Crankcase explosions Under Under norma normall condi conditio tions ns the atmosp atmospher here e in the crank crankca case se when when the engine engine is running contains a large amount of relatively large oil droplets (200 micron) in warm air. Because of the droplets small surface area to volume ratio, the possibility of ignition by a heat source is very low. hould overheating occur in the crankcase, say by failure of a bearing, then a hot spot is formed (typically over !00"# although e$periments have shown two seperate temperature ranges, the other between 2%0 & '00"#. ere lub oil falling on to the surface is vaporised ( in addition some is broken down to flammable gasses such as Hydrogen and Hydrogen and acetylene), acetylene), the vapour can then travel away from the hotspot where it will condense. *he condensed droplets, in the form of a dense white mist, are very much smaller (+ to 0 microns) than the original and so have a high surface area to volume ratio. -gnition by a hot spot (generally of the flammable gasses which in turn ignite the fine droplets in the mist), which may be the same on that cause the original vaporisation, is now a possibility. il mists are formed at temperatures of around '/0 o# -gnition occurs at under /00 o# *he white mist will increase in sie and density until the lower flammability limit is e$ceed e$ceeded ed (about (about /0mg1l /0mg1l is genera generally lly found found in real real situat situation ions s ), the result resultant ant e$plosion can vary from relatively mild with e$plosion speeds of a few inches per second and little heat and pressure rise. *o severe with shock wave and detonation velocities velocities of ./ to 2 miles per second and pressures pressures of '0 atmospher atmospheres es produced. *his is the e$treme case with pressures of ./ to '.0 bar more normal raising to a ma$imum of %.0 bar.
-t can be seen that following the initial e$plosion there is a drop in pressure, if the initial e$plosion is not safely dealt with and damage to the crankcase closure occurs, it is possible that air can be drawn in so creating the environment for a second and possible larger e$plosion. *he limiting factors for an e$plosion is the supply of fuel
and the supply of o$ygen, the air as shown can be drawn in by the slight vacuum created by the primary e$plosion. *he supply of fuel may be created by the passage of the shockwave shattering the larger oil droplets into the small sie that can readily combust. By regulation,non returning relief doors must be fitted to the crankcase in order to relief the pressure of the initial wave but prevent a rapid ingress of air
Vapour extraction fans *hese generally take the form of a small electrically driven fan. *hey are fitted with flame traps on the e$haust side. lthough 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 *hese when properly designed are made of about 'mm thick s%teel with a dished aspect and are capable of withstanding 2 bar pressure. *hey are securely dogged with a rubber seal arrangement.
Crankcase relief door (setting 1/15bar) 3ue to the heavy force of momentum the gas shockwave is not easily deflected. *hus 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 *he original design was of cardboard discs which provided no protection against the ingress of air after the initial e$plosion, in addition it was known for these discs to fail to rupture in the event of an e$plosion.
*he valve disc is made of aluminium to reduce inertia. *he oil wetted gaue provides a very effective flame trap *his reduces the flame temperature from /00"# to 2/0"# in 0./ m. *he ideal location for this trap is within the crankcase where wetness can be ensured. *he gas passing from the trap is not normally ignitable. *he gaue is generally 0.'mm with !04 e$cess clear areas over the valve. Specifically the regulations are; 1. 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 olume. !he outlets of these must be guard to "rotect "ersonnel from flame. #or engines between 150 to 300mm relief doors need only be fitted at either end. $elow this bore there is no requirement. !he total clear area through the relief ale should not normally be less than %.13cm & /m3 of gross crankcase olume &. 'ub (il drain "i"es to the sum" must e)tend below the surface and multi engine installations should hae no connections between the sum"s 3. 'arge engines, of more than * cylinders are recommended to hae a dia"hragm at mid-length and consideration should be gien to detection of oerheating +say by tem"erature measuring "robes or thermal cameras and the inection of inert gas. . ngines with a bore less than 300mm and a crankcase of robust construction may hae an e)"losion door at either end 5. eans of detection of oil mist fitted.
#ontinuous e$traction by e$hauster fan may be used but this tends to be costly, flame gaues must be fitted to all vents. imilarly a continuous supply of air can be used to reduce gas mist levels.
Crankcase oil mist detector (Obscuration)(set point 25! "#")
il mists can be readily detected at concentrations well below that re5uired for e$plosions, therefore automated detection of these oil mists can be an effective method of preventing e$plosions hown above is the 6raviner oil mist detector. *his is in common use in slow speed and high speed engines. *he 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. 7or this reason this type of oil mist detector is not commonly used on higher speed engines. 8odern 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 *he assembly consists of9 #xtraction fan&draws the sample from the sample points through the reference and measuring tubes via non&return valves. $otary %al%e&*his valve is e$ternally accessible and is so marked so as to indicate which sample point is on line. -n the event on e$ceeding the set point , the valve automatically locks onto that point so giving a clear indication of the locality of the fault condition.
$eference tube&measures the average density of the mist within the crankcase, as there will always be some mechanically generated mist. 'easuring tube& measures the opacity of the sample by means of a photoelectric cell as with the measuring cell. *o e$clude variables in lamps a single unit is used with beams directed down the tube by mirrors. *he photoelectric cell gives an output voltage proportional to the light falling on it. -n 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. *he difference is compared to a potentiometer varied setpoint which if e$ceed initiates an alarm circuit. *he 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. *he rotary valve also has a position marked "" at which air is supplied to both tubes, and ero automatically (and manually if necessary) ad:usted at each cycle. -n addition at position ";" an average sample of the crankcase is compared to air.
Crankcase oil mist detector (ligt scatter) *he 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. ;ight scatter do not suffer from these problems, are faster reacting and do not need to set ero during engine operations.
*he relationship between the light landing on the sensor is nearly proportional to the oil mist density therefore the unit can be calibrated in mg1l. -t is possible to have the sensor and a ;<3 emiter in a single unit which may be mounted on the crankcase. everal of these can be placed on the engine each with a uni5ue address poled by a central control unit. *he 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.*his makes the system much more suitable for use with medium and high speed engines were otherwise detection would be impossible.
Crankcase doors (non relie%ing) *he older type conted of doors lightly held by retaining clamps or clips. =ith doors of this type a pressure of 0./psi would give a permanent set of about 2/mm, the doors would be completely blown off by pressures of 2 to ' psi 8odern large slow speed engines have two types of crankcase door, a large securely held heavy mild steel s5uare door which allows good access for heavy maintenance. second smaller round dished aluminium door at around $&head height which allows entry for inspection. 3ue to the curved design the door is able to withstand pressures well above the setpoint for the relief doors.
ctions in te e%ent of Oil 'ist detection *he conse5uences of a crankcase e$plosion are e$tremely serious and the greatest possible caution in the actions taken should be e$ercised. hould the oil mist detector activate an alarm condition, then personnel should take steps to ascertain if the fault is real. *hey 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. hould the bridge re5uire manoeuvrability, and it is essential that the engine be operated then consideration of evacuation of the engineroom should be made. therwise the engine should be stopped and turned on gear until cooled. *he 6raviner il 8ist 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 e$ists. Under no circumstances should any aperture be opened until the engine has sufficiently cooled, this is taken as normal operating temperatures as an e$plosion cannot occur when no part has a temperature above 2%0"# (#ool flame temperature) nce cooled the engine can be opened and ventilated (the crankcase is an enclosed space). n inspection should be made to locate the hotspot, the engine should not be run until the fault has been rectified.
Crankcase safety fitting 7or the purpose of this ection, starting air compressors are to be treated as au$iliary engines Relief valves Crankcases are to be provided with lightweight spring-loaded valves or other quick-acting and self-closing devices, of an approved type, to relieve the crankcases of pressure in the event of an internal explosion and to prevent any inrush of air thereafter. he valves are to be designed to open at a pressure not greater than !," bar. he valve lids are to be #ade of ductile #aterial capable of withstanding the shock of contact with stoppers at the full open position. he discharge fro# the valves is to be shielded by fla#e guard or fla#e trap to #ini#i$e the possibility of danger and da#age arising fro# the e#ission of fla#e. %u#ber of relief valves &n engines having cylinders not exceeding "!! ## bore and having a crankcase gross volu#e not exceeding !,' #(, relief valves #ay be o#itted. &n engines having cylinders exceeding "!! ## but not exceeding ")! ## bore, at least two relief valves are to be fitted; each valve is to be located at or near the ends of the crankcase. *here the engine has #ore than eight crank throws an additional valve is to be fitted near the centre of the engine. &n engines having cylinders exceeding ")! ## but not exceeding (!! ## bore, at least one relief valve is to be fitted in way of each alternate crank throw with a #ini#u# of two valves. +or engines having (, ), , , etc., crank throws, the nu#ber of relief valves is not to be less than ", (, , ), etc., respectively. &n engines having cylinders exceeding (!! ## bore at least one valve is to be fitted in way of each #ain crank throw. /dditional relief valves are to be fitted for separate spaces on the crankcase, such as gear or chaincases for ca#shaft or si#ilar drives, when the gross volu#e of such spaces exceeds !,' #(. Si$e of relief valves he co#bined free area of the crankcase relief valves fitted on an engine is to be not less than 00) c#"1#( based on the volu#e of the crankcase. he free area of each relief valve is to be not less than ) c#". •
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he free area of the relief valve is the #ini#u# flow area at any section through the valve when the valve is fully open. &n deter#ining the volu#e of the crankcase for the purpose of calculating the co#bined free area of the crankcase relief valves, the volu#e of the stationary parts within the crankcase #ay be deducted fro# the total internal volu#e of the crankcase. 2ent pipes *here crankcase vent pipes are fitted, they are to be #ade as s#all as practicable to #ini#i$e the inrush of air after an explosion. 2ents fro# crankcases of #ain engines are to be led to a safe position on deck or other approved position. &f provision is #ade for the extraction of gases fro# within the crankcase, e.g. for oil #ist detection purposes, the vacuu# within the crankcase is not to exceed ") ## of water. 3ubricating oil drain pipes fro# engine su#p to drain tank are to be sub#erged at their outlet ends. *here two or #ore engines are installed, vent pipes, if fitted, and lubrication oil drain pipes are to be independent to avoid interco##unication between crankcases. /lar#s /lar#s giving warning of the overheating of engine running parts, indicators of excessive wear of thrusts and other parts, and crankcase oil #ist detectors are reco##ended as #eans for reducing the explosion ha$ard. hese devices should be arranged to give an indication of failure of the equip#ent or of the instru#ent being switched off when the engine is running. *arning notice / warning notice is to be fitted in a pro#inent position, preferably on a crankcase door on each side of the engine, or alternatively at the engine roo# control station. his 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 ti#e has elapsed after stopping the engine, sufficient to per#it adequate cooling within the crankcase. Crankcase access and lighting *here access to crankcase spaces is necessary for inspection purposes, suitably positioned rungs or equivalent arrange#ents are to be provided as considered appropriate. *hen interior lighting is provided it is to be fla#eproof in relation to the interior and details are to be sub#itted for approval. %o wiring is to be fitted inside the crankcase. +ire-extinguishing syste# for scavenge #anifolds Crosshead type engine scavenge spaces in open connection with cylinders are to be provided with approved fixed or portable fire-extinguishing arrange#ents which are to be independent of the fire-extinguishing syste# of the engine roo#. 41534 ul6 •
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