Nozzle assemblies

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Fleet Engineer
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Nozzle assemblies

Postby Merlyn » Wed Mar 29, 2017 2:16 am

Saw what appeared to me to be a split MAN nozzle assembly on Martins Twitter which set the old brain off with yet another very occasional and intermittent bad starting problem with an old ( pre CR set up ) inline pump going way back over the years.
Very intermittently this engine would have a bad starting problem.
Always particularly the case from a cold start up and especially when left overnight or longer.
No stop control solenoid to bypass here, it's the old cable stop control setup.
Backing off the injector pipes at the injectors its blowing bubbles.
i.e. Air in the system.
Wait for the next bad start up, ( it's flying over so not a cranking problem ) and back off the fuel injector pump inlet pipe on the top gallery and note no air present whatsoever.
Back off the injector pipes and lo and behold air present.
How can this be?
Cut the fuelines into the HP injector pump and as usuall when fault finding air introduction in a system insert a length of plastic piping so you can see exactly what's going on.
Waiting for the next bad startup ( which can take several days ) you note no air whatsover in the top gallery fuel input piping.
Backing off a few injector pipes it immediately starts firing on the remaining cylinders, lock the pipes off and straight away in all all cylinders.
Never a problem under load or a hot restart.
So the pumps generating air, but how?
Off with the fuel delivery valve holders using your spline tool to check the poppet valves seats and their reseating compression springs.
No broken springs here but some look weak.
As a broken poppet valve compression spring has caused you problems in the past concerning misfires you naturally renew all springs.
Examining the valve seats and the mating seat faces you observe one seat in particular has not been seating correctly owing to an indent and therefore damage to the seating arrangement and possibly passing fueloil when in the shut position.
The three flutes that hold the poppet valve central also display scoring marks.
You therefore decide to replace all delivery valve holders.
Giving the engine time concerning the intermittent bad start ups the problem is much better but it's still there.
So, you decide the fault lies from ( and including ) the injection pump to the injectors areas.
So out with all the injectors, onto the pop tester and observe the spray patterns and the break off lift pressures.
All reasonably ok and within the limits.
One of the older hands notices a very fine haircrack in the nozzle body on one injector, it's not exposing the pintle and is difficult to see.
However he remembers that this has happened to him before in his career and talking about this and comparing notes it's decided that a strong possibility exists here of the engine " pumping up the top gallery "
At the time ( way back when in the sixties ) this had never happened to me although I have encountered it since but very very infrequently.
450 plus psi is ( sometimes only ) being pushed through the haircrack of the nozzle assembly, bypassing the top gallery delivery valve holder and introducing air into the top gallery thereby causing an intermittent bad start situation owing to the offending engine cylinder pumping up the injection pump top gallery thus causing the situation whereby the engine is being attempted to be started on air and not fueloil which of course is a no go situation.
It's decided as the injectors are high houred to replace them all and from that day on the problem disappeared entirely.
So yet another very important lesson learnt here and although not encountered again for many years pictures online of what I thought was a MAN nozzle assembly triggered it all off again but with the advent of CR it will ( over the years ) fade into the background no doubt as the years go by and be " oh that old setup "
The MAN nozzle?
Good challenge for Mr Belzona methinks.
Remembering The Good Old days, when Chiefs stood watches and all Torque settings were F.T.

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Big Pete
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Re: Nozzle assemblies

Postby Big Pete » Wed Mar 29, 2017 11:28 pm

I had a slow speed MAN nozzle crack on on one ship, the type that had a valve body, a transmission disc, the valve, a second transmissiion disc and then an atomiser, with a "Nozzle Nut" to hold the atomiser on and a second one to hold he valve on.
In this case the symptoms were an intermittent water flow in the return sight glass for the valve cooling, when the the cylinder fired Gas was pushed into the valve cooling system. Exhaust temps and peak pressure normal, and no oil returning with the cooling water, spotted it during night time rounds and we were arriving first thing in the morning so left it untill we were alongside and changed the whole injector in the morning.
It is always better to ask a stupid question than to do a stupid thing.

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