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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