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]

Good luck with your composition sheet. I tried to get some defined limits from my gas distributor and they don't have any hard limits, what you get is what you get. The various levels of suppliers will blend the gas to keep it near the distributor's contract values but there's no guarantee on what comes out the end of the pipe. I did get last year's high and low BTU values from my distributor but that doesn't mean it's going to be the same this year. It "should" be good gas but unless you sample yourself there's no way to tell.

 

[waross]

I am surprised that no-one has mentioned valve rotation. International Harvester used to instruct users to remove the positive valve rotators for propane service. Any valve rotation and the valve will grind its way into the seat.
And you may consider reviewing your loads and reporting in kW instead of KVA. Not the same. Depends on the characteristics of the load. KVA describes the capacity of the alternator but kW describes the power of the motor and the load on the motor. Generator/alternator KVA is typically 125% of engine kW. That's the rating. The actual values depend on the load and its characteristics.
kW will never by more than KVA but KVA is often more than kW. Sometimes a lot more.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ebarba]

Bill, The engine integrator says the heads have been modified for NG-Propane operation by using specific material as well as oversize components, but I don't know if they remove the valve rotators, I'll check with them next monday.

On the other hand, I've been reporting kVA power because, as far as I know, the engine will have to supply it and not only the real component (kW). It made sense to me, since the total energy (which should be real+reactive) has to come from somewhere and that's the prime mover. I'm not an expert in electricity issues... can someone confirm this?

Ernesto.

 

[Lou Scannon]

ebarba if the integrator you're talking to isn't [/i]Buck's Engines[/i], you might try them as well for some more real world experience.

 

[Fmangas]

Ebarba
Be careful of anyone who says they change the heads or any components on a new engine. This will immediately void the GM warranty. I have came across the integrator you are talking about. I have taken there heads apart and they have standard GM parts.

The heads that come from GM are already equipped with hardened seats, different valves, and different springs.

 

[ebarba]

Thanks for the warning! Don't worry I would never trust modified engines. So far, the integrators say they use industrial engines with heavy-duty heads, but everything comes from the GM factory and has GM's warranty.

 

[turbocohen]

I wrote to a colleague who uses the GM industrial blocks for continuous service to move natural gas at high altitude. The engines are low compression port injected closed loop lean burn turbo 8.1L @ 2800rpm & 0.6 bar boost. They nearly extended the life of the heads in comparison to the naturally aspirated stochio setups by going with lean burn turbo and the long blocks last an extra 1000 hours at best. The heads are the std GM ng/lp spec parts without rotators.
They retrofit a wide deep sump oil pan with a 15 qt oil reservior.

The lean setup saves fuel but they did not care, the lean closed loop setup is more reliable and costs less per hr.

 

[ebarba]

turbocohen, very interesting info. What is the life they're getting from the engines? Have any of the engines you're talking about been dismissed yet at their end of life?

We are not sure of using the engines in lean burn mode, as NOx will increase and we'll be near the permitted emission limit, but closed loop makes a lot of sense to me...

On the other hand and as promised, here's some feedback from distributors and integrators:

Integrator 1: "Engines at those conditions will last 10 years, if properly maintained"
Integrator 2: "Standar Engine life is 25000 hrs, at your conditions and with proper maintenance, you should get at least 18-20k hrs"

Both integrators said no head replace would be necessary...

Local distributor 1: "About 10000 hrs before you replace the head, then another 10000 hrs"

Genset manuf. 1: "17000 hrs, you will have to swap heads at 8000hrs"

Genset manuf. 2: "Don't even think about using the 8.1L turbo at 120kVA (they offered Doosan engine instead and said it'll last >20000hrs). The 8.1L nat. asp. will do for 60kVA and will last >13000 hrs"

My conclusion is that integrators trust the engines more than the distributors/genset manuf... maybe because the genset manuf. don't know the engines as well as the integrators. Maybe it's just a very unusual application we have here.

I'll try to knock on GM's door directly, but they are not having a great time lately, so I'm affraid it'll be harder than usual.

Thanks to everybody!

 

[MikeHalloran]

What assurances did you get >in writing<?




Mike Halloran
Pembroke Pines, FL, USA

 

[turbocohen]

Meant to say they nearly extended the life of the heads to that of the block..

One grenaded at 2K due to a coolant leak but they run about ~9k hrs, annual replacement. They run substantially derated for longevity.

As for lean burn, if the system is well designed then Nox is low without need for much aftertreatment. Real low. They use a large commercial three way cat for nmhc due to high oil consumption. One of the keys to longer valve life is to let the valve guides leak more than usual and bleed off a little air from the turbo to increase crank case ventilation which is not a problem with port injection.

My invlovement has been limited to the fuel system. The injectors do not seem to fail due to wear but to keep the engine running they filter and dry the fuel with a regenerating dessicant system that costs as much as the engine/compressor.

I think the life expectancy would be less with a genset in your application. Just my 0.02 dinars

 

[ebarba]

To Mike: as you may guess, no integrator nor distributor sent any written info, just information over the phone. The genset manufacturers sent e-mails with the expected life on it, but no reference was made in the formal quotations.

Once againg, real life experiece (turbocohen's) suggests that the information from the integrators and genset mfgr. is at best not reliable...

 

[MikeHalloran]

If it ain't written down, it didn't happen.

If TBO or uptime is that important to you, maybe it's time to find or develop a spreadsheet to balance out the lifecycle costs of:
- extended warranty
- service contract
- preemptive replacement/ planned downtime
- on condition maintenance
- more expensive engines
- fancier monitoring equipment
- cost of unplanned outages, including collateral damage

Surely someone has done it before.

It's all a crapshoot anyway, but if you've got an MBA beating on you, well, they love spreadsheets.



Mike Halloran
Pembroke Pines, FL, USA

 

[Bribyk]

This thread seems dead but I came across this link today that's on topic & may have some additional info.

http://www.swri.org/4org/d03/engres/spkeng/sprkign/454Cyl.htm

 

[ebarba]

For me this is VERY interesting info, I don't know what you think about it, but this tells me that a 496 engine should be able to deliver about 94kVA (75kW) for 15000 hours before it'll need overhaul. Not bad!

Thanks!
Ernesto.

 

[bcs5274]

here in the gas field we use gm industrial engines for running wellhead compressors, the 5.7l gm have run continuos under 50% load for 4 yrs or aprox. 22,000 hrs with no failures, under heavier loads less of coarse, no cylinder wall or bearing issues, (oil changed once a month,clean dry fuel) valves last more or less depending on fuel, use a higher detergent oil to prevent sludge build up, we use 15w40 oil,with a custom over size pan, my customers have had issues with the 8.1 running to lean on the back 2 cylinders because of intake design, i will check to see if this issue has been cured, if you did heads every 10,000 hrs and a long block every 20,000 you won't have any problems, running a constant rpm makes a big difference on life of these engines