Corrosion on ID Fan shaft

[sgoel]

We have a ID fan installed at the outlet of an APH connected to a Fired heater. This heater fires sour gases and liquid fuel with S content of 1% approx.

Estimated dew point of flue gas is 145 deg C. APH outlet is maintained at 175 deg C and above.

Inspite of this we find repetitive corrosion of the ID fan driver shaft. The metal loss is in the form of a grove slightly away from the place where it enters the ID fan.

What could be the reason for such an occurance to take place. Any help will be appreciated.

thanks and regards

 

[pablo02]

Have you eliminated rubbing between the shaft and the seal? Also, have you eliminated stray currents (small arcing) from a static voltage diffence between the shaft and the housing? [Is the grooving smooth or ragged?]

As I recall, our ID fans generally had a slight negative pressure at the shaft seal, what's the ambiant air conditions and is it pulling any abrasives and/or chemical agents in along the shaft?

 

[sgoel]

Pabolo

The grooving is about 100 mm away from shaft seal. You are right , ID fan is under negative pressure and is pulling ambient air through the seal. Air at our location is clean, hence we have eliminated that possibility.

The grroving is ragged in nature. Could you please elaborate a bit more on elimination of stray currents.

Thanking you in anticipation.

regards

 

[pablo02]

Another few questions -- is the grooving on the outside of the fan housing, or inside? Is there anything near the grooving (closer than the 100mm to shaft seal? If so, how close and any other details?

It was an off the wall thought that maybe the fan was generating a strong static voltage that was discharging from the shaft to the housing (or vice versa) causing some pitting -- if this is the case (again, a guess at this point), using a copper braided drain wire between the two (dragging on the shaft) should eliminate the static voltage...

you mention repetitve corrosion -- has the shaft been repaired by undercut and built up and this is the edge of the undercut?

 

[odle1128]

The air being pulled into the system may be causing a local cool spot where average dewpoint conditions do not apply? The question is, what is the dewpoint of the mixture?

How accurately do you know the dew point of the process air? (mean and standard deviation)?

Robert Odle