Crankcase Explosion – Explanation and Prevention (With Pictures)
Introduction
A crankcase explosion still remains a cause of concern in most large two and four-stroke engines. It is one of the most dangerous types of explosions that can occur in marine engines. It can claim lives as well as disable the propelling capacity of a vessel leaving it stranded wherever it occurs.
SOLAS regulations to prevent crankcase explosions came into place after a particularly devastative explosion that claimed the lives of 28 people onboard MV Reina Del Pacifico in 1947.[1]
Since then, many detection and preventive measures have been added to ships to eliminate or at least reduce the risk of this phenomenon. In this article, we shall take a look at what crankcase explosion is, how it occurs and what can be done to prevent it.
What is a crankcase explosion on a ship?
Crankcase explosion refers to the explosion that occurs in the crankcase of an engine due to the ignition of oil mist. A crankcase of an engine has all the necessary requirements for an explosion. We can understand it better with the help of a concept known as the fire triangle.
For any explosion/fire to occur, it needs three components. These are heat, fuel and oxygen and together, these three form what is known as the fire triangle.
However, oxygen and fuel need to exist in the right ratio for the fire to ignite. If it is either too lean (more oxygen, less fuel) or too rich (more fuel, less oxygen) it will not ignite.
This is why, even though the three components are always available in an engine crankcase, an explosion does not occur until the right air-fuel ratio is obtained. Let us take a look at the sequence of events that leads to a crankcase explosion.
How does a crankcase explosion occur?
As we have seen, a crankcase explosion requires three components: Heat, Oxygen and Fuel. Oxygen is always present in the crankcase through fresh air. Fuel in the case of crankcase explosion is lube oil vapour, and the necessary heat to ignite the air-fuel mixture is present when a hot spot develops.
Before we get to the sequence of events that lead to a crankcase explosion, we must understand what a hot spot is.
What is a hot spot and how does it occur?
A hot spot is any region inside the crankcase of an engine that has an unusually high temperature due to some defect or damage. It can occur for a variety of reasons. Some leading causes of hot spot development are:
- Bearing failure
- Blow past
- Scavenge fire
- Overloaded engine
There are other causes too (crankshaft misalignment, poor quality lube oil, insufficient clearance, etc.) but they are not as common. When such conditions occur in the engine, they can cause local hot spots that have a temperature range between 200 to 400 degrees Celsius.
The flash point of the main engine crankcase lubricating oil is about 220 degrees Celsius. So when a hot spot occurs it first evaporates the surrounding oil into vapour. When this oil reaches relatively colder areas of the engine, it condenses and forms a white mist that is capable of explosion.
As this process continues and the air-fuel mixture present inside the crankcase reaches within a certain limit (known as the combustible range between LEL and UEL), a small explosion occurs initially. This explosion is known as the primary explosion.
The primary explosion sends a shock wave through the engine causing a sudden increase in the pressure inside the crankcase. This pressure may cause the development of cracks in the engine structure.
This is why we have crankcase relief valves fitted into the main engine. When the internal pressure of the crankcase increases between 0.2-1.0 bar, the valve lifts and relieves the pressure inside the crankcase. It also prevents the ingress of fresh air as it is a non-return valve.
However, leaky piston glands, cracks and other sources may reintroduce air and fuel into the crankcase which may result in a secondary explosion. The secondary explosion is far more severe than the primary explosion.
This explosion can result in a huge loss of property and is very likely to be fatal to the personnel around it. The ship crew must do everything in their power to prevent both types of explosions.
Actual images from a crankcase explosion
Crankcase explosion safety devices
Engine manufacturers have built many safety devices into marine diesel engines to prevent the occurrence of a crankcase explosion. These devices must be maintained well and observed diligently to detect the formation of hot spots and oil mist in a crankcase to prompt timely action.
Some of the common safety devices are:
- Oil mist detector
- Crankcase relief valve
- Crankcase exhaust fan
- Breather pipe
- Lube oil temperature sensor
- Bearing temperature sensor
Oil mist detector (OMD)
The oil mist detector takes continuous samples from different areas of the engine crankcase and examines it for oil mist concentration. If it reaches a certain threshold, the oil mist detector gives an alarm and alerts the engine room crew.
Some engines may even slow down automatically to dead slow rpm upon receiving an OMD alarm.
Crankcase relief valve
The relief valves built into the side of the crankcase prevent internal pressure build-up as well as fresh air ingress into the crankcase. It also prevents the exit of flame outside of engine by using a flame arrester. If a crankcase explosion occurs, the complete flame arrester must be replaced.
Crankcase exhaust fan
A crankcase exhaust fan can remove the unwanted gases and oil mist that may have accumulated inside. This is an important safety device to keep gas concentration under safe limits and prevent crankcase explosion.
Breather pipe
Another effective way to prevent pressure build up in the crankcase is to have a breather pipe to prevent pressure build up inside the crankcase. It usually has a flame trap to prevent spread of fire to other areas.
Lube oil temperature sensor
The lube oil temperature can give important indications of rising lube oil temperatures and indicate the presence of a hot spot in the engine structure.
Bearing temperature sensor
In some cases, SOLAS also recommends the use of bearing temperature sensors to detect bearing failure which can lead to a hot spot and cause a subsequent crankcase explosion.
How to prevent crankcase explosion?
There are many tell-tale signs of an impending crankcase explosion which, when spotted on time, can help prevent it. Some of these signs require special instruments while others merely need careful observation.
The entire engine room crew must be familiar with these signs and symptoms for safe and incident-free operation of the main engine.
Crankcase explosion signs without any instruments:
- Check for hot spots with touch or infrared thermometer
- Irregular engine running
- Abnormal sound or vibration in the main engine
- Emergence of dense white mist or its smell from breather pipes or drain cocks
Crankcase explosion signs with instruments:
- Increase in bearing lube oil temperature
- Increase in bearing temperature
- Oil mist detector alarm
- Operation of crankcase relief valve and exit of smoke, sparks or oil
- Poor quality of crankcase lube oil
- Excessive clearances between mating parts such as piston/liner, piston rod/stuffingbox, engine bearings. etc.
To sum it up
The issue of crankcase explosions still persists. Between 1993 and 2001, a total of 143 crankcase explosion incidents were reported to Class LR[2]. At the time, 20% of the world’s ships were under LR.
Extrapolating to 100%, we can say that there were about 715 cases of crankcase explosion in these 11 years or 65 cases per year. But these statistics only talk about reported incidents where the damage was bad enough that it had to be reported to the class.
In reality, the numbers could be much higher and easily reach over 3 incidents per week. Even today in 2022, crankcase explosion remains a serious threat in diesel, gas and dual-fuel engines. It is a very real possibility even in engine rooms that are maintained exceptionally well.
But the fact remains that a crankcase explosion is a preventable incident. If the ship crew keeps a watchful eye on the different causes that can result in a crankcase explosion, it is possible to prevent explosions. Immediate and effective measures must follow detection to stop the condition from worsening.
We shall talk about these measures in a subsequent article on our website. Stay in touch with us through our social media on LinkedIn and Instagram to stay updated with the latest in the shipping industry.
References:
- J.G. Atherton, The scientific investigation of marine fires and explosions, Mar. Technol. Soc. J. 46 (6) (2012) 129–141.
- D. Woodyard, Pounder’s Marine Diesel Engines and Gas Turbines, Butterworth-Heinemann, 2009.
