Unearthing The Root Cause Of Vibration Issues

The site was unable to diagnose the root cause of this problem and the unreliability and resulting unavailability of this critical equipment was causing significant stress on the new plant. The company heard through another fertilizer plant that a global aftermarket pump service provider had been able to diagnose and remediate their pump problems with success. The site reached out to this aftermarket pump service provider to help identify the issue and provide solutions.

The pump was shipped to the provider’s repair shop to perform a thorough disassembly, cleaning, and inspection (DCI). The engineering team reviewed the original pump design and historical vibration information provided by the plant. Between this review and the DCI results, sufficient evidence was gathered to perform a root cause analysis (RCA) that explained the aberrant behaviour of the problem pumps. The pumps were then re-engineered to remove the diagnosed problem and provide the plant with reliable operation.

Background

The UAN solution pumps are formed of a single stage VS4 vertically suspended pump design and are installed in a process condensate tank. The pump is submerged in the UAN solution and takes suction through an axial suction nozzle equipped with a suction strainer.

The pump has a radial discharge nozzle located at the impeller centreline; this discharge continues through an elbow to a length of vertical pipe that exits the process condensate tank.

Above the impeller, a series of column pipes and lineshafts connect the hydraulic assembly to the motor assembly located on the tank top flange. The lineshafts are supported by three guide bushings located in column spiders.

The impeller was designed with back rings and balance holes to greatly reduce the total axial downthrust of the pump. Calculations show that the hydraulically generated downthrust is very low and almost all of the axial thrust is attributed to the static weight of the rotor.

Because of particulate in the fluid, the bushings are provided with a clean lubricating flush. The flush line is taken off the discharge line and runs through a cyclone separator. It is then injected through ports at each bushing location.

The high vibration observed at the site was accompanied by abnormal noise at the pump, usually indicative of either looseness or broken parts. The site communicated that extensive damage to the pump had been observed on a previous refurbishment.

The evidence

When the pump was disassembled, it was evident that there were significant problems at the motor bearing and guide bushing locations. The motor ball bearing cage had become unseated and was damaged. Inspection of the mechanical seal revealed damage to the sealing faces.

The DCI also revealed elliptical wear on the guide bushing bores that was biased to one side, indicating a misalignment problem. Galling was observed at the impeller and case rings. Dimensional analysis of the components revealed major excursions from recommended tolerances, including:

• Excessive runout of the motor stand faces – runout was measured at 0.018 in. and should be a maximum of 0.002 in.
• Excessive clearance at the motor stand register to the baseplate – clearance was 0.015 in. and should be a maximum of 0.002 in.
• Motor flange face runout was 0.003 in., outside of the recommended maximum tolerance of 0.002 in.
• Lineshaft coupling tolerances were specified to a substandard class and resulted in excessive runout.
• Column spiders that hold the guide bushings had excessive fit-ups at the registers and face runout, bore concentricity, and other tolerances were all outside of recommended maximum values.
• Site inspection of the sole plate revealed that it was not level.

The aftermarket pump service provider performed a RCA that took into consideration the DCI results, original pump design, vibration spectra, and operating conditions to reveal the likely causes of degradation. Issues identified by the RCA included:

• The motor coupling was the incorrect design.
• The guide bushing bores were running against the lineshaft couplings instead of the shaft or a shaft sleeve.
• The flush line could be optimised to provide the guide bushings with cleaner fluid.
• Excessive manufacturing tolerances and clearances led to misalignment of the rotor and non-centreline compatibility of the stationary wear component bores.

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