Forced-Draft Fan Vibration and Combustion Instability
A packaged fire-tube boiler rated at 20,000 lb/h is equipped with a forced-draft fan and a modulating burner. The fan is driven by a 50-hp motor through V-belts. Over the past two weeks, the fan has developed a noticeable vibration at high firing rates, and the boiler has experienced occasional flame instability (flame flickering and a low-pitched rumble) when the burner is at 80–100% capacity. The stack oxygen reading fluctuates between 2.5% and 5.5% even though the O2 trim system is in automatic. The fan bearings were greased last month, and the belts were replaced six months ago. The boiler room is at sea level, and the combustion air intake is ducted from outside. The operator has been increasing the excess air manually to stabilize the flame, but the stack temperature has risen from 380°F to 430°F. The plant needs to maintain steam supply for the next 48 hours before a scheduled maintenance window.
1. List the probable causes of the fan vibration and explain how each could contribute to the combustion instability.
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Probable causes of fan vibration: (1) Belt misalignment or uneven belt tension after the recent belt replacement; this can cause axial and radial vibration at 1× and 2× belt frequency. (2) Bearing wear or contamination despite greasing; worn bearings produce high-frequency vibration and can cause shaft misalignment. (3) Fan wheel imbalance due to dirt buildup or erosion; this produces vibration at 1× rotational speed. (4) Loose foundation bolts or resilient mounts; this can amplify vibration. (5) Shaft misalignment between motor and fan; this produces 1× and 2× vibration. (6) Resonance: the fan speed may be near a natural frequency of the foundation or ductwork, causing high vibration at a specific speed. Each of these can affect combustion: vibration can cause the fan output pressure to fluctuate, leading to varying combustion air flow. If the fan is surging or operating near the stall point, the air flow becomes unstable, causing the flame to flicker and rumble. The fluctuating O2 reading (2.5–5.5%) indicates that the air/fuel ratio is not stable. The operator's manual increase in excess air raises the air flow, moving the fan operating point on its curve; if the fan was already near the stall region, more air could push it further into instability. The rumble is likely combustion-driven oscillation (low-frequency instability) caused by the interaction of the fuel/air mixture and the combustion chamber acoustics, aggravated by unstable air supply. The vibration may also be due to a failing fan bearing that is causing the shaft to wobble, which changes the fan blade tip clearance and reduces efficiency, leading to lower air pressure at high firing rates.
2. What immediate checks and adjustments should the operator make to safely operate for 48 hours, and what are the risks of each?
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Immediate checks: (1) Measure vibration at the fan bearings in horizontal, vertical, and axial directions; compare to baseline. If vibration exceeds 0.3 in/s peak, consider reducing load. (2) Check belt tension and alignment using a straightedge; adjust if misaligned, but avoid over-tensioning which can damage bearings. (3) Inspect fan wheel for dirt or damage through an inspection port; if accessible, clean. (4) Check foundation bolts for tightness. (5) Verify fan inlet damper and outlet damper positions; ensure they are not hunting. (6) Check the O2 analyzer calibration; a faulty reading could cause the operator to chase the wrong problem. Adjustments: (1) Reduce maximum firing rate to 80% until maintenance; this lowers fan speed and moves it away from the unstable region. (2) Put the O2 trim in manual and set a steady excess air of 3–4% O2 (about 15–20% excess air) to avoid the trim chasing fluctuating readings. (3) If the fan has a variable inlet vane, ensure it is modulating smoothly. (4) Consider temporarily reducing combustion air temperature if the outside air is very cold, as cold air increases air density and fan power. Risks: Reducing load may not meet steam demand; operating at reduced excess air could increase CO emissions or cause flame impingement; adjusting belts without proper alignment could worsen vibration; cleaning the fan while running is unsafe. The operator must monitor vibration, bearing temperature, and flame stability continuously and be prepared to shut down if vibration increases suddenly or flame instability becomes severe.