Drum Shell

Function

The drum shell confines the flame and hot gas from the burner, creating the high-temperature environment needed to drive off surface and interstitial moisture from aggregate before bitumen coating. Shell stiffness is critical: out-of-round deformation changes flight loading, disrupts aggregate curtaining, and forces the riding rings to wear unevenly. Because the shell is permanently in contact with abrasive aggregate, hot gases, and thermal cycling, its wall thickness and plate grade directly govern service life. A compromised shell causes heat loss, irregular mix temperature, and unplanned plant downtime.

Construction & quality

Fabricated from IS 2062 Grade B / ASTM A36 structural steel plate, 10–14 mm nominal thickness in the drying zone, with 12–16 mm boiler-quality plate (IS 2002 / ASTM A516 Gr 60) in the burner-end hot zone. Plates are cold-rolled on a hydraulic three-roll bending machine to within ±1 mm roundness tolerance per metre of diameter. Longitudinal seams are full-penetration butt-welded by MIG/SAW process and inspected by magnetic particle testing at shell junctions. Both end flanges are machined flat to within 0.5 mm for accurate chute and hood seating. External surface is grit-blasted to Sa 2.5 and primed with heat-resistant paint rated to 250 °C; the hot zone receives a high-temperature aluminium paint rated to 450 °C. Riding-ring seating areas are turned on a lathe to H7 fit tolerance.

When to replace

• Visible bulging, waviness, or dishing between stiffening rings indicating thermal fatigue or weld failure
• Through-wall pitting or thinning measured by UT gauge below 60% of original plate thickness
• Out-of-round deviation exceeding 5 mm causing uneven riding-ring contact and abnormal trunnion wear
• Longitudinal cracks at shell-to-riding-ring weld zones, especially at the hot end
• Persistent hot spots on the outer surface indicating internal flight bond or refractory failure