Russ
Lifetime Supporting Member
Hi,
Had some interesting data gained last night and could use some feedback on what it might indicate. This is on a pull through system. The fan is located by the exhaust stack of the oxidizer. It pulls air through a combustion chamber/ceramic media on a 3 tower unit.
In july 2004 the 400hp motor/fan were replaced with a 700hp motor/fan. The only duct modification was done to the breech section. The outlet duct was neither enlarger nor lengthened to accomodate the new equipment.
About 10 months ago a vibration sensor on a motor base started indicating the fan/motor was experiencing high vibration levels. This was happening when the VFD controlling the motor was running within a certain speed/freq range (80-85%). When operating at speeds above and below the range the vibration levels were good.
Initial inspections indicated that the vibration seemed to be coming from the breech side of the fan. Some insulation on the breech had fallen off (about 1/2 on one side of the square duct). However the breech didn't seem to vibrate.
In December of last year I trended the vibration levels at .5% incrememnts from 80%-86%. When the VFD was at 81% (49.2hz appromately) the unit experienced very high vibration levels
This past weekend an air straightener was added to the inlet to the fan.
When initially starting up the unit it ran at 25% until shortly after the burners in the chamber lit.
At this point I ramped it from 25% to 80% (manually at the vfd). The highest vibration level reached was 0.113ips. From this speed until I reached 85% the vibration level never exceeded .090isp.
It looked like the straightener had resolved the problem.
However at 85.5% (until I stopped at 88%) the vibration level got slightly higher (into the .130s range). While not much of a concern it was a different pattern than I had seen previously.
By this point the outlet duct and fan bearings were getting near normal operating temperatures (bearings 140s-170F, outlet between 150-220F).
To make sure I had accurate frequencies to tie into VFD speeds (I had noticed that the manual speed demand/freq were slightly off) I started recording the actual freq's for the various speeds).
When I went back through the 80-85% speeds I found that the vibration levels had increased dramatically. They weren't it the .085 to 0.09ips range any longer. They had dramatically increased and were hovering in the .300s-.525ips range.
It seems that temperature is a factor. I'm not sure if bearing temps or the outlet temp is the culprit. The fact that it's only occurring during certain speeds is also interesting.
While I can't take the heat out of the bearings, for testing purposes I can take it out of the outlet manifold/fan (by various means).
Has anyone seen anything like this before? I'd like any feedback.
The fan is by Twin City. It turns out the fan had been previously sold and returned. It was sold to it's current owner in 2004, without the curren owner's knowledge of its history. I've asked Twin City for the history of this fan, and while they promised to get it to me it has been over 6 months.
I've checked the fan for cone clearance, loose bolts, cracks in the wheel. Haven't found anything conclusive. The only I have found is that I'm tightening motor base anchor bolts every few months. Which is unusual. But with the vibration issue it doesn't seem out of place.
Had some interesting data gained last night and could use some feedback on what it might indicate. This is on a pull through system. The fan is located by the exhaust stack of the oxidizer. It pulls air through a combustion chamber/ceramic media on a 3 tower unit.
In july 2004 the 400hp motor/fan were replaced with a 700hp motor/fan. The only duct modification was done to the breech section. The outlet duct was neither enlarger nor lengthened to accomodate the new equipment.
About 10 months ago a vibration sensor on a motor base started indicating the fan/motor was experiencing high vibration levels. This was happening when the VFD controlling the motor was running within a certain speed/freq range (80-85%). When operating at speeds above and below the range the vibration levels were good.
Initial inspections indicated that the vibration seemed to be coming from the breech side of the fan. Some insulation on the breech had fallen off (about 1/2 on one side of the square duct). However the breech didn't seem to vibrate.
In December of last year I trended the vibration levels at .5% incrememnts from 80%-86%. When the VFD was at 81% (49.2hz appromately) the unit experienced very high vibration levels
This past weekend an air straightener was added to the inlet to the fan.
When initially starting up the unit it ran at 25% until shortly after the burners in the chamber lit.
At this point I ramped it from 25% to 80% (manually at the vfd). The highest vibration level reached was 0.113ips. From this speed until I reached 85% the vibration level never exceeded .090isp.
It looked like the straightener had resolved the problem.
However at 85.5% (until I stopped at 88%) the vibration level got slightly higher (into the .130s range). While not much of a concern it was a different pattern than I had seen previously.
By this point the outlet duct and fan bearings were getting near normal operating temperatures (bearings 140s-170F, outlet between 150-220F).
To make sure I had accurate frequencies to tie into VFD speeds (I had noticed that the manual speed demand/freq were slightly off) I started recording the actual freq's for the various speeds).
When I went back through the 80-85% speeds I found that the vibration levels had increased dramatically. They weren't it the .085 to 0.09ips range any longer. They had dramatically increased and were hovering in the .300s-.525ips range.
It seems that temperature is a factor. I'm not sure if bearing temps or the outlet temp is the culprit. The fact that it's only occurring during certain speeds is also interesting.
While I can't take the heat out of the bearings, for testing purposes I can take it out of the outlet manifold/fan (by various means).
Has anyone seen anything like this before? I'd like any feedback.
The fan is by Twin City. It turns out the fan had been previously sold and returned. It was sold to it's current owner in 2004, without the curren owner's knowledge of its history. I've asked Twin City for the history of this fan, and while they promised to get it to me it has been over 6 months.
I've checked the fan for cone clearance, loose bolts, cracks in the wheel. Haven't found anything conclusive. The only I have found is that I'm tightening motor base anchor bolts every few months. Which is unusual. But with the vibration issue it doesn't seem out of place.