Diesel engine inlet valves pitted (eroded)

[21121956]

Hello everybody:

In our power plant we run with large Diesel engines burning Bunker C.
Lately, we have found that the inlet valves show the classic pitting of erosion for cavitation (althoug, of course, vacuum can not build up in a turbocharged engine inlet ports). The affected areas go just from the edge of the sealing surface (which is in PERFECT condition) to the elbow between the valve head and the stem.

Furthermore, the area of the stems that remain in contact with the valves guide, presents a lot of linear scratches.

The questions: what could it be the source of such a damage? If the seal area is in perfect condition, can we only machine the damaged areas and reinstall the valves?

I need a hand from my friends of this forum

Thanks

 

[franzh]

I doubt if you could ever have manifold vacuum induce pitting of valves, that is usually a function of hydrodynamics, plus as you say, this engine would never see vacuum anyway, so this is mute.
The only time I have seen this type of pitting is with sulfur compounds in either lubricating oil or fuel. I would assume we can rule out fuel since it is not introduced in the inlet tract. Have you done an oil analysis?
Franz

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[ivymike]

No phase change from liquid to vapor and back, no cavitation.

The pits sound like corrosion. I assume you're not using EGR - is the assumption correct? Does your engine "backflow" gas from the combustion chamber into the intake? Has your oil become acidic? Is there any other source of sulphuric (or other) acid in the intake? Perhaps in the intake air stream (contaminated prior to ingestion)? How about salts or saltwater in the intake?

 

[SomeYahoo]

Sounds like something in the intake air to me also ivymike. Contaminants on the valve stem could also cause the linear scratches on the valve stem.

 

[21121956]

Thanks to all for your help. I really appreciate it.

Responding to your questions I can say:
- Our engines not use EGR system.
- The last laboratory report for the lube oil does not
shows any parameter out of rule, i.e.: TBN, water
content, etc.
- I can not assure that there is not contaminants or a
source of sulphuric acid in the intake air, but being
our plant a new one, I wonder, why untill now more than
a year of commercial operation this problem appears in
some (not all) engines that take the air from the same
ambient? It would be possible to think about a bad batch
of valves?

Once again, thanks.

 

[franzh]

I would suggest an analysis to determine if the pitting is abnormal. What does the manufacturer say?
Are all of them on one engine bad or just a select few? If all the engines are drawing air from the same source we can eliminate ambient environmental concerns. If the oil analysis is within parameters we can eliminate that too.
This is an engineering concern as well as an operational concern. Severe pitting can lead to localized stress points, potentially leading to valve head separation.
One thing else, if the engines are run in sequences with a significant down time, condensation can form in the intake tract, forming rust pockets in areas which would break loose and be ingested. I saw this once where a system had 6 Cat gensets, with 4 online at any given time while the other two were brought up on a load demand basis. The two offline gensets were alternated. Visible rust was seen in the intake tract.
Franz

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[MikeHalloran]

Are the engines' exhausts completely isolated from one another?



Mike Halloran
Pembroke Pines, FL, USA

 

[21121956]

Mike:

The engines exhausts (two groups of 8) are arranged in such a manner that the 8 stacks (chimneys) are installed side by side, separated aprox. 6 feet between each other; then, the plume of each chimney after a few feet from the chimney´s outlet forms a large plume.

So, the exhausts up to that point are independents.
The distance between each group of eight stacks is aprox. 290 feet.

Thanks

Ricardo

 

[MikeHalloran]

Multicylinder engines having the usual amount of valve overlap will ~always have one open flow path from exhaust to intake when they are stopped.

So, if the engines all draw air from a common plenum, and the plenum's inlet is significantly restricted, then the idle engines will have gas drawn through them in the reverse direction from the distal end of their stacks. If the stacks happen to be in the plume of a running engine, they will draw in exhaust gas.

Cooled exhaust gas is wet and corrosive.

I'm not saying that's what's happening, but it's a possibility.



Mike Halloran
Pembroke Pines, FL, USA

 

[icrman]

I would contact the manufacture, and ask what the material is the valves are supposed to be made of.
I would tend to guess the material and valves have been manufactured (out sourced) to a country where control over the processes aren't monitered very close. I have seen some interesting things with some of these imported materials.
Such as pockets of corrosion in stainless steel etc.
I just don't think they are as accurate with metal chemistry in some of these other countries. Since it is occuring in some engines and not all, I would suspect that.

Also you never mentioned the condition of cylinders and pistons. Scoring and scratching usually means, FOD.
What about after coolers? And the possibility of some leakage there?

 

[ivymike]

what's FOD?

 

[21121956]

icrman:

The cylinder liners and piston crowns in the cylinders where we found these damaged valves, remain in good conditions, free of scoring or scratching.

At this moment, with the analysis performer at site and with the help of the friends of this forum, we are almost agree that the possible source of this problem is a bad batch of valves; because the problem is not a generalized one.

Thanks

Ricardo

 

[TheBlacksmith]

FOD - Foreign Object Damage Usually denotes objects being sucked into jet enginee intakes with undesirable consequences.

Blacksmith

 

[icrman]

FOD can be in any thing that is mechanical or assembled by any means. Not just Jet engines.

Sounds like your on the right track Ricardo.