Arrangements required for ensuring safe engineering watch when carrying dangerous cargo.
Routine Work during watchkeeping:
Soot Blowing – Soot deposits are un-burnt carbon particulates which accumulate on the gas side of the boiler. Unnecessary buildup of soot deposits impedes heat transfer and causes exhaust pipe uptake fires. Soot blowing a boiler is done once every 12 hours using pressured steam or air. Soot blowers are operated manually or driven by an electric or pneumatic motor.
Pumping Out bilges through OWS – Water and oil leaking from the pump glands and other machinery are collected in the engine room bilge. The content of the bilges are passed to the OWS to separate the oil and to ensure that the water being pumped out does not content more than 15 ppm of oil. Each operation must be recorded and completed operation must be signed by the Engineer in charge and Chief Engineer.
Turbocharger Blower washing – Depending on the maker’s instruction, the blower side of the turbocharger may be water washed daily to remove the deposits accumulated during operation.
Transfer of Oil from Bunker Tank to Settling Tank – Fuel is transferred from the Bunker tanks to the settling tank and water and impurities are allowed to settle. This oil is then purified using purifier before transferring it to the Service tank. All line valves must be correctly opened /closed for a smooth operation.
Draining of Air bottles – This is a very important routine operation. Water in the compressed air causes corrosion in the air bottles and the pneumatic control systems.
De-sludging of Purifier – Usually the purifier will run in auto-mode. If they are run in manual mode, then de-sludging operation must be carried out manually once in a watch to remove the sludge deposits accumulated in the bowl.
Boiler Blow down – Boiler is blown down atleast once in a day to get rid of the suspended and dissolved solids. This is done by using the upper scum valve and bottom blow-down valve.
Boiler water tests – Boiler water is tested for alkalinity, chlorides and dissolved solids once in a day by following correct procedures. Treatment chemicals are added according to the test results.
Other routine jobs include – draining of lube oil sumps off water, LO analysis, incineration, cleaning or changing of filters, etc.
Q. What is Broaching? Explain the effects with suitable sketch.
Broaching
Broaching:- when a steep following sea causes the vessel to ‘surf’ forwards controllably, the bow tends to ‘dig’ into the wave ahead, decelerating the vessel rapidly.
The forces on the stern will cause the stern to swing violently to the left or right and the vessel will come to rest broadside to the waves. A rapid “broaching” may cause a capsize.
Q) Explain why regular testing of water in Auxiliary Boilers is desirable.
Ans:- Boiler water tests are
tests to find a particular chemical compound or excess chemical compound in the
boiler water. If you do the boiler water test correctly and regularly you can
find excess compound or impurities in the boiler water and prevent following
factors:-
Prevent corrosion in the boiler feed system by maintaining the boiler water’s alkaline condition.
Prevent scale formation in the boiler tubes.
Remove dissolved gases such as oxygen from water.
Control sludge formations which prevent carry over with the steam.
Determine the amount of impurities by which we can select appropriate treatment.
Exercise careful control over boiler treatment chemicals.
Maintain and provide residual reserve of chemicals.
Improve efficiency, thus increasing boiler life and service period of boilers.
For economical operation of boiler.
Q) For each test normally carried out, state – Reasons for making the test.
Ans: – Different Boiler Water
Tests and its Purpose:
Hydrate Alkalinity – For testing the presence of hydrates.
Condensate PH – Measuring the acidity or alkalinity of the water.
Chloride ppm CI Test- measure the chlorine content or sea water presents.
Phosphate – For testing the presence of phosphate compounds. It is important to maintain minimum of 10 ppm of phosphate.
Deha/Oxygen test – For testing oxygen and Chemical content.
Neutralized Conductivity – For testing the presence of ferrous ions and metal contents.
Feed Water Hardness – For testing the presence of carbonate compounds.
Q) For each test normally carried out, state – Acceptable values for any particular type of auxiliary boiler.
Acceptable values for any particular type of auxiliary boiler
Q) Action required
when measured values differ appreciably from desired values.
Ans:- When measured values differ from acceptable standards, a corrective action chart is to be referred and suitable action taken accordingly. This chart is reproduced:-
Action required when measured values differ appreciably from desired values
Q) Sketch & describe the Reverse Osmosis system for the production of drinking water on board the ships.
Principle of Reverse Osmosis – (Figure-1)
Reverse Osmosis:-
One of the methods of producing fresh water is by using the principle of Osmosis. Osmosis term is used to describe passage of pure water from one side off a semi-permeable membrane into a salt or other solution on the other, with the result the salt solution is diluted but the water remains pure.
Reverse Osmosis is a water filtration process, which makes use of semi-membrane materials. Salt water on one side of the membrane is pressurized by a pump and forced against the material. Pure water passes through but not thee salts see figure 1.
Pressure required to force the pure water through, is called osmotic pressure & requirement of osmotic pressure is higher if the salinity of salt water is higher. For production of large amounts of pure water, the membrane area must be large and it must be tough enough to withstand the pump pressure.
Also, as continuous supply of sea water is used as feed, the salinity of feed is steady, than the osmotic pressure required to force fresh water through is also steady, and steady pressure is taken from a continuously running pump, see figure 2. Semi permeable membrane may be provided by a bunch off cartridge as shown in figure 3.
Reverse Osmosis System for continuous supply of Fresh Water (Figure -2)
Cartridge used for Reverse Osmosis (Figure -3)
Q) State –
Pre-treatment used with RO equipment.
Ans:- Due to the low
temperature of operation in low pressure evaporators, the fresh water produced
by such a FWG may be harmful to drink as it may contain bacteria. It may not
even contain the minerals that are needed. Therefore, it needs to be treated,
in order for it to be potable. The treatment of fresh water can be done using
the following methods:
Chlorine Sterilization: The sterilization by chlorine is recommended and it may be done by an excess dose of chlorine provides by sodium hypochlorite in a liquid form or by adding calcium chloride granules. The chlorine content may be upto 0.2 ppm, for it to be effective. The water is then de-chlorinated in a bed of activated carbon to remove the excess chlorine. Any colour, taste and odour, which were present in the water, will also be removed by the carbon.
Silver Ion Sterilization: In this method, silver ions are injected into the distillate, by means of a silver anode. This method of sterilization is effective since silver is toxic to the bacteria present in the water. Unlike chlorine, the silver ions do not evaporate. The amount of silver ion released into the water is controlled by the current and the silver ion content may be upto 0.08 ppm, for it to be effective.
Ultraviolet Light Sterilization: A temporary but immediately effective sterilization is by means of UV light. Chlorine and silver ion methods although long lasting, change the taste of the water and require efficient carbon filtration to remove the odour. UV light on the other hand does not cause any physical or chemical change in the water. This method uses UV lamps to produce short wave radiations that destroy the bacteria, viruses and other organisms in the water. This method is usually used on the discharge side of the water storage tank so that the water is sterilized immediately before use.
Sterilization by Ozone: Use of ozone for sterilization is very effective. It is an effective oxidant. However, the equipment is costly and has high running cost.
An example of treatment of fresh water produced by the FWG is shown in the flowchart below. The system may be different on different ships. The end result should however be that the produced water made available for drinking is slightly alkaline, sterilized, clear and good in taste.
Flow chart of pre-treatment of Drinking Water
Q) State – The
Post-treatment necessary.
The Permeate (product water) from the above process may contain small quantities of dissolved salts and is suitable as drinking water if the salt content is below 250 ppm, well within the WHO limit.
However, if the water is to be used for boiler feed, it has to be completely free from salts and the product water may require further treatment to get rid of small quantities of salts by Ion Exchange Treatment.
Q) State some built in measures by which steering gear mechanism can be kept operational in the event of breakage of rudder actuator, pipe failures, motor burn out etc.
Ans:- Measures against steering gear failure: Some general requirements for steering gears, based on the various regulations and SOLAS 1974, are given below:
Ships must have a main and an auxiliary steering gear, arranged so that the failure of one does not render the other inoperative. An auxiliary steering gear need not be fitted, however, when the main steering gear has two or more identical power units and is arranged such that after a single failure in its piping system or one of its power units, steering capability can be maintained. To meet this latter alternative the steering gear has to comply with the operating conditions of paragraph 2 — in the case of passenger ships while any one of the power units is out of operation. In the case of large tankers, chemical tankers and gas carriers the provision of two or more identical power units for the main steering gear is mandatory.
The main steering gear must be able to steer the ship at maximum ahead service speed and be capable at this speed, and at the ship’s deepest service draught, of putting the rudder from 35 deg on one side to 30 deg on the other side in not more than 28 secs. (The apparent anomaly in the degree of movement is to allow for difficulty in judging when the final position is reached due to feedback from the hunting gear which shortens the variable delivery pump stroke.) Where the rudder stock, excluding ice strengthening allowance, is required to be 120 mm diameter at the tiller, the steering gear has to be power operated.
The auxiliary steering gear must be capable of being brought speedily into operation and be able to put the rudder over from 15deg on one side to 15deg on the other side in not more than 60 sees with the ship at its deepest service draught and running ahead at the greater of one half of the maximum service speed or 7 knots. Where the rudder stock (excluding ice strengthening allowance) is over 230 mm diameter at the tiller, then the gear has to be power operated.
It must be possible to bring into operation main and auxiliary steering gear power units from the navigating bridge. A power failure to any one of the steering gear power units or to its control system must result in an audible and visual alarm on the navigating bridge and the power units must be arranged to restart automatically when power is restored.
Steering gear control must be provided both on the bridge and in the steering gear room for the main steering gear and, where the main steering gear comprises two or more identical power units there must be two independent control systems both operable from the bridge (this does not mean that two steering-wheels are required). When a hydraulic tele-motor is used for the control system, a second independent system need not be fitted except in the case of a tanker, chemical carrier or gas carrier of 10000 gt and over. Auxiliary steering gear control must be arranged in the steering gear room and where the auxiliary gear is power operated, control must also be arranged from the bridge and be independent of the main steering gear control system. It must be possible, from within the steering gear room, to disconnect any control system operable from the bridge from the steering gear it serves. It must be possible to bring the system into operation from the bridge.
Hydraulic power systems must be provided with arrangements to maintain the cleanliness of the hydraulic fluid. A low level alarm must be fitted on each hydraulic fluid reservoir to give an early audible and visual indication on the bridge and in the engine room of any hydraulic fluid leakage. Power operated steering gears require a storage tank arranged so that the hydraulic systems can be readily re-charged from a position within the steering gear compartment. The tank must be of sufficient capacity to recharge at least one power actuating system.
Where the rudder stock is required to be over 230 rnm diameter at the tiller (excluding ice strengthening) an alternative power supply capable of providing power to operate the rudder, as described in paragraph 3 above, is to be provided automatically within 45 seconds. This must supply the power unit, its control system and the rudder angle indicator and can be provided from the ships emergency power supply or from an independent source of power located within the steering compartment and dedicated for this purpose. Its capacity shall be at least 30 minutes for ships of 1O,OOO gt and over and 10 minutes for other ships.
The electrical power distribution system on board a ship is designed so as to provide a secure supply to all loads with adequate built in protection for the equipment and operating personnel. The general scheme of a ship’s electrical power system is common to nearly all ships.
Both the auxiliary and emergency services are supplied by the Main generators during normal operating conditions. In event of emergency, only the emergency services are supplied by the Emergency generator.
The below figure shows a typical electrical distribution system of a vessel.
The main generators are connected to the main bus bar via air circuit breakers. The main bus bar supplies 440V directly, 220V via transformers and 24V DC via transformers and rectifiers.
The main bus bar is connected to the emergency switch board via the tiebreaker. Emergency generator is also connected to the emergency switch board.
Arrangements are also made for shore supply to be connected.
Q) Explain how paralleling of generator’s carried out.
Ans:- Parallel Operation of Generators:-
Depending upon the capacity and the electrical load, more than one alternator can be connected to the common Bus bars. The connecting process is called ‘Synchronising’ i.e. enabling the parallel operation of the alternators.
Following conditions must be fulfilled for paralleling the Alternators:
Voltage must be same.
Frequency must match.
Phase sequence must be correct.
In short, the incoming alternator should have the same parameters as the running alternator(s). If the speed of the incoming machine is different, the ‘governor control switch’ should be used to adjust the speed.
Departure from the above conditions will result in the formation of power surges and unwanted electro-mechanical oscillation of rotor which will damage the equipment.
To carry out the paralleling operation, following devices are provided:
Synchroscope
Lamps (Dark/Bright)
Following are the steps to carry out with paralleling using a synchroscope:
Check voltages are same.
Check frequency of incoming generator is same as running generator.
Put on synchroscope and see that pointer turns slowly in clockwise direction (may require to adjust speed of incoming generator using the ‘governor control switch’ for this).
When the pointer is at 11 o’clock position, close the breaker of incoming generator.
Now the load is equally distributed among the generators.
Advantages of Parallel Operating Alternators
For maintenance or inspection, one machine can be taken out from service and the other alternator can keep up for the continuity of supply.
Load supply can be increased.
During light loads, more than one alternator can be shut down while the other will operate in nearly full load.
High efficiency and operating cost is reduced.
Ensures the protection of supply and enables cost-effective generation.
Q) For a fully automatic provisions refrigeration system incorporating a number of rooms: Sketch a line diagram detailing the devices incorporated into the refrigeration system to protect the machinery and equipment against malfunction.
Main Components of Refrigeration Plants
Main Components of
Refrigeration plants:- Any refrigeration unit works with different components inline to each
other in series. The main components are:
Compressor: Reciprocating single or two stage compressor is commonly used for compressing and supplying the refrigerant to the system.
Condenser: Shell and tube type condenser is used to cool down the refrigerant in the system.
Receiver: The cooled refrigerant is supplied to the receiver, which is also used to drain out the refrigerant from the system for maintenance purpose.
Drier: The drier connected in the system consists of silica gel to remove any moisture from the refrigerant.
Solenoids: Different solenoid valves are used to control the flow of refrigerant into the hold or room. Master solenoid is provided in the main line and other solenoid is present in all individual cargo hold or rooms.
Expansion valve: An Expansion valve regulates the refrigerants to maintain the correct hold or room temperature.
Evaporator unit: The evaporator unit acts as a heat exchanger to cool down the hold or room area by transferring heat to the refrigerant.
Control unit: The control unit consist of different safety and operating circuits for safe operation of the refer plant.
Compressor
safety devices: The compressor is protected by three safety switches:-
The OP switch or Oil Differential Pressure switch compares the measured lubricating oil pressure to the Suction (crankcase) pressure. Should the differential pressure fall below a pre-set minimum (about 1.2 bar) then the compressor will trip and require a manual reset to restart. A time delay is built into the circuit to allow sufficient time for the lubricating oil pressure to build up when starting before arming the circuit.
The HP or High Pressure switch, is fitted to the outlet of the compressor before the isolating valve. On over pressurization (dependent on the refrigerant, up to about 24bar bar for R22) the switch will trip the compressor and a manual reset is required before restart.
The LP or Low Pressure switch when activated (at about 1 bar for R22) will trip the compressor and require a manual reset before the compressor can be restarted.
This normally takes the form of an LP cut out pressure switch with automatic reset on pressure rise. The cut out set point is just above the LP trip point says at about 1.4bar.
An adjustable differential is set to about 1.4bar to give a cut in pressure of around 2.8 bar. The electrical circuit is so arranged that even when the switch has reset, if no room solenoid valves are open the compressor will not start. This is to prevent the compressor cycling due to a leaky solenoid valve.
In addition to this extra LP switches may be fitted which operate between the extremes of the LP cut in and cut out to operate compressor unloaders.
Some modern systems contain a rotary vane compressor with variable speed (frequency changing) control.
Q) For a fully automatic provisions refrigeration system incorporating a number of rooms: Explain how critical temperature restricts plant operation and how these limitations overcome?
Ans:- The Critical Point:- The critical point is the point above which:
The gas will not liquefy by the action of pressure alone. This is an important temperature for refrigeration systems which rely on the change of state for heat transfer.
The gas will not liquefy by cooling alone.
Working of Ship’s Refrigeration Plant:-
The compressor acting as a circulation pump for refrigerant has two safety cut-outs- Low pressure (LP) and High Pressure (HP) cut outs.
When the pressure on the suction side drops below the set valve, the control unit stops the compressor and when the pressure on the discharge side shoots up, the compressor trips.
LP or low pressure cut out is controlled automatically i.e. when the suction pressure drops, the compressor stops and when the suction pressure rises again, the control system starts the compressor. HP or high pressure cut out is provided with manually re-set.
The hot compressed liquid is passed to a receiver through a condenser to cool it down. The receiver can be used to collect the refrigerant when any major repair work has to be performed.
The master solenoid is fitted after the receiver, which is controlled by the control unit. In case of sudden stoppage of compressor, the master solenoid also closes, avoiding the flooding of evaporator with refrigerant liquid.
The room or hold solenoid and thermostatic valve regulate the flow of the refrigerant in to the room to maintain the temperature of the room.
For this, the expansion valve is controlled by a diaphragm movement due to the pressure variation which is operated by the bulb sensor filled with expandable fluid fitted at the evaporator outlet.
The thermostatic expansion valve supplies the correct amount of refrigerants to evaporators where the refrigerants takes up the heat from the room and boils off into vapours resulting in temperature drop for that room.
This is how temperature is maintained in the refrigeration plant of the ship.
A safety system includes alarm, cut offs, and trips which safeguards the machinery and its parts from getting damage.
Q) For a fully
automatic provisions refrigeration system incorporating a number of rooms: What
is the effect on the environment of the release of refrigerants into the
atmosphere?
Ans:- Environmental and
Safety properties of Refrigerants:- At present the environment friendliness
of the refrigerant is a major factor in deciding the usefulness of a particular
refrigerant. The important environmental and safety properties are:
Ozone Depletion Potential (ODP): According to the Montreal protocol, the ODP of refrigerants should be zero, i.e., they should be non-ozone depleting substances.
Global Warming Potential (GWP): Refrigerants should have as low a GWP value as possible to minimize the problem of global warming. Refrigerants with zero ODP but a high value of GWP (e.g. R134a) are likely to be regulated in future.
Total Equivalent Warming Index (TEWI): The factor TEWI considers both direct (due to release into atmosphere) and indirect (through energy consumption) contributions of refrigerants to global warming. Naturally, refrigerants with as a low a value of TEWI are preferable from global warming point of view.
Toxicity: Ideally, refrigerants used in a refrigeration system should be non-toxic. Toxicity is a relative term, which becomes meaningful only when the degree of concentration and time of exposure required to produce harmful effects are specified. In general the degree of hazard depends on:
Amount of refrigerant used vs total space
Type of occupancy
Presence of open flames
Odor of refrigerant, and
Maintenance condition
Flammability: The refrigerants should preferably be nonflammable and non-explosive. For flammable refrigerants special precautions should be taken to avoid accidents.
Chemical stability: The refrigerants should be chemically stable as long as they are inside the refrigeration system.
Compatibility with common materials of construction (both metals and non-metals)
Miscibility with lubricating oils: Oil separators have to be used if the refrigerant is not miscible with lubricating oil (e.g. ammonia). Refrigerants that are completely miscible with oils are easier to handle (R12).
Transverse thrust is the tendency for a forward or astern running propeller to move the stern to starboard or port. Transverse thrust is caused by interaction between the hull, propeller and rudder. The effect of transverse thrust is a slight tendency for the bow to swing to port on a ship with a right-handed propeller turning ahead.
Transverse thrust is more pronounced when propellers are moving astern.
When moving astern, transverse thrust is caused by water passing through the astern-moving propeller creating high pressure on the starboard quarter of the hull, which produces a force that pushes the ship’s stern to port. Rudder angle can influence the magnitude of this force.
The Ship Handler should be aware of the variable effect of transverse thrust. As water flow over a ship’s hull changes, so does transverse thrust. The difference is most noticeable in shallow water. For example, a ship that turns to starboard in deep water may well turn to port in shallow water. Also, the magnitude of the force will change and, by implication, there will be a range of water depths for which the bias may be difficult to predict, something that is especially true when a ship is stopping in water of reducing depth.
Transverse thrust is often used to help bring the ship’s stern alongside during berthing. When a propeller is put astern on a ship moving forward at speed, the initial effect of transverse thrust is slight. However, as the ship’s forward motion decreases, the effect of transverse thrust increases.
It is essential for a Ship Handler to understand just how much effect transverse thrust has on his particular ship.