AKRAMANUJAN.ORG23rd NOVEMBER 2019 FUNCTION 4(28)

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NOTE:- I am once again repeating the request for questions from other centres so that it helps you all in preparing for the orals. you can send hand written scanned sheets of questions to my e-mail given at the head. Please treat this as urgent

(181) the boiler water alkalinity is decreasing gradually. What can be the reason.

Answer:- If the alkalinity reserves are depleting gradually , it means that the sodium hydroxide is being consumed.

Sodium hydroxide can combine with any vegetable oil and that is the process for making soap. Sodium hydroxide can also combine with the fuel oil which is distantly related to vegetable oil to form a black emulsion. Hence it indicates that oil is leaking into the boiler through a leaking steam coil in one of the oil tanks.

(182) During manoeuvring, a unit cylinder liner is cracked . How will you find out and what action you will take immediately.

Answer:- Assuming the ship is sailing away from the port the following action will be carried out:

*The fuel to the unit can be cut off by opening the puncture valve.

* The jacket cooling water to the unit and the cylinder outlet valves must be kept shut to prevent water entering the liner.

* The indicator is kept open to relieve the build up of pressure..

The engine will be run on one unit less until the ship has come out of port limits and anchored safely after which the work for renewing the liner can be commenced

(183) WHAT IS reamer? If it has to be used on the lathe, how will you attach it.

Answer:- The reamer is a milling tool corresponding to a drill. When a hole is drilled or punched out, the reamer is used to remove the burrs. When used on the lathe it is fixed in the centering tool stock at the other end of the lathe . The reamer has a socket head and hence can be fitted easily in the tool head.

R.K.Dutta and V.Roy.

(184) What is the meaning of first order moment .

Answer:- The first order moment is the primary couple caused by the inertia of the reciprocating masses in the engine. If the vector sum of all these moments becomes zero the engine is not affected, but if it results in a small unbalanced couple it gives rise to structural vibrations of the engine frame and hence not desirable.

(185) Engine vibrates severely during service speed. What can be the reason.

Answer:- The main reason for torsional vibrations of the engine shafting system is that the power produced in each unit as well as the peak pressures in each unit are not identical. This anomaly can be resolved by taking indicator cards and draw cards of all the units and by adjusting the fuel racks or fuel settings of the fuel pumps in a progressive manner by trial and error the anomaly can be resolved. On arrival in port all the engine foundation bolts will be tightened , since they may have loosened by the vibration. If possible the tie-rods tensions will be verified in the manner given in the manual.

If after all this the problem is not resolved , then the cause is external, the propeller blades are damaged(bent). This can be verified when the ship is in ballast and anchored in a port.

(186) Describe the different methods of defrosting the reefer chambers.

Answer:- There are three methods of defrosting the chambers:

* By circulating the hot gas through the coils. In most domestic fridge systems a by-pass line cutting out the condenser, receiver and expansion valve is provided so that the hot refrigerant after the compressor can be circulated through the evaporator coil. This method is fast.

* By keeping the chamber doors open defrosting is achieved by natural heat flow from the atmosphere and surroundings. This is a slow process being natural.

By pouring hot water on the evaporator coils the ice gets melted . This is faster than the natural method

(187) What are the reasons for fouling of the T/C on the exhaust side.

Answer:- Fouling of the turbocharger turbine blades and nozzle blades occurs especially during low load operations of the main engine , when the combustion of the fuel is not optimised leading to oily carbon soot accumulating in the blades and nozzle blades of the turbocharger. This fouling also occurs on the finned tubes of the forced circulation heat exchanger of the waste heat section of the boiler. The fouling in the finned tubes can cause back pressure of the exhaust gases and thereby reduce the speed of the turbine.

(188) What is accumulation pressure test and when is it done on the boiler.

Answer:- Repeat question. Refer to question number (164) KOLKATA section.

(189) Explain the procedure to cut out one unit of the engine , if the piston in the liner is seized.

Answer:- Repeat question . refer question numbers (111) and (112) CHENNAI centre.

MAY 2016 Mr SARKAR.(190) Describe the cascade system . Answer:-

When multiple sensors are available for measuring conditions in a controlled process, a cascade control system can often perform better than a traditional single-measurement controller. Consider, for example, the steam-fed water heater shown in the sidebar Heating Water with Cascade Control. In Figure A, a traditional controller is shown measuring the temperature inside the tank and manipulating the steam valve opening to add more or less heat as inflowing water disturbs the tank temperature. This arrangement works well enough if the steam supply and the steam valve are sufficiently consistent to produce another X% change in tank

temperature every time the controller calls for another Y% change in the valve opening .However several factors could alter the ratioof X to Y or the time required for the tank temperature to change after a control effort. The pressure in the steam supply line could drop while other tanks are drawing down the steam supply they share, in which case the controller would have to open the valve more than Y% in order to achieve the same X% change in tank temperature. Or, the steam valve could start sticking as friction takes its mechanical toll over time. That would lengthen the time required for the valve to open to the extent called for by the controller and slow the rate at which the tank temperature changes in response to a given control effort

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A better way

A cascade control system could solve both of these problems as shown in Figure B where a second controller has taken over responsibility for manipulating the valve opening based on measurements from a second sensor monitoring the steam flow rate. Instead of dictating how widely the valve should be opened, the first controller now tells the second controller how much heat it wants in terms of a desired steam flow rate

The second controller then manipulates the valve opening until the steam is flowing at the requested rate. If that rate turns out to be insufficient to produce the desired tank temperature, the first controller can call for a higher flow rate, thereby inducing the second controller to provide more steam and more heat (or vice versa).

That may sound like a convoluted way to achieve the same result as the first controller could achieve on its own, but a cascade control system should be able to provide much faster compensation when the steam flow is disturbed. In the original single-controller arrangement, a drop in the steam supply pressure would first have to lower the tank temperature before the temperature sensor could even notice the disturbance. With the second controller and second sensor on the job, the steam flow rate can be measured and maintained much more quickly and precisely, allowing the first controller to work with the belief that whatever steam flow rate it wants it will in fact get, no matter what happens to the steam pressure.

QUESTIONS CONTRIBUTED BY LOKESH SAKUNAM

FUNCTION 5

 01 Describe the Zener diode characteristicsAnswer:- Ordinary diodes are forward biased and work in the forward direction. They have a large forward current flowing through them with a negligible voltage drop across them. If we operate an ordinary diode in reverse biased, it conducts insignificant current until the voltage applied across them exceeds the reverse breakdown voltage. Once that happens, large current flows through the junction and the diode may get destroyed. The Zener diode is a particular type of diode that solves this problem. We operate a Zener diode in reverse biased conditions, and this diode does not get damaged even when the voltage across this exceeds the reverse breakdown voltage. Let us learn about this exciting and unique kind of diode. Zener diodes are heavily doped than ordinary diodes. They have extra thin depletion region. When we apply a voltage more than the Zener breakdown voltage (can range from 1.2 volts to 200 volts), the depletion region vanishes, and large current starts to flow through the junction. There is a crucial difference between an ordinary diode and a Zener diode. The depletion region regains its original position after removal of the reverse voltage in Zener diode whereas in regular diodes, they don’t, and hence they get destroyed.Here is the Zener diode symbol

Let us now look at the Zener diode characteristic:

A graph of current through vs the voltage across the device is called the characteristic of Zener diode. The first quadrant is the forward biased region. Here the Zener diode acts like an ordinary diode. When a forward voltage is applied, current flows through it. But due to higher doping concentration, higher current flows through the Zener diode. In the third quadrant, the magic happens. The graph shows the current vs voltage curve when we apply a reverse bias to the diode. The Zener breakdown voltage is the reverse bias voltage after which a significant amount of current starts flowing through the Zener diode. Here in the diagram, VZ refers to the Zener breakdown voltage. Until the voltage reaches Zener breakdown level, tiny amount of current flows through the diode. Once the reverse bias voltage becomes more than the Zener breakdown voltage, a significant amount of current starts flowing through the diode due to Zener breakdown. The voltage remains at the Zener breakdown voltage value, but the current through the diode increases when the input voltage gets increased. Due to the unique property of Zener diode, the depletion region regains its original position when the reverse voltage gets removed. The Zener diode doesn’t get damaged despite this massive amount of current flowing through it. This unique functionality makes it very useful for many applications.

2 what is zener potential ,inverse break down voltagediff b/w avalanche and zener diode

Answer:- It is the full strength reverse voltage when current flow is reversedThe difference between Zener and avalanche breakdown include the following.

The Zener breakdown can be defined as the flow of electrons across the p kind material barrier of the valence band to the evenly filled n-type material conduction band.

The avalanche breakdown is an occurrence of raising the flow of electric current or electrons in insulating material or semiconductor by giving the high voltage.

The depletion region of the Zener is thin whereas the avalanche is thick.

The connection of the Zener is not-destroy whereas the avalanche is destroyed.

The electric field of the Zener is strong whereas the avalanche is weak.

The Zener breakdown generates electrons whereas the avalanche generates holes as well as electrons.

The doping of the Zener is heavy whereas the avalanche is low.

The reverse potential of the Zener is low whereas the avalanche is high.

The temperature coefficient of the Zener is negative whereas the avalanche is positive.

The Ionization of the Zener is due to Electric field whereas the avalanche is the collision.

The temperature coefficient of the Zener is negative whereas the avalanche is positive.

The breakdown voltage (Vz) of the Zener is inversely proportional to temperature (ranges from 5v to 8v) whereas the avalanche is directly proportional to temperature (Vz > 8V).

After the breakdown of the Zener is voltage remains constant whereas the avalanche is voltage vary.

The Zener breakdown V-I characteristics have a sharp curve whereas the avalanche doesn’t have a sharp curve.

The breakdown voltage of the Zener decreases when the temperature increases whereas the avalanche increases when the temperature increases.

Thus, this is all about Zener breakdown and Avalanche Breakdown. From the above information finally, we can conclude that generally there are two different breakdowns are distinguished based on the concentration of doping bias in the PN-junction. Whenever the PN-junction is doped highly then the Zener breakdown will happen whereas the avalanche breakdown will occur due to lightly doped PN-junction. Here is a question for you, what are the VI-characteristics of Zener breakdown and Avalanche Breakdown?

3 where are zener diode used  and explain its application on boardAnswer:- Since the voltage dropped across a Zener Diode is a known and fixed value, Zener diodes are typically used to regulate the voltage in electric circuits. Using a resistor to ensure that the current passing through the Zener diode is at least 5mA (0.005 Amps), the circuit designer knows that the voltage drop across the diode is exactly equal to the Zener voltage of the diode.What is residual magentism and where it is used in ship?

when the power is switched off to an electro magnet, it will retains some magnetic power, and the retained magnetic power is called residual magnetism. Example, standby generator. Once the generator is started the residual magnetism in the field produce small amount of voltage and then the same voltage is fed to the field to increase the magnetic field which will produce more voltage than earlier rotation and so on untill the rated voltage is produced. If there is no residual magnetism in the magnetic field, the generator will not produce any voltage.In a generator, residual magnetism, also known as 'remanence' or remanent flux, is due to the hysteresis of the magnetic material in the core. When the current is removed, the magnetic flux often remains at some non-zero value. This is a characteristic phenomenon of ferromagnetic materials.

The residual voltage after a generator has been stopped is due to the capacitance of the insulation system. On medium voltage generators, this capacitance can store a fairly significant amount of charge, and will give an unpleasant shock if you happen to contact it before being discharged.

The other answerer mentioned that it is residual magnetism which is responsible for some form of excitation. In large generators, the field excitation is practically always externally controlled. In medium and smaller generators, a permanent magnet exciter is used to generate the current for field excitation. And, while permanent magnets do offer the most obvious form of residual magnetism, their magnetism is not dependent on generator currents, as implied by the earlier answerer.

4 How does boiler feed water controller  works ,explain set value,desire value and type of float.Answer:-

A feed controller of the float type is as sketched above. (Fig.61) 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. The float is

a hollow metal ball. The set value is also the desired value.

5How does differential pressure transmitter work

Answer:- Differential Pressure is not measured with reference to a specific reference pressure (see Fg.4).

Unlike Gauge or Absolute pressure transmitters, Differential Pressure Transmitters do not attempt to fix the reference. Importantly an increase in differential can be the result of increasing one of the pressures or decreasing the other.  An increase in differential pressure would occur if P1 became smaller OR if P2 became larger. In a similar way, a decrease in differential pressure would occur if P1 became larger OR if P2 became smaller. The differential pressure measurement is not concerned whether the lower of the two pressures is at a vacuum, atmospheric or some other pressure. It is only interested in the difference between the two. Asshown

in Fig4 The fundamental of differential pressure measurement is established.

6 PID controller,where used on board.what is diff b/w PI and PID controller.explain derivative action

Answer:- The rudder operation utilizing the floating lever in the hydraulic steering gear is a perfect example of a PID controller. The D function is contributed by the telemotor coil springs at the receiver end. Difference between PD and PID is one is a proportional and differential controller whereas the other is a proportional plus integral aplus differential controller In a controller to avoid Off set the integral control is introduced. That control is not in a proportiona control. The derivative control is added to the proportional control to reduce .Derivative action:- The action of a controller whose output signal is proportional to the rate at which the derivative is changing .

7What is over shoot explain with its characteristic curve. :-Answer:- The over shoot occurs in two step control usually used in the starting and stopping of the hydrophore system for domestic sanitary or fresh water supply on board. The main disadvantage is that it cannot sense an increase or decrease in demand . It chases the control only when it comes to the end position and change is caused by crossing over to the other side or beyond. This is called over shoot. The two humps shown above the top dotted line ( end of control) is the over shoot.

overshoot

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