High vibration caused by Wind Force?

Hi All,

Please explain or share if you have any articles regarding the wind force affect the Fin fan coolers. We have huge number of Fin fan coolers in our LNG plant. Some of the Fans particularly making lot of troubles and We suspect the wind force is the cause for this high vibration.

Data:

The Maximum Wind velocity is 40 m/s. We have both Induced and forced draught Air coolers (Fin fans). But Forced draught fan has the history of  more high vibration and broken blade history. It is onshore and wind velocity is considerable. The blade pass frequency is in uptrend, last reading was 8.5 mm/s RMS.  Post inspection of broken blades, catastrophic damage, we noticed that the blade tip rubbed on the cowling. But fan did not trip, senors are good. might be vibration not transmitted to the bearing housing where permanent velometer is  fixed. This blade rub on cowling is noted in the wind flow direction.

Questions:

1. Could you please refer me an articles on Fin fan vibrations caused by Wind force.

2. How, the process of wind force affecting the fan vibration , technically, engineering?

3. How we can solve the issue.

4. Also other fans are noted that beat phenomenon is major issue and I have noticed that the fan been designed such that Motor speed and the fan blade pass frequency match. Can wind force stall or increase the the blade speed and 10%  difference make beat?

 

Thanks guys

 

 

Original Post

Post inspection of broken blades, catastrophic damage, we noticed that the blade tip rubbed on the cowling..... This blade rub on cowling is noted in the wind flow direction. 

I can see why you might think this contact between fan blade and cowling is a cause. BUT, I wonder if that contact might possibly be an effect of the failure (rather than an initiator). Specifically, if a fatigue crack at root of the blade propogates slowly, then when it is maybe halfway accross the blade rotates out a little (and possibly tilts) to cause contact. I know that may seem intuitively unlikely (why would a cracking fan blade experience ductile deformation prior to failure?), but I'm pretty sure we found one that way on one of our large transformers oil cooler fans (a very close call). That was back in the 1990s when we didn't save pictures of everything unfortunately. Maybe inspection of your failure pieces could shed light on that scenario: If only one fan blade would show evidence of rubbing on the outer perimeter, then that suggests the failure originated with that blade;  If all fan blades show rubbing on the outer perimeter, then maybe it was a factor common to all blades such as the wind effect you are postulating.

I have worked on a lot of air-cooled condenser fan sound and vibration problems. The ACC units are the big brother of fin-tube heat exchangers with fans up to about 40-ft diameter. They are typically propeller-type blades with vertical shaft, and they may be arranged as induced or forced draft. For your questions:

1) Search: Fin-tube heat exchanger fan vibrations caused by Wind

https://www.bing.com/search?q=...473EB0FEE05F369C705B

 2) Wind speed and direction upon at fan inlet causes change in mean flow, inlet flow distortion, and turbulence. The greatest effect, besides reducing fan performance, is to raise sound and vibration levels at the blade pass frequency (blade rate). This can cause false alarms and trips on vibration monitors and sound alarms or community noise complaints. This same wind effect causes dynamic torque (torsional vibrations) that can damage drive train (belts, bearings, gears and couplings. Excessive blade bending and shaft torsional vibrations can also cause blade and shaft structural failure.

3) Correctly designed inlet and discharge stacks, solid barrier walls and porous screens can mitigate wind effects. There are several vibration control measures that can be considered on a case-by-case basis.

4) Multiple fans can "Beat" at 1xSS and at Blade Pass frequencies. Vibrations from adjacent fans interact through the structure, while sound is airborne interaction. Induction motor speed varies with fan load, and fan load can be affected by wind speed and direction. Variable speed fans can be a real nightmare for a sound and vibration person; or for me just good for business!

Walt

 

From preventive side , I would suggest to consult OEM to reset the parameters to suit operational requirements especially  blade angle, tip clearance, belt tension ( if these are belt driven) and cowling eccentricity. 

From predictive maintenance view point, reduce the frequency for vibration data collection to keep an eye on emerging failures. I guess you have protection based vibration system installed , if yes then consider reducing the trip limit.

Also, use some NDT on blades and hubs ( ideally UT) as a site acceptance testing before installation or annual maintenance. 

In terms of reliability by design try exploring  the idea of case hardening of fan blades and/or alternative materials for fan blades and hubs. 

 

Velbond,

1) Is the machinery new or has it been in service for sometime?

2) Huge number means.....100?

3) Do ALL the fans look the same, same model, same mounting structure, blade count, materials of construction?

4) Forced draft has history of failures, can you explain. Are fan blades/structures different on these unitsDoes fan blow air upward or does is suck air through? Blades should not crack, this is a clue

5) My assumption is the rubbing blade was after the fact the blade failed/cracked? With no vibration data or images of failed blades, lots of assumptions being made.

6) If vibration sensor is not tripping, where is the sensor located in relation to the bearing locations/mounting structure

7) Are the fans belt driven? If so then yes, high winds could load the fan to cause the belts to slip, if driven by gearbox, then motor amps would also increase with the high winds but not likely to slip

8) Wind loading of a structure can cause it to deform, if the supporting structure of the fan is weak, if the unit has many solid panels vs expanded metal, limited support/mounting of the entire fin fan assembly, long flimsy fan blades, dirty exchanger not allowing air to pass thru, thermal expansion of the entire assembly depending on temp of exchanger

9) I have also had experience with motor speed and blade pass being within 100 rpm of each other with excessive vibration. The resolve to my issue, belt drive fan 300 rpm with 6 blades, 1725 motor speed, made the gap between the underside of the fan blade to the top of the motor larger by moving to different motor mounting holes

Dave

My Two Cents worth:

1) Windy conditions cause the air velocity across the fan inlets to increase, which in turn causes the static pressure available at the fan inlets to decrease. (Lookup Bernoulli's Principle to see why).

2) The reduced static pressure can cause the fans to stall prematurely as compared to non-windy conditions.

3) The porous screens as mentioned above, cause the wind velocity to decrease at the fan inlets, which in turn increases the static pressure at the fan inlet and results in less fan vibration and an overall measurable increase in cooling performance due to increased air flow.

4) We have experienced this on at least two of our more recent projects. One is in Canada, the other is in California.

The wind screens greatly reduced fan vibration levels and fan trips. The cooling performance was measurably better during windy conditions.

Glenn

Glenn Miller posted:

My Two Cents worth:

1) Windy conditions cause the air velocity across the fan inlets to increase, which in turn causes the static pressure available at the fan inlets to decrease. (Lookup Bernoulli's Principle to see why).

2) The reduced static pressure can cause the fans to stall prematurely as compared to non-windy conditions.

3) The porous screens as mentioned above, cause the wind velocity to decrease at the fan inlets, which in turn increases the static pressure at the fan inlet and results in less fan vibration and an overall measurable increase in cooling performance due to increased air flow.

4) We have experienced this on at least two of our more recent projects. One is in Canada, the other is in California.

The wind screens greatly reduced fan vibration levels and fan trips. The cooling performance was measurably better during windy conditions.

Glenn

Hi Glen, Thanks. This is a valid point, 2 million worth! Pressure only making the fan stall or resonate the blade tip what ever making Beat, I guess and this beat join with adjacent fan create large vibration amplitude.

I specified the Galebreaker screens for a power plant ACC in Canada. There was a reduction in fan blade pulsation vibrations and some airflow performance improvement during high winds. Screens can be beneficial depending on what the major problem is that needs to be solved! I have not seen any feedback on machine reliabilityimprovement. There were a lot of gearbox issues compounded by VFD variable speed operation.

Walt

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