Marine boilers play a vital role in powering vessels, providing heat and energy necessary for various operations at sea. However, the occurrence of oil contamination within these boilers can have severe consequences, jeopardizing both the safety and efficiency of maritime operations. In this blog post, we will explore the causes, impacts, and essential steps to address marine boiler oil contamination, ensuring the smooth operation of marine vessels.
Understanding Marine Boiler Oil Contamination
Marine boiler oil contamination refers to the unwanted presence of oil or oil-related substances within the boiler system. This contamination can arise from multiple sources, including fuel supply contamination, leakage from seals or pipes, improper maintenance practices, or even accidental spills. Regardless of the source, the effects of oil contamination can be detrimental and require prompt attention.
Many of you must’ve been heard or encounter an oil contamination of the boiler due different machinery or systems breakdown. The contamination has an immediate and consequential impact on boiler operation and is needless to say that the adverse effects of oil contamination on boiler steel and the plant can never be deemed overemphasized. In some situations, large areas of heat transfer lead to the development of cracks, which in turn leads to a loss of the material’s integrity. The majority of them lead to costly repairs that require a significant amount of time and effort, such as the replacement of pressure parts, chemical-mechanical cleaning, and downtime.
The most prevalent sources of oil pollution detected on boilers come from leaking heating coils fitting in fuel oil tanks, fuel/lube oil heaters, cylinder lube oil from reciprocating steam engines for pumps, and heating coils in DB tanks allocated for sludge/waste oil tanks. It is not unheard of for cargo tank heating coils and tank cleaning heaters installed on the cargo side to be a contributing factor in some instances of contamination.
On the other hand, main propulsion boiler plants that use a segregated saturated steam system as their primary source of heating medium have the lowest risk of oil contamination.
Minor cracks in HFO heating coils or in other heaters in the engine room or in other places on the ship where steam is utilized for heating can sometimes be the cause of oil leaks that become more severe over time. There are several different pathways by which oil can seep out of an engine room in today’s advanced machinery. It is not usually possible to see very thin coatings of oil. According to past experiences, oil pollution at a level of 15–20 ppm (parts per million) will not be noticeable. If there is an oil pollution level of 25 parts per million, then the steam drum will collect roughly 12 kilograms of oil every single day. The boiler has a capacity of 20 tons of steam per hour. There is a potential for localized overheating of the material and potential damage to the boiler if there are thin layers of oil on the tubes of the boiler or on any of the other directly heated surfaces of the boiler.
Impacts of Marine Boiler Oil Contamination
Foaming and carryover in oil-fired boilers due to increased tension at the water surface are two of the immediate effects of oil contamination. Other immediate effects of oil contamination include malfunctioning boiler water level controls and even protective shutdown systems. Carry-over of water and moisture with the steam may even reach the intensity of priming in the worst-case scenarios, wreaking widespread destruction on consumers such as turbines, super heaters, steam pipework, and associated gaskets.
The presence of severe oil pollution causes a decrease in the rate at which heat is transferred through the steel of the boiler, which contributes to the metal temperature being greater than the design value. Even an oil film or deposit as thin as 0.5 millimeters thick on the water side of an auxiliary boiler rated at 7 bar (g) can easily increase the metal temperature on the furnace side from the design value of 250 degrees Celsius to well above 600 degrees Celsius under normal operating conditions. This occurs when the metal temperature on the furnace side exceeds the design value of 250 degrees Celsius. Because of this, there is a domino effect that leads to an exponential decrease in the material’s yield strength. This continues until the pressure parts that are subjected to active heat transfer fail.
In circumstances in which the decrease in strength does not result in an immediate failure, the boiler steel may nevertheless be subjected to a time-dependent creep zone that is difficult to evaluate (if the temperature is higher than 380 degrees Celsius), unless alloying is taken into account during the design stage.
In the case of exhaust gas water tube boilers with an extended surface area that forms part of the system for the generation of steam by forced circulation, this may, in the worst cases, lead to soot fires due to a lack of heat transfer from the gas side and a rise in the temperatures of the metal due to the uncooled boundaries. In addition, this may cause a decrease in the efficiency of the steam generation system. As a result of the differential expansion of the overheated tubes in comparison to the shell, smoke tube exhaust gas boilers are prone to developing cracks on the tube terminations (see the image below for an example).
It is also important to be aware that other long-term impacts include local corrosion of the area that has been exposed to the acidic nature of oil deposits. This is something that should be kept in mind. When hydrocarbon deposits are subjected to high temperatures in the presence of water, they have a propensity to undergo an acidic transformation.
Addressing Marine Boiler Oil Contamination
As an immediate precautionary measure, derating the boiler’s steam generating capacity by reducing the firing rate/heat input in conjunction with the design working pressure is highly recommended.
Depending of the degree of oil contamination on boiler, sometimes can lead to a requirement of the permanent restoration of the heat-transfer surfaces on the water and steam side prior to the boiler being put back into service.
To mitigate the negative impacts of marine boiler oil contamination, it is crucial to follow a systematic approach. Here are the key steps to be taken:
- Detection and Confirmation: Regular monitoring and analysis of water samples can help detect contamination. Suspicious characteristics or abnormal levels of impurities should be investigated further to confirm the presence and extent of the contamination.
- Isolate the Contaminated System: To prevent the spread of contamination, it is necessary to isolate the affected marine boiler system. This may involve shutting down the boiler or bypassing it temporarily until the issue is resolved.
- Identify the Source: Thorough inspection and investigation should be conducted to identify the root cause of the oil contamination. Addressing the source is essential to prevent its recurrence and implement appropriate preventive measures.
- Clean the System: Cleaning the contaminated marine boiler system is vital to remove oil residues and contaminants. The cleaning process typically involves flushing the system with specialized cleaning agents or solvents designed for marine boiler systems.
- Rinse and Drain: After the cleaning process, the system should be thoroughly rinsed with fresh water to eliminate any remaining cleaning agents or loosened contaminants. Complete drainage is necessary to ensure a clean and oil-free environment.
- Inspect and Replace Components: Inspect all components of the marine boiler system for damage or residual contamination. Replace compromised or heavily contaminated components to ensure the integrity and reliability of the system.
- Test and Restore: Thorough testing of the marine boiler system is necessary to ensure its proper functioning. Conduct pressure tests, verify temperature control, and monitor performance indicators to restore optimal operation.
- Preventive Measures: Implementing preventive measures is crucial to minimize the risk of future oil contamination incidents. This includes regular maintenance, monitoring, and sampling of the marine boiler system, along with adequate crew training in boiler operation and maintenance best practices is essential.
Boiling out the water side of the boiler using recommended chemicals and/or mechanical cleaning are normal procedures undertaken to facilitate satisfactory cleaning. This may be additionally supported by hardness checks and a hydrostatic pressure test at 1.5 times the design working pressure to ensure the expected safety factor at the design temperature.
Boil-out chemicals are highly caustic. Caustic Soda ash will produce a violent flash if introduced to water too rapidly. It is needless to say that the crew involved into this operation and handling the chemicals must wear protective equipment like, goggles, gloves, aprons, and an emergency shower should be nearby. Vinegar can be used as an antidote.
Below there is a recommended boil-out procedure that may be followed:
- If the boiler is equipped with prismatic type gauge glasses, replace them with the temporary boil-out glass to prevent chemical attack on the operational gauge glasses.
- Remove all manholes and handholes’ covers to verify that tubes and nozzles are not plugged with foreign materials.
- Wire brush any heavy scale on drum surfaces and remove them out.
- Close all inspection openings such as manholes, handholes, etc.
- Fill the boiler to the lower level of the water gauge glasses.
- Blow-down valves, scum valves, and gauge cocks should be checked and closed.
- Add the chemical and water mixture to the unit slowly and in small amounts to prevent excessive heat and turbulence. Add the mixture through the chemical feed or feed-water connections to a level just above the bottom of the gauge glass.
- Fire the boiler at a very low firing rate.
- When the boiler begins to produce steam (as seen through the open vents), allow the unit to steam freely for at least four hours. Watch the level in the gauge glass and always maintain normal water level (midpoint of the gauge glass). It will be necessary to add more boil-out solution when the water level falls.
- Close all vents.
- Keep the drum pressure at aprox.1 bar.
- After 8 hours, increase the pressure up to 20 % of the nominal working pressure.
- Continue boil out for at least 48 hours. During this period, open the blowdown valve intermittently and drain an amount of solution equal to one-half of the gauge glass every eight hours. Then refill the unit to the midpoint of the gauge glass with the boil-out solution.
- The boil out procedure should be continued until clean blowdown is observed.
- If clean blow down is observed, the boiler should be stopped and cool down gradually.
- Drain the water in accordance with local, national and international rules and regulations.
- Flush the boiler with clean water for at least 2 times.
- Fill the boiler with clean distillated water and start the firing up procedure.
- Start using treatment chemicals as per manufacturers’ instructions.
For stubborn oil deposits or heavy contamination, manual mechanical cleaning may be required. Use appropriate brushes, scrapers, or cleaning tools to remove the oil residues from internal surfaces, tubes, and components. Be careful not to damage the boiler surfaces during this process.
It’s important to note that the above steps provide a general guideline, and the specific cleaning process may vary depending on the type of marine boiler and the severity of contamination. It is recommended to consult the vessel’s operational and maintenance manuals, follow manufacturer recommendations, and seek expert assistance to ensure a safe and effective cleaning process tailored to your specific marine boiler system.
In conclusion, marine boiler oil contamination poses a significant threat to the efficiency, safety, and environmental sustainability of maritime operation. Timely detection, isolation, and remediation are vital to address this issue effectively. By following a systematic approach and implementing preventive measures, vessel operators can safeguard their marine boilers, reduce operational costs, and ensure a safe and smooth voyage. Prioritizing the elimination of oil contamination is not just an imperative for individual vessels but also for the overall well-being of the marine industry.
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