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Techno Fysica BV is a Dutch-based engineering company, highly specialised in solving all kinds of problems related to the dynamic loading of machinery and installations. For this, several measuring and analysis techniques are used. In a series of articles, we will give examples of what can go wrong in the engineering and operating of vessels, and what errors lie at the root of the damages observed.

 

 

 Damage to turbo-blower foundation due to high vibrations

 
 
A number of 5700 kW diesel generator sets suffered from repeated damage to the connection of the turbochargers to the engine blocks.
At first, the vibration level of the turbocharger with standard mounting brackets was too high after which the brackets were replaced by custom made items with increased stiffness.
After modification, the bolts connecting brackets to pedestal fractured. The bolts were lengthened where possible to decrease the vibratory stress in the bolts.
When the main diesel generators (MDG) were in service, the brackets started to crack. On MDG’s where the brackets did not crack, the welds of the pedestal on which the brackets are mounted started to crack.
 
Measurements were carried out and it was discovered that the firing frequency of the engine exactly matched the natural frequency of the turbocharger on it’s foundation in transverse direction which resulted in resonance and  led to vibration levels up to 120 mm/s which exceeds the allowable level by a factor of 2.5.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

racked bracket of a turbocharger

 

 

 

 

 Cracked pedestal of a turbocharger
 
  
It took a considerable amount of measurements, reports and discussions to convince the engine builder to design another type of pedestal and brackets to achieve the required level of reliability. Finally, a modified pedestal was designed, including modified brackets, which was installed on the MDG’s.
Vibration measurements showed that the resonance speed was shifted outside the operating speed range, that local stresses were reduced because of a better distribution of forces  into the pedestal (more bolts) and that stress raisers near welds were eliminated.
Due to these measures, vibration levels were reduced by a factor of 3 and further damages, up till now, have not occurred.
 
It is, however, interesting to notice that, five years later, the introduction of a new type of turbocharger on modern engines of the same manufacturer lead to exactly the same problem.
 
 

 
Resonance curves
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Transverse vibrations of a turbocharger
before (upper) and after modification
(lower) of the supports and pedestals.
The natural frequency of the turbocharger on it’s brackets coincides with the firing frequency of the engine and results in vibration levels up to 200% of the allowable level. When this conclusion was drawn, the manufacturer of the engine said that vibration levels could not be reduced, that the ship owner had to live with it and see if the installation would live up to it’s expectations regarding reliability and maintainability.
 
From the point of view of an independent consultant such as Techno Fysica, this reaction is inappropriate for a ship owner who has to exploit the ship for at least 20 years with the assumed degree of availability without unnecessary downtime and the consequential financial losses.
 
These measurements were not taken because somebody asked for them but because we were there for another reason and simply knew were to look in spare time for potential problems, based on earlier experience with similar installations.
 
 
 
 
 
For more info, please contact techno Fysica BV in the Netherlands at:
 
Techno Fysica BV
p.o. box 351
2990 AJ   Barendrecht
The Netherlands
Tel.  +31 180 620211
Fax. +31 180 620705
info@technofysica.nl
www.technofysica.nl
 

 

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