Compressor oil/water separators

By 31 May 2018

As indicated above, the Environmental or other Agency will probably require that the condensate be treated to separate the oil and water so that the water may be discharged in the normal way and the oil sent for disposal or recycling. One other option that might be considered is for the whole of the condensate to be collected by a disposal company (at say 10p per litre), but this is likely to prove more expensive than installing a separator.

Even allowing the condensate to settle for hours in a settling tank does not guarantee that the water underneath the oil is safe for discharge. This is because modern compressor oils have a tendency to emulsify and settlement alone is insufficient to produce a liquid free from contamination.

In these circumstances an oil/water separator, should be installed. All the automatic condensate drains in the system, from the air receiver, dryers and filter can be ducted to one separator as shown in the system of Figure 9. Separation is achieved in three stages as illustrated in Figure 10: the first stage relies on gravity separation; the oil forms a layer on the surface of the settlement tank to be collected in a separate oil tank; the water beneath the oil is then filtered to remove solid impurities; and finally passes through an activated carbon filter to remove traces of oil remaining. It should then be pure enough for discharge into the main drainage system.

The oil can be periodically drained off when the container is full, and the filters can be replaced without shut-down of the system. The oil can be sent for processing.

Different separation techniques apply according to whether the oil is present as an emulsion or as separable mixture. A convenient way of deciding this is to take a sample of the condensate in a jar and leave it overnight. If the oil is clearly sitting on top of the water and the water is comparatively clear, the condensate is separable. If it remains mixed and there is little separation, it has to be treated as stable emulsion. An emulsion separator incorporates a membrane filter which has to be replaced at regular service intervals, although some separators include an automatic back-flushing cycle to extend the service intervals. The appropriate oil separator should be chosen in consultation with the separator manufacturer.

Examples of the two types are shown in Figures 11 and 12.

For efficient working the separator must be correctly rated to the compressor capacity and the moisture content of the air. A higher rated separator is required if a dryer is installed.