The AIT was considered less than 900 K and ID was less than 16 millisecond. Therefore. ID at temperature of 573K (300℃) can be calculated as 0.37 sec conservatively from Equation (1). Accordingly, at the foregoing temperature ignition must occur if fuel oil mist exists.
Fuel oil will ignite first then lubricating oil will combust, in case that both fuel oil and lubricating oil exist. And it is noteworthy that if there were only fuel gas in manifold the flame would not propagate along the pipe, because the gas originally at manifold would be swept into the dead end and no distribution of combustibles along other potion of the manifold.
3.3 Why the Moment of Astern Is Dangerous
As mentioned in -2, the all accidents occurred at the moment of astern order after a long voyage. What is the reason? Two things can be considered.
(1) Temperature rise in air in manifold after a long voyage will make mist generation easier.
(2) Temperature rise will also make the temperature after compression much higher as shown in Figure 7.
Figure 7. The Relation between Initial Temperature and Final Temperature
As shown in Figure 7, the final maximum temperature will rise two times more than the rise in initial temperature. This result was calculated from same sample in 3.2.
4. PREVENTIVE MEASURES
4.1 Elimination of Combustibles
The ideal means to protect starting device against explosion would be complete elimination of combustibles. Filter between air compressor and air reservoir and filter between crankcase and starting air control valve may reduce the penetration of lubricating oil and system oil into air manifold to a certain extent, but it would not be perfect. A monitoring device for temperature of air in manifold may help detect the gas leak from cylinder due to malfunction of starting valve. And periodically careful inspection of inside air manifold can avoid deposition of combustibles.
4.2 Improvement of Design
Since the carryover of combustibles cannot be perfectly prevented, reduction of maximum temperature during charging process by improving design is considered practicable. To keep the final gas temperature lower the pressure rise rate would play an important role. Figure 8 shows the relation between the final temperature and pressure rise rate obtained from Equation (12) by changing the pressure rise rate while keep other parameter in sample explained in 3.2 unchanged.
Figure 8. Retation between Final Gas Temperature and Maximum Pressure Rise Rate
The relation between maximum pressure rise rate and maximum valve area and valve opening time are shown in Figure 9 and Figure 10.
Figure 9. Relation between Maximum Valve Area and Maximum Pressure Rise Rate
Figure 10. Relation between Valve Opening Time and Maximum Pressure Rise Rate
4.3 Rupture Disc as Emergency Means
To take measures like those described in 4.1 and 4.2 needs more time for making consensus among engine builders and ship owners. On the other hand, according to [11], when pressure rise rate due to explosion is less than 10 MPa/s, a rupture disc have relief area more than 0.025V2/3 can curb the maximum pressure nearly equal to the level of the static rupture pressure.
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