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Copyright © 2011 by Simtronics Corporation. All rights reserved worldwide.
Overview
Crude oil from the Manifold is brought into one end of the Three Phase Separator, D-001 and enters the upper section of the vessel which contains a baffle to minimize the disturbances from slugs of oil and water entering with the feed. Oil, water and gas in the feed flow under this baffle into the main separation compartment of the Three Phase Separator.
Gas flashes and separates into the vapor space at the top of the vessel, while immiscible water and oil separate by gravity and form two liquid layers in the separation compartment. The separation compartment is bounded by another baffle extending from the bottom of the vessel to halfway between the top and bottom of the vessel. Separated water and oil collect on the feed side of this baffle. The water/oil interface level is controlled at halfway up the separation baffle (which, therefore, is one quarter of the height of the vessel) by LIC-002 which adjusts the takeoff flow of water from the bottom of the separation compartment. The flow of water is indicated on FI-005. The design flow is 2.0 MBPD. The water contains small amounts of oil and is taken offsite to remove the oil before disposal.
Separated crude oil floats on the water phase and accumulates and spills over the separation baffle into the collection compartment of the Three Phase Separator. The level of collected oil in this compartment is controlled by LIC-001 which adjusts the flow of oil taken from this compartment. The flow of crude oil is indicated on FI-004. The design flow of oil is 16.2 MBPD. Selection of the destination of the crude oil can be made using HS-008A and HS-008B.
Note that LIC-002 indicates 100% when the water/oil interface level in the separation compartment is at the height of the separation baffle. If the interface level goes higher than 100%, water will flow over the separation baffle and mix with oil on the collection side. In this situation, an additional load of water will be placed on the downstream separator.
The separation of water out of the oil is not complete as it takes a very long residence time in the separation compartment for this to occur and also because there is agitation of the liquid phases due to the high rate of initial gas separation in the Three Phase Separator. Larger droplets of water will coalesce and fall to the bottom of the separation compartment more easily than smaller droplets. Given the residence time of oil and water in the separation compartment, small droplets of water will be contained in the crude oil in the collection compartment. Most of this water will be separated in the downstream separators.
Gas that is flashed and desorbed from the crude oil is collected in the vapor space of the Three Phase Separator and flows to the opposite end. The residence time that allows water to separate from the oil also allows entrained gas in the oil to desorb from the oil.
To make sure there are as few droplets of oil and water leaving with the gas, a demisting pad extends from the top of the vessel to the middle of the vessel over the separation compartment. This ensures the vapor must pass through the demisting pad as the level of oil in the separation compartment will be slightly higher than the liquid separation baffle. The demisting pad is made of crinkled wire mesh screen (CWMS). Any entrained droplets of liquid will impact the wires and the droplets will coalesce and drain into the oil phase of the separation compartment.
The pressure in the Three Phase Separator is controlled directly. Check valves are installed in the lines to prevent back flow of gas to D-001. When changing the gas lineup, the lineup of the separated crude oil using HS-008A or HS-008B must be changed accordingly to ensure controllability of the oil level by LIC-001.
In the event the pressure in D-001 builds, an automatic vent to the flare system is provided. PIC-005 will open at 1,800 PSIG to avoid over pressuring D-001. Also, D-001 is outfitted with a pressure safety valve (PSV) set at 2,000 PSIG.
In high pressure mode, D-001 operates at a pressure of 1,627 PSIG and a temperature of 162 DEG F while producing 35.2 MMSCF/D of gas.
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