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Explain the term water hammer indicating possible effects on the system ?
Water hammer occurs when steam is admitted into a cold pipeline. The steam condenses producing both water, and a vacuum (when the water seals the pipeline from the steam supply). This vacuum causes the water plug to be drawn into the closed end of the pipe with increasing velocity producing high impact forces on the pipework. This impact force can be high enough to rupture the pipeline.
State how the risk of water hammer may be avoided ?
Water hammer is avoided by slowly admitting the steam into the cold line, and draining/venting the cold line to minimise the vacuum forming, and assist in draining the condensate.
State with reasons why the testing and treatment of boiler water is essential ?
Testing of the boiler water is important to determine that:
• The level of chlorides is within acceptable limits (below 200 ppm) to prevent hard scale and pitting
• The reserve of boiler chemicals within the boiler water is accepted (P Alk above 100 ppm, and Phosphate above 20ppm)
Treatment of the boiler water with chemicals will ensure:
• Alkalinity reserve against boiler water space corrosion
• Scale build-up is minimised
• Enough coagulant levels exist to minimise sludge build-up within the boiler
• The strength of the boiler drum is maintained
State with reasons how boiler problems may be minimised if no water treatment is immediately available and a steam supply must be maintained ?
Boiler problems would be minimised by:
• Reducing the steam load and demand on the auxiliary boiler to reduce firing temperatures
• Ensuring hotwell feed temperatures are at the recommended 95°C to minimise oxygen levels in the feed
• Minimise blowdown of boiler water, whilst ensuring chloride level is not excessive, to avoid loss of the boiler chemicals
Why wear down in main bearings is critical to the condition of the crankshaft and propeller shaft system on ships ?
The operational stresses present within the crankshaft is influenced by the bending stresses present. If the crankshaft alignment is completely straight then the bending stresses are minimal, but should uneven main bearing wear down occur, then these bending stresses will greatly increase.
When the main bearings wear down, the alignment of the crankshaft to the propeller shaft will change. This wear down will increase the bending stress on the propeller shaft, and the external moment at the engine / shaft interface, increasing the crankshaft stresses.
When the crankshaft is subjected to these high stresses, the small defects present within the shaft could develop and propagate into cracks that could lead to shaft failure.
Why total reliance is placed on frictional grip in conventional built up crankshaft on main engine ?
Frictional grip is the usual method of crankshaft construction for built-up crankshaft. This grip is subjected to the full torque of the engine output, and hence subjected to high torsional stresses. If there are any defects at this joint, then the resulting stress concentration would trigger a crack and possible shaft failure. To minimise the possibility of such defects, then only a frictional grip is permitted under Classification Rules for crankshaft construction, hence pins, keys, etc are not allowed.
Why oil holes are given large fillets in crankpins and journals ?
Defect growth from a small surface defect into a crack that propagates through the shaft material require high levels of stress. Such levels of stress are possible when the stress is concentrated at section changes. The oil hole will inherently increase the local stress levels, and thus to minimise these increases, the oil hole will have a significant radius at the surface. The size of this radius will significantly influence the local stress levels, and should be closely monitored during crankshaft construction, and possible crankpin surface repairs by grinding.
What are the causes and effects of torsional vibrations in crankshaft ?
Torsional vibrations are inherent within diesel engines, due to the varying torque produced by the piston and crank arrangement from each cylinder. This torque variation is further compounded by the arrangement of the firing order of the crankshaft. The effects of such vibrations is to increase the shear stress and hence total stress levels carried by the crankshaft in service, when other stresses such as bending and combustion loads are present.
What is meant by the term critical speed and why it can be a problem on ships ?
The critical speed of a shaft occurs when the shaft rotational speed is at or close to resonant conditions. In this condition the torsional vibration of the shaft increases greatly, and will impose very high shear stress on the crankshaft. These levels of stress could even cause crankshaft failure.
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