Page 133 - Petroleum and Gas Field Processing

and hits the inlet diverter, where the bulk separation of the gas from the
liquid takes place. The gas flows upward through the gravity settling
sections which are designed to allow separation of liquid droplets down to
a certain minimum size (normally 100 mm) from the gas. The gas then
flows through the mist extractor, where the smaller liquid droplets are
removed. The gas leaves the separator at the top through a pressure
control valve that controls the separator pressure and maintains it at a
constant value.
The liquid flows downward through a downcomer and a flow
spreader that is located at the oil–water interface. As the liquid comes out
of the spreader, the oil rises to the oil pad and the water droplets
entrapped in the oil settle down and flow, countercurrent to the rising oil
phase, to collect in the water collection section at the bottom of the
separator. The oil flows over a weir into an oil chamber and out of the
separator through the oil outlet valve. A level controller controls the oil
level in the chamber and operates the oil outlet valve. Similarly, the water
out of the spreader flows downward into the water collection section,
whereas the oil droplets entrapped in the water rise, countercurrent to the
water flow, into the oil pad. An interface controller that operates the water
outlet valve controls the water level. In the design shown in Figure 3, a
chimney must be provided, as shown in the figure, to allow the gas
liberated from the oil to rise and join the rest of the separated gas and,
thus, avoid overpressurizing the liquid section of the separator. The use of
the oil weir and chamber in this design provides good separation of water
from oil, as the oil has to rise to the full height of the weir before leaving
the separator. The oil chamber, however, presents some problems. First, it
takes up space and reduces the separator volume needed for the retention
times of oil and water. It also provides a place for sediments and solids to
collect, which creates cleaning problems and may hinder the flow of oil out
of the vessel. In addition, it adds to the cost of the separator.
Other methods of level control are also available. Figure 4 shows a
schematic of a separator where an oil–water-interface controller and a gas–
oil-interface controller control the water and oil levels, respectively. Figure 5
shows yet another method of level control. In this design, an external water
column equipped with adjustable weir is connected to the water section of the
separator. The column is also piped to the gas section of the separator to
establish pressure equilibrium between the water column and separator. A
simple level controller controls the height of the water in the column, which,
in turn, controls the height of water in the separator. This eliminates the need
for an oil–water interface controller and avoids the potential problems
associated with such controllers. This design, however, takes additional
space and adds additional significant cost.

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.