Drycleaning solvent can occasionally become unstable during distillation.

The layer of foam on top of the boiling solvent, which is normally 5 – 15cm deep, will then expand to fill all the free space above. The heavily contaminated foam may then rise up the vapour pipe, entering the water separator and turning its contents a dark, muddy brown.

This is commonly known as a “blackover”. If it is not checked immediately, a large volume of contaminated solvent flows into the distilled solvent tank. However, regular machine monitoring should increase the chances of early detection and may avoid the need to distill the tank solvent again.

If contaminated solvent overflows to the working tank, the solvent in the tanks must be distilled and the water separator may have to be drained and cleaned.

The most common cause of blackovers is an overfull still where there is not enough space above the boiling solvent so that the foam is forced up the vapour pipe. Other causes include too high a steam pressure and water in the solvent. Silicone from items such as rainwear can also cause foam.

The recommended steam pressure for perc distillation is 65 – 70psi. If excessive water gets into the still, possibly from a leaking steam jacket or air heater, or if the steam pressure is too high, then the solvent boils more vigorously, possibly ending in a “blackover”.

If the unstable solvent is caught in time, turning off the still and allowing it to cool for a few minutes and/or reducing the steam pressure should prevent the problem developing.

Anti-foam products can be used to deal with persistant unstable distillation. Some cleaners use an anti-foaming product in the still as a preventative measure. A house brick in the bottom of the still is also said to help stabilise the boiling process.