Page 169 - Petroleum and Gas Field Processing

downcomer and exits through a flow spreader located slightly below the
water–oil interface to water wash the oil–emulsion stream. Water washing
helps in coalescing the small water droplets suspended in the oil. The oil
and emulsion flow upward, exchanging heat with the heater fire tubes,
then through the coalescing section. The coalescing section, normally
packed with porous material such as hay, is sized to provide sufficient time
for the coalescence of the water droplets and their settling out of the oil.
The treated oil is then collected from the treater.

Horizontal Treaters
This type of treater is normally used in centralized multiwell-treating
facilities (GOSP). The oil and emulsion stream is introduced to heating
section of the treater near the top where gas is flashed, separated, and exits
the vessel at the top through a mist extractor. The liquid is made to flow
tangent to the inside surface of the vessel and falls below the water–oil
interface, where it is water washed. Water washing causes coalescence and
separation of free water. The oil þ emulsion rises up, exchanging heat with
the fire tubes, and flows over a weir into an oil surge chamber. The hot oil
þ emulsion leaves the oil surge chamber near the bottom of the vessel and
enters the coalescing section of the treater through a flow spreader, which
ensures that the oil flows evenly throughout the length of the coalescing
section. The oil flows upward, where it is withdrawn from the vessel
through a collector. The spreader–collector system allows the oil flow to
be vertical. This section of the treater is sized to allow sufficient retention
time for the coalescence and settling of the water out of the oil. The
separated water is removed from the treater at two locations: one at the
bottom of the heating section and the other at the bottom of the
coalescing section. Interface level controllers control both outlet valves.

Gunbarrel Settling Tanks
Gunbarrel tanks are large-diameter vertical tanks operating mostly at
atmospheric pressure. They are generally used for small fields where no or
minimum heating is required for separation of the emulsion. When heating
is needed, the most common way is to preheat the oil and emulsion stream
before it enters the tank.
The oil þ emulsion stream enters the tank at the top (where gas is
flashed and separated) into a downcomer. It leaves the downcomer through a
spreader located below the water–oil interface and rises vertically upward,
flowing through the large cross-sectional area of the tank. As the oil þ
emulsion rises, it is first water washed to coalesce the water droplets. Then, it
is retained for a sufficient time in the settling section to allow for the

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