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Motion Amplification Services

12/11/2020

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About the Author
Christian A. Smith 
is a vibration specialist with Nelson Electric Motors tasked with developing and improving systems, practices, and programs. He holds a Bachelors of Science in Mechanical Engineering from Lamar University, ISO Category III Vibration Analysis Certification, Level I Infrared Thermography Certification, Optical Gas Imaging Certification, and a Failure Analysis Certification.
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Acid-Water Tank Level Case History

12/9/2020

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By: Christian A. Smith (Published via Vibrations Magazine)
A gearbox was replaced after being reported for significant gear wear.  After it was replaced and ran for about 7 hours, operations reported high vibration and temperature. The Jensen bevel gear style gearbox is driven by a 13 HP vertical motor at ?? rpm and outputs a 3-blade mixer at ?? 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% 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 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 was performed before the agitator was reinstalled, it was found to be severely misaligned.  Following the Falk Torus Coupling manual, we performed an alignment using a straight edge and feeler gauges to a better tolerance than called for.
            After the alignment issue was corrected, the equipment was restarted, and high vibration, temperature, and movement were unfortunately still with us.  The equipment was again shutdown and 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 start up when the tank was at 50,000 gallons and shut down at 45,000 gallons.  Jensen Mixers 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” 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 around 0.19 ips-pk and the temperature decreased.  It was determined that the low tank level (< 48” level) was the source of the problem causing premature gearbox issues such as the worn bushings and gear wear.


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About the Author
Christian A. Smith 
is a vibration specialist with Nelson Electric Motors tasked with developing and improving systems, practices, and programs. He holds a Bachelors of Science in Mechanical Engineering from Lamar University, ISO Category III Vibration Analysis Certification, Level I Infrared Thermography Certification, Optical Gas Imaging Certification, and a Failure Analysis Certification.
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Jet Water Pump Case History

12/9/2020

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By: Christian A. Smith (Published via Vibrations Magazine)
​            An East Texas oil refinery’s Delayed Coker Unit utilizes a high-pressure water pump in its Coke Drum operation. A Coke Drum is a pressure vessel that utilizes heat and pressure to refine hydrocarbons into lighter products such as, gasoline, diesel, and jet fuel. The drum serves as the final step in the cracking process. Heated hydrocarbons are fed into the drum which build up along the drum’s wall. A high-pressure water jet is then used to cut the coke from the walls and allowing it to fall into storage. A failure was witnessed on this 12-stage barrel style pump.
            It was routine for operations personnel to perform visual inspection on this pump once per shift, it was this visual inspection that first alerted engineering personnel of a potential deficiency. Operation’s visual inspection noted that the reservoir sight glass was cloudy in appearance and led to a sample being collected. As you can see in Figure 1, the sample confirmed the cloudy appearance which is typical of water contaminated oil. It was also noted that large metallic-like flakes were in the sample and was initially suspected to be wear from the journal bearing. Although the oil was changed laboratory analysis was requested and confirmed suspicions of water contamination at 13,584 ppm along with 215 ppm of ferrous debris and 23 ppm Iron content. Microscopic analysis of the ferrous debris indicated a heavy number of rubbing wear particles, a moderately high number of cutting and sliding wear particles, and moderately heavy number of silicate particles were detected.
            With the pump’s oil condition poor vibration analysis was conducted. Initial vibration readings indicated no significant increase in Acceleration or Velocity amplitude on the motor bearings. Motor spectra indicated no frequencies of concern. The pump inboard bearing indicated a moderate increase in amplitude across all point with the highest being on the inboard in the horizontal direction, from .054 in/Sec Peak to .217 in/Sec Peak. The pump outboard bearing indicated significant increase in amplitude across all points with the highest being in the horizontal direction from .051 in/Sec Peak to .411 in/Sec Peak. As you can see in Figure 2, time waveform data indicated severe impacting in the horizontal direction both on the outboard pump bearing and outboard oil pump. Once per revolution impacting could be seen to an amplitude of 29.39 G True Peak-to-Peak on the journal bearing and 145 G True Peak-to-Peak on the oil pump. Amplitude was monitored through the 14 day period which trended within the same ranges.
            In addition to the oil condition, Operations personnel noted a drop in discharge pressure from 1300 psi to 1100 psi. While the engineering team gathered historical maintenance records it was discovered that the pump was routinely sent out for OEM inspection during the Coker Unit’s 2-year outage schedule. During the 2013, 2015, and 2017 inspections the OEM recommended weld overlaying the barrel and milling down to tolerance specifications. Unfortunately, each routine outage was only 4-7 days in duration the OEM recommendations would take at minimum 28 days to complete thus the recommendations were not followed and the pump was returned to service. As you can see in Figure 3, the image taken in 2015 indicated severe erosion in both the suction head and barrel of the pump.
            During the next 14 days until repairs could be made the engineering team conducted daily checks including visual inspection of the oil reservoir, vibration analysis, and infrared thermography. The infrared thermography indicated a maximum heating of 122°F on the thrust pad location and 114°F on the journal bearing location after a 1 hour run. An exchanger on the oil system was found to be leaking water into the system which resulted in the reservoir’s oil being changed on a twice a week basis.
            Unfortunately, the Coker Unit did not include a spared sister for this pump and no replacement spare parts were available at the refinery. It was determined that a temporary fix could be made during a 12-hour period in between cutting cycles. The engineering team determined the best course of action would be to inspect the outboard journal bearing, replace the oil pump, and perform other minor repairs. These actions were to be a stop gap measure while a new pump could be ordered as a drop-in replacement to solve the barrel erosion issue. During the repairs it was discovered that the oil pump’s gears were almost completely destroyed due to the pump’s axial thrusting. As you can see in Figure 4, the journal bearing indicated significant rubbing wear on the bottom half and slight wear on the top half. This was due to the outboard shaft opening up tolerance and dropping down creating an internal misalignment from the pump inboard to outboard bearings. No misalignment was found across the motor to pump coupling. The journal bearings were replaced, oil pump replaced, and oil system exchanger blocked in. The resulting repair resulted in a decrease in Acceleration to 3 G True Peak-to-Peak and no impacting.
            A preliminary root cause investigation listed Human Error as the primary cause in this pump’s failure. It was found that the equipment’s maintenance history was being kept in multiple locations and did not utilize any central computerized maintenance management system (CMMS). Current plant personnel did not have the background knowledge of OEM recommendations and the proper resources to prevent failure of this pump. This case history should serve as an example in how human influence can cause critical equipment failure and should be avoided through culture, practices, procedures, and systems.

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About the Author
Christian A. Smith 
is a vibration specialist with Nelson Electric Motors tasked with developing and improving systems, practices, and programs. He holds a Bachelors of Science in Mechanical Engineering from Lamar University, ISO Category III Vibration Analysis Certification, Level I Infrared Thermography Certification, Optical Gas Imaging Certification, and a Failure Analysis Certification.
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  • Home
  • Services
  • PRODUCT LINE
    • ELECTRIC MOTORS
    • COMPONENTS & PARTS
    • PUMPS
    • CONTROLS
    • IRRIGATION
    • CONDITION MONITORING
  • Blog
  • CONTACT US
    • OPELIKA
    • SYLACAUGA
    • ALEXANDER CITY