On my container vessel the crew observes increase in vibration of cooling sea water pumps after main engine reaches 71-74 RPM?
In November 2013 we received similar question from our Client. We send our Service Engineer to perform vibration measurement of machines and additionally global hull vibration.
To fully understand the source of problem measuring points for hull were chosen as follow: stern tube, aft and forward middle bearing, main engine free end, steering gear room, aft centerline, port and star board side bridge wing and foundations of csw pumps.
Analysis of vibration measurement of mentioned machines showed no particular problem with either el. Motors or pumps. However the RMS values of vibration velocity were showing results above ISO and maker limits focusing us on collected data form global hull measurement.
Vibrations results - SB bridge wing vertical axis
Results from points along shaft line, from points on the bridge and from aft part of the vessel showed no symptoms of low rigidity of hull and superstructure. It was noticable that there exist three main engine revolution speeds except restricted at which overall measured vibrations were rising up, but they were still within acceptable limits.
Vibrations results -foundation of csw pump vertical axis
Vibration result of csw pump foundation revealed the real source of the problem. After main engine reaches 71rpm low rigidity of foundation and lack of constructional intermediate bearing supporting pump shaft were allowing the overall construction to vibrate heavily. It was recommended to stiffen the foundation or if possible change the pumps type.