Noise is next

There are numerous national and international codes that recommend and, in some cases, demand that noise levels are kept below certain limits. This applies not just to noise levels outside a ship but also to the noise levels in various parts of the ship. This has placed greater demands on the engine designer to produce an economical and reliable engine that will meet noise emission regulations both now and in the future.

MAN B&W has recently issued a paper which looks at the definition of what noise is in relation to its two-stroke engines and to go on to discuss the primary noise sources and types of engine-related noise emissions, noise level limitation and the current situation in relation to noise.

The popular definition of noise is an undesirable sound which in some cases can be damaging to heath, if the level is high enough. The primary sources of engine noise on a two-stroke engine emanate from the turbocharger air and gas pulsations, exhaust valves, fuel oil injection systems and, to a certain extent, the chain drive. MAN B&W says that the best way to reduce engine related noise is to reduce the vibration energy of the source, or if this is not feasible or adequate, to attenuate the noise as close as to its source as possible.

MAN B&W uses computer models to provide its customers with data on each of its two-stroke engine models. This is backed up by both theoretical calculations and actual measurements. The company?s constant-pressure turbocharged two-stroke diesel engines are, unlike the former impulse turbocharged engines, equipped with a large exhaust gas receiver located between the gas outlets of the cylinders and the turbocharger(s).

Because of its location close to the noise source, the gas receiver also functions as an exhaust silencer which, says the engine manufacturer, in particular attenuates the low-frequency gas pulsations. MAN B&W says that an absorption gas silencer will normally be adequate to keep noise levels on the bridge below 65dB(A) as it attenuates the dominant frequency ranges. It advises that as the exhaust gas arrangement itself can generate noise, the exhaust gas silencer should be inserted as close to the funnel as possible.

Airborne noise

The average airborne noise level in an engine room will be about 105dB(A), depending on the engine type and size. However, this is likely to rise to about 110dB(A) near a turbocharger. The turbocharger also has a dominant influence on the total average airborne noise level – an influence that has been increasing of late because of the demand for higher engine powers.

One of the problems is that it is often the maximum noise level measured in an engine room that has to meet the specified requirements, which can require the addition of extra noise reduction measures to reduce the maximum noise levels by 3-5dB(A). However, MAN B&W says that it would be extremely difficult to meet stricter requirements with regard to maximum engine room noise levels of for example 105dB(A) instead of 110dB(A), especially in view of the influence of sound reverberations and the noise emitted by other machinery.

The possibility of reducing the noise from an existing engine is greatly limited because the noise stems from many different sources and, because the noise transmission paths – through which vibrational energy is transferred from one area to another through the engine ? are numerous, says the engine builder.

However, it does explain that the transmission of airborne noise from the engine room to other locations such as the accommodation areas for example, normally has no influence on the actual noise level in these locations.

Noise excitation

Vibrational energy in the engine propagates through the engine and into the bedplate flanges and is then transferred to the ship?s tank top and outwards into the vessel?s structure, which starts to vibrate and thus emits noise.

MAN B&W says that pulses from the combustion process and the reciprocating movement of the pistons are among the sources, which can generate this vibrational energy. However, the vibration velocity level in a two-stroke engine is, on average about 15-29dB lower than in a four-stroke engine. The vibration velocity levels in the diesel engine feet can, with the aid of empirical formulas, be used to calculate the excitation velocities and therefore the sound pressure levels in the accommodation quarters and shipyards or their consultants usually have a copy of these at their disposal.

The limits are usually defined between owner, shipyard and engine builder, or indirectly by referring to national or international legislation, such as See-Berufsgenossenschaft or IMO specifications. These specifications have, in general, resulted in a considerable reduction in the noise levels in newly built ships, especially in accommodation areas, says MAN B&W.

Noise levels in accommodation areas have been considerably reduced by the introduction of the floating floor, which has reduced noise levels to 45-65dB(A) or lower. Similar to those required in passenger ships, says MAN B&W.

In conclusion the engine manufacturer says that the noise emitted by the engine?s exhaust gas and the structure borne noise excited by the engine are so low that keeping within the requirements for the bridge wing and the accommodation areas will not be a problem. However, it does point out that the airborne noise from the engine in the engine room is so high, that, in some cases, there is a risk that the noise limits for the engine room cannot be met, unless additional noise reduction measures are introduced. MAN B&W says therefore that, in the future, it must be expected that it will be very important, from a marketing point of view, to develop an engine with reduced airborne sound levels.

Oil dialysis for diesels

The kidney cures waste oil

problems at a stroke

The Harada Corporation in the UK is in the process of launching an oil dialysis system for lubricating oil into the European market. While the system is not new in Japan it is new to Europe and John Cashen, general manger for Harada Corporation Europe says Wärtsilä is currently in the process of testing the system.

The Nephron system, as it is called, has been available on the Japanese market for several years with over 1,000 units using the system on ships from fishing vessels and tug boats to containerships operated by Mitsui OSK, NYK and Sanko Lines. Cashen explains that a majority of these larger vessels are operating two-stroke engines and that scuffing problems on these vessels have been considerably reduced.

The Nephron system is designed to recycle lubricating oil almost permanently and keeps the oil purity consistent during operation. Kobe Diesel claims that one engine in operation has not had its oil changed for over 16 years with the oil remaining in good condition and having only been supplemented with a small amount of additional oil.

The Sumimoto Research Institute in Japan has developed the technology because it believes that oxidation of lub oil is negligible, that it is heat resisting and that because of this can be used semi permanently if kept clean by speedy removal of sludge.

The institute claims that foreign particles up to one micron in diameter can now be filtered out and that additives such as alkali, consumed during operation, can be replenished. The institute says that the system not only protects the engine but also protects the environment as the oil consumption and disposal is reduced. It is also claimed that the time between engine overhaul and the life expectancy of parts will be prolonged.

The system acts similar to that of a kidney and uses a new type of filter that uses many sheets of filter paper piled up in a cylindrical case. The paper sheets are circular in shape with a hole at the centre. Lub oil sludge is removed in the course lub oil flow from the outer periphery towards the centre. The sludge is accumulated on the filter paper.

Considering what this could mean for oil companies Cashen says that they have been extremely reluctant so far, although he claims that Shell has tested the system in Japan and that the results that came back were “very interesting.”

The same system can also be used on fuel oil to reduce contamination allowing cleaner pumps and injection valves to offer improved reliability.