Page 53 - Industrial Plants

CFD plays a key role

in the optimization of finger type

slug catchers

A slug catcher is quite a complex piece of equipment, usually
sized using simplified procedures. The drawback of this method
is the need for conservative assumptions which hinder the
achievement of a competitive design. CFD simulation provides a
completely new design approach that can lead to significative
savings in terms of investment cost and equipment footprint

Francesco La Motta, Process Leader, Oil&Gas and Biogas Projects,
Alessandro Boffa, Senior Process Engineer,
OILTECH srl

A

routine problem is the presence of
liquids in pipelines that transfer
untreated gas from gas fields to
processing plants. In this case,
the liquid arrival rate is irregular
rather than constant. Intermittent
flows can reach much higher peaks than average
flows. Obviously, the downstream liquid processing
facilities cannot be sized for these peak conditions
and a liquid separator is required to separate
these “liquid slugs”, accumulating them and
slowly releasing them to the downstream gas
processing plants. Slug catchers solve this
problem by acting as a buffer for the liquid
flowrate and providing a temporary storage for the gas flow. For example, during the “ramp-up”
volume of liquid when peak flow is reached. phase at the pipeline start up, the higher gas
The draining rate of a slug catcher is set to empty velocity can contribute to part of the liquid
the equipment in time before the next liquid peak accumulated inside the pipeline being carried
(or “slug”) arrives. over.
In a pipeline transporting natural gas and • The pigging operation displacing all the liquid
associated liquids (light condensates and/or from the pipeline into the slug catcher in a
water), the amount of liquid found at the pipeline relatively short time.
outlet is determined by the following three factors:
• The continuous liquid production under Normally, pigging gives the maximum slug volume
steady flow conditions, depending on the that must be taken into account for slug catcher
gas-liquid equilibrium governed by sizing. The liquid volume accumulated in the
composition and operating conditions. pipeline and displaced during pigging mainly
• The intermittent or transient liquid production depends on:
due to variation of liquid inventory inside the • Composition
pipeline caused by a significant increase in • Operating pressure and temperature

IndustrIal Plants - September 2021
51