Nautical Engineering Answers Part—2

Excerpt from U.S. Coast Guard Proceedings of the Marine Safety & Security Council magazine. Nautical Engineering Queries Answers Part—2.

1. Coast Guard regulations (46 CFR) concerning shutoff valves located inside fuel oil tanks state that the valves __________.

A. shall be arranged for

Incorrect answer. See explanation for choice “D”. Valves installed in the inside of fuel tanks must be

arranged for remote control.

B. must be made of steel

Incorrect answer. 46 CFR 56.50-60(D)(2) states: “Valves for local control outside the tanks must be made

of steel, ductile cast ironASTMA395, or a ductile nonferrous alloy having a melting point above 1,700°F.”

C. must be power-operated

Incorrect answer. See explanation for choice “D”. Shutoff valves located inside the fuel tank are to be arranged for remote control only, and are not required to be power-operated.

D. may be made of cast iron

Correct answer. 46 CFR 56.50-60(d)(2) states: “If valves are installed on the inside of the tank, they may

be made of cast iron and arranged for remote control only.”

2. The distance between a generator and its load is 100 feet. What would be the approximate total voltage drop across a two-wire supply cable if the current was 5.5 amperes and the resistance of the wire was 2.525 ohms per 1,000 feet?

A. 0.5 volts

Incorrect answer.

B. 1.38 volts

Incorrect answer.

C. 1.90 volts

Incorrect answer.

D. 2.77 volts

Correct answer. Solution is as follows:

Resistance of wire per foot: 2.525 Ω ÷ 1000 feet = 0.002525 Ω / ft

Total resistance of 200 feet of wire: 200 ft (0.002525 Ω / ft) = 0.505 Ω

Voltage drop across wire cable: V = (I)(R) = 5.5 amperes(0.505 Ω) = 2.77 volts

3. In a closed feed and condensate system, the drain from the second-stage air ejector returns directly to the __________.

Note: A steam jet ejector is a type of air ejector used to remove air and other non-condensable gases from a condenser. The ejector has no moving parts, and receives the energy

to operate from pressurized steam that creates a “pumping action” as it passes through the ejector. Air ejectors are generally multi-stage, consisting of several

ejector elements arranged in series.

A. auxiliary condenser

Incorrect answer. In a two-stage air ejector unit, saturated non-condensable gases removed from the condenser are initially drawn into the suction chamber of the first-stage air ejector. The gases become entrained in the first-stage ejector steam jet, and the mixture is discharged into the shell of a heat exchanger called the intercondenser. Condensate discharged by the condensate pump passes through the intercondenser tubes and condenses the mixture in the shell. Water formed from the condensing mixture is returned to the condenser via a “U”-shaped loop seal.

B. loop seal

Incorrect answer. See explanation for choice “A.” To prevent air and other non-condensable gases from the

intercondenser being drawn back into the condenser, the intercondenser drain line is fitted with a water sealed “U”-shaped loop.

C. atmospheric drain tank

Correct answer. Gases remaining in the intercondenser shell are drawn into the suction chamber of the second-stage ejector and become entrained in a second jet of steam. The steam and gas mixture is then discharged into the shell of a heat exchanger called the aftercondenser. Condensate discharged by the condensate pump passes through the aftercondenser tubes and condenses the mixture in the shell. Water formed from the condensing mixture is under a slight positive pressure and drains by gravity to the atmospheric drain tank. Non-condensable gases are vented to the atmosphere.

D. de-aerating feed tank

Incorrect answer. The de-aerating feed tank’s operating pressure prevents its use as a direct return for the low-pressure drains of the air ejector.

4. Which of the turbocharging systems listed operates with the least average back pressure in the exhaust manifold?

Note: In a diesel engine, the two methods utilized for transmitting the energy in the exhaust gases to drive the turbocharger are the constant-pressure system and the pulse

system.

A. constant volume

Incorrect answer. Constant volume is the term used to describe combustion in a gasoline engine. Refer to the “OttoCycle” for spark-ignition engines.

B. constant-pressure

Incorrect answer. In the constant-pressure system, the exhaust gases from the individual cylinders are discharged into a large common manifold. Since the pressure in the manifold tends to be the average of the cylinder outputs, the turbocharger is provided with a fairly constant-pressure gas supply.

C. pulse-pressure

Correct answer. The pulse system permits operation of the turbocharger with the least average back pressure in the exhaust manifold. With the pulse system, the exhaust gases from each cylinder or group of cylinders are admitted directly to the gas turbine through a short exhaust pipe. As a result, the flow of gases to the turbocharger “pulsates.” The turbocharger is designed to utilize both the velocity and pressure energy in the pulsating gases

D. radial flow

Incorrect answer. Radial flow is the term used to describe the direction of gas flow in a turbocharger.

Full article is available at http://www.uscg.mil/proceedings/winter2010-11/


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Winter 2011 Nautical Engineering Questions Part—1

Excerpt from U.S. Coast Guard Proceedings of the Marine Safety & Security Council magazine. Nautical Engineering Queries Questions Part—1.

1. Coast Guard regulations (46 CFR) concerning shutoff valves located inside fuel oil tanks state that the valves __________.

A. shall be arranged for local control

B. must be made of steel

C. must be power-operated

D. may be made of cast iron

2. The distance between a generator and its load is 100 feet. What would be the approximate total voltage drop across a two-wire supply cable if the current was 5.5 amperes and the resistance of the wire was 2.525 ohms per 1,000 feet?

A. 0.5 volts

B. 1.38 volts

C. 1.90 volts

D. 2.77 volts

3. In a closed feed and condensate system, the drain from the second-stage air ejector returns directly to the __________.

A. auxiliary condenser

B. loop seal

C. atmospheric drain tank

D. de-aerating feed tank

4. Which of the turbocharging systems listed operates with the least average back pressure in the exhaust manifold?

A. constant volume

B. constant-pressure

C. pulse-pressure

D. radial flow

Stay tuned to see the answers to these Nautical Engineering Queries.

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Nautical Engineering Queries Answers Part—2

Excerpt from U.S. Coast Guard Proceedings of the Marine Safety & Security Council magazine. Nautical Engineering Queries Answers Part—2.

1. Amortisseur windings are installed in a synchronous motor to __________.

A. reduce eddy current losses
Incorrect answer. To reduce eddy current losses, the core of the synchronous motor stator is built up from many thin steel sheets that are insulated from each other with a coating of varnish.
B. produce a higher power factor
Incorrect answer. To adjust the power factor of a synchronous motor, a DC exciter varies the amount of current to the rotor field windings. Low values of field current result in less hold-in strength and a lagging (lower) power factor. Conversely, high values of field current result in greater hold-in strength and a leading (higher) power factor.
C. provide a means for starting
Correct answer. The amortisseur winding is a squirrel-cage winding consisting of copper bars embedded in the rotor pole faces, and is used to start and accelerate the synchronous motor to near synchronous speed.
D. eliminate arcing between
Incorrect answer. Any arcing would occur at the DC exciter circuit breaker when opened. To prevent this, a “field-discharge resistor” converts the energy stored in the magnetic field of the rotor to heat energy that is harmlessly dissipated to the atmosphere.

Note: Synchronous motors are utilized in applications in which constant speed is essential, or where the power factor of a system must be maintained at a high level. Large machines that are in continuous service for long periods of time operate more efficiently when driven by synchronous motors.

2. Coast Guard regulations (46 CFR) define several acceptable means of closure for ballast and fuel oil tank vents. One of the acceptable means is by the use of a/an __________.

A. manually operated ball
Incorrect answer. 46 CFR 56.50-85(a)(7)(i) states: “A ball check valve where the ball float, normally in the open position, will float up and close under the action of a submerging wave.” A ball float check valve that will float up and close under the action of a submerging wave is an automatically operated valve.
B. automatically operated
Correct answer. 46 CFR 56.50-85(a)(7)(ii) states: “A hinged closure normally open on the outlet of the return bend, which must close automatically by the force of a submerging wave …”
C. permanently installed
Incorrect answer. A permanently installed canvas hood over the vent would prevent proper venting of the ballast and/or fuel oil tank under normal operating conditions.
D. corrosion-resistant wire screen I
ncorrect answer. A corrosion-resistant wire screen is a permeable material, and would not prevent the entrance of water into the ballast and/or fuel oil tank from a submerging wave or other source.

3. A continuous blow is used to __________.

A. regulate the density or salinity
Correct answer. A continuous blow, as the term implies, is the continuous removal of water from the boiler via a tapped connection close to the boiler water surface. A continuous blow allows for the regulation of the salinity of the boiler with minimal loss of water and heat from the boiler.
B. remove scum from the surface
Incorrect answer. A surface blow is used to remove scum and light solids from the surface of the boiler water via a tapped connection at the boiler water surface.
C. permit air to escape while
Incorrect answer. Venting of the boiler through the “aircock” permits the escape of air from a cold boiler when raising steam. The “aircock” is a high-pressure globe valve installed at the highest point of the steam drum.
D. remove sludge from the bottom
Incorrect answer. A bottom blow is used to remove heavy solids and sludge via a tapped connection at the bottom of the water (mud) drum.

Note: Boiler blowdown is the removal of water from a boiler to control boiler water parameters within prescribed limits to minimize scale, corrosion, and carryover.

4. What type of engine lubrication oil filter system sends filtered oil directly back to the high-pressure discharge
manifold?

A. centrifugal purifier system
Incorrect answer. The centrifugal purifier filtering system is a “sump”-type filtering system. The purifier is supplied lubricating oil from the engine sump, purifies same, and then discharges the clean oil back to the engine sump.
B. bypass system
Incorrect answer. In a “bypass”-type filtering system, a portion of the oil discharged by the lube oil supply pump is continuously passed through filter(s) and then discharged back into the sump. To ensure that sufficient oil is supplied to the engine bearings, the amount of oil passed through the filter(s) is limited through the use of a flow-restricting orifice.
C. shunt system
Correct answer. In a shunt-type filtering system, oil taken from the engine sump by the lube oil supply pump is discharged first into a strainer, then through a filter and cooler, and finally to the high-pressure discharge (supply) manifold. To ensure that an adequate flow of oil will be delivered to the engine at all times, the filter and strainer are fitted with pressure relief valves.
D. batch system
Incorrect answer. The “batch” system of filtering lubricating oil is a reclamation process performed periodically. When the engine oil has become too contaminated, it is drained and the system refilled with fresh oil. After the drained oil has been permitted to settle, any water or contaminants are removed through filtering and/or centrifuging. After the reclamation process is complete, the oil is stored for reuse.

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Full article is available at http://www.uscg.mil/proceedings/articles/90_EngineeringAnswers.pdf.

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Nautical Engineering Queries Questions Part—1.

Excerpt from U.S. Coast Guard Proceedings of the Marine Safety & Security Council magazine. Nautical Engineering Queries Questions Part—1.

1. Amortisseur windings are installed in a synchronous motor to __________.

A. reduce eddy current losses
B. produce a higher power factor
C. provide a means for starting
D. eliminate arcing between the stator and the rotor

2. Coast Guard regulations (46 CFR) define several acceptable means of closure for ballast and fuel oil tank vents.
One of the acceptable means is by the use of a/an __________.

A. manually operated ball check valve
B. automatically operated hinged closure
C. permanently installed canvas hood
D. corrosion-resistant wire screen

3. A continuous blow is used to __________.

A. regulate the density or salinity of boiler water
B. remove scum from the surface of boiler water
C. permit air to escape while raising steam in a cold boiler
D. remove sludge from the bottom of the water drum

4. What type of engine lubrication oil filter system sends filtered oil directly back to the high-pressure discharge
manifold?

A. centrifugal purifier system
B. bypass system
C. shunt system
D. batch system

Stay tuned to see the answers to these Nautical Engineering Queries.

Subscribe online at http://www.uscg.mil/proceedings/subscribe.asp