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OBJECIVE QUESTIONS FOR MMD CLASS 4 EXAM
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Q - In transistor why emitter is highly doped. (Asked in Feb & August 2014 in MMD Mumbai & Kolkata)
1)Transmit more power
2) Reduce resistance
3) For charge flow
4) First terminal of transistor
Ans – 3– For charge flow
Explanation by Marine Expert - Emitter is heavily doped to provide large no of majority charge carriers & base and collector are lightly doped to accept these charge charge carriers from emitter. If they are also heavily doped then the charges will not flow towards the base or collector then the transistor becomes inactive
Important Note from Expert – Many people have the misconception about the answer to this question is 2 that is Reduce resistance which is also right to some extent because highly doped means more impurities and more impurities means more conducting electrones which means less resistance but our intention of doping is not to decrease the resistance at the barrier. The main purpose is to increase the charge flow rate.
Q - Reason for reduced rated capacity of battery. (Asked in March 2014 & Feb 2015 in MMD Cochin & Kolkata)
1) Weak electrolyte
2) Short circuit in cell
3) Old age
4) Any of above
Ans – 4 –Any of Above
Explanation by Marine Expert– Short Circuit is internal phenomenon which effects the rated capacity of the battery but in this question we are more concern about that weak electrolyte affetcs the rated capacity of the battery. Electrolytesallow ions to move between the electrodes and terminals, which allows current to flow out of the battery to perform work.
Importance of Electrolyte in the battery - Batteries convert chemical energy directly to electrical energy. A battery consists of some number of voltaic cells. Each cell consists of twohalf-cellsconnected in series by a conductive electrolyte containing anions and cations. One half-cell includes electrolyte and the negative electrode, the electrode to whichanions(negatively charged ions) migrate; the other half-cell includes electrolyte and the positive electrode to whichcations(positively charged ions) migrate.Redoxreactions power the battery. Cations are reduced (electrons are added) at the cathode during charging, while anions are oxidized (electrons are removed) at the anode during discharge.[11]The electrodes do not touch each other, but are electrically connected by theelectrolyte. Some cells use different electrolytes for each half-cell. A separator allows ions to flow between half-cells, but prevents mixing of the electrolytes.
Q – Why Flywheel of Ref. AC compressor is connected through the motor with the belt and in case of MAC it is directly attached through the coupling no requirement of belt.(Asked in Feb,May,August 2014 and Feb 2015 of MMD Mumbai)
Ans – Explanation by Marine Expert – To understand the logic behind this just consider capacitor of the electrical circuit which perform the same function as just like the flywheel of any engine means give the energy when it is required.
If liquid goes in compressor since it is incompressible it will be unable to get compress so Ref comp are belt driven so that it slips during this event but if it is directly attached to the motor so there may be the possibility that before the time it stops all the internal parts will be cracked but at the same time in MAC there is a option of drainage of water after each stage so that not even the single drop should be carry away with air to the next stage of compression.
Q–During the time of Turbocharger running, what washing not to be done.(Asked in August 2014 and Jan 2015 in Kochi & Mumbai)
- Dry washing on blower side.
- Wet washing on blower side.
- Dry washing on turbine side.
- Wet washing on turbine side
Ans – 2& 3
Explanation by Marine Expert – This is the tricky question as we all know we normally perform the blower side water washing and turbine side grit washing @min 65 % load on the engine. But there are some engine in which we perform dry washing on blower side as recommended by manufacturer in the manual.
But at the same time we never ever perform the wet washing on turbine side at the time when the engine is running because it badly effect the performance of the turbocharger, when engine is operating than all carbon deposits which are attached to the blades will change to sticky nature and the performance of the turbocharger starts going down and on the long you found the pitting marks on the blade of turbocharger.
Q–Why we kept power factor 0.8. (Mostly Asked in all MMD Centres)
Ans – Explanation by Marine Expert - With reference to PF less then 0.8 means reactive load ( inductive reactence is more).
If it is shore supply then electric distribution company will object because we pay for low KW and burns more KVA.
That’s why we need capacitor as PF improver.
If generator then it is not economical,wastage of energy due to more inductive load ( means power loss in ac circuit is more.)
Note- Power factor is the ratio between the KW (Kilo-Watts) and the KVA (Kilo-Volt Amperes) drawn by an electrical load where the KW is the actual load power and the KVA is the apparent load power. It is a measure of how effectively the current is being converted into useful work output and more particularly is a good indicator of the effect of the load current on the efficiency of the supply system.
Power Factor = True Power/Apparent Power
NOTE-All current flow will causes losses in the supply and distribution system. A load with a power factor of 1.0 results in the most efficient loading of the supply and a load with a power factor of 0.5 will result in much higher losses in the supply system.
Q – Which all certificates are issued under chapter 11 of Solas.(Asked in August 2014 in Kolkata MMD)
Ans -International ship Security Certificate(ISSC)
Ship Security Plan and associated Records
Continuous Synopsis Record (CSR)
Q–If remote and local stop of M/E doesn't work, what else we can use. (Asked in Oct 2014 & Jan 2015 in Mumbai & Kolkata MMD)
Ans – Explanation by Marine Expert - Operate one of emeregency stop in control system like lube oil low pressure shutdown or spring or control air or you can operate puncture vlv if available(which operate the air piston inside the puncture valve assembly to avoid any injection of fuel inside the unit) andin the last you can wrap the cloth around turbo charger air filter.
Note by Marine Expert – this type of question is mostly asked in MMD objective only the thing that you have to keep in mind is that to stop the main engine which is your ultimate goal so always try to opt that option which fullfill your requirement in case of emergency.
Q - A F.O. purifier bowl is not closing properly and leaking slightly. You have completely overhauled the purifier, replaced the seal rings and everything seems alright within the bowl, but still it is leaking. What could be the most probable cause?
(Asked in Dec 2014 & March 2015 in Mumbai & Kolkata MMD)
A. Fuel oil density is too high
B. Friction pads worn out and slipping on the friction pulley
C. Oil temperature too high
D. Incorrect size gravity disc
Ans – B
Explanation by Marine Expert – Some time MMD try to put the answer to the question in such a way that all the answers seems to be the same and it is very hard to understand which is most appropriate answer to choose at that time you have to check which answer seems to be different from the other just in this case all the option no . A,C & D causes the purifier to overflow but only option B has the possibility that bowl is not closing properly and leaking slightly because if the friction pads get worn out than required speed can't be attained in purifier which means in other words proper centrifugal force is not attained ,if proper centrifugal is not attained than bowl won't lift up and the closing of bowl happens due to PR. Of closing water + centrifugal force.
Q - The gasket and the broken studs have been replaced on a tank manhole cover. Which of the following methods is satisfactory for testing the repair. (Asked in Nov,Dec 2014 & Jan 2015 in Kochi & Kolkata MMD)
a) Pressurize the tank with 10 psig air, soap the repaired area, watch for visible signs of leakage or bubbles
b) Fill the tank with water via the ballast pump until the reading corresponds to the maximum depth of the tank
c) Hose test the repaired area with a minimum of 100 psig water pressure
d) Fill the tank via the ballast pump until water flows from the vent line opening on deck
Ans – A
Q - By using which of the tank we can change the draft but not changing trim of the ship
A) peak tank
B) DB tank
C) deep tank
D) top side tank
Ans – C – Remaining all other tank can change the trim of the Vessel.
Q - Which of the following will result in automatic stopping of IG blower(Asked in Sept 2014 and Feb 2015 in MMD Kochi,Kolkata nad Chennai)
A) low water level in deck seal
B) Oxygen content above 8%
C) Scrubber tower low water level
D) scrubber tower high water level
Ans – D
Explanation by Marine Expert – In IG system there always the alarm for Opion A,B & C because these all are not that much critical but Option D is very critical related to the operation of IGG plant because if scrubber tower high water level alarm came it means may be your ovb valve is not fully open because it contain two valves in the ovb line one is hydraulic and one is normal butterfly valve and manufacture gives the shutdown for this alarm because if water level keeps on increasing then sea water goes all through the line and from there it will go to the tanks and contaminate the cargo which may degrade the quality of the cargo which is one of the worst scenario.
Q - During cleaning of the sea water suction filter, it was found that at some places the mesh size has increased either due to corrosion or due to mechanical Damage. What action is required. (Asked in 2013 & 2015 MMD Mumbai)
A. No action is required, it is quite normal for sea water filters to get
corroded and it will not effect the pump in anyway
B. Keep the pump which is having corroded filter on standby use while keep
using the other pump. Meanwhile order for the new filter
C. The pump can be run without filter for a short time, while the filter
is being repaired
D. Permanent repairs to the filters should be carried out before putting
it into service, or it should be replaced with good spare filter
Ans –D
Explanation by Marine Expert – Option A and C affect the Pump at long run because you are trying to bypass the safety of the pump which is not acceptable if you have only one sea water pump in stand by mode and Option B is also not right because instead of rectifying the error you try to avoid it, so only one option is left which is D.
Q – Why Relief v/v is provided in condenser of refeer system.(Asked in August 2014 MMD Kolkata)
Ans-Explanation by Marine Expert – If ur condenser not cooling properly there will be chances of developing excesspressure because of this pressure your hp will activated if hp will not work at that time this relief vlv comes in action and remove excess pressure so it prevent us from serious damage as a secondary arrangement after hp trip.
OR
Condenser tubes gets excess pressure when there is air in system or the compresser has to do more work when sea water temp is high in certain areas which leads to increase in pressure thus for ensuring safety since HP trip failure chances are very common onboard.
Q - What will happen to the motor current if the discharge valve of a centrifugal pump is completely shut off? The current will(Asked in MMD Kolkata in March 2014)
A. Not change
B. Increase to a very high value
C. Increase slightly
D. Decrease to no-load value
Ans – D
Explanation by Marine Expert – As we know we can run the centrifugal pump with the discharge valve shut and with closed discharge valve pump does not perform any work . Impeller is just churning the liquid , no fluid transfer there, so no load condition and hence the Option is D.
Q - Which of the following statements concerning antifriction bearings installed on pumps is true. (Asked in May 2014 in MMD Goa)
1. The inner race should be free to turn on the shaft
2. The outer race should be free to turn its housing
3. Alignment is not a critical factor in their installation
4. They are usually pressed on to their shafts
Ans – 4
Explanation by Marine Expert – Answer to this question is Option 4 because antifriction bearing means Roller bearing which is usually pressed on to their shafts.
Q - Lantern ring in centrifugal pump is located:
A. Around the impeller
B. Between gland packing
C. Around submerged bearing
D. Around the casing ring
Ans – B
Explanation by Marine Expert - It is not always possible to use a standardcentrifugal pumps stuffing boxto seal the shaft of a centrifugal pump. The pump suction may be under a vacuum so that outward leakage is impossible or the fluid may be too hot to provide adequate cooling of the packing. These conditions require a modification to the standard stuffing box.
One method of adequately cooling the packing under these conditions is to include a lantern ring. Alantern ringis a perforated hollow ring located near the center of the packing box that receives relatively cool, clean liquid from either the discharge of the pump or from an external source and distributes the liquid uniformly around the shaft to provide lubrication and cooling. The fluid entering the lantern ring can cool the shaft and packing, lubricate the packing, or seal the joint between the shaft and packing against leakage of air into the pump in the event the pump suction pressure is less than that of the atmosphere.
Q - BJT transistor used as an amplifier it operats in-(Asked in April 2014 in Mumbai MMD)
A.Cut off
B.Wellsaturation
C.Well insaturation
D. Active region
Ans –D
Explanation by Marine Expert- In order to use the transistor as an amplifier, it must be operated in the active mode. The BE junction is forward biased whereas the CB junction is reverse biased. Figure below shows both n-p-n and p-n-p transistors biased in the active mode of operation.
Many people have the misconception about the Option A, but the BJT is fully off in this state. In the cutoff mode both the base emitter as well as collector base junction is reverse biased. The BJT is equivalent to an open switch in this mode.
Q- The discharge head of a centrifugal pump will vary directly with the(Asked in Goa & Mumbai MMD in July 2014)
1. Change in impeller diameter
2. Square of the impeller diameter
3. Cube of the impeller diameter
4. Impeller efficiency for large changes in the size of the impeller
Ans – 2
Explanation by Marine Expert - The static head corresponding to any specific pressure is dependent upon the weight of the liquid according to the following formula:
A centrifugal pump imparts velocity to a liquid. This velocity energy is then transformed largely into pressure energy as the liquid leaves the pump. Therefore, the head developed is approximately equal to the velocity energy at the periphery of the impeller. This relationship is expressed by the following well known formula:
We can predict the approximate head of any centrifugal pump by calculating the peripheral velocity of the impeller and substituting into the above formula. A handy formula for peripheral velocity is:
Q- In a centrifugal pump sealing between gland packing and bearing housing is provided by_____(Asked in MMD Mumbai & Kolkata 2014)
1. Shaft seal
2. Water flinger
3. O-ring
4. Lantern ring
Ans – 2
Explanation by Marine Expert - A water flinger is fitted on the shaft outboard of the stuffing box or mechanical seal to prevent leakage from the stuffing box following along the shaft and entering the bearing housings. They must fit tightly on the shaft. If the flingers are fitted on the shaft sleeves instead of on the shaft, ensure that no water leaks under the sleeves.
Q - For any pump maximum theoretical lift is
1. 10.3 feet
2. 10.2 meter
3. 10.5 meter
4. 12.0 meter
5. None
Ans –1-
Explanation by Marine Expert- There's a practical limitation as to how far you can lift a column of water: the vacuum you cause in the barrel by lifting the pump handle certainly causes atmospheric pressure to force the water up the pipe - but only to the point where the weight of that column of water equals the weight of the atmosphere pushing down on the surface of the water source. It may be shown that the maximum height that water can be lifted is about 34 ft/10.3m, at sea level. However, various factors limit this in practice to about 25 ft/7.7m. So if the water source is deeper than this, the simple set-up as described above can't cope. The answer is to put the barrel deep down into the borehole, or even below the water level, and perhaps add an outlet valve - so forming a so-called "lift & force pump". This allows water to be brought up from much greater depths, but the deeper the barrel, the more difficult it is to get at for maintenance.