MOTOR & MEP ORAL QUESTIONS & ANSWERS PART-7

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Q What Is Glazing?

Ans– Glazing describes oil burned and baked onto a surface. Glaze is insulative, very hard, polishes to a high shine, wears piston rings, and can be removed only with an abrasive material or a cylinder hone. A glazing is the result of surplus cylinder lube oil being left behind by the piston rings because they have not been “pushed hard enough” against the cylinder bore surface to remove most of the oil. When excess oil is left behind it is burned onto the bore surface and forms a very hard glazed smooth finish.  Light or under load running will always result in a glazed bore. The time taken to glaze a bore will vary according to the lubricating oil used, the load applied, the length of time run under light load, and local air temperature. Any engine with a glazed cylinder bore will always use lots of oil and will cause complete engine failure if not quickly and correctly repaired.

Engines (using correct grade and quantity of frequently changed cylinder oil) that run at variable speeds and changing loads very rarely get a glazed.

Q What is Scuffing?

Ans-Piston ring scuffing is consists of local micro welding, or material adhesion, between a piston ring and a cylinder liner. Various causes of Scuffing are following

1. Lack of Lubrication- If the engine doesn’t have enough oil, the cylinders will not be properly lubricated. As the ring rubs against the liner of the cylinder, dry spots will cause it to scuff. Symptoms of scuffed piston rings include dirty exhaust and excessive oil consumption.

2. Engine Dry Starts- If an engine has sat idle for even a short time, it may have a “dry start” when next used. This occurs because the existing lubricant sheen in the engine has dried up, and these dry parts of the engine run without lubricant in the first few seconds after the engine starts before lubricant is fully circulated. If an engine is dry-started after a new piston ring is installed, scuffing could result. To avoid this, lubricate the ring before installing it or add a secondary engine oiler that squirts oil into the engine before start-up.

3. Liner Surface Glaze- Engine cylinders that are overheated by insufficient lubrication can break down the little bit of oil that is present, which then accumulates as a glaze on the interior of the cylinder. This glaze can scuff the edges of a piston ring as it scrapes by, as the glaze won’t allow oil to flow properly in the cylinder. Getting rid of this may require a professional reconditioning of the cylinder.

4. Liner Roundness Faults- If the liner of a cylinder is uneven, some points will rub against the piston ring and cause it to scuff. Here, too, a reconditioning of the cylinder may be necessary.

Q what certificate does boiler survey come under?

Ans– Boiler survey comes under cargo ship safety construction certificate.

Q What are the safeties of Steering Gear?

  • Hydraulic tank oil low level
  • Hydraulic oil temp too high
  • Self staring after power failure
  • One motor fed from emergency switch board
  • Overload alarm
  • Single phasing alarm
  • Short circuit
  • No volt alarm
  • Auto change over alarm
  • Power failure alarm
  • Relief valve
  • Bypass valve
  • 35 degree: Telemotor limit switch
  • 35.2 degree: Telemotor stopper
  • 36 degree: Rudder angle limit switch
  • 36.5 degree: Rudder stop
  • 37 degree: Mechanical stop

Q What are the safeties of electric winch/windlass?

  • Overload & Short circuit protection
  • Remote stop
  • Electromagnetic brake
  • Hand brake
  • Restart Timer delay
  • Slipping clutch- commonly mounted in electric windlass. Mounted between motor and gear box or directly in gear box. This avoids inertia of driving motor being transmitted to the gear system in the event of shock loading. Such shock loading occur when anchor pulled hard into the hawse pipe.
  • Cable stopper
  • Torque limiting device

Q What are the safeties of hydraulic winch?

  • For Hydraulic motor- overload and short circuit protection
  • Hydraulic oil low level alarm
  • Hydraulic oil high temperature alarm
  • Electromagnetic brakes- release by electromagnetism. If no power then release by an emergency screw given for releasing it. -Fail safe arrangement

Q What are main air compressor safeties?

Relief valve- Fitted after every stage to release excess pressure developed inside it. The setting of the lifting pressure increases after the every ascending stage.

Bursting disc-A bursting disc is a copper disc provided at the air side of the compressor. It is a safety disc which bursts when the pressure exceeds over the pre-determined value.

Fusible plug-Usually located on the discharge side of the compressor, it fuses if the air temperature is higher than the operational temperature. The fusible plug is made up of the material which melts at higher temperature.

Lube Oil low pressure alarm & trip-If the lube oil pressure goes lower than the normal, then alarm is sounded followed by a cut out trip signal to avoid damage to the bearings & crank shaft.

Water high temperature trip- If the intercoolers are choked or the flow of the water is less, then the air compressor will get over heated. To avoid this situation high water temperature trip is activated which cut offs the compressor.

Water no-flow trip- If the attached pump is not working or the flow of the water inside the intercooler is not enough to cool the compressor then moving part inside the compressor will get seized due to overheating. A no flow trip is provided which continuously monitor the flow of the water & trips the compressor when there is none.

Motor Overload trip- If the current taken by the motor during running or starting is very high then there is a possibility of damage to the motor. An overload trip is thus installed to avoid such situations.

Q What are the engine room crane safeties?

  • Limit switch for all 6 directions-fwd, aft, port, starboard, lowering and hoisting.
  • Mechanical stoppers in case limit switch fails.
  • Overload trip.
  • Electromagnetic brake. Release by availability of electric power.- Fail safe arrangement
  • Rail guard over the pulley
  • Mechanical locking arrangement for rough weather
  • Locking arrangement and hook for safe carriage of load.
  • Clear marking of Safe working load on crane.
  • Breakers at various places. One in engine room and another in ECR.
  • It shall be operate in 50 list and 20 trim.

Q What are the checks carried out after Over speed trip?

  • Check over speed device.
  • Check governor
  • Check fuel pump rack
  • Open crankcase and check the condition inside
  • Check bottom end bolts. It is better to change them once over speed trip has taken place.
  • Over speed limits for AE-15% and for ME-20% .

Q Fresh water generator not producing enough water,what is your action

  • Check the vacuum of the fwg,
  • Leakage possibilities from the pr gauge, vent cock, distillate pump seal, hole in the ejector, or  chockage in the ejector, from the flanges, condenser pipes leaking
  • If the temp of the shell is too high, could be the reason of chocked condenser tubes, or short cycling of the jacket water from the end cover

Q What checks will you carry out on an a main sea water pump

Ans- Check for cavitation, bearing condition, condition of the casing, wear ring clearance, condition of the seal, pitting on the shaft

Q. What is compression ratio?

Ans– The compression ratio can be defined as the value obtained by dividing the total cylinder volume by the clearance volume.

Compression ratio is generally between 12 and 18; however, it depends on the design of the engine. Compression ratio outside this ratio would either prevent the engine from starting or lead to other problems.

Marine engines with smaller cylinders will have higher compression ratio.

Higher compression ratio will lead to high thermal efficiency thus more engine power. But limitations lies with higher nox generation and whether engine will able to start.

Q. One Screw Pump

Ans- One screw pumps are referred to as Progressive Cavity Pumps, sometimes called PC Pumps or Worm Pumps. These are generally not considered in the Screw Pump family because the rotor used in these pumps is not shaped like a typical screw, but rather a twisted round shaft. A Progressive Cavity Pump has one shaft that is slightly twisted in the shape of a screw, and is housed in a pumping chamber that is usually rubber lined.

Q. Show the reason for the staying of any flat surfaces in a pressure vessel. How can the use of stays be avoided?

Ans- When a force is applied to a curved plate as shown in Fig. 1, internal forces are set up which enable the plate to withstand the force without undue distortion.


Fig. 1.- Stress in a curved plate

The bursting stress can be resolved into perpendicular components, one of which will oppose the force. The surface will bend until this component balances the pressure. It will then be found that the surface is in the form of an are of a circle. When the pressure acts upon a flat plate, it will tend to bend the plate until equilibrium is obtained. Thus to prevent undue distortion the flat plate must be very thick or supported by some form of stay. It follows that if the use of flat surfaces can be avoided in the design of a pressure vessel there will be no need to fit internal stays. Thus pressure vessels are often given hemispherical ends but, if this is not possible, any flat surfaces must be stayed or of sufficient thickness to resist the pressure without undue distortion.

Q. Why generator Connecting rod have I OR H SECTION?

Ans- The connecting rod in a medium speed 4 stroke engine is subject to an inertia whip loading due to the mass of the con rod swinging about the piston pin. (Because of the lower speed of a 2 stroke engine, the whip loading is not large enough to influence the design of the con rod) Added to this, the inertia loads due to the mass of the reciprocating parts cause a stress reversal from high compressive stress (during power and compression stroke) to a low tensile stress between the exhaust and inlet strokes.

This loading of the rod influences its design, and to withstand the loading described above,  connecting rods are often forged from a manganese molybdenum steel in an I or H section which reduces its mass from one made of round section steel (and thus reduces the whip loading) while maintaining strength.

Q. Naval brass vs Admiralty brass

Naval brass is the classic marine, high-strength and corrosion-resistant alloy containing 60 percent copper, 0.75 percent tin and 39.2 percent zinc. It is widely used in marine construction where strong, corrosive-resistant and hard material is required and is suitable for both salt and fresh water applications. Naval brass is used in propeller shafts, marine hardware, decorative fittings, shafting, turn buckles. There are also many industrial applications, such as welding rods, condenser plates, structural uses, valve stems, balls, heat exchanger tubes, aircraft turnbuckle barrels, dies.

Admiralty brass is a version of Brass that contains 70% copper, 29% zinc and 1% tin. Admiralty Brass has good corrosion resistance and is specially used for fresh, salt or brackish water application. They are usually used as condenser tubes, Evaporator and Heat Exchanger tubes, Distiller tubing in oil refineries, heater equipment as well as other industrial processes.

Q. Boiler turndown ratio?

Ans- Boiler turndown is a ratio of capacity at full fire to its lowest firing point before shut-down. Old boilers may have only two firing positions, low and high. Newer boilers fire over a wider range of capacities. Depending on the controls, there may be fixed setting points or fully variable settings. If a 1 million BTU boiler can fire as low as 100,000 BTUs, then it has a 10:1 turndown ratio.

Q. What is Critical Speed Range (or Barred range)

Ans- Critical Speed Range (or Barred range)

  • is the range of speed at which resonance may occur.
  • Resonance is a phenomenon experienced when the engine’s operating frequency coincides with the natural frequency of the hull which results in vibrations of higher amplitudes than normal which is very dangerous.
  • The critical speed range should be passed as soon as possible.
  • In order to avoid operation of the main engine in critical (or barred) speed range, a quick-pass function is provided.

Q. How Generator bottom end bearing clearance taken?

Ans- Generator bottom end bearing clearance can be taken using a lead wire as it is not possible to put feeler gauge due to crank throw.