[Possible Causes?? of ] Cracks in exhaust system
[Possible Causes?? of ] Cracks in exhaust system
(OP)
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
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





RE: [Possible Causes?? of ] Cracks in exhaust system
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.
RE: [Possible Causes?? of ] Cracks in exhaust system
--> 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
RE: [Possible Causes?? of ] Cracks in exhaust system
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
RE: [Possible Causes?? of ] Cracks in exhaust system
Find a cranky old mechanic.
Mike Halloran
Pembroke Pines, FL, USA
RE: [Possible Causes?? of ] Cracks in exhaust system
Because I think:
lower fuel pressure --> leaner Burn --> lower temperature
higher fuel pressure --> richer burn --> higher temperature
Any suggestion(?)
Thanks
EP
RE: [Possible Causes?? of ] Cracks in exhaust system
Engineering is the art of creating things you need, from things you can get.
RE: [Possible Causes?? of ] Cracks in exhaust system
really appreciate your responses.
Thanks
EP
RE: [Possible Causes?? of ] Cracks in exhaust system
"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
RE: [Possible Causes?? of ] Cracks in exhaust system
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
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RE: [Possible Causes?? of ] Cracks in exhaust system
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.
RE: [Possible Causes?? of ] Cracks in exhaust system
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)
RE: [Possible Causes?? of ] Cracks in exhaust system
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
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RE: [Possible Causes?? of ] Cracks in exhaust system
Sorry if I made some confusions.
RE: [Possible Causes?? of ] Cracks in exhaust system
RE: [Possible Causes?? of ] Cracks in exhaust system
I'll try to get it as soon as possible, whether it's rich, stoich, or lean burn.
thanks
RE: [Possible Causes?? of ] Cracks in exhaust system
What does it mean? Is it stochiometric, rich or lean?
Where can I reference it to?
thanks
EP
RE: [Possible Causes?? of ] Cracks in exhaust system
A rich burn engine might have ~4% oxygen in the tailpipe - that could be the source of your "4.5", maybe...
RE: [Possible Causes?? of ] Cracks in exhaust system
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.
RE: [Possible Causes?? of ] Cracks in exhaust system
I just posted the engine spec.
RE: [Possible Causes?? of ] Cracks in exhaust system
RE: [Possible Causes?? of ] Cracks in exhaust system
RE: [Possible Causes?? of ] Cracks in exhaust system
Most NG engines will increase exhaust temp if the mixture goes leaner than design, because of slow burn (NG is particularly prone) and late heat release = higher exhaust temp.
Engineering is the art of creating things you need, from things you can get.
RE: [Possible Causes?? of ] Cracks in exhaust system
Mike Halloran
Pembroke Pines, FL, USA
RE: [Possible Causes?? of ] Cracks in exhaust system
yeah, and anything under 5% excess O2 is counted as a "rich burn" engine in the vernacular. Common configurations of "rich burn" engines are "catalyst" settings (~0.5% O2) and "best bsfc" (2%-4% O2). I think I mentioned before that "rich burn" ng engines are typically operated on the lean side of stoich.
so anyway, if exhaust temps are too high, check the following:
- A/F ratio correctly?
- engine load too high?
- exhaust restriction too high?
- wrong ignition timing (for fuel MN used)
RE: [Possible Causes?? of ] Cracks in exhaust system