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During the process of compressing air, atmospheric air along with water vapor and atmospheric contaminants (hydrocarbon or chemical vapors), are drawn into the compressor intake. Additionally, the compression chambers of most compressors require oil for lubrication, sealing and cooling. Once compressed, the air flows into an aftercooler to remove the heat of compression. As the air cools in the aftercooler, water and hydrocarbon vapors will condense. Additional condensation takes place as the air is further cooled in the piping and air dryers. This condensed moisture must be removed from the compressed air system to prevent damage to downstream components and processes. Drain valves are installed on moisture separators, coalescing filters, air receivers, air dryers and drip legs to remove this condensate from the compressed air system. The condensate should be piped from automatic drain valves to oil/water separators to remove the oil from the condensate prior to discharge to a drain.
Environmental regulations strictly prohibit the discharge of oily wastes and chemicals, including the condensate drained from a compressed air system. Because of these requirements, municipalities regulate the discharge of compressor condensate to surface water, wastewater treatment facilities, and sanitary sewers.
Compressor condensate must therefore be either collected or treated prior to disposal. An oil/water separator can be used here to remove the oil from the condensate.
Collection involves the drainage of the condensate into drums or storage tanks. The drums or tanks are then hauled away to an approved disposal facility.
Transportation, storage, and disposal costs (TSD) can exceed $500 for a single 55-gallon (208 liter) drum. A typical 25 hp (18 kW) compressed air system will generate approximately 20 gallons (76 liters) of condensate in one (1) day. Eleven 55-gallon (208 liter) drums are required to dispose the condensate produced in only one (1) month of operation resulting in a cost of $5,500. This is a significant disposal cost.
Since the condensate is approximately 95% water and 5% oil, oil/water separators have been developed to reduce or eliminate the amount of oil in the condensate.
An oil/water separator system, installed on the 25 hp (18 kW) compressed air system example above, can reduce the number of 55-gallon (208 liter) drums from eleven (11) to less than one (1). Condensate disposal costs will therefore be reduced by $5,000 per month. Payback on an oil water separator is typically much less than one year.
Types of Oil Water Separators
Oil/water separators are available in four basic technologies:
- chemical adsorption
- gravitational separation
- mechanical separation
- vaporization separators
Chemical Absorption Oil/Water Separators
Chemical absorption oil/water separators (fig. OWS1-1) are filled with a chemical media developed to attract the oil while repelling water molecules. Depressurized oily condensate drains into the chemical absorption separator were the oil is bonded to the media. Clean water then flows to drain.
The life of the chemical separator depends on the quantity of oil in the condensate. The oil concentration in the condensate may vary from 40 ppm to more than 600 ppm. Actual oil concentration is dependent on the type and condition of the compressor, the type of oil and ambient relative humidity.
Since absorption capacity is approximately 50% of the media bed weight, a 15 gallon (57 liter) absorption separator will capture 7 - 8 gallons (26 - 30 liters) of contaminants.
Gravitational Separation Oil Water Separator
Gravity separation is accomplished by flowing the condensate into a settling tank. Oil is skimmed off the top and water is pumped off the bottom of the oil/water separator tank. The water removed from the bottom of the tank still contains oil. The percent depends on the demulsibility of the oil. Gravitational separation devices are simple and will separate free oils that have migrated to the top of the settling tank. Gravitational oil water separators are not effective on oils that have emulsified in the water since the oil does not naturally separate from the water.
Mechanical Separation Oil/Water Separator
A coalescer performs mechanical separation through pressure drop, torturous path, and oleophobic attraction. The pressure drop across the coalescer causes some of the oil to drop out of phase and separate. Pores create a torturous path through the coalescer and oil droplets adhere to the coalescer and combine coalescing. To enhance the oil's collecting and forming droplets and draining off the coalescer element an oleophobic (oil resistant) filter media is used. The combination of these factors make coalescers more than 99% effective oil separators. The actual efficiency of the coalescer will depend on the type of oil being separated. Some synthetic lubricants will not be removed by a coalescing element. Coalescer element's foul over time and require replacement based on pressure drop.
Activated charcoal adsorbs oil and most synthetic lubricants. The condensate is passed through a charcoal chamber. The activated charcoal adsorbs the oil reducing the amount of oil in the condensate. Activated charcoal chambers foul over time and require scheduled replacement.
Mechanical oil/water separators are usually a combination of a gravitational settling tank with a coalescer element. Charcoal is also typically used as a polishing bed to remove oils not separated by the coalescer.
(Note: Since the condensate will contain a mixture of water and lubricating oil, be sure to drain all condensate in a manner approved by all federal, state and local regulatory agencies. Oil/water separators are available to assist in this function.)
Vaporization Oil Water Separators
Vaporization oil water separators utilize an external heat source, such as electric or steam heaters, to boil-off the water. The remaining oil is then drained into a container for proper disposal. Vaporization separators will efficiently separate almost all oils, however attention must be given to the materials of construction of the oil/water separator since the oily waste may be corrosive.
Installation and Maintenance
Installation of the oil/water is typically quite simple. The oily condensate from drain valves is piped to a depressurization chamber/vessel to reduce the pressure to atmospheric pressure. The oily condensate then flows into the oil/water separator. Oil water separators are normally installed indoors to prevent freeze-up of the water during wintertime operation. If outdoor installation cannot be avoided, contact the manufacturer for approval and installation requirements.
- Drain collected oil as necessary. Properly dispose all collected oil as required by law.
- Clean the settling tank or boiling tank with water to remove particulate build-up once a year or as necessary. Dispose of water properly.
- Change the activated carbon adsorber before it is fully saturated with oil.
- Replace the coalescer element based on Delta P.
Follow the manufacturer's maintenance requirements.
Proper Disposal of Condensate
Since the condensate will contain a mixture of water and lubricating oil, be sure to drain all condensate in a manner approved by all federal, state and local regulatory agencies. Oil/water separators are available to assist in this function.