Water and smelt coming into contact leads to a smelt-water explosion and results in very high damage to boiler with long shutdowns needed to bring back the boiler back into service. All solidified smelt has to be removed before lighting up the boiler to check the furnace floor and ensuring no damage of any form to these tubes. This takes a lot of down time for the boiler and a loss of availability.
A smelt-water explosion can happen if even a small amount of water is mixed with the molten solids at high temperature. It is to be remembered that the smelt-water explosion is purely a physical phenomenon of the water becoming steam that has a very high volume. The liquid-liquid type explosion mechanism has been established as one of the main causes of recovery boiler explosions since long back. Even a few liters of water, when mixed with molten smelt can violently turn to steam in a fraction of a second. This sudden evaporation causes an increase of volume and a 1000 – 10000 kgf/m2 pressure wave travel in a unidirectional way.
Possible areas of failure in a chemical recovery boiler causing a smelt-water explosion
- Floor tubes
- Primary air port opening
- Secondary air port opening
- All water wall tube welds
- Attachment weld
- Butt weld
- Water platen failure
- Purging black liquor gun into furnace
Recovery boiler explosions were studied by the Black Liquor Recovery Boiler Advisory Committee, and the data showed that pressure part failures were the biggest single cause contributing to chemical recovery explosions. It was noted from this study that both operational factors and fabrication factors resulted in such explosions. The main operating factor which contributed for chemical recovery boiler explosion in the 1970s was overheating of tubes due to water-side sludge and problems with the acid cleaning of water tubes. In the 1980s it was low drum level operation which resulted in waterwall tubes leading to explosion.
Smelt-water explosion damage in chemical recovery boilers studied by Black Liquor Recovery Boiler Advisory Committee indicates that huge replacements were needed.
The replacements in various exploded chemical recovery boilers covered
- All four water walls replacement
- Most of the furnace floors were replaced
- All furnace corners opened from firing floor to roof
- Backstays sheared off at corners
- Some openings were 1 to 2 m – about half way up the walls
- Water walls displaced by more than 1 meter and corrugated
- Maximum damage in front Waterwall
- Impact against walkway displaced supporting I-beams of 15 to 20 cm flange to flange and were twisted and deformed
- A concrete walkway on 5th floor moved about 1 meter
- Damage was up to roof of the boiler in all most all cases
- No screen super heater damage was seen
- One soot blower blown to adjoining building
- One furnace floor beam had a 20cm permanent deformation
These damages indicate the amount of pressure developed by the smelt-water reaction during the physical explosion.
Care to be taken to prevent tube failures in chemical recovery boilers
- Use only seamless steel tubes of standard specification
- Ensure tube manufacturing quality
- All welds in recovery boiler pressure parts to be 100% radio graphed - Shop & site welds
- Adopt correct procedure for welding
- Electrode – Material & Size
- Post heat treatment
- Stress relieving
- Do shop hydrostatic test to detect week welds
- Adopt safe operating procedure