Cooling Water System Jacket Cooling Water System and Piston Cooling Water System

Cooling Water System

Jacket Cooling Water System and Piston Cooling Water System

Cooling of Engine is required to enable the engine metal to retain their mechanical properties.

Fresh water is used as the coolant in the jacket cooling water system.

Fresh water is circulated around through internal passages within the engine

Fresh water is then cooled by sea water in a cooler.

Sea water is not used directly as a coolant because of its corrosive action and tendency of scale formation in the narrow cooling passages.

Fresh water is used as the coolant for piston cooling water system

Lubricating oil may also be used as the coolant for piston cooling system

Fuel Oil System:

Fuel Oil System for a large diesel engine has two systems

Fuel Oil Supply System and Fuel Oil Injection System

 

Fuel Oil Supply System:

This system involves supply of fuel oil from bunker/storage tanks to the main engine fuel pumps

Fuel Oil from bunker/storage tanks is pumped into the settling tank by using transfer pumps

In the settling tanks fuel oil is heated and huge amount of water is separated and drained

Fuel oil is pumped from the settling tanks to service tank by purifier / centrifuge pump through heaters and purifiers

Purifier removes water and suspended solid impurities (sludge) from the Fuel oil

Fuel oil is pumped from service tank to main engine fuel pumps using booster pumps through flow meters, mixing column/tank, heaters, viscosity regulators and filters

Reasons for doing Chemical Treatment of Cooling Water (CW) System / Jacket Cooling Water (JCW) System

Distilled Water (Demineralised Water / Water produced from FWG) is used as CW / JCW for Diesel Engines

Untreated Distilled Water absorbs Carbon-di-Oxide from the Air and becomes Corrosive

pH less than 7 is acidic and causes corrosion

pH 7 to 9, slightly alkaline, is ideal

Hardness salts like CaCOз causes scale deposit formation

Chloride and Sulphate are corrosive even in the presence of an inhibitor

Hence Chemical Treatment of Cooling Water (Distilled Water) is done for Preventing Corrosion and for preventing scale formation of the CW System and also for Effective Cooling

 

Sea water or fresh water contaminated by sea water is not used as CW. Since SW is highly corrosive and it causes deposit formation in the CW system

Rainwater is not used as CW. Since Rainwater is heavily contaminated and is highly corrosive

Tap Water (Drinking Water / Mineralised Water) is not used as cooling water. Since it causes chalk deposit formation in the CW system

However Tap Water may be used as cooling water after softening and after chemical treatment (based on the ingredients in the Tap Water)

 

Method of Chemical Treatment of CW / JCW system 

Chemical Treatment involves adding Corrosion Inhibitors / Chemicals to the CW System

Prepare the solution of Inhibitor / Chemical according to the instruction from the maker and add the solution into the expansion tank

Some inhibitors/ chemicals may be toxic and hazardous. Safety precautions should be followed while handling inhibitors / chemicals

 

Only Nitrate- Borate based Corrosion Inhibitors are added

Oil based Inhibitors are not used. Since these inhibitors adhere to the Cooling Surface which will reduce the cooling Efficiency

.

 

Testing of CW / JCW

Property of the CW changes during service due to contamination or evaporation

Therefore, CW should be checked periodically during service (once a week)

CW Sample is draw from the system

CW sample is tested using test kits supplied by the Inhibitor / Chemical Maker

In addition to testing the CW every week, it should be checked in a Laboratory once in three months

CW test results should be recorded and kept for trend evaluation.

If test result shows that the property / contents of cooling water changes suddenly or gradually, the cooling water system should be checked to trace the cause
Some of the changes may indicate the cause as follows:

Chloride content increasing:

  • Check possibility of seawater penetrating into cooling water
  • Check the system which includes sea water, for example fresh water cooler cooled by sea water

pH value decreasing or sulphate content increasing:

  • Check if cooling water is contaminated by exhaust gas
  • Check cylinder head by hydraulic pressure test

 

Tests carried out on JCW Permitted Values
pH 7 to 9
Total Hardness (CaCOз) max. 75 ppm(mg/l)
Chloride max. 50 ppm(mg/l)
Sulphate max. 100 ppm(mg/l)
Silicate max. 150 ppm(mg/l)
Residue after evaporation max. 400 ppm(mg/l)

Scavenge Fires:(What is Scavenge Fire,How Scavenge Fire occurs, Effects of Scavenge Fire,Actions to be taken if a Scavenge Fire occurs)

Scavenge Fires:

 

What is Scavenge Fire

The fire that occurs in the scavenge space (under-piston space)

 

How Scavenge Fire occurs

Cylinder Oil can collect in the scavenge space/manifold of an engine

Unburned fuel and carbon may also be blown into scavenge space/manifold due to defective piston rings or faulty timing or defective injector

All the above forms into a flammable mixture

This flammable mixture may get ignited when blow past of hot gasses occurs or when the piston becomes hot

This is called a scavenge fire

 

Effects of Scavenge Fire:

  1. Loss of engine power
  2. Exhaust temperature increases at the affected cylinder
  3. Turbocharger may surge
  4. Sparks will come out of the scavenge drains

 

Actions to be taken if a Scavenge Fire occurs:

  1. Engine should be slowed down
  2. Fuel should be shut off to the affected cylinder
  3. Cylinder lubrication should be increased
  4. Scavenge drains should be closed

A small fire will quickly burn out

If the fire persists, then

  1. Engine must be stopped
  2. Fire extinguishing medium should be injected into the scavenge manifold (through the fittings provided for this purpose)
  3. Scavenge Manifold manhole doors should not be opened
  4. Engine should be kept turning on turning gear (to prevent seizure of engine moving parts)

 

How to avoid/prevent Scavenge Fire:

  1. Engine timing (fuel injection, exhaust opening, cylinder lubrication etc) and equipment maintenance should be correctly carried out
  2. Scavenge manifold should be regularly inspected and cleaned
  3. During the inspection, if carbon or oil is found in the scavenge manifold, its source should be detected and the fault should be rectified

Scavenge drains should be regularly blown. If any oil discharges is found its source should be detected and the fault should be rectified

MARPOL Annex VI (Prevention of air pollution by ship)

Each Engine is given a EIAPP Certificate (NOx Technical Code 2008)

Each Ship is given a IAPP Certificate

EIAPP = Engine International Air Pollution Prevention

IAPP = International Air Pollution Prevention

 

Certificate is Valid  for 5 years with  intermediate surveys

 

Documents that will be checked to verify compliance to NOx emission

 

(1) Technical File = This document is prepared by the engine manufacturer and contains information about components/spare parts with IMO markings & engine parameters

 

(ii) Record Book = This is a document for recording all parameter of the Engine, details of changing of spare parts / components & engine settings that may influence NOx emissions

 

If the parameter / component / spare parts / settings do not match with Technical file or record book then the survey may include physical measurement of NOx emissions

 

Sulphur Oxides (SOx) & Particulate Matter (PM)

SOx Gases causes “Acid Rain”.

SOx gases are formed from sulphur present in Fuel Oil

 

 

Sulphur Emission Control Area

Baltic Sea (SOx), North Sea (SOx), English Channel (SOx), North America ECA (SOx, PM), United State Caribbean Sea ECA (SOx, PM)

 

  • Sulphur level in Fuel <5 % (0.5 % after 2020) (outside SECA)

 

  • Sulphur level in Fuel < 1 % for SECA

 

Alternatively an exhaust gas cleaning system (scrubbers) can be used to reduce sulphur level in the exhaust, when high sulphur fuel is used

 

Regulations – VOC [Volatile Organic Compounds ] emissions from Incinerator Exhaust

  • Incinerator must have a manufacturers operations manual.
  • Personnel should be trained to operate & should follow operations manual

Following substances are prohibited from incineration:

  • Annex I, II and III cargo residues and related packing material.
  • (Poly Chlorinated Biphenyls)
  • Garbage containing heavy metals.
  • Petroleum Products containing halogens.

– PVC (Polyvinyl chlorides) can only be incinerated in type approved incinerators.

– Exhaust gas cleaning system residues

  • Flue gas temperatures shall be monitored and should not be less than 8500C for continuous feed and should reach 6000C within 5 minutes for batch feed.

 

 

MARPOL Annex I (Regulations for the Prevention of Pollution by Oil)

MARPOL  Annex I

Regulations for the Prevention of Pollution by Oil

 

Special Areas: Special area means a sea area where for recognized technical reasons in relation to its oceanographical and ecological condition and to the particular character of its traffic the adoption of special mandatory methods for the prevention of sea pollution by oil is required

  • Mediterranean Sea
  • Baltic Sea
  • Black Sea
  • Red Sea
  • “Gulfs” areas
  • Gulf of Aden
  • Oman area of the Arabian Sea
  • North West European waters
  • Southern South African waters
  • Antarctic Area

Red Font – Special Area requirements for these areas have not yet taken effect because of lack of notifications from MARPOL Parties whose coastlines border the relevant special areas on the existence of adequate reception facilities

 

  1. Control of Discharge of Oil or Oily Mixture from Machinery Space and from Fuel Tanks of all Ships
  2. Out side “Special Areas” discharge may take place if:
  • “en route” (discharge spread over great area)
  • processed through an oil filtering equipment (15 ppm)
  • oil content without dilution < 15 ppm (parts per million)
  1. In a “ Special Area”
  • oil filtering equipment (15 ppm) should have alarm (oil content meter) and automatic stopping device (3-way valve)
  • Oil residues which cannot be discharged into the sea in compliance with this provisions shall be retained onboard for subsequent discharge to reception facilities
  1. For Ships > 4000GT and oil tankers > 150 GT (delivered after 31st Dec 1979) no ballast water shall be carried in any fuel oil tank
  2. In a ship > 400 GT constructed after 1st July 1982, oil shall not be carried in a forepeak tank or a tank forward of the collusion bulk head

 

  1. B. Equipment requirements for machinery spaces of all ships :
  • All ships > 400 GT must be fitted with oil filtering equipment (Oily Water Separator) producing and effluent with oil content < 15 ppm
  • Ships > 10000 GT shall be fitted with oil filtering equipment (15 ppm) with alarm and automatic stopping device
  • Oily Water Separators (OWS) and Oil Content Meters (OCM) (bilge alarms) shall be of the type approved by IMO
  • OWS to be tested with a stable emulsion
  • OCM to include a recording function for date, time, alarm and Operating status. All records to be stored for 18 months

 

  1. Control of Discharge of Oil or Oily Mixtures from Cargo Areas of Oil Tankers:
  1. Outside “Special Area” discharge may take place if:
  • the instantaneous rate of discharge of oil content does not exceed 30 litres per nm
  • proceeding on voyage
  • more than 50 miles from land
  • discharge monitoring and control system is used to discharge residue
  • the total quantity of oil discharged into the sea does not exceed 1/15,000 or 1/30,000 of total quantity of the particular cargo of which residue formed a part
  • the tanker is equipped with Oil Discharge Monitoring and Control System (ODMCS) (ODME – Oil Discharge Monitoring Equipment) and a slop tank arrangement
  1. In “ special Area” only discharge of clean or segregated ballast is allowed (Oil Content in discharge <15 ppm)

 

These Regulations / Provisions do not apply to the discharge of clean or segregated ballast

 

 

 

 

  1. Equipment requirements for Cargo Areas of Oil Tanker :
  1. Oil Tanker >  150 GT shall be equipped with Oil discharge Monitoring    Equipment (ODME) / Oil discharge Monitoring & Control System (ODMCS) approved by Administration,

Equipment should include – a recording device to provide   continuous record of discharge in litres per nm (rate of discharge), total quantity discharged and oil content in the discharge

Recorded details should be identifiable (with time and date)  & available for at least last 3 years

Equipment should stop the discharge Automatically when oily mixture exceeds the permitted instantaneous rate of discharge

Oil Tanker >  150 GT  shall be provided with effective Oil/Water Interface Detectors approved by Administration for determination of the Oil/Water interface in the Slop Tanks and in the other tanks where the separation of oil and water is effected and from which it is discharged directly to the sea

  1. Every crude oil tanker > 20,000 dwt delivered after 1st Jun 1982 shall be fitted with a cargo tank cleaning system using crude oil washing (COW) approved by the administration

Every oil tanker operating with crude oil washing systems should be provided with an operations and equipment manual

 

  1. Oil Record Book requirements

Part I:  For Machinery Space Operations:

For tankers >150 GT

For non tankers > 400 GT

Part II: For Cargo / Ballasting Operations

For Oil Tankers > 150 GT

MARPOL ANNEX IV(Regulation for the Prevention of Pollution by Sewage from Ships)

MARPOL ANNEX IV

Regulation for the Prevention of Pollution by Sewage from Ships

  1. Shipboard Sewage Pollution Sources
  • Drainage and other wastes from any form of toilets and urinals (Black Water)
  • Drainage from medical premises (dispensary, sickbay, etc) via wash basins, wash tubs and scuppers located in such premises
  • Drainage from spaces containing living animals
  • Other waste waters when mixed with the drainages defined above

[Regulations not applicable to the disposal of grey water (drainage from dishwasher, shower, laundry, bath and washbasin drains)]

 

  1. Application
  • new ships of > 400 GT
  • new ships < 400 GT (certified to carry over 15 persons)

(new ships: building contract or keel laid on/after 27th Sep 2003 or delivered on/after 27th Sep 2006)

  • existing ships of > 400 GT
  • existing ships < 400 GT (certified to carry over 15 persons)

 

  1. 3. Equipment (& Tank) Requirements
  2. discharge pipeline fitted with standard discharge connection (applies to all new ships contracted for construction on/after 1st Jan 2007)

and

  1. approved sewage treatment plant

                     or

comminuter / disinfection system with temporary means of storage

                      or

holding tank

An approved sewage treatment plant shall be of a type approved by the Administration, following the IMO standards and test methods

(its test results are noted on the ISPPC and the effluent does not produce visible floating solids nor cause discoloration of the surrounding water)

  • A sewage comminuting and disinfecting system approved by the Administration shall be fitted with temporary storage of sewage when the ship is less than 3 nautical miles from the nearest land
  • A sewage holding tank shall:
  • have appropriate capacity for the retention of all sewage,
  • be correctly constructed with a means to indicate visually the amount of its contents, and

-be equipped with the ship’s discharge line to port/terminal reception facilities fitted with the standard discharge connection

 

  1. Sewage Treatment System Capacity
  • no MARPOL regulations and guidelines for handling of any plant or holding tank
  • the capacity depends on:
  • type of flushing system,
  • the number of people onboard
  • Ship;s type,
  • Trading pattern, etc

 

5.(a) Control of discharge of sewage

Based on the type of equipment on the Ship

The discharge of sewage into the sea is prohibited, except when:

-The ship is discharging comminuted and disinfected sewage using an approved system at a distance of more than 3 nm from the nearest land;        –                                                    or

  • the ship is discharging sewage which is not comminuted and disinfected at a distance of more than 12 nm from the nearest land, provided that in any case, the sewage stored in holding tanks or sewage originating from the spaces containing living animals, shall not be discharged instantaneously but at a moderate rate when ship is en route and proceeding at not less than 4 knots;
  • or
  • the ship is discharging sewage using an approved sewage treatment plant

 

5 (b). Control of discharge of sewage

         Based on discharge distance

  • while operating within 3 nm from the nearest land – discharge is prohibited unless properly treated using an approved sewage treatment plant
  • discharge within 3 – 12 nm from the nearest land must either: meet the effluent requirements within 3 nm from the nearest land or the ship is discharging comminuted and disinfected sewage using an approved comminuter/disinfection system
  • discharge at a distance of more than 12 nm from the nearest land must either: meet effluent requirements within 3 nm or within 3-12 nm from the nearest land or the sewage that has been stored in holding tanks shall not be discharged instantaneously but at a moderate rate when the ship is en route and proceeding at not less than 4 knots;

 

EFFLUENTS:       BOD < 25 mg/l        T.S.S. < 35 mg/l        COLIFORMS   < 100 col/100ml

Addition of nutrients causing algal blooms and reduced oxygen levels – serious health risk to people

 

  1. Special Areas Baltic Sea The new special area requirements, which entered into force on 1 January 2013, will only take effect upon receipt of sufficient notifications on the existence of adequate reception facilities from Parties to MARPOL Annex IV whose coastlines border the relevant special area

 

(i) Chemical Sewage Treatment Plant

 

(ii) Biological Treatment Plant

Lifeboat Winch – Gravity Davits(Electric or Pneumatic drives is used for lifeboat winch)

Lifeboat Winch – Gravity Davits

Electric or Pneumatic drives is used for lifeboat winch

Electric & Pneumatic drives – Safety Features

  1. When lowering no mechanical assistance except gravity (i.e. only the weight of the boat) shall be applied
  2. The only physical work needed being release of winch hand brake (dead man’s handle – a weight is attached to the brake handle) during the lowering operation
  3. centrifugal brake provides controlled speed (36m/minute) to the lowering when hand brake is released
  4. If the operator looses balance and fall off, the hand brake gets engaged due to the weight in the handle (dead man’s handle) & the lifeboat shall remain stationary at the place of stop.
  5. A ratchet mechanism in the hoisting arrangement ensures that the drum will not reverse and the boat fall back into the water to provide safety in the event of power failure while lifting.
  6. Manual Hosting and Power Hoisting cannot be done at the same time (interlock is provided between positioning of manual hoisting lever & positioning of power hoisting lever)

Both the centrifugal brake and the hand brake remains stationary during hoisting operation. If the power fails while hoisting the boat, hand brake will hold the boat

  1. Davits should have a positive turning out moment during the whole of the davit travel with the vessel listed at any angle up to and including 250
  2. Davits should have enough strength so that lifeboat can be safely lowered to the water from the embarkation deck with its full complement of persons on board when the ship has a trim of up to 100 and is listed up to 150 either way
  3. Davits should be fitted with “tricing pendants” to bring the  boat alongside the embarkation deck, unless boat is designed to be boarded and launched directly from the stowed position (free fall fully enclosed lifeboats)
  4. Falls should be released only when the lifeboat is waterborne
  5. Limit switches  should be provided to stop the hoisting motor when boat has reached the stacking position

Windlass(Construction,Working,Classification Society Rules for Windlass and Chain Stopper)

Used for handling the Anchor – Pay Out (Drop) & Heave In (Pull Up)

Also used for handling the lines and for warping the ship alongside in a – dock (or) canal locks (or) harbour

 

Construction

It consists of (i) Primary Shaft (ii) Intermediate Shaft & (iii) 2 Main Half Shafts

These Shafts are fitted with corresponding Pinions and Gear wheels

Primary Shaft is driven by Worm & Worm Wheel.

Worm Shaft is coupled to a Hydraulic / Electric Motor

Cable -Lifters are mounted freely (not keyed) on the Main Half Shafts. Hence the cable-lifter can rotate independent of the shaft

Cable –Lifters are provided with Sprocket (projection) on its circumference. Sprockets are engaged with the links of the Anchor Chain / Cable

Brake Band (screw operated) is fitted around the Brake Drum. Break Drum is on the outer edge of the rim of the Cable-Lifter.

Brake Band is used:

(i) to control the speed of the Anchor Chain when paying out (i.e to control the speed of Cable-Lifter) and (ii) for locking the cable in a stationary position

Main Gear Wheels is keyed to the Main Half Shafts

Main Gear Wheel can be moved / slided, to and fro, laterally for Clutching (engaging) and de-Clutching (disengaging) with the Cable-Lifter

Each End of the Intermediate Shaft is connected to a Warping Drum through a Dog Clutch

 

Working

To Pay Out (Drop) the Anchor:

Engage the Main Gear Wheel with the Cable-Lifter

Use Hydraulic / Electric Motor to Slightly Heave In (Pull Up) the Anchor Chain

Remove the Chain Stopper Pawl (Guillotine Bar / Bow Stopper Pawl)

Apply/Tighten the Brake on the Cable-Lifter

Disengage the Main Gear Wheel

Release the Brake.

Control the speed of the cable paying out by using the brake.

(i.e. control the speed of Cable-Lifter by using the brake)

Apply /Tighten the brake on the Cable-Lifter when the Anchor has dropped to the desired depth.

Place the Chain Stopper Pawl in the Chain-Holding Position

Chain Stopper takes the load of the Anchor & Anchor Chain. It avoids load/ stress coming on the Windlass

 

To Heave In (Pull Up) the Anchor:

Engage the Main Gear Wheel

Start the Electric/Hydraulic Motor

Heave In (Pull Up) the Anchor /Anchor Chain by controlling the Motor

(i.e by rotating the Cable-Lifter using the Motor)

Apply / Tighten the brake on the Cable-Lifter when the Anchor is raised

Place the Chain Stopper Pawl in the Chain-Holding position

Basic Marine Engineering Book Page No. 223   Figure  13.1

Basic Marine Engineering Book Page No. 224   Figure  13.2

 

Classification Society Rules for Windlass and Chain Stopper

Windlass should be suitable for the size of Anchor Chain Cable used

Windlass should be of sufficient power

Each Anchor should be provided with One Cable-Lifter

Cable-Lifter should be connected to the Main Gear Wheel (Driving Shaft/ Main Half Shaft) by a Release Coupling (Clutch)

Cable-Lifter is to be provided with a Brake

Each Chain cable should be provided with a Chain Stopper Pawl (between the windlass and the hawse pipe)

Chain Cables from (Chain locker) Spurling Pipe should reach the Hawse Pipe through a Cable-Lifter only

Windlass should have a torque limiting device (slipping clutch)

Windlass should be able to exert a continuous pull for 30 min

(Pull – Force (Newtons) is as per regulations laid down by the Administration)

Windlass should also be able to exert 1.5 times the continuous pull for not less than 2 min

Mean Hosting Speed should not be less than 9 m / min

Windlass brake should be sufficient for safe stopping of the anchor chain cable when paying out

With Brakes Engaged and Release Coupling Disengaged – Windlass should be able to withstand a pull of 45 % of the breaking strength of the chain cable without any permanent deformation of the stressed parts and without break slip

Chain Stopper and their attachments should be able to withstand a static pull of 80 % of the breaking strength of the chain cable without any permanent deformation of the stressed parts

 

 

Electrical or Electro Hydraulic Drives is used in Windlass

 

Slipping Clutch / Torque Limiting Device

When Anchor is Heaved In (Pulled Up), the Anchor Cable gets housed in the Hawse Pipe

Some time the anchor cable comes to a sudden stop

Example – Anchor reaching the End of the Hawse Pipe

But Electric/Hydraulic motor continues to rotate due to rotational inertia

i.e one end of the machinery has come to a sudden stop  while the other end of the machinery is still rotating

This causes excessive stresses/shock load on the motor

To avoid this -Slipping clutch is fitted between the Motor and the Gearing

When the windlass gearing side shaft comes to a sudden stop – the motor side of the shaft will slip and continue to rotate

Controllable Pitch Propeller (cpp) & fixed pitch propeller. (definition and difference)

Sl. No Solid / Fixed Pitch Propeller Controllable Pitch Propeller
1 Speed of Ship is controlled by varying the speed of Engine Speed of the ship is controlled by varying the pitch of the propeller
2 Engine with varying speed is required Engine with constant speed is enough
3 Hence Shaft driven auxiliary cannot be fitted Hence Shaft driven auxiliary can be fitted
4 Astern movement of the ship is by reversing the engine {engine should be of reversing type} Astern movement of the ship is by varying  the pitch of the propeller {unidirectional engine is enough]
5 Full power of the engine is not available during astern running Full power of the engine will be available during astern running also
6 Hence Stopping time and distance is more Hence Stopping time and distance is less
7 Manoeuvring in confined water is difficult Manoeuvring in confined water is easy
8 Propeller efficiency is more Propeller efficiency is less since large diameter boss/hub is required
9 Cost is low  Cost is high
10 Cost of repair and maintenance is less Cost of repair and maintenance is more

 

Difference between reaction and impulse turbine (MEACS 2)

Difference

Sl. No. Impulse Turbine Reaction Turbine
1 Steam completely expands in the nozzle itself. Hence its pressure remains constant as it pass over the moving blades Fixed blades (guide blades) act as nozzles. Hence steam expands both in fixed and moving blades continuously as it passes over them. Thus the pressure drop occurs gradually and continuously as it pass over both the fixed and moving blades
2 Blade (steam passage) is of constant cross section area, as there is no expansion of steam. Blade (steam passage) is of variable cross-sectional area (converging type), as there is expansion of steam
3 As pressure remains constant in moving blades, the relative velocity of steam passing over the moving blades remains constant As Pressure drop occurs continuously in the moving blades (steam expands continuously as it passes over the moving blades), the relative velocity of the steam passing over the moving blades continuously increases
4 Shape of the Blades are symmetrical, hence manufacturing of blade is simple. Shape of the Blades are non-symmetrical (aerofoil), hence manufacturing of blade is difficult
5 Because of large pressure drop in the nozzle, the steam velocity is high & turbine rpm /speed is also high. Due to small pressure drop in the fixed blades (guide blades), the steam velocity is low & turbine rpm/speed is  also low.
6 Because of large pressure drop in the nozzles, the number of stages is less, for the same pressure drop. Hence the size of the turbine, for the same power output is small. Because of small pressure drop in each stage, the number of stages is more, for the same pressure drop. Hence the size of the turbine, for the same power output is large. These turbines are multi-stage turbines only.
7 Occupies less space per unit power Occupies more space per unit power
8 Suitable for small powers Suitable for medium and higher powers

Difference between Four Stroke Engine &  two-stroke engine

Difference

  Four Stroke Engine Two Stroke Engine
1. It has one power stroke for every two revolutions of the crankshaft. It has one power stroke for each revolution of the crankshaft.
2. Heavy flywheel is required and engine runs unbalanced because turning moment on the crankshaft is not even due to one power stroke for every two revolutions of the crankshaft. Lighter flywheel is required and engine runs balanced because turning moment is more even due to one power stroke for each revolution of the crankshaft.
3. Engine is heavy. Power to weight ratio is low Engine is light. Power to weight ratio is high
4. Engine design is complicated due to valve mechanism. Engine design is simple due to absence of valve mechanism.
6. Less mechanical efficiency due to more friction on many parts. More wear and tear. More maintenance is required More mechanical efficiency due to less friction on a few parts. Less wear and tear. Less maintenance is required
7. More output due to full fresh charge intake and full burnt gases exhaust.

mep is high

Less output due to mixing of fresh charge with the hot burnt gases.

mep is low

8. Power developed is less Power developed is more (almost double)
9. Gear box [to reduce speed & reversing] is connected between engine and propeller shaft Engine directly connected to propeller shaft. Engine is reversible
10. Engine consists of inlet and exhaust valve. Engine consists of inlet port and exhaust port/valve.
11. More thermal efficiency. Less thermal efficiency, as effective compression ratio is less
12. Separate cylinder L.O is not required Separate cylinder L.O is required
13. High speed & Short Stroke – Costly fuel of better ignition quality should be used Slow speed & Long Stroke – effective scavenging and cheap fuel of low ignition quality can be burnt