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.
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.