Persistent leakage at the tube-to-tube plate joint may occur due to one or more of the following reasons:

• Waterside corrosion and pitting caused by poor-quality feed water or improper boiler water treatment, resulting in wastage of the tube ends and tube plate.
• Wastage of the tube plate ligaments due to soot blowing with wet steam, leading to erosion around the tube holes.
• Unequal thermal expansion between the tube and tube plate caused by local overheating or design faults, resulting in loosening of the expanded tube joint.
• Overheating, deformation, or panting of the tube plate, which causes distortion of the tube holes and loss of a tight tube-to-tube plate seal.

Leakage at the tube-to-tube plate connection can result in the following problems:

• Excessive feed water consumption from the cascade tank due to continuous loss of boiler water.
• Continuous running of the boiler feed pump to maintain the boiler water level.
• In the case of severe leakage:
  • Low boiler water level.
  • Drop in steam pressure.
  • Continuous firing of the boiler to maintain steam demand.
• Unstable firing in oil-fired boilers due to fluctuating water level and steam production.
• Leakage of water through boiler check valves when the main engine is stopped.
• White smoke or steam issuing from the funnel, which flashes off immediately after leaving the funnel due to escaping boiler water entering the exhaust gas side.

A permanent repair involves renewal of the leaking tube and proper restoration of the tube-to-tube plate joint. The procedure is as follows:

1. Allow the boiler to cool completely and inspect the leaking tube connection.
2. Open up the boiler and identify the defective tube for renewal.
3. Cut both ends of the leaking tube approximately 50 mm away from the tube plate.
4. Remove the remaining tube pieces by:
   • Chiselling, or
   • Heating and cooling the tube end to produce shrinkage, followed by knocking it out.
5. Thoroughly clean and polish the tube holes.
6. Carry out a dye penetrant test to check the tube holes for cracks or other minor damage.
7. Ensure the diametrical clearance between the new tube and the tube hole is approximately 1.5 mm.
8. Thoroughly clean the ends of the new tube.
9. Insert the new tube and carefully expand (roll) both ends into the tube plate to obtain a tight, leak-proof joint.
10. Ensure the new tube projects at least 6 mm beyond the tube plate.
11. Form a proper bell mouth at each tube end. The bell mouth should be approximately 1 mm for every 25 mm of tube outside diameter, plus an additional 1.5 mm.

A properly expanded and bell-mouthed tube, together with sound tube holes and good boiler water treatment, provides a permanent and reliable repair, minimizing the possibility of future leakage.