MSI was contacted by a large U.S. pump OEM to perform specialized vibration testing of a single-stage centrifugal pump at their main development facility. Destined for a large petroleum refinery, the double-suction pump would be subject to coke formation in vacuum bottoms service.
The goal of MSI’s testing was to identify the root cause of the high vibration that had occurred at super-synchronous frequencies (roughly 160 Hz) when the pump was run in a factory setting over eight months using different driving motors at three different facilities. MSI performed extensive vibration testing of the pump at two different speeds, 2965 rpm and 3570 rpm, and at different flow capacities. MSI’s test data indicated that the super-synchronous structural vibration level, 0.38 in/s RMS overall, increased with the flow and speed. However, it was not related to a detectable internal pressure pulsation, acoustic natural frequency, or resonance of the pump structure. The shaft’s vibration response in displacement was discovered to be very large, about 9.5 mils pk-pk, which caused contact with the internal “wrap-around” coke-crusher wear rings. When not in operation the first bending mode of the pump rotor was detected at 98 Hz, and this natural frequency remained around 100 Hz while the pump ran.
The excitation’s source was fluid swirl at the wear ring inlet, and the resulting non-axisymmetric pressure within the "wrap-around" coke crusher wear rings. Such rotordynamic instability occurs when the cross-coupling force from the wear ring seals increases, such that it is larger than the damping that the seals and bearings together generate in the opposite direction. The phenomenon is illustrated by the shaft center-line vector plots.
Based on collaborative discussions with the OEM, swirl-breaks were designed as vanelets on the high pressure side of the pump wear ring, to eliminate the super- synchronous vibration. The ID side of the ring was machined with a slight slope and a diverging clearance (i.e. with the widest clearance at the exit of the seal at the pump’s suction) to reduce the swirl velocity in the seal ID. Also, slots were machined at four clock positions at the bottom of the wrap-around turn “pocket”, to ensure that any swirl within the wear ring OD would be disrupted before it reached the ID. After the modifications were implemented, follow-up testing at the OEM’s facility showed that the rotordynamic instability had been eliminated completely. On average, the vibration was reduced at the bearing housing and shaft by a factor of five. The vibration decrease enabled the API 610 specification’s 0.12 in/s RMS @ BEP requirement to be met, and the OEM’s Customer received a smooth running and reliable pump. After eight months of the customer unsuccessfully trying to resolve the problem on their own, MSI was able to diagnose to root cause and recommend a solution in less than one month.
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