The Cooling Tower Gearbox Sampling Enigma
Maintenance Manager, natural gas fired power plant
Q: We had had failures on our cooling tower gearboxes in the past, and we really want to be able to monitor them with oil analysis on a periodic basis. We don’t know how to sample them effectively, however. Any ideas?
- Maintenance Manager, natural gas fired power plant
A: This is a common lament for many cooling tower owners. The box is critical to the power generation process, and if you lose even one, you may have to drop the tower bank, and thus your megawatt output, which of course means less money coming in. It is a real challenge because of where the gearbox is located. It is supported by spars, in a warm and wet environment. You can’t get near the reservoir to sample when it’s running (fan spinning) and the location of the gearbox requires a big ladder or cherry picker to get a decent sample when the unit is tagged out. To make things worse, by the time you sample a non operating gearbox, the wear debris of interest has settled to the reservoir sump, resulting in data that tells you the box is looking great, or in serious trouble- when the real result is somewhere in the middle. The best samples we see are when a filter cart (small portable hand cart style) is used to assist. Consider purchasing one, and dedicate it to the cooling tower service deck. And then we suggest the following:
- Put quick disconnects on the drain and fill line access points (usually located near the gearbox motor).
- When the gearbox is operating (at least for 30 minutes), hook up the filter cart to the fill drain lines and turn on the filter cart pump.
- Circulate the oil for about 5 minutes (factor in the dead leg volume on the drain line).
- Draw a sample from the return line on the cart BEFORE the filter.
- Continue to circulate for 25 to 45 minutes depending on volume of oil present in lines and gearbox. Look to do about 50 passes through the filters.
- When finished, disconnect and move to the next box.
You are killing two birds with the one stone here. You are getting a good representative sample of the oil in the box during operation, and you are removing potentially harmful wear debris and gunk that may cause premature failure. Expect to see a dramatic increase in reliability, and less unplanned downtime of your cooling towers by using this method!
-DPW
