Crankcase Explosion Relief Valve Explained

What are crankcase explosion relief valves?

A crankcase explosion relief valve (CERV) is a safety device that is installed on medium to large size internal combustion engines. Typically, one CERV is fitted for each cylinder of an engine, but this depends upon the size of the engine. A CERV has three main functions:

• Relieving pressure in the event of crankcase over-pressurisation.
• Arresting flames to prevent them being discharged from the crankcase.
• Re-sealing the crankcase after an over-pressurisation event, preventing any air entering the crankcase. Resealing of the crankcase must occur after a crankcase explosion (pressure surge due to combustion) otherwise air may re-enter the crankcase causing a secondary explosion.

Good to know - a crankcase explosion relief valve is also known as a 'crankcase explosion door' or 'crankcase relief door', but all terms mean the same thing.

Marine Two-Stroke Diesel Engine Crankcase Explosion Relief Valves

What is a crankcase explosion?

A crankcase explosion is a sudden and violent combustion event that occurs within an internal combustion engine’s crankcase. Combustion requires fuel, oxygen, and heat, all of which can be found within a crankcase under certain conditions.

• Lubrication oil is used in large amount within the crankcase, it acts as fuel.
• Oxygen is found in air, which is always found within the crankcase.
• Heat can be provided from multiple sources, such as exhaust gases or friction (rubbing of engine components etc.).

Crankcase explosions are a rare but serious hazard that can occur in all internal combustion engines, particularly in older engines, engines that are poorly maintained, or engines operated under extreme conditions. The consequences of a crankcase explosion can be catastrophic, potentially leading to significant property damage, business interruption, and/or loss of life.

Good to know - you can visit our courses website if you wish to learn more about marine engineering topics. Our online video courses cover topics like large marine two-stroke diesel engines, fuel pumps, liferafts, ship terminology, steering gears, pumps, valves, boilers, and many other topics.

Mounted Crankcase Explosion Relief Valve (CERV)

What causes a crankcase explosion?

Several scenarios could lead to a crankcase explosion. These include:

• Worn or damaged piston rings – preventing adequate sealing of the piston within the cylinder liner. This could allow the fuel air mixture within the cylinder to pass from the combustion space into the crankcase; the fuel air mixture may then combust if it encounters a hot spot within the crankcase. On large marine two-stroke diesel engines, gases must leak past the piston rings and stuffing box prior to entering the crankcase.
• Hot spot within the crankcase – causing lubricating oil within the crankcase to vaporise (change from a liquid state to a vapour state); the vaporised oil then condenses in a cooler area of the crankcase leaving only a fine white oil mist. The aforementioned hot spot may then serve as an ignition source for the oil mist (once the oil mist cloud is large enough). Hotspots within an engine crankcase are often caused by worn crankshaft bearings due to a lack of lubrication or bearing surface irregularities. Large slow speed marine diesel two-stroke engines are fitted with oil mist detectors to detect the presence of oil mist within the crankcase. Should oil mist within the crankcase be detected, the oil mist detector activates an alarm within the engine control room.
• Blow-by within the crankcase – blow-by occurs when combustion gases leak past the piston rings and stuffing box (if installed), then into the crankcase. This can happen when the piston rings are worn, preventing proper sealing of the cylinder within the cylinder liner. This can also occur if the cylinder liner walls are damaged, which also prevents proper sealing. Blow-by typically occurs at high engine speeds or when the engine is heavily loaded. 

Good to know - a 'crankcase explosion' is sometimes referred to as an 'oil mist explosion'.

Good to know - lubrication oil must be heated to approx. 200⁰C before it vaporises, and it requires a hot spot of approx. 850⁰C for it to ignite. The oil mist droplets have a size of less than 10 microns.Good to know - a secondary crankcase explosion is usually of a much greater magnitude than a primary crankcase explosion. After a primary explosion, a vacuum within the crankcase exists, and this would draw in a significant amount of air if no CERVs are fitted.

What are the main components of a CERV?

A crankcase explosion relief valve is made up of relatively few components, these include:

• Valve Body – the main housing of the valve. It is made of a strong, durable material, such as aluminium or steel. The valve body is designed to withstand the high pressures that can occur during an explosion.
• Valve Plate (valve disc) – a thin circular-shaped piece of metal that is located inside the valve body. The valve plate is connected to a spring that keeps the valve closed under normal operating conditions. When the pressure in the crankcase increases, the spring is compressed, the valve plate moves away from the valve lid, thus effectively opening the valve.
• Valve Seat – the valve plate presses against the valve seat to close the valve.
• Flame Arrester – the flame arrester is a device that prevents flames from escaping the crankcase. It is typically manufactured as a fine mesh screen or a series of perforated plates. Flames are cooled as they pass through the flame arrester, which causes them to be extinguished prior to reaching the exterior of the engine. The flame arrester is sometimes oil saturated due to its location within the crankcase (CERV design dependent); this adds an additional flame cooling effect.
• Seal – a gasket or O-ring that is located between the valve seat and the valve plate. The seal reduces the likelihood of air or gas leaking from, or into, the crankcase.
• Spring – a metal coil that is located inside the valve body. The spring keeps the valve plate pressed against the valve seat under normal operating conditions, thus maintaining the valve in a closed position. When the pressure in the crankcase increases, the spring is compressed, and the valve opens.
• Deflector – directs any pressure pulse and flames downwards to protect nearby personnel; the gap through which the pressure pulse is directed is 120 degrees of the total valve circumference.

Good to know - a CERV is essentially a non-return valve i.e. it allows flow in only one direction.

Good to know - a CERV has no valve spindle.

Crankcase Explosion Relief Valve

How does a crankcase explosion relief valve work?

During normal operation of the engine the crankcase explosion relief valves remain closed, effectively sealing the crankcase, and maintaining the crankcase pressure within an acceptable range.

In the event of a crankcase explosion, a rapid increase in pressure occurs inside the crankcase. If the pressure is not relieved, engine seizure, component damage, and/or even crankcase rupture, may occur.

How a Crankcase Explosion Relief Valve Works

A crankcase explosion relief valve is designed to respond to any sudden pressure increase within the crankcase. When the crankcase pressure increases, it acts upon the CERV valve plate, overcomes the spring force keeping the valve closed, thus opening the valve.

Opening of the crankcase relief valve creates a passage for the high-pressure gases to escape from the crankcase; this release of pressure prevents the crankcase becoming over-pressurised, thus mitigating the effects of an explosion.

To prevent the propagation of flames from the crankcase, a CERV incorporates a flame arrester. This component cools the combustion gases and disrupts the flame front, preventing flames escaping the valve and potentially igniting surrounding combustible materials.

Once the crankcase pressure has decreased below the valve’s opening pressure set point (cracking pressure), the force exerted by the spring will cause the valve plate to press against the valve seat i.e. the valve closes, effectively sealing the crankcase and preventing any external air or gas from entering. Sealing of the crankcase after an explosion is important because otherwise fresh air may enter into the crankcase after a primary explosion, which may then lead to an additional larger secondary explosion.

Crankcase Explosion Relief Valve

What damage is caused without a CERV?

An engine experiencing a crankcase explosion without the presence of a CERV would most likely suffer catastrophic structural damage. If this scenario was to occur on a ship, it would leave the vessel without propulsion and maybe without electrical power. In particular, crankcase explosions can cause damage to the following engine components:

• Crankcase – the crankcase is the housing that encloses the engine's crankshaft and other moving parts. A crankcase explosion could rupture the crankcase, causing oil and debris to be discharged from the crankcase, potentially injuring nearby personnel and damaging the surrounding area. A failure of this kind could also lead to an engine room fire, which might cause damage to other ship systems.
• Conrod (connecting rod) failure – a connecting rod connects a piston to a crankshaft in almost all engines, however, in a large marine two stroke engine, the conrod connects to a crosshead, which then connects to a piston rod. A crankcase explosion could bend or break a connecting rod, causing the engine to seize.
• Piston damage – the pistons are the parts that move up and down within the cylinders to generate power. A crankcase explosion can damage the pistons due to misalignment, resulting in a lack of sealing between the piston rings and cylinder.

How can a crankcase explosion be avoided?

A CERV exists only to protect the engine if a crankcase explosion occurs. Crankcase explosions can be avoided by ensuring the engine is properly maintained and correctly operated. Maintenance should always follow the manufacturers’ maintenance guidelines.

Who invented the CERV?

Crankcase relief valves were developed in the 1950s by the British Internal Combustion Engine Relief Association (BICERA). BICERA was formed in 1948 in response to a series of crankcase explosions that occurred aboard ships. The organization conducted research into the causes of these explosions and developed a number of safety devices, including the crankcase relief valve.

The first crankcase explosion relief valve was patented by BICERA in 1954. The valve was designed to open and relieve pressure if there was crankcase explosion and to prevent flames from escaping the crankcase. Crankcase explosion relief valves are now a standard safety feature on almost all medium and large internal combustion engines.

Additional Resources

https://www.marinetopics.com/articles-desktop-en/crankcase-relief-valve/

http://www.machineryspaces.com/explosion-relief-valve.html

https://man-es.com/docs/default-source/service-letters/sl2009-512.pdf?sfvrsn=6ff9c151_8

https://chiefengineerlog.com/2022/10/19/what-you-need-to-know-about-crankcase-relief-valves/