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Should overheating occur in the crankcase, say by failure of a bearing, then a hot spot is formed (typically over 400'C although experiments have shown two seperate temperature ranges, the other between 270 - 300'C>. Here lub oil falling on to the surface is vaporised ( in addition some is broken down to flammable gasses such as Hydrogen and acetylene), the vapour can then travel away from the hotspot where it will condense. The condensed droplets, in the form of a dense white mist, are very much smaller (6 to 10 microns) than the original and so have a high surface area to volume ratio. Ignition by a hot spot (generally of the flammable gasses which in turn ignite the fine droplets in the mist), which may be the same on that cause the original vaporisation, is now a possibility.
Oil mists are formed at temperatures of around 350oC
Ignition occurs at under 500oC
The white mist will increase in size and density until the lower flammability limit is exceeded (about 50mg/l is generally found in real situations ), the resultant explosion can vary from relatively mild with explosion speeds of a few inches per second and little heat and pressure rise. To severe with shock wave and detonation velocities of 1.5 to 2 miles per second and pressures of 30 atmospheres produced. This is the extreme case with pressures of 1.5 to 3.0 bar more normal raising to a maximum of 7.0 bar.
Pressure within crankcase
It can be seen that following the initial explosion there is a drop in pressure, if the initial explosion is not safely dealt with and damage to the crankcase closure occurs, it is possible that air can be drawn in so creating the environment for a second and possible larger explosion. The limiting factors for an explosion is the supply of fuel and the supply of oxygen, the air as shown can be drawn in by the slight vacuum created by the primary explosion. The supply of fuel may be created by the passage of the shockwave shattering the larger oil droplets into the small size that can readily combust.
By regulation,non returning relief doors must be fitted to the crankcase in order to relief the pressure of the initial wave but prevent a rapid ingress of air
Vapour extraction fans
These generally 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, this is seen to be more than compensated by the removal of flammable vapours and the reduction in oil leakage.
Crankcase doors
These when properly designed are made of about 3mm thick steel with a dished aspect and are capable of withstanding 12 bar pressure. They are securely dogged with a rubber seal arrangement.
Crankcase relief door (setting 1/15bar)
Due to the heavy force of momentum the gas shockwave is not easily deflected. Thus any safety device must allow for a gradual change in direction, and be of the non-return type to prevent air being drawn back into the crankcase
The original design was 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.
Explosion door
The valve disc is made of aluminium to reduce inertia. The oil wetted gauze provides a very effective flame trap This reduces the flame temperature from 1500'C to 250'C 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 normally ignitable. The gauze is generally 0.3mm with 40% excess clear areas over the valve.
Specifically the regulations are;
Non-return doors must be fitted to engines with a bore greater than 300mm, at each cylinder with a total area of 115sq.cm/m3 of gross crankcase volume. The outlets of these must be guard to protect personnel from flame. For engines between 150 to 300mm 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 normally be less than 9.13cm2/m3 of gross crankcase volume
Lub Oil drain pipes to the sump must extend below the surface and 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 and consideration should be given to detection of overheating (say by temperature measuring probes or thermal cameras) and the injection of inert gas.
Engines with a bore less than 300mm and a crankcase of robust construction may have an explosion door at either end
Means of detection of oil mist fitted.
Continuous extraction by exhauster fan may be used but this tends to be costly, flame gauzes must be fitted to all vents. Similarly a continuous supply of air can be used to reduce gas mist levels.
Crankcase oil mist detector (Obscuration)(set point 2.5% L.E.L)
graviner oil mist detector
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 and 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 and for the rotory valve to reach that sample point. For this reason this type of oil mist detector is not commonly used on higher speed engines.
Modern detectors often have the detection head mounted in the probe, the probe is able to determine the condition of the crankcase and output an electrical signal accordingly
The assembly consists of;
Extraction fan-draws the sample from the sample points through the reference and measuring tubes via non-return valves.
Rotary valve-This valve is externally accessible and is so marked so as to indicate which sample point is on line. In the event on 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 mirrors.
The photoelectric cell gives an output voltage proportional to the light falling on it. In this way the opacity of the sample is measured, the voltages generated in the cell in the measuring and reference tubes are compared in an electronic circuit. The difference is compared to a potentiometer varied setpoint which if exceed initiates an alarm circuit. The alarm circuit, dependant on installation, will generally declutch the drive to the rotary valve, give an output signal to the engineroom alarm monitoring system and an output to the engine protection system causing it to slowdown.
The rotary valve also has a position marked 'O' at which air is supplied to both tubes, and zero automatically (and manually if necessary) adjusted at each cycle. In addition at position 'L' an average sample of the crankcase is compared to air.
Crankcase oil mist detector (light scatter)
The disadvantage of obscuration types is that they are generally slow to operate and suffer from inaccuracies and false alarms caused by such things as a dirty lens.
Light scatter do not suffer from these problems, are faster reacting 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 have the sensor and a LED emiter in a single unit which may 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.
having these heads mounted on the engine removes the need for long sample tubes which add to the delay of mist detection.This makes the system much more suitable for use with medium and high speed engines were otherwise detection would be impossible.
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 at around x-head height which allows entry for inspection. Due to the curved design the door is able to withstand pressures well above the setpoint 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 activate an alarm condition, then personnel should take steps to ascertain if the fault is real. They should initially assumed that it is, the bridge should be informed and the engines slowed if the oil mist detector has not already done so. Should the bridge require manoeuvrability, and it is essential that the engine be operated then consideration of evacuation of the engineroom should be made. Otherwise the engine should be stopped and turned on gear until cooled.
The Graviner Oil Mist detector indicates via markings on the rotary valve which sample point has the high readings. By inspection of the graviner, and by viewing crankcase (or thrust, gearcase) bearing readings it is possible to ascertain whether a fault condition exists.
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 made to locate the hotspot, the engine should not be run until the fault has been rectified.
Crankcase safety fitting
For the purpose of this Section, starting air compressors are to be treated as auxiliary engines
Relief valves
Crankcases are to be provided with lightweight spring-loaded valves or other quick-acting and self-closing devices, of an approved type, to relieve the crankcases of pressure in the event of an internal explosion and 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 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 and damage arising from the emission of flame.
Number of relief valves
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 valves. For engines having 3, 5, 7, 9, etc., crank throws, the number of relief valves is not to be less than 2, 3, 4, 5, etc., respectively.
In engines having cylinders exceeding 300 mm bore at least one valve is to be fitted in way of each main crank throw.
Additional relief valves are to be fitted for separate spaces on the crankcase, such as gear or chaincases for camshaft or similar drives, when the gross volume of such spaces exceeds 0,6 m3.
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.
Vent pipes
Where crankcase vent pipes are fitted, they are to be made as small as practicable to minimize the inrush of air after an explosion. Vents from crankcases of main engines are to be led to a safe position on deck or other approved position.
If provision is made for the extraction of 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 two or more engines are installed, vent pipes, if fitted, and lubrication oil drain pipes are to be independent to avoid intercommunication between crankcases.
Alarms
Alarms giving warning of the overheating of engine running parts, indicators of excessive wear of thrusts and other parts, and 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.
Warning notice
A warning notice is to be fitted in a 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 and lighting
Where access to crankcase spaces is necessary for inspection purposes, suitably positioned rungs or equivalent arrangements are to be provided as considered appropriate.
When interior lighting is provided it is to be flameproof in relation to the interior and details are to be submitted for approval. No wiring is to be fitted inside the crankcase.
Fire-extinguishing system for scavenge manifolds
Crosshead type engine scavenge spaces in open connection with cylinders are to be provided with approved fixed or portable fire-extinguishing arran
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