Question

(a) Describe THREE different patterns of tube used in exhaust gas boilers. (4)

(b) Give reasons why a tube is condemned. (4)

(c) Describe how it is replaced by a spare tube. (4)

(d) Suggest with reasons the possible consequences of allowing an exhaust gas boiler to run dry during full power operation of the main engine. (4)

Answer 1

Part (a)

Plain (smooth) tube: A simple cylindrical tube, the most common, used in fire-tube/ smoke-tube exhaust gas boilers. Exhaust gas passes through the tube, heating the surrounding water.
Finned tube: A tube with external (or internal) fins/ extended surface to increase the heat-transfer area on the gas side, improving heat recovery where the gas-side heat-transfer coefficient is low. Used in some exhaust gas economisers.
Swirl/ turbulator tube: A tube with an internal insert (turbulator/ spiral) that creates turbulence in the gas flow, improving heat transfer. Also, "corrugated" or "spiral" tubes are used to increase surface area and turbulence.

Part (b)

Corrosion/ wastage: The tube wall thinned by corrosion (especially at the gas inlet/ outlet ends and where condensation occurs) below the minimum safe thickness.
Cracking/ fatigue: Cracks in the tube or at the tube-to-tube-plate joints from thermal/ mechanical fatigue.
Leakage: A leaking tube (from corrosion, erosion, or a defect) that cannot be safely repaired.
Erosion: Erosion of the tube wall by high-velocity gas/ soot particles, thinning the wall.
Blockage/ severe fouling: A tube so badly blocked by soot/ scale that it cannot be cleaned and is ineffective.
Distortion/ deformation: A tube bent or distorted, affecting the boiler structure.
Pitting: Deep pitting that reduces the wall thickness below the safe limit.

Part (c)

Isolate the boiler (shut off the exhaust gas and water sides), drain the water, and allow the boiler to cool.
Remove the tube: Cut/ remove the defective tube from the tube plate (using a tube cutter/ saw or by drilling out the expanded/ rolled ends), and withdraw it.
Clean the tube holes in the tube plate.
Fit the spare tube: Insert the new tube into the tube plate holes, position it correctly, and secure it by expanding (rolling) the tube ends into the tube plate (using a tube expander/ roller) and/ or by welding/ brazing the ends as per the maker's method.
Pressure test the boiler (hydraulic test) to confirm the new tube is leak-tight.
Refit the boiler to service (reconnect the gas and water sides, refill, and bring back on line).

Part (d)

Overheating: With no water to absorb the heat, the boiler metal (tubes, tube plates, shell) overheats rapidly, causing thermal stress, distortion, and possible failure.
Tube failure: The tubes overheat and may collapse, crack, or burst, causing a major leak/ failure.
Damage to the boiler structure: Overheating can distort the shell, tube plates and headers, and damage the refractory/ insulation.
Risk of fire/ explosion: Overheating of soot deposits on the gas side can ignite, causing a soot fire; the boiler may be damaged or destroyed.
Loss of steam/ heat recovery: The boiler is out of service, losing the heat-recovery capability and possibly affecting the auxiliary steam supply.
Safety hazard: A sudden tube failure/ boiler rupture is a serious safety hazard to personnel and the vessel.

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