When the engine is operating under normal conditions, the atmosphere in the crankcase typically has a significant quantity of relatively large oil droplets, of around 150 – 200 microns, floating around in the existing warm environment. The chance of the droplets being ignited by a heat source is extremely rare due to the small surface area relative to the total volume of the droplets.
When a hotspot is generated due an overheating event, for example the failure of a bearing or a bearing’s lubrication failure, the temperature will generally exceed 300 ºC (as per laboratory tests oil mist is formed at a temperature of about 350 ºC). In this case the lubricating oil that spills onto the heated surface will turn to vapour, oil mists generated by being boiled off can produce particles between 3 to 10 microns. This mist is visible and is known as a blue smoke. Temperature and area of surface contact affect the rate of oil mist generation. At this stage, a temperature as low as 150°C could result in ignition. Ignition by a hotspot, which may be that which triggered the initial vaporization, is now a possibility. This results in the combustible gasses igniting, as the ignition temperature for this type of oil mist can be extremely low depending on the type of oil being atomized which in turn ignites the fine droplets that are present in the mist. For this reason, regulations require that the engine must be equipped with an oil mist detection system that will detect an oil mist before it can reach levels where it saturates the atmosphere to such an extent that there is a risk of fire. For more information about oil mist detector follow this link.
The blue smoke will continue to grow in size and density until the lower flammability limit is exceeded. The explosion that occurs as a direct consequence can range from being relatively mild, with explosion speeds of a few millimeters per second and little rise in heat and pressure, to being severe, with shock wave and detonation velocities of 2 to 3.3 kilometers per second and pressures of 30 atmospheres produced.
It is clear that after the initial explosion, there is a drop in pressure; however, if the explosion is not dealt with in a safe manner and there is damage to the crankcase closure, it is possible that air could be drawn into the crankcase, thereby creating the environment for a secondary explosion that could be more violent. This can be seen by looking at how the pressure drops after the initial explosion. The availability of fuel and oxygen are the key elements that control the size of explosions of this type; however, it is possible that air will be pulled in due to the minor vacuum that is generated by the primary explosion. It’s possible that the passage of the shockwave may break the bigger oil droplets into smaller sizes that are more easily combustible, which will result in the creation of a supply of fuel.
For this reason, all organizations involved set a set of rules in this regard. The most important is that crankcases are required to have lightweight spring-loaded valves or other quick-acting and self-closing devices installed so that pressure can be released from the crankcases in the event of an internal explosion while also preventing any subsequent inrush of air. The valves are required to have a design and construction that allows them to open rapidly and be fully open at a pressure that is not greater than 0.2 bar.
The number of relief valves varies with the engine size. For example, in engines having cylinders not exceeding 200 mm bore and having a crankcase gross volume not exceeding 0,6 m3, relief valves may be omitted. In engines having cylinders exceeding 200 mm but not exceeding 250 mm bore, at least two relief valves are to be fitted; each valve is to be located at or near the ends of the crankcase. Where the engine has more than eight crank throws an additional valve is to be fitted near the centre of the engine. In engines having cylinders exceeding 250 mm but not exceeding 300 mm bore, at least one relief valve is to be fitted in way of each alternate crank throw with a minimum of two valve. In engines having cylinders exceeding 300 mm bore at least one valve is to be fitted in way of each main crank throw.
The combined free area of the crankcase relief valves fitted on an engine is to be not less than 115 cm2/m3 based on the volume of the crankcase. The free area of the relief valve is the minimum flow area at any section through the valve when the valve is fully open.
As part of their maintenance, during running of the engine, check if there are any leaks. If a leak occurs, replace the O-ring inside the relief valve. If work involving risks of mechanical damage to the flame arrester has taken place, a visual inspection of the flame arrester should always be performed before starting the engine.
Check on the whole circumference that all the plates in the flame arrester are evenly distributed and that no local openings exist.
If one or more plates in the flame arrester are damaged, the relief valve must be disassembled and the flame arrester replaced. The complete flame arrester has to be replaced after a crankcase explosion.
The video below is self explanatory regarding onboard relief valve maintenance.
These relief valves can’t be tested and calibrated onboard due lack of means and testing equipment. The calibration is done in the manufacturing facility and test of these valves require specialized equipment as can be seen in below training video.
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Source and Bibliography:
- YouTube video training credit – Anold Kim; Informative clips; Schaller Automation
- DNV and Lloyd Register rules and regulations
- Photo credit: researchgate.net and chiefengineerlog.com