Medium Voltage Diesel Generator Frame Cracking

[kmh1]

We are working on a situation at a remote mine site where four 4160 Volt, 2 MW Caterpillar diesel generators are operated in parallel in an off-grid configuration to provide power for the site. Over the span of many months, several of the generators have been experiencing cracking of the stator frame housings for no apparent reason. Investigation has not revealed any specific cause to date.

Has anyone seen this problem or offer a suggestion on where to look for a problem? Alignment appears to be accurate and synchronizing equipment appears to be working correctly.

 

[Hoxton]

How old are they -how many years since they were commissioned?

How many hours has each one run? How many starts?

What load do they normally run at? What is the site rating? Are they debated for altitude / ambient temperature?

Can we have a photo showing where the cracks are - are they in a similar place?

 

[miningman]

Let me guess, two running at all times, one on standby and one undergoing maintenance at all times. Another educated guess is that this is a relatively new installation and is located in a part of the world where normal North American construction standards are difficult to achieve and maintain. Does the concrete floor show signs of cracking / differential settlement?? Was vibration isolation installed per spec AND good mining standards ( believe me , they are not always the same ). You may well be looking at a mechanical issue rather than electrical.

 

[kmh1]

I have attached a photo of a typical crack. They are 3516C machines and were commissioned in 2010 but I don't have specific information on hours or number of starts. They run an off-grid system so would cycle as needed based on a set priority. The crack appears in the same place on all machines. We have no historical loading information for the machines but they are not considered to be derated for altitude or ambient temperature.

 

[kmh1]

The site is in Northern Ontario and presumably the installation work was done by qualified tradesmen per certified installation drawings. The mechanical inspection did not uncover anything obvious including a review of the foundation and alignment procedures. This is installed at surface at a large underground gold mine site.

 

[kmh1]

Also the units are installed in vendor supplied modular generator enclosures with generator mounting rails and linear spring vibration isolators. They have an 1850 ekw prime rating.

 

[miningman]

OK my WAG guesses werent spot on but at four years old, if my assumption of two running at all times is close, each unit probably has around 17,500- 20,000 hours running time. This is getting really close to the 22,000 hour top end overhaul time recommended by CAT. Northern Ontario might be remote relative to civilisation but its not so remote that you couldnt get CAT on site to review the damage. They have probably encountered this elsewhere at some point in time. Plus your comment " presumably the installation work was done by qualified tradesmen per certified installation drawings" may not be a valid assumption. CAT personnel may spot something that might not be obvious to lesser experienced personnel. I still think youve got mechanical rather than electrical issues. I have run 3512s in very remote locations so I do have some operating expertise with these type of things.

 

[kmh1]

CAT done an assessment and haven't come up with an specific explanation either. Apparently they haven't seen this previously. I am thinking mechanical as well but I still haven't ruled out something related to transient oscillations between synchronized machines, possibly due to starting of relatively large fan loads.

 

[miningman]

Well personally I would never install units like these on anything other than a good solid concrete foundation......... I think there are threads on Eng Tips which refer to the required mass of concrete for an installation like this.......... but now it seems that these units are in a glorified Sea Container. Were the containers properly levelled during installation?? Was the level rechecked every spring after the thaw?? If Surveyors go out there tomorrow and determine that the units are say, say 5 degrees off of horizontal , you've lost all supplier warranty.

 

[Skogsgurra]

catserveng and waross - where are you?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[catserveng]

Hard to tell from the one picture, but do the generator feet overhang the frame?

Has a vibration analysis been performed?

When you say "alignment checked ok" how was the check performed, the feeler gauge in between the generator and flywheel housing, or with a dial indicator or laser?

Are the cross braces at the rear of the generator base bolted or welded?

I have ran into this type of failure before, was related to base and mounting issues, most recent was a site with 5 units that had originally shipped with low voltage generators and had been field fitted (by dealer) with MV generators before being shipped.

Cracks in the area your picture shows are usually due the a flimsy set of base rails, or a soft foot problem. Problems with synchronizing usually shows up as excessive wear or damage to the torsional coupling. 3516C's usually have the rubber sections and the aluminum drive ring, have these been checked? I never seen load transients or oscillations cause cracking to a generator frame, and have seen some pretty severe load swings, like on crushing and drilling equipment. Severe cyclic loading usually shows us as excessive wear on the torsional coupling, misfire and poor combustion will also cause coupling wear/failure.

Do you have a copy of the CAT Application and Installation Guide? If not get the local dealer to get you one and take a good look at the section on mounting and alignment.

My personal opinion is that current base rails for MV generator sets by CAT are not as stiff as they should be, and I really dislike the base rails with the bolted cross members, no one checks them on a regular basis and if they loosen even a small amount your get flexing of the base rails. It gets worse if the vibration isolators are not properly adjusted.

I have worked on lots of units in containers over the years, up to 2.5MW standbys and 2MW prime units in long term rentals and jobs like mine sites and utility grid support. For the most part they do ok, but as noted above, they have to be level, and supported properly to allow the base rails to maintain alignment.

The old oil field bases were massive in comparison, but took huge amounts of abuse, like dragging them off transport trucks, poorly leveled foundations (usually a hard dirt patch that mostly hit both sides evenly) and "thumbnail" leveling. But they ran and ran with not many problems.

I don't use the CAT method for aligning close coupled generators, too many times have found it not to provide good operation, my preference is a laser tool. And I always try to do a soft foot check (procedure is in the A&I Guide mentioned above). I also try to regularly do vibration measurements, especially on MV machines, as my experience is that they have more problems in the field. When you make the measurements pay attention to the frequencies below first order, low frequency vibrations at higher magnitudes than first order usually indicate the frame is bouncing on the isolators. CAT specifies limits for overall and first and second orders, but seems a lot of folks overlook the lower frequency issues until something breaks.

Hope that helps, Mike L.

 

[eboutin]

Thanks for the previous replies. I am also working on this evaluation from the mechanical side.
The bases are 3' thick, properly reinforced concrete on verified compacted gravel. The containers are framed according to Cat mounting recommendations and appear to be still structurally tight.
There is no sign that the generator to engine alignment was done, only air gap measurements that could only be at the bearing supported end away from the engine. There are simple vibration monitors on board, but they just indicate very high vibration status, not measurement.
While on site, we tried putting a generator on line to see if there were any synchronization issues and found that there were no shocks during the loading process.
Right now the most likely cause of the stator failure is the soft foot.

 

[Hoxton]

Interesting in that the photo shows the crack coming from (or to) a square edged hole in the frame? Stress point?

Please flow catserveng's recommendations sequentially, from the foundation upwards. I suspect some installation deficiency made worse by four years hard running.

 

[Hoxton]

Follow not flow

 

[waross]

Many years ago at a mine site, the construction power was supplied by gen-sets.
The crankshaft bearings were failing in about six weeks.
The office building had a large electric heating load. The load was controlled on a time proportioning system with zero point switching. The result was that the load was on for a few cycles and then off for a few cycles. This was taking out the bearings in short order.
Do you have any loads that may be cycling rapidly?
Are these Cat sets or Cat engines with generator ends from the lowest bidder?
Does the Sea-Can vibrate?

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

 

[desertfox]

Hi

The crack begins and runs from the square edged window which I believe is a stress raiser, also the orientation of the crack makes me think it is due to torsional loading/cycling either during operation and/or stop/start.

 

[squeeky]

I agree with this more likely being a mechanical issue. The fact that it is moving to corners a weld points may be a sign that it is built in stress from manufacture. My experience with pumps mounted near each other is that the standby units have more vibration damange on bearings and fitting than the running units. Are the running hours nearly the same? Then I agree with the over hang observation. Any torsion load could cause the stress seperation. Even an inepensive vibration meter could give you a comparison between machine. Please keep us posted as I have many sites running with standby Diesel Alternator sets.

 

[MikeHalloran]

The square edged hole sure doesn't help.
See "BOAC Comet".
Or just contrast it with the other hole.

That makes it susceptible to fatigue damage from anything that results in a net torque in the shell.
A more rigid foundation or skid or more mounts and/or a more careful leveling procedure might have helped the shell survive longer.



Mike Halloran
Pembroke Pines, FL, USA

 

[kmh1]

We believe it to be an alignment issue from installation and/or a soft foot on the generator. The engine/generator sets are CAT supplied in standard winterized enclosures with substantial mounting rails and foundations that meet or exceed the specifications in CAT's mounting guide and there does not appear to be any excessive vibration during operation. We have also recommended enhanced electrical system monitoring to record any possible load or start-up transients.

 

[Compositepro]

Here are a couple thoughts that may or may no help. If the frame is going to crack, it will crack where it has. If one point of attachment of this frame is more rigid than the other points more load will concentrated at the most rigid point. Since this frame is so rigid, any relative flexibility in the attachments or foundation could place almost all the load on one point.