INSTITUTION OF DIAGNOSTIC ENGINEERS

 

Text Box: WHISPERS IN THE DARK I was Chief Engineer on a twin engine medium speed diesel ship which blacked out while under way. Cause of the fault was not obvious, partially due to some inaccurate reporting of occurrences which individually had to be investigated and eliminated, a procedure which had to be carried out bearing in mind the necessity of the safety of the ship and minimum interruption to the voyage. At around dusk, the vessel was proceeding at 14 knots in calm weather, with two generators on load and no reported extraneous activities likely to cause surges sufficient to trip any generators. The first indication of loss of power was the dimming of the lights throughout the ship and the slowing of the Main Engine, rapidly deteriorating to a total blackout and loss of all power except for 24V lighting and associated safety circuits. It was reported that, although the Main Engines had slowed, they did not stop immediately even though the fuel rail had been tripped. The emergency generator started correctly but failed to come on the board. Shortly after this, the vessels situation was as follows:- The Emergency Generator was running but not on load, the vessel had only 24V lighting and no machinery was running. A third main generator (that which had not been running) was started, put on load, and power restored to main lighting and other immediately necessary circuits, the emergency generator being stopped. At this time I attempted to collate a summary of what had happened, and a report came in stating that all lighting had not been lost, the reporter being quite adamant in his insistence, despite all other reports and indications' being to the contrary. This indicated a partial supply, and in conjunction with reports that the two generators initially running had surged a number of times and not dropped off the board, immediately gave rise to a number of further possibilities, although the speed of the incident was such that the sequence of events was hard to determine. The initial investigation confirmed that all relevant fuel tank quick closing valves were open. As the third generator had started with no problem on the same valve line up, the fuel system was then checked on the other two machines. Although both machines were found to have tripped due to over speed, there was no evidence of either fuel contamination or starvation anywhere in the system, both machines starting after reducing the governor motor position below that was found to be hard over. It should be noted that the vessel was running on diesel fuel at the time. This was accepted as initial proof of the integrity of the fuel system. With all three main generators available, the main engine systems were circulated and proven correct, the main engines inspected, run up and the vessel proceeded to safe anchorage, to carry out further generator trials. Each machine was loaded and unloaded a number of times with increasingly heavy loads, but all functioned correctly at all stages, no anomalies being found. They were then loaded to the same power as reported at failure, and in varying configurations all tested for load acceptance, all functioning correctly in all aspects. Any possibility of a surge (considered very unlikely) with the non operation of preferential trips and consequent loss of power was likewise simulated, all 3 steps functioning correctly. This basic proof of full and correct operation of the Main Generators was followed by blacking out the vessel to check the operation of the emergency generator. Every time the machine performed faultlessly. At this stage, it was decided to continue the voyage and go ahead with investigation at the same time, the only difference being that both fuel service tanks were opened instead of the normal one. We had by now a number of possibilities and problems arising;- 1) What had caused the initial blackout? 2) Why had partial power remained? 3) Why did the emergency generator fail to come on the board?? 4) Why had the main engines continued to run? While we were under way, an opportunity arose to try Diagnostic Engineering 33 to replicate the main engines situation at failure At five knots, both engines were tripped i.e. the fuel was shut off, with the CPPs remaining in their pitch position (at CPP oil failure the pitch remains at the position of the last telegraph signal even if the telegraph is brought to Stop.). It was noted that the main engines continued to rotate, due to the ship's way through the water causing the pitched propellers to drive the engines. In fact the engines had merely been rotating, not running, which eliminated one of the four questions, and the first reported anomaly i.e. why had the main engines continued to run - they hadn’t! Once alongside, investigation started into the partial power supply. This had at first been considered necessary for the main engines to have continued running, but as this had been disproved; the residual lighting report was now inconsistent. The officer concerned, after considerable questioning eventually toned his report down to “all the lights in the MCR” being on when he arrived, this was the 24V system, and so proved that partial supply from the emergency generator was unlikely in the extreme. This eliminated a second anomaly, i.e. why had partial power remained available - it hadn't. We then considered the failure of the emergency generator to load up, and simulated yet again various situations at the generators. This time with one of the initial generators on the board, the engine was stopped to cause the emergency generator to cut in. The engine stopped, all the meters returned to zero, and the breaker appeared disconnected. The Emergency Generator started but did not come on load. The second generator was running off load, an attempt to close the breaker was unsuccessful, and all power remained off Investigation showed that the breaker of the first generator was still connected to the busbars. We had then a possible chain of occurrences, not the initial cause but, at least a logical chain of subsequent events. 1. For whatever reason the first generator stopped, but the breaker did not clear the busbars. 2. This caused the second generator to "stall" as it tried to motor the first. 3. This surge caused the overload trips of the second generator to operate and drop the machine off the board. 4. As the governor linkage was fully over at that time to compensate for the load increase, when the load came off, the machine over sped. 5. Because the first generators breaker had not cleared the main busbars, the emergency generator interlocks would not allow it onto the board, resulting in the emergency generator starting but not supplying power. Thus another answer, this as to why the emergency generator failed to come on the board. This seemed to be a considerable breakthrough but still not the whole answer, the surging and over speeding of the generators being still some indication of a possible fuel problem. After having resolved all the known problems, it was decided to investigate the effect of gradual and total fuel starvation, a situation that would cause surging and over speed. The supply valve at the tank to the two generators (with one tank only supplying) was partially shut, the governor increasing the fuel rack until it reached 100. The stability of the engine speed at this state is erratic, and a small increase in flow causes the engine to surge and over speed. The over speed trip operated, cutting off air for combustion and causing a gradual slow down of the engine, reducing the main switchboard power rather than breaking from it immediately. The emergency generator, sensing the voltage reduction at the main switchboard, started correctly but was unable to close its breaker owing to power still being supplied by the main generators as they were slowing. The timed sequence controlling the emergency generator was then exceeded and the total blackout occurred. This was the cause - something had caused a fuel supply failure. A cleaning rag had been left inside the tank. Disturbed by the ship's movements it apparently had intermittently blocked the outlet from the tank to the generators, causing fluctuation in the fuel flow and then total blockage for a very short time, before being flushed clear of the suction line. With the wisdom of hindsight, it is possible that investigations should have started at the fuel tank, but as there were no obvious faults on the fuel system after thorough inspection, I considered the fault more likely to lie elsewhere.  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


 

 

 

 

 

 

 

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