a large amount of water vapourising at high temperature can all produce foam in the tank. Once foam forms, it travels along the liquid level towards the oil-gas separation stage. That stage relies on gravity to separate the oil from the gas, and the entrainment of foam makes the separation unstable, the premise of the gravity separation is broken, and the qualification rate of the oil falls accordingly.
The effect on the vacuum side is more direct. When foam enters the vacuum pipework, the gas flow rises abruptly, the vacuum pump cannot keep up with its pumping speed and the pressure rises; if foam is drawn into the pump, the operating state of the pump is affected as well. In the mild case the vacuum level drops, in the severe case the machine has to be stopped and cleaned, and a restart means going through the preheating sequence again. The processing cycle of the whole batch is disrupted, and the site has to confirm the oil feed state once more.
The two signals cannot substitute for one another
The key point is that the two are independent signals: an abnormality in one does not mean that the other is normal. A normal level means that the amount of oil in the tank is within the set range; it does not mean that there is no foam in the tank. The fact that the foam detection has not triggered means that no foam was detected at that moment; it does not mean that the feed rate has not exceeded the processing capacity. Conversely, foam may be detected while the level is completely normal and the feed rate is not exceeded, simply because the gas and water content of the oil itself has produced foam.
- A level that is too high or too low corresponds to the matching of the feed rhythm against the processing capacity.
- Foam being detected corresponds to the condition of the oil and the gas-liquid separation conditions.
- Automatic defoaming and level protection handle these two cases respectively; they act on different targets and cannot replace one another.
What each protection chain protects
The over-pressure audible and visual alarm with shutdown protects the machine itself. Continuing to operate when the pressure exceeds the set range may damage the pipework, seals or filter elements, and the audible and visual alarm warns site personnel at the same time, so that the situation is not left as a silent numerical curve. Phase failure and phase sequence protection protect the motor and personnel: phase failure makes the motor run on two phases until it burns out, and a wrongly connected phase sequence makes the pump run in reverse. If both conditions are only discovered afterwards, the cost is high in either case.
Looking at these protections in one table makes it possible to distinguish which of them protect the equipment and which protect people and motors, and also to judge which item should be restored first after a shutdown and which should then be investigated. This avoids clearing the real hidden risk along with the reset, having the machine repeat the same problem under the same conditions, and it also makes the incident traceable.
| Item | Trigger condition | Protected target |
|---|---|---|
| Infrared level detection | Level leaves the set range | Feed rhythm |
| Foam detection | Foam detected | Stability of the gas-liquid separation |
| Automatic defoaming | Acts after foam has been detected | Separation stage and vacuum side |
| Over-pressure audible and visual alarm shutdown | Pressure exceeds the set range | Equipment and pipework |
| Phase failure protection | Phase failure in the power supply | Motor and personnel |
| Phase sequence protection | Phase sequence wrongly connected | Motor and personnel |
Site judgement: the detection itself or a real process problem
When one channel alarms, look at the phenomenon before acting. For a level alarm, first confirm whether the actual opening of the oil inlet valve differs from the setting, or whether foam has adhered to the detection line and air bubbles are interfering with the reading; for a foam alarm, first look at whether the airflow sound at the exhaust end has become coarser and whether the vacuum reading falls at the same time. If both appear together, the problem is most likely in the oil feed state or in the oil itself, rather than in the detection component.
Conversely, if the alarm appears while all the process parameters are normal, for example the vacuum level is steady, the pumping speed is steady and the flow is steady, and only the level reading fluctuates, the detection step should be suspected. Cleaning the detection line and the probe, recalibrating them and then checking whether the reading is stable usually takes effect faster than adjusting the process, and is less likely to shift normal parameters away from their correct values.
One further point should be mentioned: the complete functions of these detections and interlocks are normally used together with options such as a PLC with a touch screen and an online water content meter. The available alarm items and display items differ between configurations. When taking over a machine, check the alarm items and the action logic against the actual configuration first and then set the operating rules; do not copy the handling procedure from another machine.
The reason the two detection channels cannot replace one another is that they do not measure the same state at all: the level measures the quantity, the foam detection measures the presence of something that should not be there, and if either channel is missing, the site can only guess.
