The most common form of parallel plate interceptor used in oil facilities is the corrugated plate interceptor (CPI). This is a refinement of the PPI in that it takes up less plan area for the same particle size removal, and has the added benefit of making sediment handling easier. Figure 7-6 illustrates a typical downflow CPI design and Figure 7-7 shows a typical CPI pack.
In CPIs the parallel plates are corrugated (like roofing material) with the axis of the corrugations parallel to the direction of flow. The plate pack is inclined at an angle of 45° and the bulk water flow is forced downward.
The oil sheet rises upward counter to the water flow and is concentrated in the top of each corrugation. When the oil reaches the end of the plate pack, it is collected in a channel and brought to the oil-water interface.
In areas where sand or sediment production is anticipated, the sand should be removed prior to flowing through a standard CPI. Because of the required laminar flow regime all plate coalescers are efficient sand settling devices. Experience has shown that oil wet sand may adhere to a 45° slope. Therefore, there is the possibility that the sand will adhere to and clog the plates. In addition, the sand collection channels installed at the end of the plate pack cause turbulence that affects the treating process and are themselves subject to sand plugging. To eliminate the above problems, an “upflow” CPI unit employing corrugated plates with a 60° angle of inclination may be used.
For service temperatures less than 140°F, fiberglass with a steel frame is used. For service temperatures greater than 140°F, corrosion-resistant alloys or stainless steels are recommended.