Dealing with foam overflow has always been one of the most troublesome faults on site: the foam lifts, the machine stops, the tank has to be cleaned, and once it is clean the batch has to be started again. Machines with infrared level and foam detection together with automatic de-foaming intervene on their own when foam appears, and nobody has to stand and watch. But a good number of machines in use were built several years ago and have none of this, so whether it can be added has to be settled by looking at three things first.
First: can the control logic still take it
Automatic de-foaming is not finished by fitting a probe. Once the probe reports foam, there has to be an actuator to respond, there has to be control logic to connect the signal to that actuator, and it all has to stay in relation with the existing safety chain. Models with PLC smart control and a touch screen have logic that can be written and interfaces that are there to be used; with an older relay control cabinet the difficulty and the extent of the modification are both considerably greater.
One question has to be settled in particular: how the new logic sits against the existing audible and visual overpressure shutdown and the phase loss and phase sequence protection. If two sets of logic are left to act on their own after the modification, which one moves first in an abnormal condition directly defines the safety boundary, and that has to be agreed before any work starts rather than added afterwards.
Second: is there anywhere left on the tank to mount the sensing element
The elements for infrared level and foam detection have to be mounted where they reflect the real state inside the tank. Mounted too close to the oil inlet, the oil mist interferes with the reading directly; mounted too close to the oil outlet, the dip in the surface caused by extraction is counted in as well. On an older machine those positions are often already occupied by fittings and brackets, so the tank has to be opened and looked at first to confirm whether there is a position that takes the element and is not washed over by the oil mist.
The condition of the tank itself also has to be confirmed: whether the wall is deformed and whether the flange faces still seal. These points look unrelated to de-foaming, but the element will be working on this tank for a long time, and if the condition of the tank is not sound the installation will not be stable either.
Third: where does the foam go once the de-foaming action has fired
The action of automatic de-foaming is in essence the act of leading the foam away through some passage. The de-foaming unit is usually connected between the tank and the oil outlet pipework, and the foam is led into a space that can contain it. On an older machine that position has often already been altered, or the routing of the outlet pipework differs from the original design, and the foam that is led out has nowhere to go, which turns the de-foaming action into a continuous overflow.
| What to check | How to check it | When the modification is not advisable |
|---|---|---|
| Control logic and interfaces | Look at the type of control cabinet and the input and output points available | Only simple relay control with no room to extend |
| Mounting position | Open the manhole on the tank and look at the arrangement of the fittings | There is no position that is undisturbed and still reflects the real liquid level |
| Where the foam goes | Check how the tank and the outlet pipework are connected | There is no space to contain the foam once it is led out |
| The existing protection chain | Check the order between the overpressure shutdown, the phase loss and phase sequence protection and the new logic | The change would disturb the existing order of protection |
| Tank and flanges | Check for deformation of the tank wall and the sealing condition of the flange faces | The tank already needs a full overhaul |
When the money is not worth spending
If the older machine is to be replaced anyway, or if the tank and the pipework were already on the list for modification, it is simpler to put the de-foaming function into the new design. In the other direction, if the machine is in good condition, the throughput is modest and foam only appears now and then in a few batches, then adjusting the way the machine is operated and fixing the times for sampling and topping up may well be more economical than fitting a device.
It is worth recording the occurrence of foam over a period first: which oil, at what temperature, at which throughput, and how often it happens. That record both helps in judging whether the modification is necessary and serves as the comparison when the effect of the work is checked afterwards.
Adding automatic de-foaming changes whether an older machine is usable, so the conditions come before the proposal, not after it.
