Extract from
The American Society Of Mechanical Engineers


Air pollution control agencies often apply an opacity standard to lubrication oil reservoir vents. This standard is frequently set at 20% opacity. Opacity is a measure of the intensity of light entering and leaving the plume. It depends on particulate size, color, concentration and other factors. Many lubrication oil vents fail to meet this 20% opacity criteria. Even if they do meet the legally-mandated opacity criteria, they can still emit enough oil to create troublesome oil buildup around the equipment and in the general area.

So to design or select a mist eliminator we should address not only whether it will meet the opacity standard but also how much vented oil would create an undesirable mess on the ground or the roof around the vent.

It’s not always apparent how to relate an opacity standard to collection efficiency or discharge rate. But the Bay Area Pollution Control District has provided some help

Some years past the District published a formula for estimating the opacity of visible plumes. The formula is very simple – Q = 0.12 / I. I is the area of the plum point of observation. Q is the source emission at the point of observation. For these discussions we will consider the emission point as the exit of the stack.

For any given size, the formula calculates the maximum allowable emission rate (Q) required to achieve 40% opacity. The 0.12 constant in the formula is an estimate based on particle size distribution.

Assuming that the value of the constant is linear with reducing opacity requirements, one may calculate the number, and mass, of particles which would result in various opacity percentages. Of course, the calculation should be based on the assumption that the mist has the same relative particle size distribution and color (black) as was the basis for the original formula.

Using a somewhat speculative emission rates calculated for various opacities, we can then estimate the volume of oil that will still be vented out the stack. At an opacity of 5%, or 1% there could still be some oil residue to clean up, but nothing compared to what would be caused if the mist eliminator only achieved the minimum stan-dard of 20% opacity.

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