Q. Diesel Electric propulsion is now being chosen as the power plant for an increasingly wide variety of vessels.
a. Sketch a simple layout of such installation (6 Marks),
b. Explain the advantages of selecting such a plant. ( 10 Marks) Jan20
Ans: The electrical propulsion arrangement for the ship is often explained as a diesel-electric or turbo-electric system. It is characterized only by the type of prime mover. When the prime mover is a diesel engine, then it is known as Diesel-Electrical Propulsion.
Types of Diesel Electric Propulsion:-
There are two Types of Diesel Electric Propulsion
- Frequency controlled A.C Motors &
- SCR controlled D.C Motors.
Frequency controlled A.C Motor drive system were usually more cost effective below 500 H.P & SCR controlled D.C motors systems at the higher end. The offshore drilling industries favor SCR controlled DC drives.
Modern SCR & frequency controlled systems have efficiencies approaching 97% in power conversion. The selection of one over the other is an application issue.
Lay Out of Diesel Electric Propulsion in Ships
Advantages of Diesel- Electric Propulsion
1. Economic Reasons
Diesel electric propulsion is generally economical for a number of reasons:
1. Optimal utilization of fuel for the diesel engines to produce electrical power, even in partial load ranges.
2. High efficiency across the entire speed range.
3. Less maintenance costs through longer service intervals depends on the optimized operating times of diesel engines with constant speed.
4. Minimal standstill time for maintenance & service.
5. Flexible & need-oriented use of diesel generator sets in combination power plant for drives & onboard power systems.
2. Availability
Diesel electric propulsion systems demonstrate high availability for the reasons that include:
1. Modular design with small probability of total loss of the propulsion power.
2. Sharply reduced number of moving mechanical parts.
3. Proven technologies based on the decades of operating experiences.
4. Redundant drives with one propeller are also possible.
5. Designs are also possible for the maximum redundancy requirements.
3. Environmental Compatibility
Diesel electrical propulsion systems save the environment because the pollution emissions of diesel engines is decreased by operating the engine at the optimal speed and load ranges.
4. Operating Convenience
Diesel electrical propulsion is very convenient for the users, because of the following reasons:
1. Excellent dynamic response from zero(0) to maximum propelling speed.
2. Short reversing time.
3. Availability of maximum torque across the entire speed range at propeller.
4. Quite operation.
5. Minimum mechanical vibrations.
5. Flexibility
1. Flexible arrangement of the components in ship.
2. Simplified mechanical requirements for the propeller shaft.
3. Reduced space requirements in shaft system.
4. Design & engineering of propeller is independent of the drive.
5. Flexibility in the choice of the diesel engine speed.
Q. Describe the working of a single phase full-wave rectifier with a resistive load. Draw the load voltage and current waveforms. (8 marks) July 2019
Single-Phase Full-Wave Rectifier
Differing from the half-wave rectifier, the full-wave rectifier allows the input AC source to complete its current flow circuit in both positive & negative half of an AC cycle. The structure of the full-wave rectifier
There are 4 diodes in the full-wave rectifier circuit. When the AC source voltage is positive, the current flows through D1 to the load & back to the AC source via D2. When the AC source voltage is negative, the current flows via D3-load-D4 path. Either way, the current always goes across the load from the positive to the negative pole.
Figure below shows the waveforms of the input and output voltage of the full-wave rectifier. It can be considered as if rectifier flips the negative voltage to the positive side.
Q. With respect to circuit breakers:- (a). Compare the effectiveness of a current limiting circuit breaker with that of a HRC fuse (6 Mark) July 2019
- A current limiting circuit breaker exhibits a current limiting characteristic. It can reduce the peak apparent fault current to a lesser value and isolate the fault in less than a half cycle time. They can not interrupt currents higher than their nameplate short circuit rating
- Current-limiting circuit breakers, provide short-circuit protection without the need for current-limiting fuses.
- The breaker must be subjected to short-circuit testing at three levels: the threshold current, the intermediate point and the maximum interrupting rating.
- it can be more expensive to install, repair and replace.
- When breaker tripped it can be reset.
- It is most suitable for power circuit .
High Rupturing Capacity (HRC) Fuse.
- It can carry short circuit heavy current for a known time period. During this time if the fault gets removed, then it does not blow off. Otherwise, it blows off or melts.
- It is a part of the circuit which consists of a conductor which melts easily and breaks the connection when current exceeds the predetermined value.
- The fuse is the weakest part of an electrical circuit which breaks when more than predetermined current flows through it.
- It is less expensive to install, repair and replace.
- It is most suitable for electronic equipment, /PLC/ PCB etc .
- When fuses are blown, they need to be replaced
- The materials used for fuse wires are mainly tin, lead, zinc, silver, antimony, copper, aluminum etc
Q – With reference to three phase induction motors; • Explain the phenomenon of Crawling and Cogging. (6 marks) July 2019
Crawling- Sometimes, squirrel cage induction motors shows a tendency to run at very slow speeds(as low as 1/7 of their synchronous speed). This phenomenon is called as crawling of an induction motor. This action is due to the fact that, flux wave produced by the stator winding is not purely sine wave. But, it is a complex wave consisting a fundamental wave & odd harmonics like 3rd, 5th, 7th etc. In this case, motor will not accelerate up to its normal speed, but it will run at a speed which is nearly 1/7th of its normal speed. This phenomenon is known as crawling in induction motors.
Cogging- The phenomenon of Magnetic Locking between the stator & the rotor teeth is called as Cogging or Teeth Locking. Even after applying full voltage to the stator winding, the rotor of a three phase induction motor fails to start/ rotate. This condition occurs when the numbers of stator & rotor slots are either equal or have an integral ratio. The number of the stator slots equal to or an integral multiple of the rotor slots, strong alignment forces is generated between the stator & the rotor. As a result of these forces an alignment torque greater than the accelerating torque with consequent failure of the motor to start. Thus, a locking is created between the stator & rotor teeth. This condition is called as Cogging or Magnetic locking.
Q – With reference to MARPOL Annex IV:
(a). Draw a biological sewage treatment plant and explain the principle of operation. (8 marks) Jan20, July 19,
(b). Periodical maintenance and checks tests required to be done to verify the effectiveness of the above system. (8 marks) Jan20, July 19
(a) Biological Sewage Treatment Plant Raw sewage is passed to an aeration chamber and air is supplied through diffusers. This helps the action of aerobic bacteria which break down the sewage into carbon dioxide, water & inorganic waste. Figure below shows a biological sewage treatment plant.
Sewage then passes into the settling chamber. Any solids that settle out are returned via an air lift to the aeration chamber which assures that they are fully broken down. The sample applies to any surface scum. A small vane type air compressor supplies the air for the air diffusers & air lift.
The clear liquid then passes through the chlorinator where the liquid is disinfected, into the chlorination chamber. The chamber has float switches, which control the discharge pump, & a high level alarm.
Maintenance and checks
Although the sewage treatment plant runs automatically, without regular maintenance the unit will not function properly & anaerobic bacteria may help in the formation of hydrogen sulphide & methane, both of which are hazardous.
The chambers required to be cleaned out occasionally to remove any accumulated matter. The aeration diffusers should be checked to assure they are clear & that air is bubbling from them. The air lift returns should also be checked to make sure they function correctly. These generally have a clear plastic pipe so that the sludge can be seen returning to the aeration chamber.
The internal tank coating should be checked for any signs of cracking or blistering. When cleaning out a sewage unit rubber gloves & a mask should be worn. After overhaul the external surfaces of the unit & surroundings are to be washed down with disinfectant. Hands should also be thoroughly scrubbed & overalls washed.
Q – With Reference to entry into enclosed spaces onboard:
i) Define “Enclosed Space” and give examples of enclosed spaces onboard. (4Marks) Jan 20
ii) Describe hazards relating to entry into Enclosed spaces on board (4 Marks)
iii) Explain safety precaution taken prior to entry into enclosed spaces on board vessel. (8Marks) Jan 20
Answer: – i) An Enclosed Space is defined as any space of an enclosed nature
Where there is a risk of death or serious injury from hazardous substances or dangerous conditions such as lack of oxygen, toxic gas or other vapour, Limited openings for entry and exit, Unfavourable natural ventilation.
Examples:- Ballast tanks, Fuel oil tanks, Pump rooms, Cargo holds, double bottoms, lube oil tanks, Void spaces, Engine crankcases, BOW Thruster spaces, Battery lockers, Boilers, Cargo tanks, Double hull spaces, Sewage Tanks, Cofferdams, Inter barrier spaces, CO2 rooms, Paint lockers, Fresh water tanks, Spaces affected by chemical spill, Pressure vessels, Gas bottle storage lockers, spaces affected by fire, compressor rooms, duct keels, chain lockers, hollow spaces.
Enclosed space, as defined by IMO Resolution A.1050(27), means a space which
has any of the following characteristics:
- Limited openings for entry and exit;
- Inadequate ventilation; and
- Is not designed for continuous worker occupancy
ii) Describe hazards relating to entry into Enclosed spaces on board. (4 Marks)
Hazards relating to entry into enclosed spaces – There are 4 main hazards:
a) Hazardous atmosphere – There are 7 types of hazardous atmospheres:
- oxygen depleted
- oxygen enriched { by certain chemical reactions, leaking oxygen by hoses and torches( pipe in ship yard)}
- presence of toxic gases or liquids
- flammable atmosphere
- temperature extremes
- presence of dust
- absence of free flow of air
b) Physical or configuration hazard – Examples of configuration hazards include:
- slicks, wet surfaces and ladders
- very narrow openings that inhibit emergency evacuation
- risk of fall from unguarded heights
- complex arrangement of structure making illumination difficult
- surface configuration such that cleaning ahead of entry is difficult
- objects falling from deck head openings
- the pipelines and ventilation trunking running into and through the space,
- absence of railings and the presence of openings in floors. Many of this present trip and fall hazards.
- Strengthening frame work and box structures within the space may create areas where air exchange does not occur effectively when ventilated or sitting water has not been completely pumped out.
c) Changing and hazardous conditions –
- Changing conditions within an enclosed space such as water ingress, oxygen-depleting work (burning, welding), ventilation failure and vapour from paint or cleaning materials must be monitored.
- Sometimes working within an enclosed space can coincide with other activities being carried out elsewhere onboard. Potentially hazardous changes to conditions within the enclosed space caused by external factors are also crucial but less easily managed. These may be caused by the inadvertent actions of other ship or shore staff, work in adjacent or connected spaces, cargo work, pumping ballast or fuel transfer and hot work, as well as communications of work being carried out in the enclosed space, for example, when there are shift or watch changes.
d) Engulfment hazard –
- Engulfment is when the person entering is drowned, suffocated, or trapped by falling material. Loose, granular material stored in holds or tanks, such as grain, can overcome & suffocate a person.
iii) Explain safety precaution taken prior to entry into enclosed spaces on board vessel. (8Marks)
a) TOOL BOX TALK
- All parties to discuss the job to be done in the space.
- What are the hazards of the space and how can they be controlled.
- What are the hazards of the job and how can they be controlled.
b) Risk Assessment
- Document the hazards and necessary safety measures.
- Empty the space if necessary and take steps to prevent the space filling up.
- Lock out valves and pumps.
- Place notices forbidding their operation.
- Secure the space adjacent to other tanks, holds, or pipelines which if not secure could present a danger.
c) Ventilate
- The space to be thoroughly ventilated naturally or mechanically.
- Guard any openings against accidental and unauthorized entry.
- Test the atmosphere in the space for oxygen content and the presence of flammable and toxic gases or vapour.
- Do not enter until the atmosphere has been determined to be safe.
d) Permit to Work:–
Complete an enclosed space entry permits to work, confirming that
- The hazards of the job and of the space have been dealt with,
- The atmosphere in the space is safe and ventilated,
- The space is adequately illuminated,
- An attendant at the entrance has been appointed,
- Communications have been established between bridge and entry point, and, entry point and entry party.
- Emergency rescue equipment is available at the entrance and there are sufficient personnel on board to form a rescue party.
- All personnel involved are aware of the task and the hazards and are competent in their role.
Q – Describe the checks done for the atmosphere inside enclosed spaces. (4 marks) April 2019.
Ans- In confined spaces there are a number of atmospheres that are hazardous :-
Oxygen- Assure that proper oxygen levels are present.
Combustible gases- Assure that combustible gases are not present.
Toxic Gases- Assure that toxic gases are below the OSHA permissible exposure limit. Common toxic gases in a confined space could be hydrogen sulfide (H2S) & carbon monoxide (CO), but other toxic compounds could be present. RAE Systems offers monitors to assess all these gases either individually or simultaneously.
In a confined space, it is important to take samples at the top, middle, & bottom to locate varying concentrations of gases & vapors. Highly concentrated gases can collected at the top or bottom of a confined space depending on whether they are less or more dense than air. Dilute gases & vapors in the ppm range distribute evenly throughout a confined space. It is important to sample at a distance from the opening because air supply near the entrance can give a false sense of adequate oxygen presence. As the remote air monitoring is completed & the area is safe.
