Sponsored by the Association for Maintenance Professionals Join or Manage Your Profile Posting Boards Machinery Condition Monitoring and Predictive Maintenance Posts About Technologies and Techniques for Condition Monitoring Cooling Tower Balancing
Join or Manage Your Profile Posting Boards Machinery Condition Monitoring and Predictive Maintenance Posts About Technologies and Techniques for Condition Monitoring Cooling Tower Balancing|
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Hi everyone!
In checking a cooling tower,I found a high vibration amplitude of just over 1 in/s at the blade pass frequency. I read previous post on cooling tower issues and considered several possibilities including blade looseness, delamination od some type of blades, resonance, gear box output bearing looseness, etc. But if being the case and balancing is needed, is there any procedure as to where to place the balancing weights on the actual fan? Type of weights? Any comments are appreciated. |
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The last two I have balanced had a "disk" over the top of the blade hub where the gearbox output shaft came through the hub. This disk was drilled (and I drilled it some more) and bolts with nuts and washers were used as balance weights.
Dave This message has been edited. Last edited by: RRS_Dave, |
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If indeed the issue is "at the blade pass frequency" as you state, balancing will have minimal if any effect. I'd be looking elsewhere.
John from PA |
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I agree with John. Balance issues would have been at 1X output speed of the gearbox. Dave |
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Agree
Hi blade pass frequency is not an unbalance issue. Check the that the pitch and clearance of each blade is per spec (or at least consistent with each blade). If you do have hi unbalance, I found the 4 point method to be ideal in this case. I simply attached C clamps at the hub at three different blade locations (120 deg apart if possible) and found this to be very accurate and linear. The final weights were plates attached to the top surface of the hub. Before you balance, check that the water relief holes at end of blades are not blocked Regards Jim P |
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So your blade was high: what is the magnitude of 1X? Is it high and out of tolerance as well?
Can this be an aerodynamic problem and a balance problem? Are all the blades at the same tilt or angle? Are they installed properly by tracking properly one behind the other having the same foot-print in their rotational path? Answer more fundamental questions. Is 2X a factor and/or noteworthy? Etc........ Cordially, Sam |
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Cooling towers typically have low structural resonance frequencies. Often very low.
This leads to increased and sometimes excessive vibration level when an harmonic of 1*rpm, often but not necessarily blade pass frequency, coincides with a structural resonance frequency. This resonant condition can be verified by a coast down test or by running the motor at the second speed if a two-speed motor is installed. And a bump test also, although you may need a lot of energy to get the whole thing shaking. And all neighbouring fans should then be at standstill. Such a resonant condition is often the root cause. Aerodynamic irregularities like (slightly) different blade angles, impeller excentricity have less severe impact on vibration. To cope with such problems some cooling tower manufacturers make use of high-damping material for the fan stack. Or stiffening or flexing the structure can be a remedial action. |
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If you're just asking, balancing is usually done on the hub, or center section. Althought I've had good success on metal fan blades by attaching "c" clamp style weights on them.
But if you're considering imbalance causing blade pass vibration, I agree with the others, in that balancing won't have much affect. If one ( or moe) of the blades are at different pitch angles, that can cause blade pass vibration. I've also seen it when the fan isn't centered in the shroud. A couple of times, I've seen it when the fan used a large spacer shaft, which was very close to the fan itself. Is it a wooden tower? |
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So far, I have seen high blade pass frequencies under two different conditions, One when the gearbox and fan assembly were not central in the shroud, the other when the blade pass coincided with the resonance of the new carbon fibre shafts which we used. In the first instance, the assembly was centralised and realigned. In the second case, the speed of the gearbox output shaft was changed. In both cases the blade pass frequency issue was resolved.
Best regards, Joe Mc Cormack |
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Dear all I do agree with your comments but original question of balancing of blades remains to be answered.
I have balanced locally made fibre glass fan blades dia 8 ft and then balanced them by gradually adding on fibre glass repair resin with chopped strand mat. Technicals don't hit me. |
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