Stack damper closing

A typical natural-draft gas-fired process heater is shown in Fig. 20.1. Suppose we gradually close either the stack damper or the air register the flow of air into the firebox will then be reduced. If both the process-side flow and the fuel-gas rate are held constant, the following sequence of events occurs ... 

As we close the stack damper, the pressure at the convective-section inlet will increase that is, we will have less draft. This will reduce the rate of flue gas then to the convective section, minimizing the rate of air in-leakage to the convective section and the chances of afterbum occurring. However, if we close the stack damper too much, a positive pressure will develop at the convective-section inlet, which we must avoid. 

While seeking approval to by-pass the lock out of the burners and restart the afterburner, the common stack automatic continuous air monitoring system (ACAMS) alarmed at 11 26 PM. The site was immediately masked. A depot area air monitoring system (DAAMS) tube was taken for analysis at 11 38 PM and another put in its place. ACAMS readings as high as 3.63 allowable stack concentration (ASC) were obtained. The furnace was bottled up (dampers closed to slow airflow) at 11 44 PM. By 12 18 AM on May 9.2000 the ACAMS had cleared and the order to unmask given. 

Fire-tube heaters contain the combustion gases in tubes that occupy a small percentage of the overall volume of the heater. The basic components of a fire-tube boiler include a large shell that surrounds a horizontal series of tubes. A large, lower combustion tube is attached to a burner that admits heat into the tubes. The upper tubes transfer hot combustion gases through the system and out the stack. Airflow is closely controlled with the inlet air louvers and the stack damper. Water level in the shell is maintained slightly above the tubes. 

The stack damper and the secondary air registers must be used as a team to satisfy these requirements. If the stack damper is mostly closed and the secondary air dampers are mostly opened, a positive pressure can develop at the convective section inlet (Fig. 15-3). 

Of course, if the stack damper is closed too far, a positive pressure may develop below the bottom row of convective tubes, forcing hot gas out against the structural members of the heater, possibly reducing the structural integrity of the heater. 

To determine the location of leaks during a turnaround, close the stack damper slightly and ignite colored smoke bombs in the firebox. If a forced draft fan is available, turn it on. The colored smoke will escape from the leaks. 

Close the furnace stack damper consistent with obtaining a slight draft at the inlet to the convective section. 

Closed stack damper. At a reduced firing rale, the damper may have been partially closed to control excessive draft. The damprer can become stuck—or perhaps someone forgot to open it—when the firing rate was increased. Occasionally, the shaft of the damper will come loose. The operator opening the damper will see the shaft moving, but the damper itself remains closed. An X-ray will tell if this is so. 

All air doors and stack dampers should be wide open prior to purging the furnace. Primary air registers on premix gas burners should be closed prior to lighting to prevent flashback. 

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