Cooling tower Fan failures
Cooling tower Fan failures
(OP)
Hi
I'm working on the case of our cooling tower fan (Hamon-Sobelco fan) whom brake in the blade holder (good term?).
Blade holder is cracking at bolt hole (which retain blades) and advance in fatigue propagation. It is 40' diameter and 1 inch thick.
What could be the cause of failure ? Vibration ?(sure) Flow and pressure of air sucked? I don't know where to start, I'm a beginner in fan modelization.
Material is NOT A POINT TO BE CONSIDER, after fracture analysis, we conclude that fatigue failure is occuring even in stronger material.
Assembly of the blades are as this picture on the Howden site and blades are FRP.
http://www .howdencoo lingfans.c om/files/u pload/DSC0 9355%20zon der%20roes _groot.jpg
I'm working on the case of our cooling tower fan (Hamon-Sobelco fan) whom brake in the blade holder (good term?).
Blade holder is cracking at bolt hole (which retain blades) and advance in fatigue propagation. It is 40' diameter and 1 inch thick.
What could be the cause of failure ? Vibration ?(sure) Flow and pressure of air sucked? I don't know where to start, I'm a beginner in fan modelization.
Material is NOT A POINT TO BE CONSIDER, after fracture analysis, we conclude that fatigue failure is occuring even in stronger material.
Assembly of the blades are as this picture on the Howden site and blades are FRP.
http://www





RE: Cooling tower Fan failures
Do you undertsand the theory of crack initiations?
What number of cycles (revolutions) are you getting failures at?
Is this a one-off unti or do you have many units taht all fail?
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Cooling tower Fan failures
Have you performed a stress analysis.
RE: Cooling tower Fan failures
Is the crack initiating on the "saddle" side, or on the nut side? What is the direction of propagation? Is the saddle a very stiff component, or relatively flexible?
RE: Cooling tower Fan failures
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Cooling tower Fan failures
I am surprized that their design is not using through-holes on the pillow blocks (aluminum blade supports on their web site).
They may be creating quite a stress riser between the edge of their Al support and the bolt hole.
RE: Cooling tower Fan failures
RE: Cooling tower Fan failures
RE: Cooling tower Fan failures
I'm not a mechanical engineer but I agree with John... it sure looks flimsy to me. The separation distance of the two U-type clamps is about the same as the diameter of the blade stub. With this small separation distance, the force for a given moment on the blades is magnified. Longer blade stup with bigger separation distance between clamps would seem a better design. Even better continue to blade stubs to a center point and join them all together somehow. None of those are practical modifications after the machine is built, but it seems like the machine should have been designed that way to begin with.
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RE: Cooling tower Fan failures
RE: Cooling tower Fan failures
The relevant comparison of moment arms I should have talked about is the small distance between clamps compared to the large length of the blade.
If forces act on average 2/3 way out the blade, my eyeball say the moment arm between those clamps is 1/10 of that distance and the clamps will see 10x the force that was on the blade. I agree with you guys 100% the stress-riser created by configuration of the U-bolt through the plate is very important.
To understand the forces, it would be interesting to know the speed of the machine, vibration magnitude, vibration measurement point, vibration frequency content. Also might be interesting to look at it with a strobe.
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RE: Cooling tower Fan failures
The design LOOKS very weak. It fails. So, what's the problem? It seems to me to be a classic case of bad design.
Gunnar Englund
www.gke.org
RE: Cooling tower Fan failures
Your lawyers should be strongly encouraging them to do so.
Mike Halloran
Pembroke Pines, FL, USA
RE: Cooling tower Fan failures
Do the failed hubs have cracks with multiple forks, kind of like lightning bolts? That would be stress corrosion, which can happen at pretty low constant stress levels to some aluminums if they are operating in the wrong atmosphere.
Although the hub does look pretty flimsy, Howden has been around forever, so I'd expect them to have worked things out pretty well. Similar fans apper in their literature here - http://ww
RE: Cooling tower Fan failures
- Good idea about getting more distance between both u-bolts to give more stiffness to the assembly.
- Plate of blades holder was in carbon steel. Bad corrosion from sulfuric acid environment forced to change for stainless 316L. It was cracking in steel and worst in 316L. So we tried Ferralium 255 (Duplex 2205 steel). It is stronger and cracks as well. But slower.
- I'll post some picture on monday, I promise. You are so kind answering and trying to find a solution, I owe you some pictures, at least. A picture of the fracture surface will be posted, and you'll be able to see that crack propagates slowly...
- Also, I'd like to try to put some stress probe near bolts holes,to monitor the stress, on both side of the blades holder. I guess that stress is not the same on both side.
- What do you think, guys, about shotpeening the stressed surface of my plate?
- Supplier was Hammon-Sobelco,(I used Howden site because of nice picture on their site).
Waiting your suggestion. Project is on going...for year 2006.
RE: Cooling tower Fan failures
Doubling the thickness of the plate will help more than shotpeening will.
Which leaves you with the dilemma of getting longer u-bolts, or counterboring the plate for their nuts.
I'd toss them, and use through bolts, double the number of saddles, and two plates, sandwiching everything between them.
Mike Halloran
Pembroke Pines, FL, USA