The Design and Performance of CPI Oil Separators

CPI Gravity Oil Separators Designs are engineered based on the application of Stokes Law to ensure laminar flow conditions. Stokes Law describes the relationship between a particle’s velocity in a fluid and the force required to keep that particle moving at that velocity. Under these engineered design conditions, the CPI unit separates oils (hydrocarbons) from water using gravity and closely spaced parallel plates.

The term CPI, or Coalescing Plate Interceptor, is often referred to as a Corrugated Plate Interceptor, based on the original Shell design. The original Shell patent included flat plates, corrugated plates, zig-zag plates and tubes of all shapes and sizes. Many suppliers have further developed commercial products over the years that use a variety of materials, plates (flat and corrugated), tubes and other coalescing media.

CPI Oil Separators are widely used in applications such as fuel oil storage terminals, oil refineries, petrochemical plants, airports, fire training facilities, vehicle washing and maintenance workshops to separate free, non-emulsified oil from water. In fact, any situation in which process water or stormwater could be contaminated with hydrocarbons a CPI Oil Separator will likely be in use.

Oil Separators Designs

There are two internationally recognised standard Oil Separators designs:

• British Standard: BS EN858 1 & 2
• American Petroleum Institute: API-421

The design criteria generally adapted by many oil separator manufacturers is to separate oil droplets larger than 60 micron for oils with specific gravity of SG < 0.9 and water temperature T > 20 C. Under these design conditions, when the FEED Oil Concentration is < 1,000 mg/l, then the treated free oil concentration would be < 10 mg/l, which represents a 99% capture rate.

The variables which will impact on the treated water oil content are:

• Flowrate
• Water temperature
• Oil Specific Gravity

The information which is required in order to correctly design and size an oil separator for a particular installation includes the following:

• Flowrate
• Water temperature
• Oil Specific Gravity
• Feed Oil concentration
• Treated water oil concentration required (ie, discharge standard)

Over the last 20 years or so, many countries have gradually reduced oil discharge limits from a historic 10 mg/l to 5 mg/l and sometimes even lower. SWA Water Australia Pty Ltd has developed a range of CPI Oil Separators that consistently achieve 5 mg/l oil content and often achieve 1 mg/l oil content.

Plate angle

The optimal plate angle for gravity oil separators is 45 degrees. For high solids loading situations, a plate angle of 60 degrees is normally used.

Plate spacing

When the solids loading is low (e.g., TSS < 500 mg/l), plate spacing of 10mm can be used. A plate spacing of less than 10mm would be extremely unusual. When the solids loading in the feed exceeds 500 mg/l, a wider plate spacing (typically 20mm) and a steeper plate angle (typically 60 degrees) should be considered.

Impact of oil density

The lighter the oil density, the faster the oil droplet will rise and be separated. The higher the oil density, the slower the oil droplet will rise and be separated.

Heavy oils

Heavy oils are defined as hydrocarbons with SG Oil greater than 0.90. When the SG Oil exceeds 0.90, the size of the oil separator must be adjusted to compensate for the higher oil specific gravity.

Impact of Temperature on Oil Separator Performance

The standard Oil Separators designs basis is T=20°C water temperature. The higher the temperature, the faster oil droplets will rise and be separated. The lower the water temperature, the slower the oil droplet will rise to be separated. When the water temperature is T=10°C, the size of the oil separator must be adjusted to compensate for the lower temperature performance. When the water temperature is above 40°C, a smaller oil separator can be used.

The impact of varying operating temperatures and various oil specific gravity is summarised in the following table:

Temperature (°C)	SG (Oil)
	0.7	0.75	0.8	0.85	0.9
Oil Droplet Size Separated (Micron) (100%)
20	80	100	120	135	150
30	72	90	110	125	140
40	65	80	95	112	130

Separators Materials of Construction

Stainless steel is a common material for oil separators, eliminating the need for coating (painting) systems as well as galvanic corrosion, which can occur when dissimilar metals are used. The most commonly used stainless steel grade is AISI304. AISI304 stainless steel, however, has limited corrosion resistance to chlorides, which may result in “tea staining” of the stainless steel.

SS304 provides better can withstand chloride concentrations of up to 400 mg/l. When the chloride concentration exceeds 400 mg/l, SS316, Duplex or Super Duplex stainless steel should be used:

• SS304: suitable for chloride concentration to 400 mg/l
• SS316: suitable for chloride concentration to 2,250 mg/l
• Duplex SS: suitable for chloride concentration to 4,000 mg/l
• Super Duplex SS: suitable for chloride concentration to >4,000 mg/l

Customised Oil Separators

It is common practice to customise standard oil separators to the specific requirements of individual projects. Common options include:

• Integral waste oil tank, with level transmitter and visual level indicator
• Removable gas tight covers
• Access walkways
• Outlet pump chamber and pumps
• Waste oil transfer pump
• On-line oil monitor
• Treated water outlet diversion valves for out of spec water

Performance of Oil Separators

SWA Water Australia Pty Ltd has developed a range of CPI Oil Separators which normally achieve < 5 mg/l oil content and often achieve < 1 mg/l oil content. SWA Water’s “CC” Range of CPI Oil Separators has been designed to achieve treated oil content < 5 mg/l under the following Feed conditions:

Shell Jakarta CPI Oil Separator

In 2015 SWA supplied a 60 m3/h CPI Oil Separator to the Shell Lube Oil Plant at Jakarta, Indonesia. This oil separator was tested under a variety of Feed oil concentrations, resulting in the following performance:

Feed conditions:

• Flowrate: 60 m3/h
• Feed pump: Progressive cavity (non-emulsifying type)
• Oil SG: 0.90
• Water Temperature: 25C
• Oil Concentration: 10,000 mg/l
• Suspended solids: < 100 mg/l

Oil separator treated water outlet:

• Oil Concentration: < 1 mg/l

Measuring instruments:

• Flowrate: Magnetic flow transmitter on oil separator inlet pipe
• Oil Concentration: By online oil analyser & by laboratory sample testing

Example of Oil Separators

SWA Water is the world’s leading industrial wastewater treatment company. Australian owned and operated, with offices in Australia, Malaysia and Thailand, we deliver complete turnkey engineering solutions and products for wastewater treatment plants across a diverse range of industries. For more information on our wastewater treatment solutions, please contact us at info@swawater.com.au or visit our website at www.swawater.com.au.