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问答题 6分

Spark erosion is caused by a voltage discharge between the main bearing and journal surface. The cau...

Spark erosion is caused by a voltage discharge between the main bearing and journal surface. The
cause of the potential is the development of a galvanic element between the ship’s hull, sea water,
and the propeller shaft/crankshaft.

The oil film acts as a dielectric. The puncture voltage in the bearing depends on the thickness of the oil
film. With increasing engine ratings, the specific load in the main bearing is increased. This will reduce the
oil film thickness, and enable the discharge to take place at a lower voltage level.

Since the hydrodynamic oil film thickness varies through a rotation cycle, the discharge will take place at
roughly the same instant during each rotation cycle, i.e. when the film thickness is at its minimum. The
roughening will accordingly be concentrated in certain areas on the journal surface. In the early stages, the
roughened areas can resemble pitting erosion. but later, as the roughness increases, the small craters will
scrape off and pick up white metal-hence the silvery white appearance.

Therefore, to ensure protection against spark erosion, the potential level must be kept at maximum 80
mV, which is feasible today with a high efficiency earthing device. If an earthing device is installed, its
effectiveness must be checked regularly. Spark erosion is only observed in main bearings and main bearing
journals.
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3 问答题 6分
Spark erosion is caused by a voltage discharge between the main bearing and journal surface. The
cause of the potential is the development of a galvanic element between the ship’s hull, sea water,
and the propeller shaft/crankshaft.

The oil film acts as a dielectric. The puncture voltage in the bearing depends on the thickness of the oil
film. With increasing engine ratings, the specific load in the main bearing is increased. This will reduce the
oil film thickness, and enable the discharge to take place at a lower voltage level.

Since the hydrodynamic oil film thickness varies through a rotation cycle, the discharge will take place at
roughly the same instant during each rotation cycle, i.e. when the film thickness is at its minimum. The
roughening will accordingly be concentrated in certain areas on the journal surface. In the early stages, the
roughened areas can resemble pitting erosion. but later, as the roughness increases, the small craters will
scrape off and pick up white metal-hence the silvery white appearance.

Therefore, to ensure protection against spark erosion, the potential level must be kept at maximum 80
mV, which is feasible today with a high efficiency earthing device. If an earthing device is installed, its
effectiveness must be checked regularly. Spark erosion is only observed in main bearings and main bearing
journals.
4 问答题 6分
Pressurized oil atomization is used for smaller capacity burners. The nozzle is located at the end of the
oil gun. In the nozzle a number of small holes are oriented to assure the rotating flow for atomizing and
spraying the oil in the most efficient way.

As the geometry of the nozzle is fixed the velocity in the small holes and slits in the nozzle depends
upon the load on the burner. The higher load the higher velocity and thereby the better atomization. On the
other hand this means that the lower load the lower velocity and thereby the poorer atomization. Typically
pressurized jet atomization is efficient for a turndown ratio 1:2, i.e. 50 % load. At lower load, the
atomization will not be satisfying and this means non-satisfying combustion quality eventually causing soot
formations. The oil pressure varies between 0.5 and 2.0 MPa. As mentioned ensuring the correct
distribution of air and fuel is vital for the combustion efficiency. One of the most important parameters that
affects the combustion of the oil heavily is the angle of atomization that is a characteristic parameter for the
specific nozzle. For the pressurized jet burners the swirl of the combustion air is created by means of a
swirler. Depending upon the nozzle the required normal operation pressure for the pressurized jet burners is
approx. 2.0 MPa.
5 问答题 6分
For a certain provision cooling plant, the capacity regulator is generally controlled depending on
pressure, temperature or relative humidity. The pulse emitter is therefore either a pressostat(压力继电器),
thermostat or hygrostat. The switching difference of the control units should be rated as large as possible.
This applies in particular to control by means of the pressure, because change to the load in the
refrigeration circuit results in relatively quick pressure changes compared to temperature, which can in turn
lead to inadmissible oscillation of the regulator.

Under partial load, the suction volume and thus the gas speed can undergo marked changes depending on the
regulation stage. For this reason, the rating and routing of the suction line must be given particular attention.
In order to guarantee sufficient oil return, neither full load nor partial load operation may allow the gas speed
to fall below a minimum level: 4 m/s in horizontal pipe sections; 8 m/s in vertical pipes.

For this reason we recommend that vertical pipes bestaggered into two separate sections (as illustration).
As a result of the falling suction gas speed velocity in partial load operation, the pipe elbow B is
blocked by an accumulation of oil. This increases the gas velocity in pipe section A. In order to guarantee adequate
oil return, both cross sections must be rated so large that the velocity does not fall below the minimum level
in full and partial load operation. In plants with extensive pipe systems it is recommended at least for R22 and NH3
to install an additional oil separator. This reduces the oil circulation period and can have positive effects above
all for cold starts with relatively high oil ejection.
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第一章 船舶主推进装置
第二章 船舶辅助机械
第三章 船舶电气和自动化
第四章 船舶轮机管理业务
第五章 国际公约、规则
专项训练一
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