fin fan blade failure

[engineeryaq]

dear friends,
I have a variable pitch fin-fan blades damaged due to unknown reason. Can anyone answer me what would failure mechanism and suggest a suitable plan of action which must be taken in order to avoid this damage in future.

regards,
engineeryaqx

 

[LittleInch]

Maybe someone could if you could actually supply the following info:
Picture of failure / damage
size of blade
Type of damage
Power of fan
size of HX
Your guess as to what might have happened
Description of how things normally work
Location of HX
How long in service?
Do you mean pitch of the rotating fan or the long blades on the outside of the HX?
How is variable pitch changed? Manually or actuated?

There are lots of experienced people here able to help you, but they are not magicians and we cannot see or know what you do unless you tell us / show us.

It is very easy to upload documents and drawings / pictures.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[engineeryaq]

Dear LittleInch (Petroleum),

Picture of failure / damage , attached
size of blade; fan diameter 4572 mm
Type of damage, actuator rods were broken and causing the connected 2 blades to smash the housing
Power of fan; 24.8 KW
size of HX; 7120 x 12250 mm
Your guess as to what might have happened,actuator rods were broken and causing the connected 2 blades to smash the housing
Description of how things normally work, fan blades speed is variable
Location of process area
How long in service? Since 2010
Do you mean pitch of the rotating fan or the long blades on the outside of the? the long blasdessdd
How is variable pitch changed? Manually or actuated? Actuated

 

[LittleInch]

Still quite difficult to see the actual failure of the rod and what stops the top hat turning around the shaft, but my guess from that is that the rather thin looking rod has failed in fatigue at the screw which finely adjusts the position of the blades. What is the rod made from and does it show signs of corrosion. Can you take a picture of the fractured end?

I's not clear how the top cap is secured to prevent the rods being subject to bending forces or vibration. If the top cap moves around the shaft or vibrates in normal operation, the rods could be subject to high bending foces as the freedom of movement is at 90 degress to the freedom of movement of the rod at the blade end. I am quite surprised that the end connection isn't a free rose end which does not transmit forces into the rod. The connection could also have been over tightned leading to the rod having very little freedom of movement in the other direction, which it seems to need as the blade pitch turns.

Clearly any fouling or damage to the blades will cause viration which will rapidly make the rod to fail.

I would remove all the rods and look very carefully at the same location where the others have failed to see if cracks are present, but I think the design of the top bracket needs to be looked at to stop bending forces onto the rod.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[DubMac]

My suggestion would be to get a good vibration measurement/analysis outfit out there to take data and give you report. Most always the failure in cooling tower & fin-fan air cooled unit fans is in unbalance. One of these reports, with the proper vibration analyst's expertise will tell you much more than hit and miss guesses about where your unbalance may stem from.

I have found several of these outfits over the years to be very reasonable and very professional (Envibe in Houston comes to mind and there are many others just as good).