In today's world, With the ever increasing sophistication in Main Engines, their turbochargers & Exhaust Gas Boilers, the main aim of extracting maximum possible waste heat from exhaust gas has brought us to a juncture where the operation & maintenance carried out on these systems holds equal importance in terms of plant efficiency & safety. That is why it is important to understand the reasons for a soot fire.
The main reasons for a soot fire can be summarised as follows:
|
| • |
Excessive smoke emission from Engines will cause increase in deposition. |
| • |
Very low gas temperatures at the boiler tube surfaces can cause increase in soot deposition due to formation of wet soot. Exhaust gas temperatures below 165degC can cause condensation resulting in sulphuric acid formation leading to sticky soot.
|
| • |
Gas velocities through the boiler must not fall below designed minimum value- this is the main cause of increased deposition rates. Most boilers are designed with flow velocities of about 20m/s.
|
| • |
The circulation water flow velocity & the ratio of the water volumes in the boiler as compared to the system must not reduce too much as this keeps the gas temperature at the boundary of the boiler tubes below the ignition temperature of the soot. |
Countermeasures |
| • |
Efficient & regular sootblowing. |
| • |
Regular water-washing of the tubes to prevent soot build-up. |
| • |
Supply of well preheated feed water (about 140degC) during start-up & during normal operation. |
| • |
After the engine is stopped, water circulation to be continued till all tube temperatures are below 120degC, to prevent oily wet soot from catching fire. If soot deposits are excessive, then it is better not to stop the circulating water even in ports. |
| • |
Excessive smoke from the Main Engine to be avoided. |
If an initial soot fire is discovered, then the main aim is to prevent a high temperature metal fire.
In case a small soot fire does occur |
| • |
Stop Main engine & cut off air supply to the fire. Auxilliary blowers off. Turbocharger air filters covered. Main Engine exhaust valve spring air supply to be on. |
| • |
The water circulation to be kept on, provided no continuous water loss is observed. |
| • |
Soot blowers are never to be used for putting off the fire. |
| • |
If multi-level water washing equipment are fitted then same may be used provided that copious amounts of water can be supplied.
|
Normally, small fires can be put off with just oxygen starvation. However, the boiler soot deposits must be washed down to ensure against recurrence of a fire.
|
