Separator low temperature analysis

Combining gFLARE and CFD to analyse the effects of jet impingement

Oil–Gas separators – such as knock-out drums – must be able to withstand high two-phase flowrates that may enter through several nozzles, at very cold temperatures for approximately 15 minutes.

Even when the large vessel size provides sufficient heat capacity that average wall temperatures stay well above material embrittlement temperatures, effects such as cold jet impingement on vessel walls and liquid droplet entrainment can create local cold spots on the vessel's walls that threaten its integrity.

Case example: HP Flare system KO drum

The design integrity of the KO drum is assessed for a 15 minute blowdown event – identified as the limiting case – where very cold fluid enters through nozzle N2 and relatively warm fluid enters simultaneously through nozzle N1.

Oil-gas separator limiting caseLimiting case: very cold fluid enters through N2 and relatively warm fluid through N1

Assessing the impact of cold jets

Simulations performed with PSE's gFLARE Oil–Gas separator model, applying validated heat transfer coefficient correlations for free and forced convection, show that in general the vessel wall temperature will stay well above the metal embrittlement temperature limit (-46°C).

However, to assess the impact of the cold jets of fluid entering at the vessel's nozzles, PSE employs its Combined gFLARE–CFD Oil–Gas Separator technology.

Oil-gas separator inlet flows through nozzles: top and side views – Click to enlargeOil-gas separator inlet flows through nozzles: side and top views

The high velocities around the nozzles result in localised areas with very high heat transfer coefficients and, in the case of Nozzle N2, low fluid temperatures.

Oil-gas separator fluid temperatures with distributed approach – Click to enlargeOil-gas separator fluid temperatures

Combining gFLARE and CFD

PSE's Combined gFLARE–CFD Oil–Gas Separator technology is an efficient and extensively tested solution that can accurately describe the transient nature of the feeds to the drum and the consequential dynamic response of the vessel walls (at all locations throughout the vessel).

A model based on this technology accounts for the complex fluid flow behaviour in the vessel (computational fluid dynamics) and employs rigorous fluid thermodynamic description of the gas and entrained liquid droplets entering at each nozzle.

The results indicate where local cold spots can be expected on the vessel wall. The transient wall temperature profiles show (i) a case of gas only feed to the vessel and (ii) a case when there is a small amount of entrained liquid in the feed.

Oil-gas separator minimum temperatures over time – Click to enlarge
Oil-gas separator minimum temperatures over time for cases (1) and (ii)

The direct impingement of what is a relatively small amount of liquid droplets (< 0.5% of the total feed at N2) has a disproportionate impact on calculated wall temperatures.

Conclusions

For this vessel design, a detailed dynamic calculation indicates that the wall temperature stays above the minimum vessel temperature if only gas phase heat transfer is considered.

However, when the effect of direct liquid droplet impingement on the vessel wall is included a significant low temperature risk is seen.

Alternative vessel designs and operation modes are assessed via sensitivities. In this case, redirecting the nozzles so that cold gas/liquid flow is not sent towards the wall increases the temperature of the walls around the nozzles by 12°C, resulting in a safe vessel design.

Apply high-fidelity dynamic modelling to your separator analysis

The use of a rigorous model-based safety approach based on our gFLARE technology and coupled process and flare network models can result not only in improved process safety but also an improvement in operations and a considerable reduction in CAPEX. Read our case studies to find out how Oil & Gas companies are saving billions.

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