Need advise for Niigata 8MG 25 BX

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Need advise for Niigata 8MG 25 BX

Postby EiKuD » Fri Aug 16, 2013 9:23 pm

Hie experts out there, I have a tug equipped with 2 sets of Niigata 8 MG 25 BX, it's a 40-year old tug, don't ask me why it is still in service, owner's sentimental tug....anyway, let keep to my point.

I have a hydro knock on one of the unit, con rod bent, liner bottom part broken etc, for those of you who are not familiar with this engine, it's 4 stroke medium speed.

now, boss wants to know the root cause of it, EVERYONE in the department knows is water knock, but question is WHERE DOES THE WATER COME FROM....alright i know you have bundles and bundles of ideas rushing up now and you are so ready to type them out one by one, but before you unleash your ideas, let me tell you our investigation finding first.

1) Scav. Manifold is filled 80% with FRESH WATER. Manifold position is below cylinder head, saying this is to let you know water must have come from cylinder head side, and not from the manifold to the head (water cannot flow anti gravity).

2) Thinking that water must be from T/C or Air cooler ? nah, T/C is water cooled but pressure test the casing no leaks (pressure test with hot water as well), air cooler is sea water cooled, also pressure tested no leaks.

3) Cylinder head - everyone knows water is from here, BUT ! pressure test the head, no leaks. Valves (suction / exhaust) seat rings are uncooled type. Injector sleeves are water cooled but no leaks from there during pressure test. Cooling water connector - 4 pcs per unit O-rings were badly squeezed and damaged though...HOWEVER, even the o-rings were the culprit, water will only leak OUT but not INTO combustion chamber, chamber is sealed with protruding land of cylinder head + cylinder head gasket (brass) !

4) Say if the sealing arrangement is compromised, then when engine runs, we should get a gas leak, but engine was running fine......with white smoke emitting though. I know I know, this is an indication of water leaking in but where does the water come from ?

God it kills so much of our brain cell, every time the top guys came in for root cause we cannot convince them, we were shot down by many questions which made our investigation look frail.

I need advise from you experts out there man, I really do, please provide constructive ones, rule out the sabotage part or the god factor, we are all technical guys and we require hard evidence to show those morons on the top...c'mon guys, help me out...

:cry: :cry: :cry:

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Re: Need advise for Niigata 8MG 25 BX

Postby JK » Sat Aug 17, 2013 10:42 am

not sure where you are, but in high humidity conditions, a lot of water can come out of the air at the aftercooler due to the compression and subsequent cooling.
Do you check the drains frequently to see if you have a water build up?

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Re: Need advise for Niigata 8MG 25 BX

Postby JollyJack » Sun Aug 18, 2013 7:17 am

Agreed, the aftercooler outlet air temperature must be above the dew point, otherwise the entrained water in the air will pour out, in a humid climate. Keep the scavenge temperature above 32C.
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The Dieselduck
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Re: Need advise for Niigata 8MG 25 BX

Postby The Dieselduck » Sun Aug 18, 2013 9:27 am

You say fresh water, as in coolant fresh water, or, distilled, sea water? Maybe test the water, if you treat your coolant water then is should be colored - chemical residue when dried. If it taste like sea water... and if its not either than as JK and JJ say, condensation, but this would normally only occur when running, which is kinda of strange that it would do damage to a running engine. Most likely its building up during shutdown, with damage done during start up. Actually this is what the root cause of the Carnival Splendor engine room fire was as determined by the USCG.

My two cents.
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Re: Need advise for Niigata 8MG 25 BX

Postby JK » Mon Aug 19, 2013 2:50 am

If your leak is from the JW cooling, the first indication would be a drop in your expansion tank or the expansion tank overflowing doing operation. If this is not happening, your system is tight.
80% full is a lot of water. Has the boat been shut down for an extended period of time and just been restarted?

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Re: Need advise for Niigata 8MG 25 BX

Postby JFC » Mon Aug 19, 2013 10:32 am

I had an experience similiar to this with a older MTU engine that had a water cooled exhaust manifold and the water only leaked into the engine when the engine was at operating temperature and running. Somehow the damage opened up When warmed and "pumped" water in. Took a long time to find, but eventually the failure got worse!! These things never ever fix themselves.

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Re: Need advise for Niigata 8MG 25 BX

Postby JK » Mon Aug 19, 2013 12:29 pm

I have heard of a cracked liner doing that. When the engine was shut down, everything stayed at level, when it was running the header tk would overflow because exhaust gases were pushing through and pressurizing the system.
The engineer who told me about it said they had a bugger of a time finding the problem.

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Re: Need advise for Niigata 8MG 25 BX

Postby Vegman » Thu Aug 22, 2013 1:30 am

Good Day Eikud,
A few questions to assist in fault finding and get ideas.
I am assuming the hydro lock happened upon start up ???
How long had the engine been shut down for , a few hours, more than a day?
Have you had any torrential rain recently, Is it p0ssible that rain water could have leaked into the exhaust uptake somehow?

I am guessing that you dont make it a practice to turn the engine with indicator cocks open or blow over on air with indicator cocks open before starting. If that is possible, you should consider making it a practice.

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Re: Need advise for Niigata 8MG 25 BX

Postby D Winsor » Sat Aug 24, 2013 3:57 pm

You also described a knock from the engine which leads me to think the problem may not be water leakage issue but fuel leakage from an injector that is cracked, leaking or stuck open.
Fuel can leak into a cylinder through an open or badly leaking injector if the static head pressure from the day tank or an electrically driven booster pump exceeds the spring pressure of the injection pump delivery valve spring when the engine is stopped and cause the knocking you hear when the engine is running, Excessive fuel leakage will wash away the lubrication between the piston and the liner causing a piston seizure and possible crankcase explosion which will cause the damage to the piston & connecting rod you described
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