Marine Diesel Engine Exhaust System Mutes New Viking Super Ferry

By George Backwell at December 15, 2012 05:56
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Noise, a ‘pollutant’ and thus injurious to health, though endured as a feature of the job without question by engine room personnel since steamships came along, will be restricted by revised SOLAS ll-1/3 maximum noise limits applicable to all new ships from July 2014. Keeping ahead of the game, Wärtsilä, came up with an appropriate remedy for machinery space noise with their patented ‘Compact Silencer System’ (CSS) which is installed in the  new Viking Lines’ cruise ferry Viking Grace.

CSS technology minimises engine noise emissions, in particular in the most disturbing low frequency spectrum, and has given the Viking Grace a degree of silent operation appreciated by crew and passengers as well as people living on the shores of the peaceful scenic route through the Turku and Stockholm archipelagos.

'Viking Grace': Photo credit Viking Lines

The noise generated by a reciprocating engine encompasses the entire frequency spectrum. As the power output demand increases, the exhaust noise becomes louder, particularly in the low frequency region. The firing frequency of medium-speed engines (which power Viking Grace) is within that low frequency range, and typically dominates the noise spectra.

Noise from the combustion and turbocharger is generally in the high frequency area, which are effectively dealt with by conventional silencers, but low frequency noise often causes the greatest annoyance and travels further . The CSS consequently pays special regard to attenuate noise over those frequencies.

Wärtsilä Compact Silencer System

The design principle puts the emphasis on the desired noise level on board, rather than the conventional way of subtracting the transmission loss of a standard silencer from the exhaust noise spectrum of an engine.

The slender design of the Wärtsilä CSS features multiple elements along the exhaust pipe, from the engine room to the exhaust outlet. The intention with Wärtsilä CSS is not to replace one conventional silencer element, but to effectively design the complete exhaust system. Interestingly, the acoustical properties of the system can be optimized according to the desired sound level requirements on bridge deck, in harbour, etc.

Conventional Silencer versus CSS: Image courtesy of Wärtsilä

Conventional silencers usually have both absorptive and reactive chambers, but the reactive chamber can generally only be effective at one frequency. With the Wärtsilä CSS, this limitation is overcome by tuning its elements to match the engine over the noise range.

The three standard Wärtsilä CSS elements are composed of a reactive element for attenuation of lower frequencies; a resistive element-absorptive silencer to deal with higher frequencies; and a combination element of both reactive and resistive elements.

Wärtsilä say that the advantage of the CSS is its low back pressure, which is of importance in view of an increasing demand for exhaust after-treatment systems that typically feature high back pressure.

The slender design and its minimal 'footprint' offer flexibility in distributing the elements within the exhaust system, as  evident in the schematic above.



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