An LNG tanker vessel had been suffering repeated failures of seawater pumps, with the damage on each occasion resulting in the need to replace the pump completely at a cost of a few thousand dollars on each occasion.
MBVI permanent monitoring units were installed and after several months, one unit started showing some changes in behaviour. The two main features were a drop off in power factor, and the development of a hump in the spectrum around the vane pass rate of the pump.
When the power factor had dropped off to 80%, repair was advised. It was discovered that internal corrosion of the pump had destroyed the flow straighteners on the pump inlet leading to poorer efficiency. Wear was found at the replaceable wear rings which had been leading to reduced pumping – and hence the drop off in power. And crucially, corrosion had created a hole right through the internal section of the pump casing from the high pressure to the low pressure zones. Each time the impeller vane passed this point, the pressure wave created by the vane was able to partially escape through this hole, leading to a pressure pulse at vane – pass frequency. Because of flow turbulence, this was not at a single frequency exactly matching vane pass rate, but was in a broad spread of frequencies around this frequency.
The pump was repaired at low cost: the hole was repaired by cold-weld resin (Belzona) and the wear rings were replaced as a routine. The total cost of the repair was a few hundred dollars – less than one tenth of the typical pump replacement cost.
Interestingly, this failure had not been detected by hand held vibration monitoring, even on the day the pump was taken off line for repair. This is explained by the fact that the effect of the pressure pulses had an impact on the torque required to turn the shaft, but did not create a radial signal of the sort that could be detected by an accelerometer.
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