Natural gas fired engines driving gensets - life expectancy

[ebarba]

Hello All,

A couple of gensets will be operated at a constant load for 1500 hours a year on a 16h/day scheme. Engines are GM industrial powertrain Vortec 5.7L (350 cu.in), 8.1L (496 cu.in) and 8.1LT (496 cu.in, turbocharged), fired with Natural Gas and load is about 80%, 65% and 75% of standby rating respectively (see below).

The question is: what would be the expected engine life at the above conditions? By expected life I mean the time between major overhauls, with a decrease in the initial output power of no more than 10% to 12%. Of course, very close maintenance programs would be performed on these units. Is it nonsense to expect 13000 to 15000 hrs engine life, before having to overhaul it?

Additional application data:

GM Vortec 8.1LT-based genset
Actual Load=120kVA
Rated Standby power=163kVA
Rated Prime power=144kVA

GM Vortec 8.1L-based genset
Actual load=65kVA
Rated Standby power=94kVA
Rated Prime power=101kVA

GM Vortec 5.7L-based genset
Actual Load=60kVA
Rated Standby power=69kVA
Rated Prime power=75kVA

I've been trying to find an answer to this during the past three weeks, but I only got confused replies from prospect suppliers.

Does anybody know where I can find this info or has an answer for my question? Any help will be greatly appreciated.

Thanks!
Ernesto.

 

[Bribyk]

Sounds like you need better suppliers, they should be in the best position to know expected life. Have you tried to get a hold of GM about this?

Assuming these are 1800 rpm units, you're running 100-140 psi BMEP on these engines. Turbocharged engines built to run continuously on natural gas typically operate at 145-175 psi BMEP and the heads/top ends will last 16,000-24,000 hrs. I don't know if they've beefed up the heads for this type of service but 15,000 hrs sounds a little high to me.

We do rebuild these type of engines, I'll talk to some people around the shop and see if they've got a typical lifespan for you.

 

[rbulsara]

Please do not post the same question in multiple forums. Instead provide a link here to the other forum you posted the same question, as itsmoked suggested in other forum. So that all answers are in one place.

 

[Fmangas]

Ernesto
Using your numbers (1500hrs X 60MPH =90000 miles per year)This 5X what a normal auto should see in a year.

I work for a GM industrial engine distibutor and the GM factory warrentee is for 36 months or 3500 hours.

We have some engines that have been running 24/7 for 3 to 4 years with no major problems. So,I don't think it's unreasonable to expect 7500 to 10000 hours. I think that 13000 to 15000 hours is quite a lot to expect.

If there is any thing I can do to help please let me know

 

[ebarba]

Bribyk: Unfortunately, both genset manufacturers gave me completely different figures. One stated "about 10000hrs" and the other one "17000hrs".

All engines are 1800rpm, but there's something I don't understand: you say the heads/top ends will last 16000 to 24000 hrs, so why 15000 seem too much to you?

More or less the same question to Fmangas: 3 years @ 24/7 wouldn't mean 26000 hrs?

thanks!

 

[Bribyk]

The heads that will last 16-24000 hrs are on engines that were specifically made for the fuel and continuous operation (Caterpillar, Waukesha, Superior, etc...). Even then the heads are an issue and very touchy about what oil you use and the quality of the gas. These GM engines are being run fairly hard at a low speed (compared to automotive, almost lugging).

Word around here is about 18 months to 2 years is best case for these engines and this may include a cylinder head swap or two in that time. Usually, the valves recess into the head which bottoms out the lifters and wipes out the cam shaft. At that point, for the price of these engines they are pulled out and replaced with an exchange long block. 15,000 hours doesn't sound unreasonable but that will likely include a top end overhaul.

 

[Fmangas]

The standard GM warrentee is for 3 years or 3000 hours and thats what we limit our liability to.

I used the 3 years @ 24/7 as an example of what the engines are capable of. Will all GM engines last this long no, If there taken care of (Regular preventive maintenance, quality fuel, and properly adjusted fuel system)They are quite capable of a long life.

As to why the heads are usually the first to go Natural gas and LPG are dry fuels. There is no cooling property as there is with Gasoline. This tends to make the entire combustion area hotter then normal. If the fuel quality varies and the fuel system isn't adjusted to compensate. The exhaust seat gets too hot and starts to transfer metal from the seat to the valve. When the valve cools the metal particles fall off. This is when valve recession occurs.

 

[Bribyk]

Don't take this the wrong way, Fmangas. I'm not knocking your product in any way, just giving our experience with these applications. All of the gas engines have valve and head issues.

 

[Fmangas]

I completely agree with you everyone has head and valve problems. There usually caused by poor maintenance, poor quality fuel, and lack of proper fuel system adjustments.

If you take care of the above problems on any engine you generally will get a good life span.

 

[ebarba]

The units will be operated with natural gas from the a distribution network supplying some steel mills and other industrial activities in the area. Everybody says the gas is of good and constant quality (but I don't have any figures or quality parameters).

Assuming we change oil every 250hrs, install double oil filters and oversized cooling system, plus we monitor the systems remotely for temp. gas consumption, power output, running hours... will you trust these units will last up to 13000hrs? What would be the expected head life in the above scenario?

I'm pushing both prospect suppliers to give a clear indication of expected life and I'm asking a couple of other manufacturers for quotes and info... will post any relavant data I'll get.

Thanks to both of you for your help!

 

[MikeHalloran]

You're basically asking for ten years TBO for piston engines in two-shift duty.

I think that's, um, overly optimistic.

I'd distrust anyone who said it was likely.







Mike Halloran
Pembroke Pines, FL, USA

 

[Bribyk]

In industrial terms, these engines are cheap. Around here they are rarely monitored and an operator will usually just stop by to see if they're still running or not and whether there's oil in it. Don't go overboard trying to make them last forever, there's a point of diminishing returns where it's cheaper just to junk it and swap in a new longblock. We rarely rebuild them here, just buy a longblock and swap over the auxiliary and custom parts.

 

[itsmoked]

Is there some reason you don't just put in one gas turbine? That'll get you those hours no problem, and with little maintenance too.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ebarba]

Yes, I know these are not the perfect engines for the job, but the combination of required power output and working hours a year seem to be in "gray area" where automotive-derivate engines are too stressed and industrial engines are to expensive (the application requires a low cost solution, but it still has to make financial sense).

We also analyzed the microturbines, but they seem to make sense financially for applications with more than 5000hrs/year.

I'm getting the sense I'll have to choose between using "cheap" engines and hope they don't fail too often or use industrial engines and not maintain them as thoroughly as I should...

 

[j79guy]

Bribyk nails it on the head, around here, in Northern British Columbia, there are a good number of GM industrial Vortec, and Ford industrials running small gen sets, and when they stop running, a replacement long block is put in, and old one tossed in the recycle bin. Sounds wasteful, but this makes the most economic sense here.
You have to pay attention to your fuel composition. If the Butane levels are relatively high, then the engines can detonate, and obviously, not survive for long. If the fuel you are proposing to use is "sales gas", make sure it indeed is, and have a composition test done. This will help determine optimum ignition timing. As the engines "age", the savvy field mechanics here will retard the timing a bit at each maintenance inspection to ward off detonation. At the end of their usefull life, the power will drop off to the limit of acceptability, but at least they won't ping to death.
Same goes for A/F ratio. Have it checked once and a while, as the turbocharged 8.1L is quite sensitive to detonation, if run on the rich side.

Robin Sipe.

 

[itsmoked]

Also consider using all GM Vortec 8.1L-based gensets.

Not a 5.7. Then one long block will go into any one of them.

Alternatively could you get a deal on 4 sets - one as a spare.

Then if one needs service there is no panicked rush required.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ebarba]

Firts of all, thanks to everybody, you've been of great help (much more than the suppliers!)

Robin: do you have any figures of how often you change the longblocks and what is their % load with respect to prime power? This info would be extremely helpful.

So far the manufacturers either don't give a straight answer or just stick to the book, i.e. they just say the engine can't be operated continuously above 65% rated prime power.

One of them said they can endorse using the genset 8hrs/day at 100% rated prime power, but above that point they want me to downrate the genset. He didn't however tell me what would be the expected life of the engine should I make it work at our conditions...

So far, the informal information I have points to what Brybik said: maximum 6000 hrs total life and one head swap in between

Later today I'll call a major GM engine integrator to see what they tell me, I'll post the reply.

Thanks again,
Ernesto.

 

[j79guy]

ebarba; Tough one to pin down, due to so many variables, but here where I live, on average, the automotive based industrial longblocks are changed out every 1.5 years. (If run 24/7, with regularily scheduled maintenance.) In your case @ 1500 Run Hours/Year, perhaps you might expect eight years service, IF the units are run on "good" fuel, and maintained well.

Robin Sipe.

 

[ebarba]

Thanks Robin, that is very valuable data!

Yesterday I called one of the biggest GM engine integrators in the US, they told me they really don't see any problem with the units lasting up 6000 hrs, but they want me to talk to another person today.

Here in Italy the gas quality should be good, but I'm asking the distribution company for a composition sheet to be sure

 

[Fmangas]

ebarba
If you haven't already, you may want to talk to the guy's at Rama Motori. They have experience in your country with Industrial (GM,John Deere, and Briggs) gensets