The dump tank type LACT systems that are now being installed by Phillips Petroleum Co. are vastly different from the LACT units that were installed a few years ago. The dump tank type LACT system or today embodies many refinements or the earlier systems as well as entirely new concepts. This development is the result or the thought and research that has gone into the system as well as the experience attained from the operation of dump tank LACT systems. Phillips Petroleum Co. presently has 35 dump tank type LACT units in operation, through which a total or approximately 42,000 BOPD are sold to various pipe line companies. This paper will discuss the development or rive or the components of the present system. These components were developed for use in dump tank systems; however, several are also applicable to positive displacement meter systems. The operational sequence of the LACT unit will be discussed briefly so that the various components can be related to the entire system.
A flow diagram of the LACT unit is shown in Fig. 1 and the operational sequence is as follows. As a starting point it is assumed that the meter tank is empty and that the level in the surge tank is at Level 1. As the surge tank fills from Level 1 to Level 2, the monitor pump, which operates continuously, pumps a stream of oil through the BS and W monitor cell to check the quality of the oil. The circulating pump starts whenever "bad oil" is detected. The transfer pump starts when the level in the surge tank reaches Level 2 provided, of course, that the HS and W monitor is indicating merchantable oil. The meter tank fills and the transfer pump continues to run until after the oil flows over the weir and Level 4 indicates wet. The level outside the weir rises because a valve is installed in the overflow line to restrict the flow of oil from the space outside of the weir. when Level 4 goes dry, Valve 4 opens and the metered volume between Valve 4 and the upper weir is delivered to the pipe line. Level 5 indicates when the metering tank is empty. The procedure described above is repeated until the surge tank level has been lowered to Level 1. Two pertinent points to note are that the entire stream of oil pumped into the metering tank passes through the BS AND W monitor cell and that the metering tank can be maintained full for several minutes, if needed, to allow time for the measurement of certain properties of the oil in the tank. This is accomplished by circuitry which delays the opening of Valve 4.
Liquid level floats were installed in the surge tanks of the earlier systems to provide electrical signals when the oil reached the various levels. This method required that several holes be either cut or drilled in the surge tank and that numerous wires be run from the surge tank to the control panel. In the later LACT units the surge tank floats have been replaced by a control system which utilizes the pressure exerted by the head or oil in the surge tank. The system consists of a diaphragm-type pressure multiplier located in an outlet near the bottom of the surge tank and connected by capillary tubing to a pressure indicator which is located at the control center. The diaphragm-type pressure multiplier multiplies the tank pressure 14 times and transmits this pressure through the capilliary tubing to the indicator-located at the control center. This multiplication of tank pressure increases the accuracy and reliability of the pressure indicating instrument. There are three adjustable mercury switches located on the rotating shaft of the pressure indicator. These switches serve the same functions as the floats formerly installed in the surge tank. The advantages of the pressure system are: the installed cost is less; Levels 1; 2 and 3 in the surge tank are more easily adjusted; and the level in the surge tank can be read directly on the indicator scale.
The design of the dump tank system has been improved in many ways but the most significant progress has been made in the data handling equipment.