Calming Turbulence at an Acid-Water Tank Agitator
Christian A. Smith
Reliability Testing Services, Inc. and TNT Reliability
A gearbox was replaced after being reported for significant gear wear. After it was replaced and ran for about seven hours, operations reported high vibration and temperature. The Jensen bevel gear–style gearbox is driven by a 13 horsepower vertical motor at 1,770 rpm and outputs a three-blade mixer at 432 rpm. The entire mixer assembly is mounted onto the side of a large acid and water mix tank. Movement could be seen with the naked eye, and high temperature felt with nylon gloves.
Initial vibration readings were highest on the motor in the horizontal axis around 0.66 ips-pk, which was 126 percent higher than previous readings. The dominate vibration across the coupling was 29.5 Hz (1,770 cpm) and 59 Hz (3,542 cpm), which was 1x and 2x the input turning speed (see Figure 1). A significant amount of low frequency peaks were also evident along with a raised noise floor, which led to suspicions of flow turbulence. Dual channel phase analysis was conducted across the coupling, which indicated close to a 180-degree phase difference; this led to inspecting the coupling for misalignment. Although an alignment had been performed before the agitator was installed, it was found to be severely misaligned. Following the coupling manual, we performed an alignment using a straight edge and feeler gauges to a tolerance better than called for.
After the alignment issue was corrected, the equipment was restarted, and high vibration, temperature, and movement were unfortunately still present. The equipment was again shut down and this time sent out to be inspected. The output shaft and propeller were checked for excessive run out and imbalance—both were within tolerance. The gearbox had excessive backlash and a severely worn bushing despite the short operation of the gearbox; both were corrected, and the equipment returned to service.
With only minor issues found to the equipment, investigations into the process and operation of the machine were performed. It was determined that the agitator had an interlock only allowing startup when the tank was at 50,000 gallons and shutdown at 45,000 gallons. The agitator OEM recommends maintaining a 48-inch level above the agitator at all times, which turned out to be above the current interlock. The startup level of 50,000 gallons was about 15 inches too low and the shutdown level of 45,000 gallons was almost 24 inches too low (see Figure 2). The interlock level was adjusted up to 62,000 gallons for both startup and shutdown to insure the agitator was never run below optimal tank level (48 inches above agitator).
After these adjustments were made to the tank level system, the agitator was reinstalled and started up with no issues. Overall vibration was significantly lower at around 0.19 ips-pk, and the temperature decreased. It was determined that the low tank level (< 48-inch level) was the source of the problem, causing premature gearbox issues such as the worn bushings and gear wear.
Editor’s Note: This case history illustrates the importance proper tank levels have on the performance and reliability of agitators. Inadequate tank levels can and often do result in excessive forces due to turbulence on these machines. We measure the effects of excessive turbulence as high and random low-frequency vibration at the agitator. Ensuring the tank level is at or above OEM recommendations when operating these machines ensures the hydrodynamic forces around the agitator are equal or relatively equal resulting in lower turbulence, lower vibration levels, and better reliability.
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Fig. 1: The spectrum from the motor coupling end in the horizontal direction. The frequencies below 30 Hz are 7.2 Hz (432 cpm, 1x rpm agitator) with side bands of 7.2 Hz, 14.4 Hz (864 cpm, 2x rpm agitator), and 21.6 Hz (1,296 cpm, agitator blade-pass), which are related to 1x, 2x, and 3x output turning speed. The highlighted tallest single peaks are 29.8 Hz (1,771 cpm) and 59 Hz (3,542 cpm), which are 1x and 2x input turning speed. A significant amount of low frequency peaks (< 30 Hz) are evident with a raised noise floor (turbulence).
Fig. 2: A model done to compare the previous shutdown level (45,000 gallons) to the new shutdown level (62,000 gallons).
Christian Smith is a reliability specialist Reliability Testing Services, Inc., and TNT Reliability Services assigned to Delek Oil’s Tyler Refinery. He holds an ISO Category III Vibration Analysis Certification, Level I Infrared Thermography Certification, and Optical Gas Imaging Certification. He supports Delek Oil, performing a multitude of reliability methods such as vibration collection and fault analysis, online condition monitoring using Bently Nevada 3500 systems, single plane field balancing, and advanced vibration analysis on fixed and rotating equipment. He also consultants on high speed turbo machinery balance and startups.