ArticleCooling Tower Fan Blade Diagnostics
by Tony Wofford
This paper is a brief case history of how cracked blades on a cooling tower fan were detected and a fan failure likely avoided by the use of vibration analysis. Vibration sensors were properly installed on the gearbox driving the fan and the vibration levels routinely monitored. When a significant change in levels was noted by the PDM group, and dominant vibration at the fan speed determined, a thorough inspection by maintenance was requested. This inspection found significant cracks in two of the fan blades that undetected, would have certainly resulted in a catastrophic failure of the fan.
“Permanently-mounted accelerometers were installed on the gearbox of the chemical preparation cooling tower center ID fan. The cost of approximately $500 for parts and labor allowed remote data collection. Initial baseline vibration data were collected when process allowed the fan to be operated. The motor is rated for 1,190 RPM but usually operates around 700 RPM. It is coupled to a drive shaft that in turn is coupled to the input shaft of the gearbox (no carrier bearing). The gearbox is a double reduction unit with a 7.97:1 ratio and the typical configuration for a cooling tower top output shaft.
PdM route vibration surveys were conducted every month, but the fan was not used for about a year. No significant changes or problems were noted until a month later. Trend plots (Figure 1) showed drastically increased vibration levels (from 0.139 to 0.510 IPSpk) at one measurement point (used for both the high-speed and intermediate-speed measurements) on the gearbox in the axial direction (parallel to the input shaft).
The source of this increased vibration was expected to be related to the high-speed input shaft (fluctuating around 846 RPM). However, spectral analysis (Figure 2) showed that the dominant source was the operating speed of the vertical output shaft (100 RPM).
Possible causes were unbalance due to dirty or damaged fan blades or a problem with the gearbox mounting structure. The latter would allow the gearbox to rock in a direction parallel to the input shaft. A physical inspection was conducted as soon as possible prior to the major downtime to determine the cause of the high vibration and to plan a repair.
Inspection a month later showed two adjacent fiberglass blades cracked at the root. The most serious crack was approximately 12 inches along the length of the blade and approximately five inches around a 22.5 inch circumference (Figure 3). Liquid penetrant testing confirmed that the cracks were relatively deep and possibly all the way through the blade wall. The depth and length of the cracks and the rapid increase in vibration from the fan speed — apparently from the fan blade flexing at the crack(s) during rotation —led to the conclusion that at least one blade was on the way to catastrophic failure.
Both blades were replaced with care to maintain the balance of the entire assembly and then retested. The new spectral data (Figure 4) showed that vibration from the output speed of 93.75 RPM was significantly less. Trend data also showed the return to normal levels (Figure 5).”
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