In the vacuum degassing process the oil is atomised and evaporates violently, and foam cannot be avoided. The traditional arrangement is to have somebody watch the sight glass and open the defoaming valve when foam appears. The job itself is not difficult; what is difficult is that it requires someone to be there all the time, and the one thing the site lacks most is manpower.
What foam leads to
Foam is a problem in two directions. First, foam has a large volume and a high water content, and when it is drawn off by the vacuum pump it can carry moisture into the pump chamber, which emulsifies the vacuum pump oil; the pumping speed and the ultimate vacuum of the pump then fall, and the degassing capability of the whole machine follows them down. Second, once the foam builds up beyond the oil level it overflows, and if it overflows, all the purification work done before has to be started again.
There is also an effect that tends to be underestimated: the foam itself occupies part of the usable volume in the tank. Where the oil volume is small, a thick layer of foam is enough to bring the actual throughput down noticeably, while the operator assumes that the machine is not working. Looking at the thickness of the foam layer together with the actual flow rate often explains a good many questions about efficiency.
How infrared detection and automatic defoaming work together
The ZYD-100 uses infrared level detection and infrared foam detection together with an automatic defoaming solenoid valve. The detection unit monitors the liquid level height and the foam condition in the flash evaporation tank continuously; as soon as the foam goes beyond the set range the control program gives a signal, the automatic defoaming solenoid valve opens the exhaust path and discharges the surplus gas into the vacuum system. The whole process runs without manual intervention. The problem that occurs most often during manual attendance, nothing happening while someone is watching and an overflow as soon as they walk away, is removed this way.
Two more safeguards are needed for unattended running
Automatic defoaming is only one part of it. Truly unattended operation requires at least three types of detection to be online at the same time:
- Level detection: to judge the amount of oil in the tank and to prevent the pump from drawing in air at a low level or the tank from overflowing at a high level.
- Foam detection: to judge whether the defoaming action keeps up with the rate at which the foam is actually produced.
- Pressure protection: to give an audible and visual alarm and stop the machine on overpressure, so that the strainer is not damaged by overpressure and so that contaminated oil is not pushed further downstream.
In addition, phase-loss, phase-sequence, overload and thermal relay protection form the electrical safety baseline. The point of this combination is that the machine can run continuously for long periods, needing only the planned inspections during the run and no dedicated operator watching it.
The limits of the automation should be stated as well: where the oil is dark in colour, contains carbon particles or a high level of impurities, the judgement of the foam detection is affected, and for such oils the level and the foam condition still have to be confirmed by an operator.