Electrical Distribution System Answers | Assessment Answer

Answer:

1 – Give a critical analysis of why the ship’s Low Voltage (LV) power electrical distribution system is of the “Insulated neutral type”

The major requirement ashore is the safety for human. The priority onboard is not at all the safety of the machinery, but the priority is to keep the essential machinery in continuous operation. If because of any earth fault, any of the essential components stops working there are a heavy chances of collision, fire or grounding of the ship. Hence, to avoid such instances insulated neutral type system is used.

When using earthed distribution system, if earth fault occurs, it may lead to short circuit across the ship and will blow up the fuses. This short circuit takes place because of double break insulation which allows the connection of the conductors.  As these conductors are connected, a very high current flows through which by-passes the loads. In earthed distribution system, only one earth fault is enough to create a hazardous situation.  Whereas, in Insulated neutral system, if one earth fault occurs, this will not affect the operation of the circuit and the machinery will operate continuously. If second earth fault takes place, then these two cumulative will result in short circuit and then the process will stop. Thus, two earth faults will be required in Insulated neutral type system to create any such destruction which doesn’t happen most likely. It can be concluded that Insulated neutral type system is more effective when compared with any other system. This is reason why Insulated neutral type system is preferred over earthed distribution system.

2 – evaluate the importance of programmable logic controllers PLCs to the machinery control and monitoring systems on-board modern tonnage.

The machinery control and monitoring system which was previously used onboard doesn’t possess the extensive I/O arrangements which are the essential requirement of the current days. The current PLC (Programmable logic controller) gives the access to the programmer to develop and make changes according to the situation. The uses of actuators and sensors have evolved in the recent past just because of the use of PLC’s.  The PLC’s operates the magnetic relay/solenoids, hydraulic cylinders and also the electric motors. The previously used machinery and monitoring system uses the form of instruction list programming which was based on the stack-based logic solver in which the difficultly level of tracking circuit problem was immense. However, after the arrival of PLC’s this problem have been eradicated completely. The PLC’s uses a simple ladder logic which appears as simple electric circuit diagram.

Almost all the operations onboard are done using PLC’s. These include processing power, storage, data handing, motion control, sequential relay control which are still evolving day by day.

The second importance of PLC’s are that it can be armored in extreme conditions too; like, cold, heat, dirt and moisture whereas this  is not feasible when we talk about machinery control monitoring system. The PLC’s used onboard are different when compared to other PLC’s which are used for production purposes or light work. Very high precision PLC’s are incorporated within the mechanism of the ship so that there are no any chances of any damage. They are highly customized in order to be operated efficiently and accurately. Any error in the PLC will lead to severe destruction to the system as well as the human present within the system.

The next is the cost at which it replaces the other mechanism. PLC’s are very economical when compared with the machinery and monitoring system.

3 – explain how the electrical power is maintained to the vessel’s essential operating services if the total generating capacity of the power plant is suddenly reduced, for example by the loss of one of the diesel generators.


There are various situations in which the supply from the power plant is suddenly reduced; however this doesn’t affect the operation of the vessel. Let us consider the situation of loosing of one generator. As we know that generator is considered to be the heart of the electrical design, it is quite evident that loss if single generator should have also affected the operation of the vessel’s operation. The generators possess the ability to work on its own and can manage the large system loads swings. There should always be special consideration for the additional operations which might occur suddenly.

 As per the International maritime regulation, at least two of the generators are required for the electrical power system. Out of which one is driven from the propeller shaft and for the other one, it must have its own prime mover. Considering this situation if one is driven from the shaft of the propeller and the other fails, then this will make the ship non-compliant according to the International regulation. Therefore, in almost all ships the owner opts for three generators. Out of which one will be used for the normal sea load and the other two will be kept as reserve and on standby mode for any uneven fluctuation of loads or any specific maintenance work.

As the generator fails, it automatically actuates the standby generator and the electricity is supplied. As the frequency and the voltage are settled, the electricity is supplied to the necessary equipment and machinery one after another. Further, if this standby generator fails to start because of any constraint such as not settling the frequency and voltage, the emergency generator starts up and provides the required power to the machineries.

4 – 0.5 kg of ice at -5 oC is put into a vessel containing 2 kg of water at 15oC and mixed together, the result being a mixture of ice and water at 0oC. Calculate the final masses of ice and water, taking the water equivalent of the vessel as 0.15 kg. The specific heat of ice is 2.04 kJ/kg K and the latent heat of fusion is 335 kJ/kg.

m1 = 2 kg

the initial mass of water

t1 = 15 °C

the initial water temperature

m2 = 0.5 kg

the mass of added ice

t2 = −5 °C

the temperature of added ice

Let’s solve it out process by process

Heat required by ice to change its temperature from -5 to 0 degree Celsius

                       

                         = 0.5 × 2040 × (-5 - 0)

                         = 5100 J

Heat required by ice to change its phase

                        =

                         = 0.5 × 335000

                         = 167500 J

Heat rejected by water to attain temperature of 0 degree Celsius                      

                         = 2 × 4186 × (15 - 0)

                         = 125580 J

Upon further calculation we found out that the final mixture of ice and water will contain 2345 g of water and 155 g of ice.

5 – Explain the main properties of “Superheated steam” and give the reasons why it is used in a marine steam plant where Turbines are used

The steam which is at higher temperature than that of its boiling point such that it can be cooled by some amount and will not affect the state of it is called as superheated steam. That is, it will not be converted to a mixture of liquid and saturated vapor even if the temperature is lowered by some value.

Superheated steam incorporates with tremendous internal energy which is used for the rotary motion through expansion of it against the reciprocating pistons and turbine blades. The most important property of superheated steam is the ability to release internal energy without decreasing the temperature to the condensation temperature of water vapor.

There are various reasons because of which superheated steam is only used in the marine steam plant and are as follows:

If we use water vapor, which generally contains some droplets of liquids it will be difficult to compress the water vapor as the liquid droplets are incompressible at those pressure at which the turbine operates.

Superheated steam doesn’t changes its state even after releasing its internal energy, whereas if the steam is not superheated, then after releasing its energy it will take form of liquid droplets which will incorporate with steam and strike the mechanical components of the turbine. This may force the mechanical components to fracture, crack or bend.

Using superheated steam ensures that throughout the passage from the turbine to all its mechanical components of the engine, it remains as a compressible gas and will not damage any parts of the engine.

6 – Describe how the thrust from the propeller is transmitted through the shafting and associated equipment, to the hull of the ship

The transmission of thrust from the propeller to the other equipment is governed by the thrust block, propeller shaft and the stern tube. In other words, the transmission of energy and the control of torque from the thrust block to the stern tube are controlled by these components only.

The thrust block present just aft of the main engine and its purpose is to transmit the energy to the structure of the ship which is produced by the rotating propeller. The thrust block contains metals pads which are wedge shaped with oil groves into them. This prevents them from wearing because of the rotation of the drive shaft. There is an oil reservoir at the bottom through which the sea water is circulated. Next, the power is transmitted to the propeller shaft which is connected to the main engine flywheel through the thrust block, which is further passed along the tunnel of the shaft. The stern tube is at the end of the transmission system, it seals the propeller shaft to the aft hull. 

7 – Explain how sulphur dioxide comes to be a common chemical found in the composition of the exhaust gases of ocean going ships

There are various exhaust gases which are emitted from the exhaust of the ship engines. However, the Sulphur dioxide is one of the common chemical compounds which are found.  There are several reason because of which this happens but the most important one is the “leftovers”. The presence of leftover in the engine means that the combustion is not complete. Generally, the fuels used in the ships engines are gasoline which contains greater amount of Sulphur when compared with other fuels which are used. This helps in creating Sulphur dioxide at the time of combustion of fuel.

In most of the cases, this Sulphur dioxide is oxidized by the catalyst and SO3 is formed which further react with water and forms Sulphuric acid, whereas in some of the case hydrogen sulphide gas may also be formed. This process given a smell of rotten egg and a black thick smoke is generated. This is the reason why we see black smoke coming out of the exhaust of the ship’s engine. This is one of the major concerns of the water transporters as well as the living being on this planet. It is very necessary to keep a close eye on the total emission of Sulphur dioxide. The US government has already passed the regulatory according to which the % of Sulphur dioxide shouldn’t be greater than 3.6% of the total emission. This can also be controlled by desulfurization of the flue gas, which are being used in almost all the factories and plants across the globe.  During this process, the flue gases are allowed to pass through a close chamber where calcium hydroxide is sprayed. The SO2 react with it and form calcium hydroxide, calcium sulfate and calcium sulfite.

8 – Explain the term “hydrodynamic lubrication” and give the reasons why it can fail.

The term hydrodynamic lubrication is given when there is no direct contact between shaft and the bearing material. A wedge or layer of oil supports the shaft rotating within the bearing.

The principle used in hydrodynamic lubrication system and because of which the film supports such high load is similar to the aquaplaning of the car on the wet road surface. The tyre of the car slides over the water accumulated between the road and the tyre. This happens because the water accumulates faster than the time required by the weight of the car to remove that water from between and the car floats on the water surface. The same principle goes with the hydrodynamic lubrication system. A wedge is created between the two surfaces because of the relative motion between them. This wedge shaped zone creates high pressure film between the surfaces.

Consider the fig, when the journal is at rest, there is a direct contact between the bearing because of load “W”. When the journal starts moving, it will climb the bearing surfaces and after certain speed the lubricant is pulled a wedge shaped and pushes the journal to the other side. Hence, it is necessary that continuous and sufficient lubrication is provided to the system irrespective of whether supplied under pressure or not.

There are various reasons because of which the failure of hydrodynamic lubrication can fail and they are as follows:

  1. The weight of the journal is much higher such that it doesn’t allow the lubrication to form a wedge like structure.
  2. If the speed of the shaft is too slow, then this will not allow the lubricant to form a film between the two surfaces of the contacting material.
  3. Hydrodynamic lubrication can only take place if continuous and sufficient lubricant is provided to the system.

9 – Complete a line drawing showing the 5 main components, and associated pipework, found in a “Basic refrigeration system”. In 300 words explain the operation of the system

The five main components of the basic refrigeration cycle are as follows:

  1. Evaporator
  2. Condenser
  3. Heat Exchanger
  4. Compressor
  5. Expansion Valve

Working Principle: Consider the beginning of the cycle at the evaporator. Low pressure liquid is fed into the evaporator which absorbs heat, expands and converts into low pressure gas at the exit point of evaporator. This low pressure gas is fed into the compressor from where it is then further pumped to condenser after increasing the pressure of the gas. In some of the refrigeration cycle, accumulator is also used in addition which doesn’t allow any liquid droplet to pass to the compressor. It is advised to have accumulator in almost all the system subjected to load variations. Then this gas is transferred to the condenser where it is condensed (heat is removed from the gas) and becomes a high pressure liquid.

Between the suction line and the liquid line the heat exchanger is placed which also is an optional one, it may or may not be present depending on the system design. After the condenser, this high pressure liquid gas is passed through the expansion value which controls the flow of the refrigerant into the evaporator. It majorly depends on the evaporator load, higher the evaporator load higher is the flow of refrigerant to the evaporator. 

As this high pressure liquid enters the evaporator, it is subjected to lower pressure because of the suction of the compressor. This forces the refrigerant to evaporate and for which it takes the heat from the air passing over the evaporator. And, this help in achieving the desired temperature and this cycle continues as long as desired temperature is maintained.

Thermostat is also used in the refrigeration system so that it can break the electrical circuit of the compressor motor and stops it as soon as the desired temperature is achieved and starts it as the temperature increases a little bit.

10 – Using a combination of drawings, compare the action of a ship’s propeller/rudder combination with that of a “podded” drive system, when a ship is turning to Port. Name all the main parts required and explain their function.

Rudder which is placed on the aft of the ship is not sufficient enough to turn the ship on its own. It can be understood from the size of the rudder to that rudder being such a small part in the ship will not be able to turn the ship which is infinity times bigger than it.

Consider a helmsman turning the ship towards the starboard. It gives some angle to the rudder. Because of this a rudder is force is generated which is directed towards the transverse direction of the ship. This will cause to attain a sway velocity towards the port side. However, this sway is so negligible in comparison to the turning of the ship that it is hardly felt. This only helps in creating a drift angle towards the starboard. Now, let’s consider the rudder at the bow side. Now, if rudder is turned toward the starboard, the rudder force will still be in the direction of the port. This rudder direction towards the port and the net hydrodynamic moment would cause the ship to turn towards the port.

The podded drive system is still finding its way into the marine industry. The mermaid system from kamewa and Azipod system from Finland have both made up to the cruise industry recently and holding on to the market. It has been known since hundreds of year that narrow channels and confined water are always a challenge to the captain in maneuvering the ship. However, the existence of Podded drive system has somehow decreased the traumatic condition of the captains.

In rudder propulsion system, a complete set of propeller and controlled pitch propeller is used to maneuver a ship, whereas in this system it’s just a mechanism which will maneuver the ship with less effort. This newly designed mechanism doesn’t only work efficiently, in addition in conserves energy and money to a greater extent.

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