We have two Forced Draft Fans, rated at 1500 HP, 1188 RPM. serving a 150 MW Boiler. A week ago the fans were started after a period of two months shutdown. As soon as the fans picked up speed, there was heavy vibration in both the fans and the vibration broke the cooling water fittings inside the housings of the outboard bearings of both the fans causing heavy damage to the bearings. The temperature of the outboard bearings did not rise as the cooling water had mixed with the lube oil. On inspection of the fans it was observed that there was a heavy coating of black grit everywhere in both the fans. The coating was so much that we could not rotate the actuator of the inlet vanes. The fans were balanced and put back in service. The black grit has come from the Air heater wash which was done during the shutdown period. The question is whether the unbalance in the fans was caused due to the runner having been coated by the grit in the Airheater wash water.

Yes

The fan shaft may also have taken a temporary bow that coincided with the the grit buildup on bottom of fan wheel. I guess the preheater wash job was a little out of control!

My opinion is the grit "coating" was, by itself, not enough to cause the kind of imbalance that broke the cooling water lines, unless the lines were copper or aluminum, or thin-wall pipe. I agree with Walt that a shaft bow is the more likely cause. If the fans run "hot" and were and simply shutdown without the benefit of cooling, this is a very likely scenario. If the fans don't run hot, it is less likely that you had a gravity bow, but still a possibility. Just my opinion.

I work at a nuke and we don't need any fans for our boilers (steam generators)... so needless to say I have no experience with these. I would definitely trust guys like Rusty and Walt.

Just a question - Are the dampers supposed to reposition automatically? If so, is it possible that erratic damper operation (unbalanced among two, or wrong position) due to the binding mentioned would cause large fan vibs?

Pete made a good comment regarding the dampers/VIV that were probably not operning properly. The combination of two fans operating in parallel that can cause surge and poor flow that can cause stall could cause high vibrations. The dominant frequency would not be at 1xSS either. Chris, the vibrations were high, but was a spectrum analysis performed?

Getting back to the possible unbalance from grit coating. I have seen this effect on a similar size ID fan that had a leaking steam reheater coil upstream. The flyash that escaped the precipitator combined with the steam to form hard deposits on fan rotor causing unbalance.

A two month boiler outage is pretty long. I guess the spray job on the air heater turned out to be pretty expensive!

Spectrum Analysis was done after replacing the bearings but before balancing and the dominant frequency was observed to be at 1*SS. The fans are in service now after having been balanced. We intend doing an inspection of the runners during the forthcoming annual outage. We also will have a procedure in place to inspect the fans after each airheater wash. I thank everyone for their input.