Process Description
Oily water from the Oil Terminal process sections is sent to the Oily Water Surge Tank T-801 which serves as the first stage for separating entrained oil from the oily water. The residence time of the water in T-801 allows some of the oil to coalesce and float to the top of the water phase in T-801. The drained water from T-801 flows by gravity to the Skim Pit SU-821.
Coalesced oil in T-801 rises and floats on top of the water phase until it accumulates high enough to reach the height of the drainpipe for the recovered oil. Oil flows through this pipe by gravity to the Skimmed Oil Tank T-811. Accumulated oil in T-811 is pumped by Skimmed Oil Pumps P-811A/B as needed to the Slop Tank T-121.
Water from T-801 flows through a channel on the inlet side of Skim Pit SU-821 which is below grade. The water flows through a settling chamber in SU-821 where more oil coalesces from the water. Floating oil is drained by gravity to the Recovered Oil Sump SU-831. The water from the settling chamber then flows into the pumping chamber of the Skim Pit SU-821. The submersible Skim Pit Pumps P-821A/B pump water to the Dissolved Air Flotation unit for removal of residual amounts of oil from the water.
Recovered oil from the Skim Pit SU-821 and from the Dissolved Air Flotation Unit are collected in Recovered Oil Sump SU-831. Oil is pumped by submersible Recovered Oil Pumps P-831A/B to the Slop Oil Tank T-121.
Oily water from the Oil Terminal process following sections flow through isolation valve XV-801 and into the Oily Water Surge Tank T-801:
- 1ST Stage Separator D-201
- Electrostatic Desalter D-211
- 2ND Stage Separator D-221
- LP Condensate Recovery Separator D-321
- Slop Oil Tank T-121
T-801 is a fixed-roof tank and serves as the first stage for separating entrained oil from the oily water. The residence time of the water in T-801 is about 20 minutes which allows some of the oil to coalesce and float to the top of the water phase in T-801. The water drainpipe is inserted into the water phase about halfway up the tank height. The drainpipe has an upward looping bend outside the tank so that the water within the tank will not flow out until the water level in the tank is at the height of the bend.
A small-diameter vertical pipe open to the atmosphere at the peak of the bend (not drawn) ensures there is no vacuum created in the drain line. This prevents the drain line from becoming a siphon. The drained water from T-801 flows by gravity to the Skim Pit SU-821.
Coalesced oil in T-801 rises and floats on top of the water phase until it accumulates high enough to reach the height of the drainpipe for the recovered oil. The height of this pipe is about 1.5 feet above the height of the top bend of the water drainpipe. Oil flows through this pipe by gravity to the Skimmed Oil Tank T-811.
Instrumentation
T-801
Hand switch XV-801 opens and closes inlet isolation valve XV-801.
LI-801 indicates the level of water in T-801. This level will be around 86-87% during normal operations.
FI-801 indicates the flow rate of water from T-801 to SU-821.
FI-802 indicates the flow rate of oil from T-801 to T-811.
T-811
Hand switch XV-811 opens and closes inlet isolation valve XV-811.
LAHL-811 indicates the level of oil in T-811. It is used by control interlock I-811 to start and stop the Skimmed Oil Pumps P-811A/B. LAHL-811 zero-point range is at an elevation just above the maximum range point of LI-812. LI-812 indicates the water interface level in the lower part of T-812.
Hand controller HIC-812 adjusts the position of control valve HV-812. HV-812 is normally fully closed and is opened to drain water from T-811 to SU-821 as needed.
P-811A/B
Hand switches MOV-811A/B open and close the suction MOVs (motor operated valves) on P-811A/B, respectively. Hand switches HS-812A/B are used to select whether its respective pump is manually or automatically started. Normally, HS-812A is in the AUTO position while HS-812B is in the MAN position.
Therefore, P-811A will be automatically started by control interlock I-811 when LAHL-811 reaches its high alarm setting (80%) and will automatically stop P-811A when LAHL-811 reaches its low alarm setting (20%). In normal operation, the pump operates every few hours for about 20 minutes.
Hand switches HS-811A/B start and stop the P-811A/B motors, respectively. These switches can be operated irrespective of the position of their AUTO/MAN switch.
SU-821
Hand switch XV-821 opens and closes inlet isolation valve XV-821.
FI-822 indicates the flow of recovered oil that flows form SU-821 to SU-831. LIC-821 controls the level of water in the pumping chamber of SU-821 by adjusting the position of control valve LV-821.
P-821A/B
Hand switches HS-821A/B start and stop the motors of P-821A/B, respectively. Normally, only the “A” pump is in operation.
FAL-821 indicates the flow of water from P-821A/B through LV-821. FAL-821 also is used by interlock I-841 in the DAF Unit.
SU-831
Hand switch XV-831 opens and closes inlet isolation valve XV-831. LAHL-831 indicates the level of oil in SU-831. It is used by control interlock I-831 to start and stop the Recovered Oil Pumps P-831A/B.
PU-831A/B
Hand switches HS-832A/B are used to select whether its respective pump is manually or automatically started. Normally, HS-832A is in the AUTO position while HS-832B is in the MAN position. Therefore, P-831A will be automatically started by control interlock I-831 when LAHL-831 reaches its high alarm setting (80%) and will automatically stop P-831A when LAHL-831 reaches its low alarm setting (20%). In normal operation, the pump operates every six hours for about 20 minutes.
Hand switches HS-831A/B start and stop the motors of P-831A/B, respectively. These switches can be operated at any time, irrespective of the position of their respective AUTO/MAN switch.
FI-831 indicates the flow of water from P-831A/B to T-121.