After reviewing many boilers with the nearly identical control strategy, I believe there are some substantial issues relating to the simultaneous fan speed and damper control for the control of combustion air flow, not the least of which is a scenario where an explosion can occur. And, while the likelihood of an explosion may be unknown, the use of the strategy for combustion air flow control is fundamentally flawed.
Exploring The Phenomenon
The boiler burner is a parabolic system, pressure drop increasing to the square of the flow through the boiler. A damper will merely increase the pressure drop which pushes the system operating point up that curve, increasing the pressure on the fan while reducing the flow from that fan.
This is true so long as the operating point is on the right of the maximum pressure point of the pressure versus flow fan curve. The control air flow by a damper to the left of this point is nearly impossible since the slope is positive, which means opening the damper will result in a decrease in flow from the fan and closing the damper will result in an increase in flow. Operation in this range typically results in a flame out at synchronous speed which is set as the limit of operation (otherwise known as turndown ratio).
Closing the damper a full speed to cause a pressure drop of 5"wc will decrease the air flow by 18kCFM. To reduce the flow by that same 18kCFM requires the damper to impart just 1.7"wc of pressure drop on a fan running at 50 percent speed. Therefore, the attempt to control air flow by simultaneously controlling fan speed and damper opening is tenuous at best.
Assuming that the opening of the damper is repeatable (the linkage slop making that assumption unreasonable), the control of flow at lower fan speeds is exponentially more sensitive to damper opening, which means that the smallest of damper movement will result in incredible increases or decreases in air volume flow.
Moreover, the flat curve of a slowed fan, makes the transition into the positive slope of the fan curve substantially more likely than that of a synchronous fan. The propensity of the damper to cause operation near or at the maximum pressure point of the fan curve increases the likelihood for the fan to operate on the positive slope portion of the pressure versus flow curve.
In exploring the affects of this phenomenon within a boiler, let's assume that the boiler is operating below its setpoint O2 AND the fan operating point fell on the left side of the curve during the ramp down of the boiler. The O2 trim signal would kick in, opening up the damper (increasing the fan speed would result in the point maintaining its location on the positive slope). However, because the operating point was initially at the left of the maximum pressure point of the fan curve, the volume flow rate of combustion air is reduced.
The lower combustion air results in filling the firebox with CO and unburned fuel. The damper continues to open to a point where the opinion would allow the greatly higher air flow on the negative slope of the fan, instantaneously filling the combustible filled firebox with enough air to cause an explosion.
Please bear in mind that most low NOx burners have a tendency to produce vibrations regardless of the combustion control.
When the burner is a stage fuel burner, the design of which injects fuel through opposing spuds into the firebox in the attempt to lower NOx by stretching the flame over a longer path within the firebox. This design is predisposed to flame detachment, which is exasperated by stratification of recirculated flue gas which reduces flame speed.
As the flame detaches from the burner front, fuel is injected into the firebox accumulating in pockets which combust instantaneously upon reaching the detach flame. We typically redesign the burners to sharply reduce the tendency for flame detachment. (The stage fuel approach for lowering NOx is just one of many approaches sold which have no basis in fact, as a old ring burner will actually get lower NOx with the same amount of FGR.)
*Boiler operators are encouraged to request a fan analysis and supporting documents from Benz Air Engineering.
Providing A Solution
There seems to be an increasing number of installations that have similar control schemes. After reviewing several boilers that have experienced furnace explosions, it seems most likely that the combustion air flow strategy is the cause. If the combustion air flow strategy using a simultaneous fan speed and damper control is not the cause, then it seems to be, at the very least, a contributing factor.
We recommend that any such installations be evaluated before other flawed solutions are implemented to alleviate the symptoms of a considerable problem. For example, one operator had installed buckstays in an attempt to alleviate the noise from combustion vibration, which resulted in concentrating the vibration within the boiler. It was our conclusion that this concentration most likely contributed to the fatigue fractures of their superheater tubes.