PTQ Q2 2022 Issue

(more negative draft) brings higher fuel pressures to compensate for the heat wasted by heating the extra excess air. These higher fuel pres- sures would alter the characteristics of the flames, causing them to strike directly on the tubes of the radiant section, leading to localised coking of the process fluid inside the tubes. Higher draft levels could also cause an increase in the flue gas tempera - ture in the convective section, which could, in turn, cause the deteriora - tion of tube extended surfaces. Many operators and their super- visors operate the heater in a ‘wasteful’ mode because doing the opposite (i.e., operating with air deficiency) could have imme - diate catastrophic results for the equipment. Draft and excess air deficiency: hazardous operation Exceeding the lower limits for the draft conditions (>-0.05in H 2 O) would result in insufficient air intake, thus limiting the combustion reaction and increasing the proba- bility of flame failures (‘flameout’). The accumulation of unburned fuel inside the furnace could result in the complete suffocation of the equipment, or more concerning a safety hazard, as any untimely entry of air could cause an explo- sion, resulting in a process or per - sonal safety incident, damage to the equipment, loss of production, or even injury to personnel. Heater tuned-up operation: a joint responsibility The correct or tuned-up operating window for a direct fired heater is within the specifications set by the equipment designer. These typi- cally define draft as -0.10in H 2 O and excess oxygen as 3.0%, wet basis (WB) on the radiant arc for design and normal conditions. Alternatively, some manufacturers use 20% excess air. Operating under conditions of minimum flow or minimum processing ‘load’, design - ers specify higher excess air levels to ensure greater mixing between the air and the fuel. These parameters defined in the previous paragraph for draft and excess oxygen are essentially oper-

Too open

Controlled Too close

Stack damper

Damper opening

Flue gases

Excess O (%) 5.0 4.0 3.0 1.5 0.0


Tuned up

Low A/C ratio Hazardous

Process scenarios

High A/C ratio

(Targets met)

Onset of incomplete combustion Sub-stoichiometric

Excessive fuel consumption Energy losses through ue gases High CO & NOx emissions

Furnace draft & excess O

combustion (”ooding”) Afterburning

Draft (in. HO)

-0.20 -0.15 -0.10 -0.05 0.0

Burner registers

Registers opening


Too close

Controlled Too open

Figure 3 Draft and excess O 2 control tools

• Competent and skilful operators Operating a natural draft heater under optimal conditions requires, in the first instance, competent and skilled operators who fully under - stand the operation of the equip- ment and the mechanical tools available for its control: the stack damper and the burner registers. This understanding, supported by the continuous measurement of draft values and excess oxygen at the top of the radiant arch, will be sufficient to keep the heater tuned up within the margins defined by the equipment designer, based on plant operational experience and on changes or replacements of com- ponents (such as burners) that may have taken place. • Proactive and dependable engi- neers and supervisors The partic- ipation of experienced engineers or supervisors with professional backgrounds is also essential within the proposed scheme to exercise reliable and proactive leadership of the different operator shifts. Structuring a group of process heater engineers (PHE team) could be a reasonable starting point for a successful programme of heater efficiency improvements and envi - ronmental awareness. The evalua- tion and monitoring of direct fired heater performance is very demand- ing but also a very challenging task for heater engineers due to the direct impact of this equipment on the economy of the refinery.

ating guidelines. In practice, the operator has a certain margin of manoeuvre, which could be per - haps less efficient from a thermal perspective but must still be opera- tionally safe. The safety margin for draft is usually defined as -0.05in H 2 O). In the case of oxygen, it could be between 1.5-1.0% WB, although the safest option is to have a carbon monoxide analyser (CO, ppm) to anticipate the onset of incomplete combustion in time. Figure 3 summarises the operating methodological tools used to con- trol process scenarios of a natural draft process fired heater. A stack damper too open and/or burner reg- isters too closed will generate high draft as well as high excess oxygen levels. On the other side, a damper too closed and/or burner registers too open will produce low draft and excess oxygen readings. A con- trolled draft and air input environ- ment (i.e., a tuned-up heater) are crucial to controlling CO 2 emissions from petroleum refineries. Key elements of a continuous operational heater tuning (COHT) programme A programme of this nature should be based on competent operators, proactive engineers, and supervi - sors. Site managers and high-level corporate executives should pro- vide clear and positive support, and employees should be stimulated and motivated in different ways.

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