ACC Fan U-bolts

[Ockert Augustyn]

We’ve noticed an unusual amount of loose ACC fan U-bolts on one of our stations. All bolts are torqued to the correct OEM values when installed but we have to continually keep inspecting them and tighten them again if found to be loose. The same fan type runs on the auxiliary cooling (mech. wet cool type) and these never have to be inspected as they don’t go loose. We’ve had multiple fan failures due to blades coming loose eventually.

My question is whether other plants experience the same kind of problem or do your bolts/nuts stay fixed during long periods of operation. Thanks.

[nick_romano]

Is the wet cool type running at the same speed? Do you know the blade natural frequency? The ratio between the BNF and the fan RPM can be tricky and causing troubles.

[fbadia]

How long have these fans been in service?
How often do you have to re-torque the U-bolts?

We’ve never had this issue, but we do an annual PM to check blade pitch, tracking and bolt torque.

[Ockert Augustyn]

Nick, I agree BNF/RPM is one possible cause (as it is a bit close for comfort) but we can’t do too much about that now – it’s been running for 25 years like this. We have to typically re-torque every 3 months and it occurs across all fans (young and old).
We’ve found that it becomes worse with windy stretches which is kind of expected but I was just wondering if this is common among other users.

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