(3) In November 1987, the ship Y equipped with a 12-cylinder 4-stroke V-type engine of 13239 kW lost the manoeuvrability due to the breakage of the starting air manifold caused by explosion in starting air piping during passing through the Singapore Channel at 9 knots. The explosion almost coincided with dead slow astern order for avoiding a potential collision with a former Soviet flagged ship. The junction pipe between the air mains and the portion from the cylinder cover to flame arrester at branch air pipe in No. 7 and No.12 were blown off.
(4) In Novenber 1997, the ship I equipped with 6-cylinder 2-stroke engine of 21917kw lost the manoeuvrability due to breakage of the automatic starting air stop valve and starting air control valve caused by explosion during ahead-stop-astern trial before entering the port of Kawasaki. The explosion almost coincided with the first astern order after engine stop. Figure 2 shows the broken bolts fixing the lower cover of the automatic starting air stop valve. Judging from the strength of these bolts, the instantaneous pressure could reach a level more than 30 MPa, namely, more than 10 times of initial pressure. This meant that there was a very high possibility that a detonation occurred. Furthermore, according to the chief engineer reports, the accident took place before injection of fuel oil, hence the backfire from cylinder can be ruled out as original cause of the explosion.
Figure 2. The Broken Bolts Fixing Lower Cover of Automatic Starting Air Stop Valve
(5) In March 1998, the ship D equipped with 6-cylinder 2-stroke engine of 21917kw lost the manoeuvrability due to breakage of the starting air control valve caused by explosion during ahead-stop-astern trial before entering the port of Shimotsu. The explosion was reported occurred at third astern order after engine stop.
3. CAUSE DETERMINATION
Detonation can roughly be categorised into gaseous detonation and film detonation. Gaseous detonation is a phenomenon of propagation of flame through detonable gas mixture. Film detonation is a phenomenon of propagation of flame along a pipe coated with a detonable liquid, such as oil. Film detonation, unlike gaseous detonation, does not require the distribution of detonable gases along the propagating pass. Detonable gases can be generated from detonable liquid while flame propagates in case of film detonation. Literatures have shown that lubricants can be ignited in the charging process by rapid compression and that a flame could propagate through the pipe if the pipe wall were coated with the lubricants. [2, 3]
At the present, we have no sufficient data to identify what kind of detonation or just ordinary explosions occurred in these accidents mentioned above. However, explosion in air manifold must contain three separate physical processes in the order listed below.
(1) Ignition of combustibles in the presence of oxygen.
(2) Flame propagation through the air manifold.
(3) An explosion or may be detonation of sufficient energy to burst the air manifold and some valves connected to it.
Without ignition there can be no flame propagation and then explosion. Therefore, first of all the ignition phenomenon will be focused on in this paper.
Auto-ignition temperature (AIT) for a given combustible material is defined as the temperature at which the material would ignite automatically. However AIT is affected by many factors, such as concentration of vapour of combustibles, volume of container, piessure, contents of oxygen, medium, etc.
Ignition delay (ID) for a given combustible material is defined as the period between the beginning of exposure to high temperature and the realisation of ignition. Semenov [4] has advocated the following equation for expressing the relation between ID and ambient temperature.
where
τ: ignition delay (sec.)
T: ambient temperature (K)
E: nominal activation energy (kJ/mol), approximately = 167.4 kJ/mol
B: constant
Accordingly, the relation could be illustrated in Figure 3.
Figure 3. Relation between ID and Ambient Temperature
It was postulated that if the gas temperature in the air manifold reached the auto-ignition temperature of the combustibles, at local pressure condition, and the duration of higher temperature than auto-ignition temperature was longer than the ignition delay of the combustibles, ignition would occur. Accordingly, analysis hereafter will be focused on possible combustibles, their auto-ignition conditions, and maximum instantaneous temperature in starting air manifold in rapid compression model.
However, many experimental results in literatures [5] indicated that there were considerable scatters in auto-ignition temperature and ignition delay under an 'identical condition'. In this sense, ignition should be considered as a probabilistic phenomenon. Hence discussion here should be understood as a general trend or qualitative result rather than deterministic conclusion.
