Page 54 - Industrial Plants

• Gas and liquid properties (density, viscosity, liquid is drained to the downstream facilities. A
surface tension, etc.) slope (1% minimum) is required to promote liquid
• Fluid dynamic (gas and liquid velocity) drainage from the fingers to the outlet.
• Pipeline geometry (pipeline length, elevation There are a number of different designs for
profile, presence of “pockets”, etc.) finger-type slug catchers. In what follows, some
key solutions are listed together with their
Interaction between these factors is quite applications.
complex. Predicting slug volume is normally
done using a dynamic mono-dimensional
multiphase flow simulator which assesses the Three-Phase Slug Catcher
time-dependent behaviour of the pipeline. Sometimes, in addition to gas-liquid, separation of
Transient modelling is essential to reduce the the aqueous phase (heavier) from the liquid
uncertainty of slug catcher design conditions hydrocarbon phase (lighter) is needed. This can be
and to reduce overdesign margins. achieved in an additional section where liquid is
There can be different types of slug catchers accumulated, avoiding turbulence for a residence
depending on gas and liquid flows and slug time adequate to reach HC-water gravity separation.
volume. This section could be an additional liquid header
If the slug volume is not very high (100 m3 is an installed below the main one, or an additional
approximate limit value), a horizontal vessel is secondary bottle installed below the other bottles,
the preferred choice. In practice, a horizontal dedicated to liquid-liquid separation.
two-phase separator is selected on account of a
significant slug accommodation allowance
between normal liquid level and high-level alarm. Double-Level Slug Catcher
Occasionally, an alternative to this solution is a The double-level slug catcher is an alternative
vessel with two horizontal barrels in a stacked solution allowing for the reduction of the total length
configuration, connected by two or more of the slug catcher. The equipment consists of two
downcomers. This solution has several sections at two different levels: fingers in the upper
advantages, especially in the case of high design section are dedicated to gas-liquid separation, while
pressure, by reducing vessel diameter. fingers in the lower section are designed for slug
When high slug volume occurs, the “finger-type” accumulation. Downcomers connect the upper to
or “multi-pipe” slug catcher should be considered. the lower section for liquid falling.
In a “finger-type” slug catcher, the inlet stream is
distributed into four or more parallel pipes
(typically between 28” - 60” in diameter). Secondary “Fingers” or “Bottles”
As a rule of thumb, the number of “fingers” In some cases, the slug volume to be accumulated
depends mainly on the gas flow rate, while the can be so large that it requires a total finger length
finger length depends on the slug volume to be exceeding the space available for the slug catcher
accommodated. Generally, each finger has a installation. Under these circumstances, the slug
diameter equal to or larger than the arriving accommodation capacity can be increased by
pipeline, ensuring that the gas, distributed adding a certain number of “secondary” fingers to
among the parallel fingers, slows down. This, the “primary” fingers. “Primary” fingers are those
coupled with the slope of the fingers, promotes connected to the inlet stream and composed of the
gas-liquid separation. “separation length” (where gas flow occurs) and
The first segment of fingers is focused on phase “accumulation length” (where slug is stored), while
separation: the length of this “separation section” the “secondary” fingers are only connected to the
is sufficient to allow the liquid droplets that are liquid outlet header so that they can be filled from the
greater than the limit size to fall into the liquid liquid header, providing additional capacity for slug
layer at the finger bottom in less time than that storage.
required for the gas to cover the length itself. At
a distance from the inlet, equal to (or higher than)
this calculated “separation length”, each finger is Slug Catcher Design Criteria
provided with a gas riser which collects the gas As previously discussed, many design alternatives
flowing towards the gas outlet header. The are available. Based on input data, designers must
fingers are placed downstream of the gas risers find the best solution for optimal sizing and cost
and are intended to catch the liquid slugs. The reduction. Perhaps, more than for other types of
fingers end in an outlet liquid header from which equipment, designers of slug catchers can

IndustrIal Plants - September 2021
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