Medium Voltage Diesel Generator Frame Cracking
Medium Voltage Diesel Generator Frame Cracking
(OP)
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.
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.





RE: Medium Voltage Diesel Generator Frame Cracking
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?
RE: Medium Voltage Diesel Generator Frame Cracking
RE: Medium Voltage Diesel Generator Frame Cracking
RE: Medium Voltage Diesel Generator Frame Cracking
RE: Medium Voltage Diesel Generator Frame Cracking
RE: Medium Voltage Diesel Generator Frame Cracking
RE: Medium Voltage Diesel Generator Frame Cracking
RE: Medium Voltage Diesel Generator Frame Cracking
RE: Medium Voltage Diesel Generator Frame Cracking
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Medium Voltage Diesel Generator Frame Cracking
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.
RE: Medium Voltage Diesel Generator Frame Cracking
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.
RE: Medium Voltage Diesel Generator Frame Cracking
Please flow catserveng's recommendations sequentially, from the foundation upwards. I suspect some installation deficiency made worse by four years hard running.
RE: Medium Voltage Diesel Generator Frame Cracking
RE: Medium Voltage Diesel Generator Frame Cracking
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
RE: Medium Voltage Diesel Generator Frame Cracking
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.
RE: Medium Voltage Diesel Generator Frame Cracking
RE: Medium Voltage Diesel Generator Frame Cracking
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
RE: Medium Voltage Diesel Generator Frame Cracking
RE: Medium Voltage Diesel Generator Frame Cracking