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Table 1 - Results of the dewaxing heaters ∏1/∏2/∏3 thermal and fluid-dynamic re-rating at “Design Case”
Item ∏-1 ∏-2 ∏-3
Case Design Re-Rating Design Re-Rating Design Re-Rating

Process Fluid Velocity Too low Too low Too low

Coke Risk Formation Yes Yes Yes

Liquid/Vapour Yes Yes Yes
Separation

Radiant Tube Flow
Regime Slug/Plug/Annular Plug-Annular/Annular Slug/Plug/Annular
Flue Gas Draft Velocity Not optimised Not optimised Not optimised

Close to max operating
Calculated TMT temperature N/A Too high

Outlet Temperature Not reached Too low Too low

Specified
Convection Section N/A efficiency Specified efficiency
not achieved
not achieved
Feed Flow Rate N/A Uneven N/A
Distribution

heaters issues. The study included a thermal and
fluid-dynamic re-rate of the dewaxing fired heaters,
to compare the original design with actual operat-
ing conditions, simulate and analyse the heaters
operating parameters, to find out the cause of such
serious heater tubes integrity problems (figure 2).
Thermal and fluid-dynamic re-rating

The customer provided input data considering
three different oil feedstocks (FEED 1, 2 & 3), burn-
ers fuel gas firing, fired heaters design conditions,
available documentation and actual operating con-
ditions to analyse. Considering the three different
oil feedstocks (FEED 1, 2 & 3) and fuel gas as feed
to burners, current performance against the speci-
fied one was verified.
A “Design case” and a “Maximum allowable load
case” study for the dewaxing heaters was carri-
ed out. The “Design case” was based on the fired
heaters original design conditions in order to iden-
tify design weak points. The “Maximum allowable
case” was studied on actual operating data to as-
sess the heaters required design changes at actual
operating capacity.
Heaters “Design Case” study
outcomes
The results of the dewaxing heaters ∏1/∏2/∏3 Fig. 3 - Evidence of the effect of vapour/liquid separation:
thermal and fluid-dynamic re-rating at “Design coke stratification caused by liquid/vapour separation on
Case” are reported at table 1. radiant inner tube

Heaters “Design case” study acceptable fluid mass velocity for such a service.
Results discussion - Fluid mass Such a low value leads to coke built-up and liq-
velocity uid to vapour separation inside the tubes. The low
fluid mass velocity promotes coke built-up in fired
The calculated process fluid mass velocity in all heaters causing higher coke laydown than coke
the three heaters resulted well below the minimum spalling. Furthermore coke deposition is enhanced

58 Impiantistica Italiana - Marzo - Aprile 2018