FEBRUARY 2017

MARINE ENGINEERING KNOWLEDGE (MOTOR)TIMEALLOWED - 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

Q1. Describe the main engine shaft engine generator arrangement with an electronic system for frequency correction. Describe the operation of the generator arrangement so sketched.

Answer:- Static frequency converter for a shaft generator

The converter system shown in sketch serves the shaft generator of a ship with a fixed-pitch propeller and a large main-engine speed range. The shaft generator must supply full output over the permitted speed range, and to achieve this at the lower end (i.e. down to 40% of the rated speed), it is overrated for higher speeds.

The a.c. shaft generator itself is a synchronous machine which produces alternating current with a frequency that is dictated by variations in engine speed. At the full rated r.p.m., frequency may match that of the electrical system.

The output is delivered to the static converter, Which has two main parts. The first is a rectifier bridge to change shaft generator output from alternating to direct current. The second part is an inverter to change the d.c. back to alternating current, at the correct frequency.

Alternating current from the shaft generator, when delivered to the three-phase rectifier bridge, passes through the diodes in the forward direction only, as a direct current .The smoothing reactor reduces ripple. The original frequency (within the limits) is unimportant once the supply has been altered to d.c. by the rectifier. The inverter for transposition of the temporary direct current back to alternating current is a bridge made up of six thyristors. Direct current, available to the thyristor bridge, is blocked unless the thyristors are triggered or fired by gate signal. Gate signals are controlled to switch each thyristor on in sequence, to pass a pulse of current. The pattern of alternate current flow and break constitutes an approximation to a three-phase alternating current.

Voltage and frequency of the inverter supply to the a.c. system must be kept constant within limits. These characteristics are controlled for a normal alternator by the automatic voltage regulator and the governor of the prime mover, respectively. They could be controlled for the shaft alternator inverter by a separate diesel-driven synchronous alternator running in parallel. The extra alternator could also supply other effects necessary to the proper functioning of an inverter, but the objective of gaining fuel and maintenance economy with a shaft alternator would be lost. Fortunately the benefits can be obtained from a synchronous compensator (sometimes termed a synchronous condenser), Which does not require a prime mover or driving motor except for starting. The compensator may be an exclusive device with its own starter motor or it may be an ordinary alternator with a clutch on the drive shaft from the prime mover.

The a.c. generator set that fulfils the role of synchronous compensator for the system shown is at the top right of the sketch. The diesel prime mover for the compensator is started and used to bring it up to speed for connection to the switchboard. The excitation is then set to provide the reactive power, and finally the clutch is opened, the diesel shut down and the synchronous machine then continues to rotate independently like a synchronous motor, at a speed corresponding to the frequency of the a.c. system.

A synchronous compensator is used with the monitoring and controlling system, to dictate or define the frequency. It also maintains constant a.c. system Voltage, damps any harmonics and meets the reactive power requirements of the system and converter, as well as supplying, in the event of a short circuit, the current necessary to operate trips. The cooling arrangements for static frequency converters included . The provision of fans as well as the necessary heat sinks for thyristors.

Q2. A. If an auxiliary diesel generator over-speeds and runs away while off the load, explain: - i. How it can be stopped; ii. What is likely to be the reasons for the failure? B. Give-details of what cheeks are made after the machine has been stopped: (i) Mechanically, (ii) Electrically.

Answer:- To stop the engine shut the fuel inlet valve and for more speedy action cover the air inlet filter with a piece of canvas cloth. . This will bring the engine to a speedy stop.

Reasons for failure :- The fuel control rod is controlled by the engine governor which connects the rod to the individual fuel pumps through a rack and pinion arrangement. The quantity of fuel oil delivered to the engine is controlled by the angular position of the plunger in relation to the inlet hole. The rotation of the plunger is controlled by the rack connected to the control rod through a coil spring. It is possible that the plunger is tight in the barrel and hence is not able to turn the plunger the required amount. The movement of the fuel control rod is taken up by the spring without moving the plunger the required amount and hence the engine does not come to a stop. The concerned fuel pumps must be dismantled and the plungers eased in their barrels. If defective the plunger barrel sets must be renewed.

The mechanical checks required are complete crank case inspection of all units This is required to ensure that all fastenings of the running gear have not slackened.

The electrical checks required are the following:

(1) The excitation circuit to be checked for any failure of diodes in the rectification circuit. This can be caused by high voltage developed when engine over speeds.

(2) The anchor bolts of the armature holding device should be hammer tested for any breakage The magnetic force or reaction becomes suddenly large when voltage build up shoots up due to over speeding

(3) The air gap clearance should be taken and recordedQ3. The analysis of oil may be used as a method of monitoring the condition of the equipment that it lubricates A. Explain briefly how shore analysts might test the oil; B. State the type of information that would be expected; C. Give possible reasons for an excess of: - i. Iron, ii. Copper, iii. Antimony Iv. Tin, v. Silica

Answer:- (A) The physical & chemical characteristics of an in-service oil are obviously linked back to the specific type of oil, its age and the conditions under which it operates. These tests are typically carried out in highly automated specialised laboratories. Only a small volume of oil is needed- typically less than 250 ml for a full Routine Analysis, and fully automated equipment can be used. This makes Routine Analysis quick, easy & economical to run. Normally the test method used will be according to conventional ISO or ASTM standards but where in-house specialised test methods are used these can have the advantage that the tests are specifically designed for their relevance to ‘used oil’ based on many years field experience. In cases of a dispute the ISO or ASTM methods are used as the referee method. Routine Analysis test kits, which contain all the equipment & containers necessary for taking the samples are delivered to the installations at regular intervals. Additionally, many kits now contain prelabelled sample containers and pre-paid express mail postage bags. Most laboratories now operate on a 24 hour testing turnaround time from the time of sample receipt.

(B) “Routine Analyses” will typically include: 1) Viscosity 2) Water content 3) Base Number (BN) or Alkalinity reserve 4) Insolubles 5) Flash Point 6) Elements (measuring the concentration of additives and levels of wear metals, etc.)

(C) (1) and (2) The presence of appreciable quantities of iron in the used lub-oil sample indicates wear of crank pins, journals and crosshead pins. When bottom end bearings and cross head bearings are opened up for inspection , the pins should be calibrated. If this information is kept in mind, then at the following inspection activity of any top-end or bottom-end bearing the calibration should be carried out and wear rate assessed. Also the condition of the bearings assessed and if condition of the bearing is bad exhibited by excessive wear down the bearing halves should be renewed. The presence of copper, antimony and lead indicate worn particles of bearing material found in the lub-oil sample.

(3)The presence of silicon suggests liner wear or piston ring wear particles. Silicon is an essential alloying ingredient of cast iron and liners as well as piston rings come in contact with the lub-oil especially if the engine is a trunk piston engine where the same crank case lub - oil is used for cylinder lubrication. Cylinder liner and piston ring wear may be excessive and could be caused by inadequacy of required alkalinity in lub-oil and if necessary the lub oil shoud be renewed or grade suitably changed.

Q4. During a routine crankcase inspection a main engine top end bearing is found to be wiped and subsequent inspection shows that the pin is badly scored. A. Explain in detail the action which should be taken to enable the engine to be safety operated so that the vessel may reach a port where effective repair facilities are available. B. State with reasons the factors which influence the speed at which the engine may be safely operated.

Answer:- The main engine being a multi- cylinder in-line engine with all units identical, the affected unit is cut out from operation in such a manner that connection between affected piston and crank is removed , so that the engine operates with one unit less, at a proportional lower speed and power to reach port. The details of this arrangement is described under.

. The steps required to disconnect piston from crank are as follows:

(1)Taking all due precautions the crank case doors of the affected unit opened ,crank case cleaned sufficiently to enable persons to work safely in the crankcase. Turning gear to be engaged and kept ready only.

(1) The defective cross head bearing is temporarily refitted only for the purpose of turning the crank to TDC for dismantling the bottom half of bottom end bearing.

(2) The bottom half of bottom end bearing is dismantled in this position and removed from crank case.

(3)Guide bars or guide pins are fitted on the guides, and additional supports by way of two chain blocks and suitable slings suspended from both ‘A’ frames connected to the crosshead.

(4)Crosshead top half bearings dismantled and removed. The turning gear turned, properly supporting connecting rod using chain blocks ,so that connecting rod is disconnected from the crank shaft. The connecting rod is taken out through the crank case and left properly supported outside the main engine.

(5) Exhaust valve actuating gear is disconnected and the hydraulic line for actuation is suitably drained so that there is no back pressure on the line. The roller is lifted and held free from the cam.

(6) Lub-oil supply and outlet lines at crosshead are suitably disconnected and blanked.

(7) Fuel pump roller of affected unit lifted off the fuel cam so fuel pump is suitably dis engaged and fuel supply to the unit cut off.

(8) Indicator cock kept open so no pressure build up in affected unit.

(9) The starting air connection to the unit air starting valve completely disconnected and suitably blanked.

(10) Jacket cooling water supply to the unit may be suitably cutout.

With the unit now completely disconnected, the engine will operate with one unit less, which will induce heavy torsional vibration amplitudes. To reduce the effect of these vibrations the engine must be operated at a lower RPM where the vibration amplitudes are reduced considerably

Q5. A diesel generator when fitted in a machinery space which is periodically unmanned may be equipped with monitoring alarms of the exhaust temperatures. Discuss the relative merits of A. Individual cylinder maximum temperature alarms; B. Individual cylinder maximum and minimum temperature alarms; C. Individual cylinder maximum temperature alarm and an alarm for any two cylinders exhaust temperatures deviating more than 35°C. Explain how arrangement C can be provided for.

Answer:- (A) The higher temperature limit alarm is a minimal safety requirement to warn the watchkeeper of the damage to the liner and piston of the concerned unit , so that he can take suitable corrective action. It can be considered as a minimal requirement Automation.

(B) By providing for a low temperature alarm for all units the system is made more sensitive to provide signs of improper combustion in the units where the low temperature alarm is sounded , so that the operation of the engine is made more efficient .(c) The requirement for maximum and minimum indicating alarms for any two units is shown in the sketch given under. Assuming a deviation of+/- 350C , and the normal temperature is assumed 3500C the maximum temperature is 3850C and the minimum temperature is 3150C . These are the upper and lower temperatures for which the visual and sound alarms are provided.

The maximum temperature indicating device is a bimetal relay set for operation at 385 0C and that circuit is from +ve terminal through bimetal relay connected to the indicator 1 on panel and back to -ve terminal. This will give an alarm and indication by lamp on the panel when the temperature reaches the set value.

The low temperature alarm and indicator to indicate poor combustion or failure to fire is operated by a thermistor resistance . the behaviour of the thermistor is opposite to a metallic resistor whose resistance increases with temperature rise.

Themistors are made of powdered mixtures of metallic oxides such as manganese, nickel, cobalt, iron and copper and sintered (heating under pressure) and then shaped in rods beads or discs. When used as a resistor their resistance decreases with rise in temperature.

Referring to the sketch , the circuit is made from +ve terminal to thermistor an connected to the movable contact lever and from there to the three contact positions that are shown on the sketch.

(1) When the engine is just started and running at low speed, the exhaust temperature is sufficiently low and the resistance of the thermistor is high which makes the holding solenoid weaker than the spring and the contact is made on the extreme right side lighting the red lamp in the panel to show tha the circuit is on.

(2) As the engine speed reaches the service load the exhaust temperature reaches 350 0C if the operation is normal and in this condition the spring force becomes weaker than the hold on coil force. The lever shifts to the extreme left position and the green light will appear on the panel indicating normal combustion quality in the unit.

(3) Now if for some reason the combustion quality is poor or the firing has failed, the exhust temperature of the unit will reduce and when it reaches about 315 0C the lever takes a middle position contacting the alarm circuit . The circuit is made through the lever , the mid contact point and the indicator on the alarm panel and to the -ve terminal. This mid contact is made wider so that it starts operating as soon as the temperature falls below the normal operating temperature namely 3500C and will stay steady at any lower temperature.

Q7. Describe the process of inspecting the running gear of a main diesel engine. What is the purpose of such an inspection and what defects may be found?

Answer:- The process of inspecting the running gear of a main diesel engine is commonly called crank case inspection and is a routine practice carried out at regular intervals of three months The scope of this inspection is as listed :

· The clearances of the crosshead bearings measured and recorded.

· The clearance of the bottom end bearing measured and recorded.

· The clearance of the main bearing measured and recorded.

· All locking devices of all bearings checked and confirmed secure.

· The tie-rod pinching screws checked for tightness . Any broken pinching screws to be renewed.

· The crank bearings pinched to ensure free lateral movement.

Procedure:-

· The crank case doors to be opened only after about 20 minutes after the engine is stopped.

· All the crank spaces of all the units to be thoroughly swiped out so that nobody slips inside.

· Obtain propeller clearance and engage turning gear.

· Measurements and checking can commence now as it is safe.

· The position for measuring crosshead bearing oil clearances is crank on TDC and clearance measured on top half bearing and pin.

· The position for measuring crank bearing clearance is BDC and the clearance is measured between pin and bottom half bearing.

· The position for measuring main bearing clearance is to place the crank webs at half stoke on starboard side when the webs do not interfere with the long feeler gauge.

· After al;l checks have been carried out in all the units, the lub oil pump is started and the oil flow from all bearings spilling out to be noted. The engine is turned for about two revolutions during this verification

Q8. Explain the term ‘cascade control’ and sketch such a system suitable for use with a main engine jacket cooling water system. Show the variation of pressure and temperature at major points of the system.

Answer:- 3 

Cascade control loop is simply a cascade of two single control loops. Cascade control is used for process with slow dynamics like temperature, level ,and humidity.

Cascade control can be usefully applied to any process where a measurable secondary variable directly influences the primary controlled variable.

In a single control loop, the controller’s set point is set by an operator. For example, consider the heat exchange process here.

Fig 1

The water in the cooler iscooled by the sea water flowing through the tubes inside the cooler. Temperature of the fresh water flowing out of the cooler is measured, controller compares the temperature with the set point. And controller operates the inlet valve of the sea water flow into the cooler..

However due to the change in the out let sea water pressure, the sea water flow rate may change, though the control valve is at the same position. This will affect the amount of heat exchanged and the temperature at the fresh water outlet. It will take time to detect the temperature change due to the outlet pressure. And thus for the control action,the above problem can be overcome by controlling the flow rates, by using the cascade control loop.

Fig 2

In cascade control system an additional controller is used to check the effect of disturbance at the inlet flow. This additional controller works as the inner loop of the control system.

The primary controller, which is the temperature controller gives the control action as the set point of the secondary or flow controller. Flow controller gives the final control action to the valve, comparing the Flow meter measurement and the temperature controller output.

The pressure and temperature at the major points of the system namely the inlet and outlet points of the two fluids through the cooler are(ref fig 2):

Fresh water inlet temperature 750C.

Fresh water inlet pressure 2.8 kg/cm2

Fresh water Outlet temperature 550C.

Fresh water outlet pressure 2.8 kg/cm2.

Sea water inlet temperature 350C

Sea water outlet temperature 500C

Sea water inlet pressure 2.5 kg/cm2

Sea water outlet pressure 2.3 kg/cm2.

Q9. Draw a line diagram of a complete feed water system for an auxiliary boiler labelling all the principal items and showing the direction of flow in all lines. B. Explain how the feed supply to the boiler is regulated; C. State what means are provided to prevent oil contamination of the feed water.

Answer:-

(b) The feed supply to a boiler is regulated by a feed controller. Most feed controllers work on the float sensor control keeping the half level in the float chamber as the desired level in the boiler. This level is the same as half the gauge glass levels. Rising water level characteristics is a feature accompanied by “swell” caused by accumulation of feed water in the boiler when steam demand has reduced. This happens in steam turbine driven ships during manoeuvring periods. The problem is addressed by the two element control described below.

Two variables are measured for controlling the feed water supply , these being steam flow and boiler drum water level. A signal is generated from the steam flow transmitter which positions the feed water regulating valve to maintain equilibrium conditions between the feed water supply and the steam generated.

This control system on its own is not capable of maintaining the level in the steam drum due to the time lag between the steam flow being sensed and the feed regulating valve operation. The water level could vary considerably during this time interval. This problem is overcome by utilising a signal from the water level transmitter which connects the signal to the feed water regulator via a computing relay. The level controller is used as a trimming device to mainrain the water level at the correct position during equilibrium conditions. Installation arrangements are shown in the figure given below.

(c) When oil contaminates the feedwater, it happens by oil heating pipes holed, through which the oil finds its way into the boiler through the observation tank as well as the cascade tank. It will be noticed when the boiler is shut down and cooled, when oil separates out in the boiler and accumulates above the water level and will be seen in the gauge glass above the water level. The oil can be removed by boiling the water in the boiler with caustic soda in the proportion of 25-35 kgs per ton of water in the boiler upto gauge glass level. The boiling is done in the manner used for warming up the boiler with the air vent open and so that no pressure is built up. Warming is continued for 18 to 24 hours and the contents (black emulsion) drained into the machinery space bilges and then dealt suitably. The boiler internal surfaces become clean with out any trace of oil.

(c) When oil