Page 131 - Petroleum and Gas Field Processing

from the water. The oil and emulsion layer forming on top of the water is
called the oil pad. The weir controls the level of the oil pad and an
interface controller controls the level of the water and operates the water
outlet valve. The oil and emulsion flow over the weir and collect in a
separate compartment, where its level is controlled by a level controller
that operates the oil outlet valve.
The relative volumes occupied by the gas and liquid within the
separator depend on the relative volumes of gas and liquid produced. It is
a common practice, however, to assume that each of the two phases
occupies 50% of the separator volume. In such cases, however, where the
produced volume of one phase is much smaller or much larger than the
other phase, the volume of the separator should be split accordingly
between the phases. For example, if the gas–liquid ratio is relatively low,
we may design the separator such that the liquid occupies 75% of the
separator volume and the gas occupies the remaining 25% of the volume.
The operation of the other type of horizontal separator (Fig. 2)
differs only in the method of controlling the levels of the fluids. The oil
and emulsion flow over the oil weir into the oil bucket, where its level is
controlled by a simple level controller that operates the oil outlet valve.
The water flows through the space below the oil bucket, then over the
water weir into the water collection section, where its level is controlled by
a level controller that operates the water outlet valve. The level of the
liquid in the separator, normally at the center, is controlled by the height
of the oil weir. The thickness of the oil pad must be sufficient to provide
adequate oil retention time. This is controlled by the height of the water
weir relative to that of the oil weir. A simple pressure balance at the
bottom of the separator between the water side and the water and oil side
can be used to approximately determine the thickness of the oil pad as
follows:
H ow H ww
H o ¼ ð1Þ
1 ð = Þ
w
o
where H o is the thickness of the oil pad, H ow is the height of the oil weir,
H ww is the height of the water weir, and o and w are the oil and water
densities, respectively.
Equation (1) gives only an approximate value for the thickness of the
oil pad. A more accurate value could be obtained if the density of the oil
in Eq. (1) is replaced by the average value of the density of oil and density
of emulsion, which depends on the thickness of the oil and emulsion layers
within the oil pad. The height of the water weir should not be so small as
to avoid the downward growth of the oil pad and the possibility of the oil
flowing below the oil bucket, over the water weir, and out with the water.

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