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Anonymous Poster

Fence Mesh to Guard Cooling Tower Fans

02/05/2010 9:10 AM

hi

we have air cooled condenser tower that has fans at 10 m height above ground.

many times in the past years fan blades sheared and fell down. i am thinking to use a wire mesh as a guard to protect any people working around.

please share any idea do you have on how we can put a guard (garden fence) below the fans without making restriction whenever the fans has to be lowered down for maintenance.

Thanks in advance

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Power-User
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#1

Re: fence mesh to guard cooling tower fans

02/05/2010 9:50 AM

I'm guessing the fan you're working on is out of service and the concern is missiles from adjacent operating fans. If the barrier is vertical it could be constructed of expanded metal with structural steel angle frames anchored to the fan structure. This would not restrict flow from adjacent fans. If the barrier is horizontal (i.e. under the fan) something more substantial would be necessary to stop a missile and have openings large enough to allow sufficient flow. A 6" X 6" grid constructed of half inch round stock would probably be adequate, but depending on the size may require additional support across the horizontal members.

Otherwise; I'm trying to think why any personnel would be working below the fan of an operating forced air cooling tower? Tag that fan out of service when workers are around.

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Anonymous Poster
#4
In reply to #1

Re: fence mesh to guard cooling tower fans

02/05/2010 11:37 AM

your guess is correct

there are total 12 fans in the tower. so there is hazard of flying missile when people are working on the adjacent out of service fan. we would like to have barrier mesh fixed under all fans but we cannot fix permenant structural steel because the fan and gearbox are lowered from down in case any maintenance has to be carried out.

i am trying to find some solution so that we can prevent the hazard but considering maintenance needs.

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#5
In reply to #4

Re: fence mesh to guard cooling tower fans

02/05/2010 12:02 PM

Make panels with the expanded metal that can be removed they do not need to be permanent structure. Only you can determine whether the extra time to remove them is worth the safety hazard of not having them in place.

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Anonymous Poster
#6
In reply to #5

Re: fence mesh to guard cooling tower fans

02/05/2010 12:23 PM

can you please further explain if possible with some dwg?

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Anonymous Poster
#2

Re: Fence Mesh to Guard Cooling Tower Fans

02/05/2010 10:22 AM

How about setting up a PM/inspection schedule for the fans/blades?!?!?!?

That should NOT be happening. There are 10,000's of fans in the world that do not break blades.

It could be a simple matter that you are OVERtightening the hub connections and cracking the blades.

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#3
In reply to #2

Re: Fence Mesh to Guard Cooling Tower Fans

02/05/2010 10:47 AM

Good maintenance procedures and periodic inspection are essential to reliability, but things still break. There is a great deal of stress on those blades and material fatigue from cyclic loading. I am an inspector and I have inspected fans of this type. Verifying blade angle, torquing of hubs and variable pitch connections, as well as blade condition. Even after this blades have sheared and done considerable damage to adjacent components before the next maintenance/inspection interval. There may be ten thousand cooling tower fans that don't break, but I don't want to be anywhere near the one in ten thousand that does.

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Anonymous Poster
#7

Re: Fence Mesh to Guard Cooling Tower Fans

02/05/2010 1:13 PM

relatively light weight panels could be constructed using heavy gage reinforcement mesh such as used in concrete road construction, and bolted in place with lag bolts.

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#8

Re: Fence Mesh to Guard Cooling Tower Fans

02/06/2010 12:08 AM

It sounds as though this is a fairly common occurance, which says there is something very, very wrong with the basic design of your system. Rather than thinking in terms of putting up a shield, you should be looking at redesigning the setup. Either the loads on the fans were underestimated, the system was not built as designed (or was modified after the initial design) or the materials used for fabricating the blades is inappropriate, or there is some fault in the manufacture of the blades. One does not, afterall, see blades shearing off airplane propellers or helicopters. You should be looking at how to prevent this.

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#9

Re: Fence Mesh to Guard Cooling Tower Fans

02/06/2010 3:12 AM

First, to build a shield, use galvanised steel tube frames covered with 1" x 1/8" chain link mesh. Do not tension the mesh, the idea is for it to deform and stretch absorbing the energy of the failed blade. Hard expanded mesh barriers often break loose under impact or bounce the blade back into the surviving blades causing more damage.

Secondly I would suggest having ALL your fans rebalanced and then get an engineer to set up harmonics tests on the mounting structures Run the test for an extended period recording the harmonics under different wind conditions. If your fan motors are controlled by VSD's there is a strong possibility that there is a fan velocity that is setting up an oscillation that is causing your blade failures. You may have to put in some gussets & or braces to the existing fan mounting structure. I have seen an out of balance fan reduce a 50 foot cooling tower to scrap metal. Fortunately it was not one of mine. On another of mine we had oscillations setting up at 343 to 345rpm that was shaking the entire building, so the VSD's were set to accelerate through 340 to 350rpm as quickly as possible...problem solved.

On any multi-fan installations I have always specified the inclusion of harmonics sensors liked to alarms and safety cut outs. A couple of hundred dollars well spent every time. Your local harmonics engineers will be best equipped to analise and specify the best solution for you.

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Anonymous Poster
#10
In reply to #9

Re: Fence Mesh to Guard Cooling Tower Fans

02/06/2010 5:29 AM

your idea seems the best solution, i guess your solution to use steel tube frame welded to the existing fan support anchored to the mesh? please confirm.

however these points come in mind why you proposed to use steel tube instead of structural steel angle.

assume the fan existing support is square area 10mx10m so this represent the total area to be sheilded, do we need to put extra horizontal support in the middle?

Best Regards

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#12
In reply to #10

Re: Fence Mesh to Guard Cooling Tower Fans

02/06/2010 10:27 AM

No, not welded in, but loose panels that are bracketed in so-that they can be removed, reconditioned or repaired easily. I cannot overstress the need to first identify why the fan blades are breaking. They are either at the end of their service life, in which case they should already have been replaced, or you have vibration/harmonics problems, you NEED TO deal with first.

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#11

Re: Fence Mesh to Guard Cooling Tower Fans

02/06/2010 9:32 AM

Based on my 25 years of experience I would want to know why you are loosing so many fan blades in the first place. First rule of safety is remove the cause. I suggest some vibration analysis, some ultra sound trending. Get your maintenance personel involved along with the engineers. Not one or the other but both. Guarding will have to be substantial to avoid possibility of injury. Go to root cause. Get the manufacturer involved. If it is a trend they should know about it.

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