MOTOR & MEP ORAL QUESTIONS & ANSWERS PART-1

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Q Boiler survey requirement as per dg shipping

Ans – Boiler Survey schedule: “2 surveys in a period of 5 years with not more than 3 years between two consecutive surveys”,

Q Cross question, when can you do the first survey

Ans – Within 30 months from the date of issue of certificate.

Q  Cross question what if I do my survey in then 10th month

Ans – Within 36 months from the 10th month, i.e. 46th month (no window period) n one more survey will be required before the end of the 60th month

Q Cross question, so how many months from then

Ans- within 14 months from the 46th month

Q What is that survey called

Ans– Renewal survey

If first survey in 18th month- next survey will be after 36 month that is in 54th month.

As per dg shipping circular. No conjunction of 2nd survey with the renewal survey.

But as per DNV circular renewal survey can be conjunction with 2nd survey. Within 15 months from the month of expiry of the certificate.

DG shipping circular

As per Merchant Shipping (Cargo Ship Safety Construction and Survey) Rules

  • For all vessels more than 8 years old, an internal & external examination of the boiler is required to be carried out at intervals of 1 year. 
  • Auxiliary water-tube boilers for which the Principal Officer is satisfied that it is being given correct feed water treatment with proper boiler water analysis shall be examined the boiler internally and externally at intervals not more 2.5 years (30 months).

Classification Rules and Draft M.S. (Cargo Ship Construction and Survey) Rules and in order to ensure uniformity with the rules of the Classification Societies, henceforth following periodicity of boiler survey is applicable:

1. Auxiliary water-tube boilers shall be examined internally and externally at intervals not exceeding 2.5 years (30months) subject to conditions specified in Note.

2. All other boilers including exhaust-gas boilers, super-heaters, economizers and domestic boilers (other than domestic boilers having heating surface of not more than 5m² and a working pressure of not more than 3.5 bar gauge) shall be examined internally and externally at intervals not exceeding 2.5 years (30months).

Survey interval in years

1. Main boilers 2.5 year– 30 months

2. Auxiliary boilers 2.5 year– 30 months

3 Exhaust gas steam generators and economisers 2.5 year– 30 months

Note : At least 2 surveys are to be carried out within any 5 years but interval between two consecutive surveys is not to exceed 3 yrs (36 months) .

The Recognized Organization during the Annual Survey shall review the following records since the last Boiler Survey:

  • Operation
  • Maintenance
  • Repair history
  • Feed water chemistry and submit report of the above to the concerned Principal Officer and in case of any adverse report same shall be brought immediately to the notice of the Directorate and Principal Officer.

Lube oil properties.

  • Base number
  • Insoluble content
  • Flash point
  • Total acid number
  • Viscosity
  • Density

Fuel oil properties.

a)  Viscosity-

  • Viscosity is a measure of fuel’s resistance to flow. Expressed in Redwood or in Cst.
  • Higher the viscosity higher will be the specific gravity.
  • The viscosity of any oil is indirectly proportional to temperature of oil.
  • Viscosity is important for handling, treatment and atomization of the fuel.
  • It also is a rough indicator of its carbon & asphalt content.
  • High viscosity fuels need proper preheating for good separator operation & heating before injection for good atomization, this characteristic usually can be handled without any problems.
  • By preheating the fuel, separation in the centrifuge is improved, but a temperature of 98°C should not be exceeded because flashing of the water in the separator may occur with resultant loss of the centrifuge water seal.
  • Caution must be exercised when heating prior to injection to temperatures above (>)135°C because cracking may occur, gases may be given off, & water may vaporize forming steam pockets in the fuel line. Insufficiently heated fuel, on the other hand, can result in the poor atomization & delayed burning, which may lead to higher thermal loading, scuffing problems, possible piston & piston ring failure, & to an increase in fuel consumption.
  • In addition to heating prior to injection, an increase in fuel injection pressure may also be required to maintain design atomization spray patterns depending on the fuel used.

b) Specific Gravity

  • Specific gravity is defined as the ratio of the weight of a given volume of the product at 15°C to the weight of an equal volume of water at the same temperature.
  • It is important for the oil purification. Therefore, as the specific gravity of fuel approaches 1.0, centrifuging becomes less effective.
  • High specific gravity indicates a heavily cracked, aromatic fuel oil with poor combustion qualities, which can cause the abnormal liner wear.
  • Rate of change in specific density with respect to water is 0.66 kg/m³ per degree Celsius.
  • Heating the fuel prior to centrifuging assist in the separation process because the density of fuel oil changes more rapidly with temperature. A viscosity decrease also helps centrifuging.
  • A maximum specific gravity of 0.991 (at 15°C) can be handled satisfactorily. Above this value the centrifugal purifier cannot repeatedly & successfully operate, due to loss of its water seal. Specific gravity for the future fuels is expected to rise to about 0.995. The major significance of this increase in specific gravity will be greater difficulty relative to water removal in settling tanks & centrifuges.

c) Carbon Residue/Asphaltenes

  • Conradson Carbon Residue (CCR) is a measure of the tendency of a fuel to form carbon deposits during combustion and indicates the relative coke forming tendencies of heavy oil.
  • Carbon-rich fuels are more difficult to burn & have combustion characteristics which lead to the formation of the soot & carbon deposits.
  • Since carbon deposits are a major source of the abrasive wear, the CCR value is an important parameter for a diesel engine. 
  • Carbon residue is the percent of coked material remaining after a sample of fuel oil has been exposed to the high temperatures.
  • ASPHALTENES are those components of asphalt that are insoluble in petroleum naphtha & hot heptanes but are soluble in carbon disulfide & hot benzene.
  • They can be hard & brittle and made up of large macromolecules of hydrocarbon derivatives containing carbon, hydrogen, sulfur, nitrogen, oxygen and, usually, the three heavy metals − nickel, iron and vanadium.
  • A high CCR/Asphaltenes level denotes a high residue level after combustion and may lead to ignition delay as well as after-burning of carbon deposits leading to engine fouling and abrasive wear and finally thermal loading of engine.
  • After burning will lead to burning of the lube oil film which leads to scuffing, cylinder wear and engine deposits.
  • Fuels with high CCR values have an increasing tendency to form carbon deposits on injection nozzles, pistons, and in the ports of two-stroke engines.
  • This causes reduction in the efficiency and performance of those components and increased wear.
  • The maximum permissible CCR value depends on engine speed. The higher the speed, the shorter the time for combustion and the more residues deposit. Hence, acceptable CCR values should decrease as engine speed increases.
  • 2-S are less affected by a high CCR than 4-S, it does contribute to increased fouling of gas ways and turbochargers, especially during low power operation or at idle. Idling should be limited to 5 to
  • 10 hours & be followed by running at full load to clear gas ways whenever possible.
  • Continued operation at reduced output can also load up gas ways with the unburned heavy fuel oils & lube oil. Here also, full load operation can help to clear gas ways.
  • The combination of higher Conradson Carbon content & higher Asphaltenes content can increase the centrifuge sludge and fine filter burdens. This can require more frequent centrifuge desludging & filter element cleaning/replacement.
  • Higher Conradson Carbon content also lowers the gross and net heating values (on weight basis) of a heavy fuel oil.
  • Asphaltenes content is particularly important in 4-cycle engines due to their higher operating speeds, smaller bore sizes, and reduced combustion time.

D) Sulphur

  • When the crude is distilled, sulphur derivatives tend to concentrate in the heavier fractions, leaving the lighter fractions with relatively low sulfur contents.
  • This characteristic of fuel oil is responsible for “low temperature” corrosion which attacks cylinder liners and piston rings, leading to an increase in cylinder liner wear. In cold corrosion the oxides in sulphur combine with condensing water vapor in the combustion chamber to form highly corrosive sulphuric acid. Some of this water is present in the fuel already, while another source of moisture may be in the intake and scavenging air.
  • Careful control of the cooling water temperature in the inlet air coolers and/or installation of a water mist separator after the air coolers should remedy the problem of condensing moisture in the intake air.
  • When operating an engine on a fuel with high sulphur content, care must be taken to avoid reaching the acid dew point temperature within the cylinder.
  • One way of controlling this is to adjust the cooling water temperature at the cylinder wall.
  • Sulphur is oil soluble, it cannot be removed from the fuel by centrifuging. It can be neutralized by the use of proper alkaline additives in the cylinder and/or engine lubricating oils.
  • It should be noted that although the sulfur content of a fuel can be neutralized by the use of cylinder lubricating oils of proper alkalinity (TBN − Total Base Number), over-treatment for sulfur (low sulfur fuel oil) can be just as harmful as under-treatment. Over-treatment for sulfur leaves an excess of the alkaline additive material free to form hard, abrasive deposits during combustion, with the resultant increased abrasive wear of the cylinder liners & piston rings. Therefore, when burning low sulfur fuel oil, the lube oil TBN should be lowered.
  • In trunk-type engines the cylinder lube oil is scraped into the crankcase by oil control rings on the pistons. This oil has been contaminated by the sulphur in the fuel. Upon entering the crankcase, the sulfur is free to combine with moisture which may collect there. The TBN of the lubricating oil is eventually lowered to a point where it is rendered ineffective in controlling the sulfur content of the fuel.
  • For 2-S engines, an increase in the sulfur content leads to slower and less complete combustion with resultant formation of more corrosive acids, more unburned carbon, and an increase in wear rates.

d) Ash/Sediment

  • The ash contained in heavy fuel oil includes the (inorganic) metallic content, other non-combustibles and solid contamination.
  • The ash content after combustion of a fuel oil takes into account solid foreign material (sand, rust, catalyst particles) and dispersed and dissolved inorganic materials, such as vanadium, nickel, iron, sodium, potassium or calcium.
  • Ash deposits can cause localized overheating of metal surfaces to which they adhere and lead to the corrosion of the exhaust valves.
  • Excessive ash may also result in abrasive wear of cylinder liners, piston rings, valve seats and injection pumps, and deposits which can clog fuel nozzles and injectors.
  • In HFO Ash can be removed by centrifuging.
  • They can form hard deposits on piston crowns, cylinder heads around exhaust valves, valve faces and valve seats and in turbocharger gas sides.
  • Effects of ash can decrease by reduction of valve seat temperatures by better cooling.

e) Vanadium

  • Vanadium is a metallic element that chemically combines with the sodium to produce very aggressive low melting point compounds responsible for the accelerated deposit formation & high temperature corrosion of engine components.
  • Vanadium is responsible for forming slag on exhaust valves & seats on 4-S engines, and piston crowns on both 2-S &4-S engines, causing localized hot spots leading eventually to burning away of exhaust valves, seats & piston crowns.
  • When combined with sodium, this occurs at lower temperatures & reduces exhaust valve life.
  • Vanadium is oil soluble. It can be neutralized during combustion by the use of the chemical inhibitors (such as magnesium or silicon).
  • Cooling exhaust valves and/or exhaust valve seats will extend valve & seat life.
  • Raising fuel/air ratios also prolongs component life. Other measures which can be used to extend component life are the use of heat resistant material, rotating exhaust valves, & the provisions of sufficient cooling for the high temperature parts.

f) Cetane number

  • Ignition quality is indicated by the cetane number.
  • Lower the Cetane Number of a fuel, poorer the fuel quality & greater the ignition delay.
  • Higher the Cetane Number more will be the aromaticity which can increase the ignition delay, & can result in hard knocking or noisy engine running, which is undesirable over long periods of time. The result could be poor fuel economy, loss of the power &, possibly, even engine damage.
  • Diesel engines operating at speeds of less than 400 rpm are much less sensitive to the fuel ignition quality.
  • Physical factors which influence ignition & burning time are the speed with which fuel droplets are atomized, vaporized & thermally cracked to form a combustible mixture.
  • Ignition & burning time can be improved by decreasing fuel droplet size and/or increasing swirl. Experience also has indicated that raising inlet air temperature can reduce the cetane sensitivity of higher speed diesel engines.
  • Cetane number is normally quoted for the distillate fuels only. A number of methods exist for approximating the cetane number for residual fuels.

Flash Point- The flash point of a fuel is the temperature at which fuel vapors can be ignited when exposed to the flame. All petroleum products will burn. But, in order for this to occur, the ratio of the fuel vapor to air must be within certain limits.

Pour point- For pumping & handling purposes, it is often necessary to know the minimum temperature at which a particular fuel oil loses its fluid characteristics.