Stack damper

Since the initial start-up of the furnace in 1995, the stack damper controlling the oxygen level had been little used, because it was very difficult to operate. Several complaints had been made to direct attention to this problem, but all to no avail. 

This accident will now be analysed according to the analysis flow chart. The first step is to identify re-occurring deviations prior to the accident. In this case the repeated complaints of the operators about the difficulty in operating the stack damper and the re-occurring flickering of the flames are identified as soft and hard precursors, respectively. 

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 ... 

The air register and the stack damper are used together as a team to optimize the heater draft. Our aim in balancing the draft for a natural-draft or a balanced-draft heater is to maintain a small negative pressure of, say, —0.2 to —0.1 in of water, just below the shock tubes (please refer to Fig. 20.5), at the entrance to the convective section. At the same time we must maintain enough air to operate on the good side of the point of absolute combustion. 

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. 

We adjust the draft with the stack damper and maintain the combustion-air level by adjusting the air registers to accommodate the adjustments made on the stack damper. 

Suppose we put the air registers back as they were and pinch on the stack damper instead. So let s suppose we restrict airflow via the stack damper until the firebox oxygen goes down from our base case value of 6 percent, to 3 percent. The oxygen in the convective section will also be reduced to, say, 5 percent. In this case, we see that the A02 is also much... 

Automatic controls on stack dampers. The stack damper is used to control the heater draft. This in turn approximately controls the O2 concentration in the firebox. 

Automatic modulation of the burner inlet air registers. This is used in conjunction with the stack damper control. The stack damper controls the draft and the O2 concentration in the firebox controls the inlet air registers. 

Confirm that all auxiliary furnace equipment is functioning properly, including all instrumentation and measurement devices open both the burner air inlet register/damper and the furnace stack damper to the fully open position. 

Problem statement What is the maximum draft pressure at the floor of a 10-m furnace operating at 800°C Presume that the control system adjusts the pressure at the top of the furnace to a draft of 3 mm w.c. via a stack damper. 

It is possible to calculate the dimensions of ports and flues so that the resistance of ports and flues will be balanced by the draft (suction) plus furnace pressure. However, good practice in automatic furnace pressure control usually necessitates a stack damper that always takes a minimal pressure drop. Therefore, the real balance is stack draft -I- furnace pressure = AP furnace exit orifice + AP stack skin friction -I- A P damper. Tables 7.2 and 13 from Prof. Trinks fifth edition list information for a few specific cases that illustrate points mentioned earlier and equations 7.3,7.4, and 7.5 below. 

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. 

If excess air can be adequately controlled with the secondary air registers, what is the purpose of the stack damper The optimum positioning of both the secondary air register and the stack damper are related. Two objectives must be satisfied if an optimum operation is to be approached Excess air in the flue gas to the convective section should be minimized, and there should be a very small negative pressure at the convective section inlet. 

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). 

If the stack damper is wide open and the secondary air registers are pinched back, too low a pressure can develop in the convective section. Figure 15—4 illustrates this condition. Depending on the furnace skin integrity, a great deal of cold air can be drawn into the heater through the holes in the metal sides of the furnace or through leaks in the roof tiles. 

One way to minimize the possibilities of afterburn is to pinch back on the stack damper and further open the burner air registers (Fig. 15-6). The resulting higher pressure (low draft) reduces the rate of air into the convective section tube banks. 

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. 

When short of air, a healer is draft-limited. (See the section on air deficiency to learn how to identify this problem.) With both the stack damper and the secondary air registers fully open, not enough air can be drawn... 

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

Sponge coke, 46—48, 70 Stack damper, 317-318, 320 process heaters, 317 Stacked paper plates, 94 Stainless steel, 416-418, 424-425 field identification of pipe, 417-418 cracking, 424-425 Start-up stack, 123 Start-up tips (sulfur plant), 134 Start-up (pump cavitation), 246-247 Start-up (sulfur recovery), 122-124 oxygen supply, 123 atmospheric vent, 123-124... 

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. 

Slowly open the stack damper until the draft gauge indicates a small negative pressure (-0.1 in. H2O) at the convective section inlet. 

The stack damper and air registers may have to be adjusted in several small increments to meet the desired targets. 

On one 50,000-B/D crude unit, steps were taken to eliminate air leakage through the furnace skin fuel use subsequently dropped by 6%. To correct excessive draft, one should slowly pinch back on the stack damper until the draft gauge indicates a small negative pressure and open the secondary air registers until the excess air target is reached. 

Draft gauge plugged CO2 in flue gas Fouled convective section Leaks in furnace skin Stuck stack damper Damage to furnace structure Smoke leaks out of convective section... 

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