Application of CFD to a Flare Drum Carryover Issue
Authors: Lewis Michjael James , Deddis Colin Robert 
Published: May 2011

Process diagnostics suggested there was an oil carry-over problem with a Flare Knock-Out Drum; the liquid level inside the vessel was observed to drop during flaring events. Further indicators of this came from a temperature increase in the vessel. Oil carry-over can lead to poor combustion and resultant damage to the flare tip.

CD-adapco worked together with a Client to find a solution to this oil carry-over problem. STAR-CCM+, the general purpose CFD software, was used to identify the cause of the carry-over; suggest potential design solutions; quantify improvements of the best designs; and to provide design information for use to size and manufacture the modifications.

The simulations showed the inlet device to be the cause of the problem, directing the incoming flow onto the liquid surface inside the drum resulting in liquid being re-entrained into the gas stream and carried over to the flare tip. The inlet device was redesigned with the aid of CFD, without reducing the residence time inside the vessel, such that the inlet gas stream was distributed throughout the drum. The result was a dramatic reduction in liquid re-entrainment.

Introduction

The flare system on production facilities is used to safely dispose of unwanted hydrocarbon fluids which arise primarily during emergencies such as to relieve excess pressure or to remove hydrocarbon inventory from the production systems. It can also be used to safely dispose of excess gas during start-up or shut down of the facility. The fluids routed to the flare system are largely composed of vapour but liquids can be present in the stream due to multiphase relief, liquid entrainment or retrograde condensation due to the temperature and pressure reduction the fluids undergo. Flare knock out drums are designed to disengage these entrained liquids from hydrocarbon fluid streams routed to the flare disposal system. This is to prevent liquid being carried over to the flare tip where it won't be fully combusted and can hence pose a hazard as "burning rain" or cause pollution as the liquids fall onto the ground or water.

The internal design of a flare knock out drum is kept as simple as possible to avoid the potential for blockages due to internals failure as this would incapacitate the flare system and prevent safe disposal of the fluids. A liquid level is normally kept in the flare knock out drum to prevent vapour break out in the liquid disposal pump suction or gas breakthrough to a lower pressure system. The flare knock out drum is usually equipped with level and temperature instrumentation to monitor the system and to take executive action to shut down total production to prevent safe operating limits being exceeded.

Under normal flaring operations, the liquid level would be expected to rise in the flare knock out drum. For one platform the liquid level was observed to fall markedly in the flare knock out drum each time there was a significant flaring event. This liquid wasn't fully combusted in the flare tip, and consequently a sheen could be observed on the sea surface following a flaring event confirming that the phenomenon wasn't an artifact of the level measurement becoming unreliable due to a disturbed liquid. A typical event is illustrated in Figure 1 where the liquid level, flaring rate and liquid temperature for the flare knock out drum are plotted. This information is quite limited in value in that it only shows what is happening but does not provide an explanation as to why the liquid level should decrease. Several mechanisms were postulated for this including liquid re-entrainment due to poor fluid flow distribution and evaporation of the liquid. Diagnostics such as neutron backscatter or gamma scanning techniques of the flare knock out drum would not provide any further insight into what was causing the drop in liquid level, rather this would probably only confirm the observations. In order to find a solution to the problem it was decided to utilize Computational Fluid Dynamics (CFD) to gain insight into the flow distribution in the vessel.

Initially, the as is design was simulated using a typical flaring event. The insight gained into the causes in the drop of liquid level was used to propose and rank several alternative designs. Finally, the most appropriate design was quantitatively compared with the original design