Under the normal conditions the atmosphere in the crankcase when the engine is running contains a large amount of the relatively large oil droplets of size (200 micron) in warm air. Because of the droplets small surface area to volume ratio, the possibility of the ignition by a heat source is very low.
Should overheating occur in the crankcase, failure of a bearing e.g, then a hot spot is formed, normally over 400°C, Experiments have shown that two separate temperature ranges exist, the second being between 270 – 300°C.
Lubricating oil falling onto the hot surface will be vaporised, in addition some is broken down to flammable gasses such as Hydrogen and acetylene. The vapour then travels away from the hotspot and condenses. The condensed droplets, in the form of a dense white mist, are very much smaller (6 to 10 microns) than the original oil droplets and therefore have a high surface area to volume ratio. Ignition by a hot spot, which may be that which caused the original vaporisation, is now a possibility; the flammable gasses igniting which in turn ignite the fine droplets in the mist.
Oil mists are formed at temperatures of approximately 350°C
Ignition at <500°C
The white mist will increase in size & density until the lower flammability limit is exceeded, normally in the region of 50mg/l. The resultant explosion can vary from relatively mild with explosion speeds of a few millimetres per second & little heat & pressure rise, to severe with shock wave & detonation velocities of 2 to 3.3 kilometres per second & pressures of 30 atmospheres produced. The latter is the extreme case with pressures of 1.5 to 3.0 bar being the norm, rising to a maximum of 7.0 bar
It can be seen that following the initial explosion a drop in the pressure occurs; if the explosion is not safely dealt with & damage to the crankcase closure occurs, it is possible that air can be drawn into the crankcase so creating the environment for the secondary & possibly more violent explosion. The limiting factors for such explosions are the supply of fuel & of oxygen, as indicated air may be drawn in by the slight vacuum created by the primary explosion. The supply of the fuel may be created by the passage of the shockwave shattering the larger oil droplets into the small size that can readily combust.
Regulations, require the fitting of non-returning relief doors to the crankcase in order to relief the pressure of the initial wave but prevent a rapid ingress of air
Regulations – Crankcase Safety Fittings
For the purpose of this Section, starting air compressors are to be treated as the auxiliary engines
- Crankcases are to be provided with the lightweight spring-loaded valves or other quick-acting & self-closing devices, of an approved type, to relieve the crankcases of pressure in the event of an internal explosion & to prevent any inrush of air thereafter. The valves are to be designed to open at a pressure not greater than (>)0.2 bar.
- The valve lids are to be made of the ductile material capable of withstanding the shock of contact with stoppers at the full open position.
- The discharge from the valves is to be shielded by flame guard or flame trap to minimize the possibility of danger & damage arising from the emission of the flame.
Number of relief valves
- In engines having cylinders not exceeding (<)200 mm bore & 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 installed; 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 installed in way of each alternate crank throw with a minimum of 2 valves. For the engines having 3, 5, 7, 9, etc., crank throws, the number of the relief valves is not to be less than(<) 2, 3, 4, 5, etc., respectively.
- Non-return doors must be fitted to the engines with a bore greater than (>)300 mm, at each cylinder with a total area of 115sq.cm/m³ of gross crankcase volume. The outlets of these must be guarded to protect personnel from the flame. For engines between 150 to 300 mm bore, relief doors need only be fitted at either end. Below this bore there is no requirement. The total clear area through the relief valve should not usually be less than (<)9.13cm²/m³ of gross crankcase volume
- Additional relief valves are to be fitted for the separate spaces on the crankcase, such as gear or chain cases for the camshaft or similar drives, when the gross volume of such spaces exceeds (>)0.6 m³.
- Lube Oil drain pipes to the sump must extend below the surface & multi engine installations should have no connections between the sumps.
- Large engines, of more than (>)6 cylinders are recommended to have a diaphragm at mid-length & consideration should be given to detection of overheating (say by temperature measuring probes or thermal cameras) & the injection of inert gas.
- Engines with a bore less than (<)300 mm & a crankcase of robust construction may have an explosion door at either end.
- Means of detection of oil mist fitted.
Size of relief valves
- 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 each relief valve is to be not less than (<)45 cm².
- The free area of the relief valve is the minimum flow area at any section through the valve when the valve is fully open.
- In determining the volume of the crankcase for the purpose of calculating the combined free area of the crankcase relief valves, the volume of the stationary parts within the crankcase may be deducted from the total internal volume of the crankcase.
- Where crankcase vent pipes are installed, they are to be made as small as practicable to minimize the inrush of the air after an explosion. Vents from crankcases of main engines are to be led to a safe position on the deck or other approved position.
- If provision is made for the extraction of the gases from within the crankcase, e.g. for oil mist detection purposes, the vacuum within the crankcase is not to exceed (>)25 mm of water.
- Lubricating oil drain pipes from engine sump to drain tank are to be submerged at their outlet ends. Where 2 or more engines are installed, vent pipes, if installed, & lubrication oil drain pipes are to be independent to avoid intercommunication between crankcases.
- Alarms giving warning of the overheating of the engine running parts, indicators of the excessive wear of the thrusts & other parts, & crankcase oil mist detectors are recommended as means for reducing the explosion hazard. These devices should be arranged to give an indication of failure of the equipment or of the instrument being switched off when the engine is running.
- A warning notice is to be pasted on the prominent position, preferably on a crankcase door on each side of the engine, or alternatively at the engine room control station. This warning notice is to specify that whenever overheating is suspected in the crankcase, the crankcase doors or sight holes are not to be opened until a reasonable time has elapsed after stopping the engine, sufficient to permit adequate cooling within the crankcase.
Crankcase access & lighting
- Where access to crankcase spaces is required for inspection purposes, suitably positioned rungs or equivalent arrangements are to be provided as considered suitable.
- When interior lighting is provided it is to be flameproof in relation to the interior & details are to be submitted for approval. No wiring is to be fitted inside the crankcase.
Fire-extinguishing system for the scavenge manifolds
- Crosshead type engine scavenge spaces in open connection with the cylinders are to be provided with approved fixed or portable fire-extinguishing arrangements which are to be independent of the fire-extinguishing system of the engine room.
Crankcase doors are generally manufactured from 3mm thick steel with a dished aspect & are capable of withstanding 12 bar pressure. They are securely dogged with a rubber seal arrangement. The Crankcase relief doors are set to operated a pressures of 1/15 bar
The high momentum of the gas shock wave is not easily deflected & any safety device should allow for a gradual change in direction & be of the non-return type to prevent air being drawn back into the crankcase.
An early design consisted s of cardboard discs which provided no protection against the ingress of air after the initial explosion, in addition it was known for these discs to fail to rupture in the event of an explosion.
In modern designs The valve disc is made of aluminium to reduce inertia. The oil wetted gauze provides a very effective flame trap This decreases the flame temperature from 1500³C to 250³C in 0.5 m. The ideal location for this trap is within the crankcase where wetness can be ensured. The gas passing from the trap is not usually ignitable. The gauze is normally 0.3 mm with 40% excess clear areas over the valve.
Vapour extraction fans
These normally take the form of a small electrically driven fan. They are fitted with flame traps on the exhaust side.
Although the fans keep the crankcase at a slight negative pressure thereby increasing the risk of air being drawn in, it is considers that the removal of flammable vapours and the reduction in oil leakage more than compensates for this drawback.
Oil Mist Detectors
Crankcase oil mist detector (Obscuration) (set point 2.5% L.E.L)
Oil mists can be readily detected at concentrations well below that required for explosions, therefore automated detection of these oil mists can be an effective method of preventing explosions. Shown above is the GRAVINER oil mist detector. This is in common use in slow speed & high-speed engines. The disadvantage of this type if system is that there is a lag due to the time taken for the sample to be drawn from the unit & for the rotary valve to reach that sample point. For this reason this type of oil mist detector is not commonly used on higher speed engines.
The assembly consists of an Extraction fan that draws the sample from the sample points through the reference & measuring tubes via non-return valves.
Rotary valve – the valve is externally accessible & is marked to indicate which sample point is on line. In the event of a point exceeding the set point, the valve automatically locks onto that point so giving a clear indication of the locality of the fault condition.
Reference tube – measures the average density of the mist within the crankcase, as there will always be some mechanically generated mist.
Measuring tube – measures the opacity of the sample by means of a photoelectric cell as with the measuring cell. To exclude variables in lamps a single unit is used with beams directed down the tube by the mirrors.
The photoelectric cell produces an output voltage proportional to the light falling on it. In this way the capacity of the sample is measured, the voltages generated in the cell in the measuring & reference tubes are compared in an electronic circuit. The difference is compared to a potentiometer varied set point, which if exceed initiates an alarm circuit. The alarm circuit, dependant on the installation, will normally pass a signal to the engine protection system causing the engine to slowdown.
The rotary valve also has a position marked ‘O’ at which air is supplied to both tubes, & zero adjusted automatically, manually if required, on completion of each cycle. In addition at position ‘L’ an average sample of the crankcase is compared to the air.
Modern detectors often have the detection head mounted in the probe, the probe is able to determine the condition of the crankcase & output an electrical signal accordingly
Crankcase oil mist detector (light scatter)
The disadvantage of the obscuration types of the oil mist detectors is that they are generally slow to operate and suffer from inaccuracies and false alarms caused by such things as dirty lenses.
Light scatter detectors do not suffer from these problems, they react rapidly and do not need to set zero during engine operations.
The relationship between the light landing on the sensor is nearly proportional to the oil mist density therefore the unit can be calibrated in mg/l.
It is possible to combine the sensor and an LED emitter in a single unit, which may then be mounted on the crankcase. Several of these can be placed on the engine each with a unique address poled by a central control unit. The results of which may be displayed on the control room.
Mounting the heads on the engine removes the need for long sample tubes which add to the delay of mist detection. The system is therefore more suitable for use with medium and high speed engines, where detection becomes extremely difficult.
Crankcase doors (non relieving)
The older type consisted of doors lightly held by retaining clamps or clips. With doors of this type a pressure of 0.5psi would give a permanent set of about 25mm, the doors would be completely blown off by pressures of 2 to 3 psi.
Modern large slow speed engines have two types of crankcase door, a large securely held heavy mild steel square door, which allows good access for heavy maintenance. A second smaller round-dished aluminium door may be placed at around cross-head height, allowing entry for inspection. Due to the curved design the door is able to withstand pressures well above the set point for the relief doors.
Actions in the event of Oil Mist detection
The consequences of a crankcase explosion are extremely serious and the greatest possible caution in the actions taken should be exercised.
Should the oil mist detector indicate an alarm condition, personnel must initially accept that the alarm is genuine. The bridge should be informed and the engines slowed if this has not already been achieved by way of the oil mist detector alarm. Emergency stations should be piped and a fire party made ready adjacent to the engine room.
Should the bridge require manoeuvrability, and it is essential that the engine be operated then due consideration should be given to evacuating of the engine room. Otherwise the engine should be stopped and turned on gear until cooled.
Under no circumstances should any aperture be opened until the engine has sufficiently cooled, this is taken as normal operating temperatures as an explosion cannot occur when no part has a temperature above 270’C (Cool flame temperature)
Once cooled the engine can be opened and ventilated (the crankcase is an enclosed space).
An inspection should be undertaken to locate the hot spot. The engine should not be run until the fault has been rectified.
The GRAVINER Oil Mist detector indicates via markings on the rotary valve the sample point with a high reading. By inspection of the GRAVINER & by viewing crankcase (or thrust, gear-case) bearing readings it is possible to ascertain whether a fault condition exists.