Increasing Vibrations in 15,000 hp Centrifugal Pump

[Amer93]

This case is for a single-stage double suction, 15,000 hp pump with sleeve bearings driven by a synchronous motor. The pump was recently overhauled as the inboard wear rings were dislodged and broken. After overhaul, the pump was shipped back to the facility and during the coupled run test, the vibrations on the inboard side were increasing gradually over 30 minutes, almost reaching the trip set limit (4 mil pk). The vibration data were analyzed were the team observed a dominant 1x on the half spectrum, no negative energy on the full spectrum, the orbits are circular in shape with truncation signs on the time-waveform, the orbits maintain their circular shape even during shut-down at low speeds near 0 rpm, and the shaft centerline show a vertical movement by ~5 thou during startup for inboard and outboard, however, the inboard shaft centerline drops by 4 thou on the inboard side only during steady state. Phase reference remains nearly the same throughout the run test. A second runt test was conducted after correcting minor soft foot issue, pipe strain, and inspection of bearings. Nevertheless, the behavior of the machine was almost exactly the same. Any thoughts or comments on the issue and possible causes?

 

[electricpete]

the vibrations on the inboard side were increasing gradually over 30 minutes

...and the shaft centerline show a vertical movement by ~5 thou during startup for inboard and outboard, however, the inboard shaft centerline drops by 4 thou on the inboard side only during steady state.

Does this pump handle hot fluid? It sounds like thermal growth that slowly changes shaft position and alters the shaft alignment which increases vibration. If that were the case then the solution could involve measuring off line to running changes and adjusting off-line alignment targets accordingly.


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(2B)+(2B)' ?

 

[Amer93]

The pump is a head shipping pump of seawater. Maximum temperature is approximately 50 deg C.

 

[JS_]

I'm curious..

What is the shaft size and clearance in the journal bearings?

What is the clearance in the wear rings?

Is it a sleeve bearing with a bronze oil ring?

What is the rpm?

 

[georgeverghese]

Direct drive or through a gearbox ?

 

[Amer93]

Thank you all for your interest in this topic.

The shaft diameter at bearings is 8.5" with clearance of journal bearings to shaft is 8-13 thou. The clearance in the wear rings is 29-33 thou. In terms of lubrication, it is a forced lubrication system with a horizontal groove on the bearing that distributes the oil over the full length of the bearing, so there is no oil ring. The pump runs at 1800 rpm rated speed and coupled to the motor with a diaphragm coupling.

 

[JJPellin]

You did not mention how the vibration amplitude changes with speed. But, run speed vibration with a round orbit and constant phase angle following repair suggests run-out or balance. The vibration is highest on the end where the wear ring was replaced. I suspect that wear ring is not concentric to the shaft center of rotation, or it changed the balance. If the radial bearing on that end was replaced, I would look for an accidental design change. The original bearing could have been a lemon bore or had a pressure dam. Vibration increasing over time could suggest a poor coupling alignment. This pump is large and probably a non-API design (foot mounted rather than split-line mounted). Alignment offset to compensate for thermal growth could be substantial.

Johnny Pellin