One of our customer ask us to help with problems on his new built vessels. The vessels were still on guaranty period, but he has to face serious problems with diesel generators exhaust lines – especially with dampers and mounting elements. Customer complained about the necessity of frequent replacement of damping elements, which were damaged even every few months. Shipbuilder together with owner decide to ask us for help in that matter, cause actions taken previously by them do not give satisfying results.
Our investigation start with data collection which allow us to narrow the search field. Mentioned vessels were diesel-electric cruisers which are quite complicate construction with four diesel (V12) generators over 10 000 kW each. Due to fact that our customer suggest problems with them we start with crosscheck of its condition. Vibration measurements were performed for each of generators in different working conditions, but results shows no abnormalities according to international standards – that have forced us to look for problems elsewhere. Next step were to focus on the vibration dampers and mounting elements directly. We decided to crosscheck all exhaust lines mounting elements and dampers to find those with possible highest readings. Portable data collector were used to spot check measurements along the lines till chimney. Based on results we found that problem exist only in first part of exhaust lines, from diesel engine to exhaust manifold.
We decided to crosscheck condition of dampers in that area by multichannel measurements of its structure, which allow us establish when dangerous vibration occurs and what is its source. Using multichannel vibration data collector we mounted sensors on each vibration damper – from both sides (engine exhaust line side and structure side). Those type of measurements allow us do not only confirm vibration level but also damping effect of each element.
Pic.1 . Exhaust line Vertical damper overview.
As like for Diesel Generators, measurements were performed in wide range of load – that’s allow us to establish which load generating the most vibrations and how their spectrum looks like. Our goal were achieved. We found that some of the dampers have very high vibration level– over 2 times the DNV limit for cracks development in steel (45 mm/s, RMS of Velocity 4-200Hz) , that’s clearly shows why dampers are damaged so frequently. Next step were to find the source of those vibrations. Detailed spectrum analysis shows that the main source of high vibrations are frequencies 20.8Hz, 25Hz and 29,2Hz. Those frequencies are related with few things –first, with diesel engine working conditions (they were found in measurements performed on DGs). Results were presented and discussed with shipbuilder representative and we found that those frequencies could be related with exhaust gases pulsation but were not clear why they produce high vibration. We started to suspect some resonances at those frequencies. We decided to perform additional type of measurements – natural frequency measurements of exhaust lines in area of interest.
That part of work were discussed very detailed with shipbuilders – we had to cooperate to perform the best possible measurements which allow us confirm the natural frequencies of the exhaust elements. Based on information obtained from them and our experience we performed natural frequencies measurements (hammering test).
Hammering test were performed by modal hammers and multichannel data collector – it was very important to get results from each point simultaneously, from same hit. Thanks to that we received information how structure responds to impacts in different directions. Hammering were performed in 3 main directions: UP-DOWN, PS-STBD and FW-AFT.
Pic. 2. Hammering position overview.
Data were recorded and delivered to office, where special software has been used to present results as vibration magnitude and phase. That type of results presentation allow to clearly identify if frequencies of interest are excited by hammering in different directions. As we suspected, we found them in collected data. Each of them exist in some cases depends on hammering position which have been noted in peaks in vibration magnitude together with phase jumps. As a results, our customer received detailed report with data presented in easy to read way. Shipbuilders used those information to propose solution.
Meantime we decided to do one more test – vibration measurements of one of the exhaust lines with mounted additional weight of 300kg. Such action (adding the weight) should allow us to separate natural frequencies from exhaust gases pulsation frequencies and reduce the vibration level on dampers. We choose one of the lines with the highest level of vibrations, and perform test at the same working conditions as previous. Performed action give very good results – vibration level were reduced from 84 mm/s to 29 mm/s (DNV limit: 45 mm/s, RMS of Velocity 4-200Hz). Thanks to performed measurements we support our customer and shipbuilders in solving problem with premature dampers failure.
Problem were founded and solution were properly indicated – all related parties were satisfied.