[Possible Causes?? of ] Cracks in exhaust system 3

[enginepuppet]

I'm having trouble with our gas engine (recip). It was shutdown because of the following indications:
1st. Engine overspeed
2nd. Turbocharger temp high high
3rd. Exhaust manifold temp high high
4th. Jacket Water surge tank level low
5th. aux water surge tank low

and we found that there are cracks spotted on several areas:
- 3 inside the exhaust manifolds
- wastegate
- and exhaust outlet

what is the possible causes of these cracks?

Ask me question and I'll give you more details.

Thnks
EP

 

[WhizzMan]

The question is, are these causes, symptoms, or both? Did your coolant levels drop because of overheating, or did the low levels make it impossible to cool the engine properly?

Cracks usually occur because of either too much heat, localized hot spots or mechanical (vibration) issues. Since you have heat problems, the cracks are most likely caused by temperatures too high, or hot spots.

Find the cause of your overheating and you will find the cause of the cracks. It could be mixture, ignition timing, too much compression or a combination of these.

 

[enginepuppet]

The question is, are these causes, symptoms, or both?
--> I got the indications from the alarm list, so i would say, it was symptoms.

Did your coolant levels drop because of overheating, or did the low levels make it impossible to cool the engine properly?
--> I think it was the first one. The engine was overheated (to be exact it occurred mainly on exhaust system), and then the coolant levels drop. And it was found that there are cracks inside the exhaust manifolds which is surrounded by the jacket water system.

My thought, these cracks caused the coolant leak, thus coolant level drop.

But why these cracks appeared? I work on this now.

Any suggestion what should I check first?

Thank You

 

[enginepuppet]

So..

I went to read the reading sheet of the engine, and I found that the exhaust temperatures were around 1200-1300F. It is 100-200F higher than the design temperature (1120F).

does the fuel pressure play a big roll? The design fuel pressure states 25-50PSI, and our engine ran with the max pressure allowed 50PSI.

Can it be one of the reason?

thanks
EP

 

[MikeHalloran]

No.

Find a cranky old mechanic.



Mike Halloran
Pembroke Pines, FL, USA

 

[enginepuppet]

No??

Because I think:
lower fuel pressure --> leaner Burn --> lower temperature
higher fuel pressure --> richer burn --> higher temperature

Any suggestion(?)

Thanks
EP

 

[gruntguru]

Most likely cause of the sequence (except the overspeed) and cracking is lean mixture. I assume AFR is not monitored? Fuel pressure set to high limit may be a clue to some previous difficulty getting the mixture rich enough.

Engineering is the art of creating things you need, from things you can get.

 

[enginepuppet]

sorry, but i don't really understand. how can lean mixture cause the sequence and cracking?

really appreciate your responses.

Thanks
EP

 

[enginepuppet]

Okay, I got it, I was wrong from the beginning

"Because I think:
lower fuel pressure --> leaner Burn --> lower temperature
higher fuel pressure --> richer burn --> higher temperature"
---> this is WRONG

It should be the opposite!
Lean Mixture --> hotter combustion gas temperature

"Lean mixtures produce hotter combustion gases than a stoichiometric mixture, so much so that pistons can melt as a result. Rich mixtures produces cooler combustion gases than a stoichiometric mixture, primarily due to the excessive amount of carbon which oxidises to form carbon monoxide, rather than carbon dioxide." [quoted from Wikipedia]

thanks!
EP

 

[patprimmer]

Not exactly correct.

Lean mixture also burns slower so there is more still burning as it enters the exhaust.

Cracks are not generated by heat as such, but by expansion from heat only where that expansion creates stress as some areas expand more than others or some are free to move to accommodate the expansion and others are restrained.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[potteryshard]

With the tenative conclusion that your engine has been running lean long enough to cause exhaust manifold cracking, also comes the thought that the exhaust valves may be damaged.

The excessive heat developed by the slow burning mixture may also explain the overspeed; with the manifold and wastegate overheated, the wastegate would likely stick in position, allowing an overspeed on load reduction.

 

[ivymike]

so when you say "gas" engine, are you talking about gasoline or natural gas or something else?

If it's natural gas, then "lean" mixture on a lean burn engine typ. results in cooler exhaust.

"Leaner" mixture on a rich burn (stoich-ish) engine could potentially give hotter exhaust, depending on the setpoint (many "rich burn" nat gas engines are actually lean of stoich by enough that leaner would mean cooler exhaust)

 

[patprimmer]

Isaac is correct. Lean and EGT depends on how lean.

Also the American custom of calling motor spirits gas for gasoline is much more prone to confusion than the British custom of calling it petrol as obviously gas and petrol are different whereas gas (LPG) and gas (gasoline) are not unless extra qualifiers are used which typically they are not. I know I will never change it. Just an observation.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[enginepuppet]

Yes, I meant natural gas engine.
Sorry if I made some confusions.

 

[ivymike]

so do you know whether it's a stoich/rich engine, or a lean burn one?

 

[enginepuppet]

we still discuss the possible causes, today or tomorrow we will visit the engine for more investigation.

I'll try to get it as soon as possible, whether it's rich, stoich, or lean burn.

thanks

 

[enginepuppet]

So I just got informed, the AFR is 4.5 (natural gas engine).
What does it mean? Is it stochiometric, rich or lean?
Where can I reference it to?

thanks
EP

 

[ivymike]

well, that answer doesn't quite fit. The stoichiometric AFR for natural gas is about 17:1. A "stoich" or "rich burn" engine would normally be in the range of about 18:1, and a "lean burn" engine would be more like 24:1. (note that both of those figures are on the lean side of stoich, despite the names)

A rich burn engine might have ~4% oxygen in the tailpipe - that could be the source of your "4.5", maybe...

 

[ivymike]

btw, being at 1300F exhaust versus 1115 is an important difference. some common cast manifold materials are good to about 590-610C, and start to lose life quickly (due to oxidation problems) above that. 700C is a real stretch for many manifold materials, if that temp is sustained for even short amounts of time.

do you have a record of boost levels? if you have a stuck wastegate and you're boosting too much, (depending on the fuel system) you could simply be running the engine at too high a power output - resulting in higher exhaust temperatures and various failures. I know with some large gas engines used w/recip compressors it is not uncommon to "accidentally" adjust the wastegate to give higher boost and ~10% extra power output, to increase gas production.

Overspeeds could point to a problem with the governing system - but you haven't mentioned what the system is.

 

[enginepuppet]

I thought stoich 17.4:1 is valid for gasoline fuel (??)

I just posted the engine spec.