DNV GL has considerable experience from surveys involving the evaluation and acceptance of repairs related to different types of defects on auxiliary boilers. Now, after about one class period (five years) of experience with the BMON class notation, this technical news summarizes some interesting observations.
Relevant for ship owners and managers as well as ship yards.
Typical boiler damages and defects
Typical boiler damages and defects Auxiliary boiler defects on pressure parts are typically related to mechanisms such as:
- Active local pitting corrosion from the water/steam side
- Overheating due to deposits, oil, scales, low water level, flame impingement, etc.
- Poor workmanship during fabrication
- Soot fires on fin/pin type water tube exhaust gas boilers
- Cold corrosion from gas side
DNV GL has also observed that most of the boiler defects reported are caused due to corrosion arising out of probable factors related to inferior water condition, most often due to in-sufficient maintenance.
Lack of a stable and passive magnetite layer (oxide) on the water/steam side of metal surfaces appears to be the most predominant contributory mechanism behind many of the reported defects.
A smaller number of defects are related to other factors or operational issues.
It is also our experience that many ships are struggling to allocate time and arrange acceptable materials and resources to repair the defects after they have occurred or are observed, making the situation even worse.
Our approach to improved boiler management
DNV GL introduced the BMON class notation in 2012 to improve the overall boiler performance in terms of deficiencies, repair, maintenance costs, reduced downtime and similar. An additional benefit was to offer an alternative, efficient and flexible survey approach. With the BMON notation, ship owners and managers can reduce vessels’ downtime in port and, at the same time, maintain their boilers optimally. Since 2012 and to date, more than 150 vessels have been granted the BMON class notation; for these vessels, we have observed zero findings related to boiler condition in service arising out of mechanisms mitigated by the notation.
Some lessons learned
Based on this experience, these are the most important lessons learned:
Enhanced focus on water treatment: By utilizing prescriptive methods to initiate and sustain a passive magnetite layer on steel surfaces, as well as by increasing the frequency of monitoring the water condition, the risk of active local and general corrosion of the internal surfaces (steam and water side) is reduced to a minimum. Furthermore, by keeping the heat transfer surfaces in an optimal condition, e.g. avoiding formation of scale and impurities, the heat transfer barriers are reduced, which leads to:
Improved fuel efficiency Avoidance of heat stresses that may result in cracks Avoidance of overheating of furnace wall, top plate, and screening tubes
Monitoring and maintenance of the boiler plant: Increased focus on maintenance and internal inspections mitigates the risk of other contributing factors which lead to, or increase the probability of, a defect (flame impingement from burner, etc.).
Optimization of feed water system design: By optimizing the feed water system design (e.g. salinometers, oil content sensor, hotwell temperature) and monitoring the differential pressure across the exhaust gas boiler, the risk of water side contamination/ excessive dissolved oxygen and defects related to the gas side is minimized.
A more flexible class survey comes as an additional benefit: Vessels with BMON class notation follow an enhanced approach on the management and follow-up of the boiler condition in service. Consequently, a part of the scope during every alternative boiler survey – which is not connected to the main class renewal survey – can be credited based on the chief engineer’s inspection report, which reflects evidence of satisfactory internal examination. This is documented for review and final acceptance by the DNV GL surveyor.
This survey arrangement results in better flexibility in terms of when and where to do the internal inspections. The remaining scope of the survey shall still be carried out by a DNV GL surveyor. Then, the survey will be formally credited upon the satisfactory review and completion of both parts.
DNV GL recommends operators reduce the probability of commonly encountered boiler defects by enhancing the focus on boiler water condition management and monitoring. Experience demonstrates that the BMON class notation leads to better boiler performance, with an accompanying benefit of flexibility of the intermediate boiler survey.
DNV GL rules for classification:
Pt. 7, Ch. 1, Sec. 6 (26) on Boiler Monitoring;
Pt. 6, Ch. 9, Sec. 5 on Boiler Monitoring – BMON.