Engine RPM for continuous duty generator

[oldfieldguy]

I used to 'know' that one did not use an 1800 RPM (or higher) engine for continuous duty generation.

I have an offshore facility that now has TWO dead 750 kw natural gas fueled generators. Both are forty years old 'big iron' 750 RPM units rated at 750 KW for when this was a major compressor station for an offshore natural gas gathering system. We no longer use it for that.

Total load is in the 200 KW range.

I am looking at replacement, but I'm finding a very small selection of NEW 250 KW units that do not have 1800-RPM engines.

Did we change our ideas about engine RPM in continuous service?

What's the thoughts these days?

old field guy

 

[TugboatEng]

High speed engines are the norm for applications up to 2500kW. They are much less expensive to purchase and operate and have completely displaced the medium speed engines in the lower power ratings.

 

[LittleInch]

Curiosity or is there a problem with the new units?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[oldfieldguy]

LittleInch-

No, I'm looking at buying replacements. I have extensive installed base of backup generation in my five-state area of responsibility, but most units are 1800 RPM units of various outputs.

I have three offshore platforms, all with oversized generators that are 20-40 years old. Both of those died on this one platform. I have one dead generator, one dead engine, the other engine is on its last legs, and the generator on the dead engine appears to have failed an insulation test.

I don't want to throw money at aged and oversized equipment, but many of the options I'm considering for replacement are 1800-RPM units. I have never installed one for continuous service, preferring 1200 RPM or below. Those are rather scare in the 200-300 KW range I need.

I'm a dinosaur. Times may have changed, so I'm soliciting opinions.

old field guy

 

[IBRCAN]

I do field service work for a major manufacturer of natural gas/diesel engines and generators. I’ve never seen anything in the 2-300kw range running at any speed other than 1800 rpm for 60 hz.
It’s not until you get into much larger units that 1200 rpm is common. Many gas units are offered in 1500 rpm versions only and utilize a gearbox to drive a 60hz generator.

 

[TugboatEng]

John Deere is the only truly reliable engine in that power range. I suggest looking for sets that have 6090 or 6135 model engines. We got 35k hours out of our fleet of 6081 engines (same as 6090). They can be in-frame overhauled. The major pain with these high speed engines are small oil pans and on engine filters which means short service intervals, typically 250-500 hours.

The 71 series Detroit Diesel is the last 1200 rpm set I have seen. A 16V-71 was rated at 350kW.

 

[oldfieldguy]

IBRCAN-
Thanks for the response. I've found a couple of 1200-RPM units on the used/surplus/rebuilt market, but nothing has made me move towards buying one.

A lot of my older stations still have original generation and those are 'big iron', heavy, slow - 750 RPM, 200 KW. Unfortunately I have none surplus to my needs.


old field guy

 

[TugboatEng]

Don't handicap yourself with those 71 series Detroits. Parts are starting to dry up and they can't really be properly in-framed. The dry fit cylinder liners fret in the bore which often required over boring and the use of oversized liners.

 

[oldfieldguy]

tug-

The 'D-word' never crossed my mind. I thought they were all diesel.

I need natural gas fueled, long legged, reliable solutions.

old field guy

 

[TugboatEng]

Dang it, I missed the gas detail. Deere had a gas engine lineup but I don't know if they do now. Cat has their 3400 series engines in that power range and those are solid units.

 

[Gr8blu]

oldfieldguy:
The overall trend has been to higher speed drivers (engines) to reduce size and weight of the train. After all, the main power generation for an offshore entity is not a paying customer, right?

That being said - pretty much all the current crop of "nat gas" drivers are running 1500-1800 rpm depending on desired output frequency (4P generators). Do they last as long as the older slow-speed stuff? Not really. Design life around 8 years, usually, because there is often the need to "up" the output right about then. Or the hull itself isn't really slated to last any longer than that.

You might get lucky, though - there are some "big iron" units still around, from the electrical side. Whether you can nab a replacement driver is a whole 'nother ball of wax!

Converting energy to motion for more than half a century

 

[catserveng]

Went thru this last year with some 300 kW old Worthington units. Unfortunately most manufacturers have little to offer in that range for packaged gensets NG fueled, at least in North America. We talked with CAT, Cummins, and Waukesha looking for something a lot more robust than an auto derivative type engine based unit.

We ended up buying industrial gas engines with an 1800 rpm rating and packaging up the sets ourselves to meet the customers desires.

We also looked at some older 300kW 1200 rpm units that were getting kicked out of California, but the rebuild cost for the older iron was pretty high, so we went with CAT 3406 gas engines and Kato tail ends.

Frankly in the end I think we ended up with a better package than what we could have bought anyway, since we were flexible on sizing cooling systems and some other support equipment, like oversized air cleaners and oil sumps. We also didn't have to live with the canned control systems that many manufacturers are using now.

If you looking for a lower speed robust unit, this is a hard to beat model, I have worked on hundreds of these over the years, and they just keep running. Not real fuel efficient or "clean", but tough for an engines its size,

https://swiftequipment.com/used-caterpillar-g379na-gas-engine-p-101183.html

Hope the helps, MikeL.

 

[TugboatEng]

I strongly second buying built to order packages over the units from the big OEM's. The big OEM units are complicated to setup, diagnose, and repair. The packaging exposes a lot of the electrical components to vibration and wear. We are using an outfit in the PNW that skid mounts the generator set and packages them with a Deep Sea Electronics controller. It doesn't get much simpler than that. They can work with you to give relay outputs to make integration into your ancient alarm system a breeze as well.

 

[itsmoked]

Maybe look out of the box?

Redox Cube

Consider a fuel cell stack. They usually run on N.G. and they're about 2x as efficient as engine driven generators. The referenced one above is 25kW/m[sup]3[/sup] and you stack as many as you need together to achieve your needed power.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[TugboatEng]

I like outside the box thinking, itsmoked.

Now you've got my gears turning. Some other options might include Capstone micro turbines. They use foil bearings and have some 30k hours between service. Solar is another more conventional option that fits in the 200kW range.

Turbines on their own can't approach IC engines or fuel cells for efficiency but if you have any air conditioning loads, Capstone pairs their package with absorption cycle refrigeration and claims very good efficiency.

Also, install is simplified as no cooling water piping is required.

 

[itsmoked]

Tug; My thinking is that if one is only seeing undesirable highspeed gensets with ~8 year lifetimes and that includes some serious wrenching at times to get anyway, then it's probably time for different technology. Micro turbines with nice lifetimes would be one focus and fuel cells would be another.

If emissions matter at all and they do to some companies that have reduced carbon footprint mandates then fuel cells would provide real reduction in that too.

I can also see adding a few extra units for N+0.2 or N+0.5 or whatever. You wouldn't even need N+1 like you would absolutely have to have for an I.C. based solution. All the fuel cells are not going to fail at once allowing more granularity in providing backups. I'd put in, say, 125% of what's needed then have them all on rotation. If one needs maintenance then it's removed from rotation.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[TugboatEng]

How would a fuel cell handle a big motor start, let's say 50% of total capacity?

 

[waross]

How would a fuel cell handle a big motor start, let's say 50% of total capacity?

Good question.
Not well.
I have been thinking about several mitigation schemes for a possible residential application.
Most motor starts are not time sensitive and may be delayed for several seconds. A cheap and dirty solution may be to ramp output up with a dummy load and use capacitors on the DC bus to supply the starting surge.
Without knowing the ramp time of the fuel cells, it is not possible to provide specifics, but I do not see any inherent contraindications to a solution when the ramp up information becomes available.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[itsmoked]

Three companies have fielded motor vehicles based on fuel cells. I suspect motor vehicles are one of the worst motor loads so I think they've solved this, probably thru capacitors and or a small amount of battery, naturally on the DC side.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[TugboatEng]

With variable load, batteries are a great solution. I like what Wrightspeed did by using a small turbine to charge battery on a garbage truck where regenerative breaking can do most of the work. Sadly, they chose the sexy turbine option when an internal combustion engine would have been much more efficient.

Fuel cells could pair well with a battery for similar variable load applications, I think. A battery would likely double the cost, though.

The answer here is that a brand spankin' new genset with a 3406 based engine but none of the other Cat accoutrements. This will cost you about $60-80k which is a quarter of what your low speed engines cost to overhaul so just install the package in such a way so it can be swung easily with a replacement and enjoy the EZ life.