JULY 2017

MARINE ENGINEERING KNOWLEDGE (GENERAL) TIME ALLOWED - 3 HOURS

Instructions 1. Answer any SIX questions.2. All questions carry equal marks.

3. Neatness in handwriting and clarity in expression carries weightage

4. Illustration of an answer with clear sketches /diagrams carries weightage

5. All unused pages of answer script must be cancelled out by two lines (X) across the page

Q1. A. Explain Creep, Brinelling, Fretting and Fretting corrosion. B. Where and why does Creep, Brinelling, Fretting and Fretting corrosion occur in a ship propulsion system?

Answer:- Repeat question . Refer Q8 EKG APRIL 2017

Q2Sketch and describe a boiler water level controller of the float operated type. State the reasons for having this mechanism on the boiler and using this controller and boiler for analogy. Explain the following terms: A. Detecting element; B. Servo motor; C. Desired value.

Answer:- A feed controller of the float type is as sketched above. This controller controls the speed of the pump by a variable resistance in the current supplied to the pump motor. The variation in resistance is controlled by the float . When the difference in the water level is low in the boiler the command signal is difference between the desired value(half level in float chamber) and the actual level in the boiler. When this

difference is high the resistance is cut out or reduced so the motor turns faster bringing up the level quickly. When the actual level in the boiler is higher than the desired level in the boiler the resistance increases slowing down the pump or in the extreme condition the motor stops because the current connection is cut off. This is the principle of operation of the controller. This type of controller is a proportional plus integral controller. The desired value is the half level in the float chamber corresponding to half the gauge glass level. The amplifier is the adjustable rheostat which controls the current. The detecting element is the float.

Q3. With regard to care of lubricating oils on board answer the following – A. What is microbial degradation of lubricating oil and how is it prevented? B. What methods are employed to ensure

correct sampling for shore based testing? C) If shore based testing results show abnormal value of water content and TBN for crank case lube oil of a slow speed main engine, what will be interpretation and subsequent action?

Answer:- Repeat question. Refer Q 3 EKG JUNE 2017

Q4. With regards to main transmission shaft flange coupling arrangements: A. Sketch a hollow type coupling bolt and the hydraulic head/nut and loading rod which are used to fit it; B. Describe how the bolt is fitted; C. State the advantage of the hollow coupling bolt as compared to the traditional type of coupling bolt.

Answer:- The Pilgrim hydraulic bolt uses the principle embodied in Poisson’s ratio to provide a calculated and definite fitting force between bolt and hole .The bolt is hollow and before being fitted is stretched with hydraulic pressure applied to an inserted rod from a pressure cylinder screwed to the bolt head. Stretching makes the bolt diameter small enough for insertion into the hole , after which the nut is nipped up. Release of hydraulic pressure allows the bolt to shorten so that (1)predetermined bolt load is produced and (2) diametrical reexpansion gives a good fit of the shank in the hole.

(c).These bolts when fitted in flange couplings and flange mounted propellers , have the advantage that they are easily removed for inspection and maintenance. Since the strain is calculated excessive stress is avoided.

Q5. With respect to the steering gear, answer the following: A. Explain with a diagram, a “fail safe steering gear” suitable for use on a tanker of more than 100000 T dwt; B. Explain the sequence of events that take place when an oil leak takes place in one of the hydraulic pipe lines.

Answer:- The single failure concept was developed with the advent of large crude oil carriers above 100,000 tons DWT and large passenger ships.

According to this concept , if the steering gear fails for any single failure of the pump ,motor or pipe leakage , The stand by steering gear pump set will come on load within 45 seconds of the failure In this system pipe interconnections are there and the change over is automatically accomplished with the right combination of the inter connections. The failure of the rudder stock is not considered since the failure of rudder stock is not probable.

The above sketch shows the basic single failure concept with interconnections in the piping system. In failure mode especially when oil level in expansion tank falls to a dangerous low level by oil leakage, an alarm is sounded and change over to the alternate system occurs within 45 seconds.

Q6. Describe with a sketch a pneumatic relay and show how feedback can be achieved when such a relay is used in conjunction with a flapper mechanism .

Answer:- The relay provides pneumatic amplification, proportional movement and reduced time lag. Equivalent in action to an electronic amplifier. If p increases then the bellows acts to close the bleed port and supply air passes, conversely if p decreases a continuous bleed to atmosphere occurs.

Amplification by a fraction of 16 can be easily arranged, for example a flapper travel of 0.01 mm causing a change of p on the bellows of .0.05 bar could give output from 1.2 to 2 bar. Flapper travel is approximately proportional by a linear relation to output signal.

Various alternatives and refinement can be added. A bellows connected to the relay output will give action proportional to output utilising negative feedback.

Q7. With respect to refrigeration gases used on-board vessels, answer the following A. Explain ozone depleting potential of conventional refrigerant gases B. Name the alternate refrigerant gases available and being used onboard. C. Explain the steps you will take to ensure that release of refrigerant gases from the plant is minimized during normal operation and during maintenance activities.

Answer:- Repeat question. Refer Q 7EKG JAN 2017.

Q8. A. State why fixed pitch propellers have a poor efficiency when going astern. B. With reference to controllable pitch propellers state: i. Why is it preferable that the main servomotor be housed in the propeller hub rather than in the shafting forward of the propeller shaft? ii. What regular maintenance and checks should be carried out to ensure maximum reliability of the gear at all times?

Answer:- (A) The conventional fixed pitch propellers are hydro-dynamically designed only for the driving face which is active in the ahead direction. Hence the propeller efficiency is best in the ahead direction. The astern motion need not be 100 percent efficient since it is needed for manoeuvring only

(2)When going astern the wake direction is forward and the flow of the wake is resisted by the underwater stern hull portion. Hence propulsive efficiency is reduced.

(b) The main servomotor for changing the pitch of the propeller is cylinder with a piston arranged within the shaft system. The piston is connected to the pitch actuating gear by the piston rod. If it is placed within the propeller shaft it reduces the piston rod length considerably and ensures oil tightness restricted within the propeller shaft. If it is placed in the intermediate shaft the design gets more complicated as can be seen in the concept sketch given below.

(2) The regular maintenance required in every dry docking is the Oil seals of the individual blade connection to the actuating X-head connection is renewed and the actuating mechanism is dismantled , inspected and worn parts are renewed.

Q9. Reverse osmosis is the modern alternative for shipboard production of drinking water. A. Describe using simple diagrams if necessary, the principle of reverse osmosis. B. (i)Sketch a line diagram showing a single

pass system for producing fresh water from sea water. (ii)Describe such a system.

Answer:- Reverse osmosis (RO) is a water purification technology that uses a semi-permeable membrane to remove ions, molecules, and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic pressure, a colligative property, that is driven by chemical potential differences of the solvent, a thermodynamic parameter. Reverse osmosis can remove many types of dissolved and suspended species from water, including bacteria, and is used in both industrial processes and the production of potable water. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. To be "selective", this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution (such as solvent molecules) to pass freely.

In the normal osmosis process, the solvent naturally moves from an area of low solute concentration (high water potential), through a membrane, to an area of high solute concentration (low water potential). The driving force for the movement of the solvent is the reduction in the free energy of the system when the difference in solvent concentration on either side of a membrane is reduced, generating osmotic pressure due to the solvent moving into the more concentrated solution. Applying an external pressure to reverse the natural flow of pure solvent, thus, is reverse osmosis. The process is similar to other membrane technology applications. However, key differences are found between reverse osmosis and filtration. The predominant removal mechanism in membrane filtration is straining, or size exclusion, so the process can theoretically achieve perfect efficiency regardless of parameters such as the solution's pressure and concentration. Reverse osmosis also involves diffusion, making the process dependent on pressure, flow rate, and other conditions.[1] Reverse osmosis is most commonly known for its use in drinking water purification from seawater,

removing the salt and other effluent materials from the water molecules.(The above explanation need not be written in the answer. Itis given only for understanding the phenomenon.).)

The above diagram exhibits the principle of reverse osmosis. When sea water is made to pass through a semi permeable membrane , at high pressure the salts dissolved in the sea water is prevented from passing through the membrane and only pure fresh water is obtained on the other side of the membrane. Hence it is a form of filtration of dissolved salts from water.