Aside from being important components for enabling designed heat transmission, boiler tubes play an important role in preserving pressure integrity in boilers. The nature of their service, as well as their exposure to heat loads and corrosive conditions, creates a demanding operational profile for the tubes in order to keep their designated life term.
Defective tubes pose an immediate risk of losing main propulsion functions in propulsion, exhaust gas, and composite boilers, and they may interfere with auxiliary and/or cargo-handling capabilities in oil-fired auxiliary boilers. The sealing of tubes by plug repairs to stop leaks and restore the boiler’s integrity is the most preferred and/or instinctive repair approach used by engineers onboard.
The common causes of tube boiler’s tube leaks are:
- pitting and local corrosion on the water side – caused by oxygen attack, high alkalinity, acid attack due to presence of carbon dioxide and carbonic acid or ammonia attack.
- cold corrosion on the exhaust gas side mainly due operation at low load.
- soot fire due high soot deposits and water poor circulation.
- stress corrosion and material fatigue – cracking near weld zones with high residual stress.
The repairing process depends on the position of the tube, size of damage and time available for repair. In case of a minor leak and if there is access to the faulty tube, the repair can be done, after boiler has been cooled down, through welding by building-up the missing material or by installing a doubler patch on the tube. Unfortunately, most of the time the damaged tube is not so easily accessible for this kind of repair and it becomes necessary to plug the respective tube and remove it from the service. The tube must be closed by inserting a metallic plug on each end of it. The plug has a conical shape (with conicity of 1/10) and its dimension depends of tube inner diameter. The conical side can be simple or threaded and usually must be inserted around 25 mm inside the tube and the outside part must be twice as much. When the plug is forcibly inserted/pressed inside the tube, the outside remaining part of the plug is around 15 mm.
After plugs have been inserted, the tube is prone to destruction through burn-out as it will be uncooled. It is imperious to have an evidence of the total number of tubes that are plugged, as a is not admissible, as general rule of thumb, to run the boiler if more than 10% of the total number of tubes are out of service. Running the boiler with more than 10 % of the total number of tubes isolated will interfere with the natural circulation of the water and can lead to complete failure of the boiler tubes. In this regard it is recommended that every time a tube is pulled out of service, the position of the tube should be marked on the boiler tube schematic diagram as will allow to easily trace the number of the non-operational tubes.
Replacement of the broken tubes is possible only with total replacement of the boiler’s tubes when the total number reaches 10 % of the total number.
By blocking the tubes due repairs will have an impact on the boiler’s performance as well as on vessel’s main propulsion. Therefore, further below will try to explain and understand this impact.
Water tube boilers with natural circulation is particularly applicable to oil-fired boilers, which rely on internal natural water circulation for effective heat transmission and to keep material temperatures below acceptable limits. Depending on the number and position of the plugged water tubes in the boiler, they interfere with natural circulation to various degrees. For example, the same number of plugged tubes may have a different influence if they are mainly situated on one area of the steam/water drum adjacent to the furnace, as opposed to being spread in other regions within the same drums.
Thicker pressure parts, such as drums, in close proximity to furnaces rely on tube circulation to maintain acceptable temperatures. If the blocking of many nearby tubes in this location is not assessed, it can be harmful for the boiler. De-rating the firing rate is one of the most effective mitigation methods for correcting for the loss or restriction of natural circulation.
Leaving plugged and uncooled water tubes near the furnace have a significant risk of “burning out” ( overheating and melting, causing damage to the furnace wall and adjacent pressure parts). Plugged tube banks on water tube exhaust gas boilers and economizers with extended heating surfaces (fins, pins, etc.) remain uncooled and are more likely to start soot fires if deposits build up and slow steaming plays a significant role in this case. Local smoldering of soot involves a high heat intensity capable of melting the uncooled metal as well as potentially damaging nearby tube banks and restrict exhaust gas flow. In the worst-case scenario, if it causes a leak in adjacent tubes, it increases the chance of hydrogen fires.
On the smoke tube boilers, if the load remains constant, plugged tubes limit the effective area for exhaust gas flow while increasing gas velocity (thermal loads) through the remaining tubes. The additional thermal load on the remaining tubes if the same evaporation rate is maintained must be reduced through reduction of heat. This can be done through de-rating the firing rate on the oil-fired boiler or through slowing down the engine rpm on the exhaust gas boilers or economizers which affects the steam generation capacity and implicitly, in worse case scenario, the safe operation of the vessel.
In conclusion, tube plugging necessitates careful monitoring and evaluation of the local and global characteristics of the boilers and associated propulsion plants. Ignoring these elements increases the potential of long-term and catastrophic damage, as well as time-consuming and costly repairs in the future. After examination, mitigation actions may include, but are not limited to, reducing the firing rate/engine load and de-rating the pressure as applicable if proposals for repairs by plugging are eventually considered for temporary/permanent acceptance.
It is important that every engineer must be familiar with monitoring of boiler’s performance, to recognize the signs of a broken tube and know which steps need to be taken in this case. The most common sign are:
- Unstable firing in case of oil-fired boiler
- Water leak through check valves when engine is stopped
- Abnormal boiler water consumption.
- Low water level in the boiler despite continuous filling
- Very dense and cloudy white exhaust smoke on the funnel
- Unable to keep steam pressure.
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