MARINE PROPULSION STEAM TURBINE

Md Redzuan Zoolfakar

 Thermodynamic Definition of Enthalpy (H): h = U + PV U = internal energy of the system P = pressure of the system V = volume of the system

 Consider a process carried out at constant pressure.  Under the principle of conservation of energy  Energy in = Energy out  Q = U + W, where work = PV ◦ Then:  Q = U + PV,  Q = h ◦ Hence,  The change in enthalpy is equal to the heat transferred at constant pressure.



 A boiler deliver 5400 kg of steam per hour at a pressure of 7.5 bar and with a dryness fraction of 0.98. The feedwater to the boiler is at a temperature of 41.5ºC. The coal used for firing the boiler has a calorific value of 31000 kJ/kg and is used at the rate of 670 kg/h.  Determine:  The thermal efficiency of the boiler

 A boiler generates 5000kg of steam per hour at 18 bar. The steam temperature is 325ºC and the feedwater temperature is 49.4ºC. The efficiency of the boiler plant is 80% when using oil of calorific value 45 500 kJ/kg. The steam generated is supplied to a turbine which develops 500kW and exhausts at 1.8 bar; the dryness fraction of the steam is 0.98.  Estimate:  1. the mass of oil used per hour  2. Enthalpy drop through turbine.

SUBCOOLED SUPERHEATED LIQUID VAPOUR P R E “WET S VAPOUR” S U R E VOLUME



P-V DIAGRAM BOILER P TURBINE RP CONDENSER EU SM VOLUME SP U R E

AB P2 CD











 Steam at a pressure of 30 bar and temperature of 250oC is fed to a steam turbine from a boiler. In the turbine, the steam is expanded isentropically to a pressure of 1 bar. The steam is then exhausted into condenser where it is condensed but not undercooled. The condensate is then pumped back into boiler, determine:  i The supplied energy to the feed water per kilogram of steam generated  ii The dryness fraction of the steam when entering condenser  iii The Rankine efficiency

 A steam turbine plant operates on Rankine cycle. Steam is delivered from the boiler to the turbine at a pressure of 35 bar, with temperature of 350 oC. The exhaust steam entered into condenser at a pressure of 0.1 bar. Later the condensate will feed back to boiler via main feed pumps. Neglecting losses, determine:  i The supplied energy to the feed water per kilogram of steam generated  ii The dryness fraction of the steam when entering condenser  iii The Rankine efficiency



Requirements of Good Boiler • Simple construction, Excellent workmanship Low maintenance cost, High availability • Accommodate expansion and contraction • Adequate steam and water space • Good water circulation • Furnace ensures efficient combustion and maximum rate of heat transfer

contd • Responsiveness to sudden load demands and upset conditions. • Easy access for inspection and repairs. • A factor of safety that meets code requirements

Heat Transfer in Boiler • Heat transfers to the water by – Conduction – Convection – Radiation • Boiler design will determine which mode is more dominant. • Radiant boilers are more robust against sudden load variations.

Types of Boiler Based on flow of flue gas Smoke/Fire Tube boilers •Low steaming capacity •Low pressure (12-15 bar) •Low temperature ( 200-300 bar) • Bigger in size due to large tube bore • Less flexibility due to large bore tubes • Poor heat transfer due to lack of circulation

Types of Boiler Water Tube Boilers •Better suited for power generation/marine applications •More compact hence saving in weight •Can withstand high pressure and temperature •Much higher thermal efficeiency •Mechanically more flexible due to small bore, bend tupes •Greater heating surface area due to small bore tubes •Quick response to sudden load variation

Water Tube Boiler •Greater mechanical flexibility, can accommodate bigger value of tube expansion. • Saving in space in consideration of steam production compared to smoke tube boiler. •Better water circulation due to natural convection •Easy of boiler tube renewal. • Tubes may act as risers or down comers depending on operating conditions of the boiler. •Large diameter external down comers ensure efficient and reliable water circulation.

Based on Furnace Design- D-Type ESD- Type

Manufacturer of Boilers •Babcock and Wilcox •Kawasaki •Combustion Engineering •Foster Wheeler •Alborg •Mitsubishi

Types of Water Tube Boilers • D-Type. • External Superheater D-Types. – ESD-I – ESD -II – ESD-III – ESD-IV

D-Type

Characteristics of D-Type Boilers • Advantages – Good natural water circulation because temperature difference between screen tubes and generating tubes is large due to location of super-heater tubes between these tubes. – Quick steam generation following sudden load variation. This is because the super-heater tubes are located in high temperature region of the furnace. • Disadvantages – Frequent failure of super-heater tubes due to tube burning

contd • Super-heater tubes burn out quickly because these are located in very high temperature region of the furnace and steam is a poor cooling medium compared to water. – The super-heater support tubes in particular suffer the most, because these are made from solid steel angle bars and no coolant flow through. – Due to high metal temperature super-heater tubes are more susceptible to slagging and deposit of molten glass like substance. – Use large amount of fire bricks and refractories.

General Construction of D-Type • Upto about 52 tons/hr steam generation. • At 60 bar pressure. • At maximum supreheat temp of 515 deg C. • Consists of two drums, steam drum located above the water drum. • Main tube bank consists of 3 rows of fire row tubes and many rows of small diameter generating tubes.

contd • Furnace has side and rear water walls. • Roof tubes are part of side water walls and are connected to steam drum. • The bottom ends of water walls connect to a header which also accommodate floor tubes to the water drum. • Rear water walls connect the water and steam drums through a top and bottom header. • The top header is connected to steam drum through risers.

Contd • Super-heater consists of U-bend tubes arranged between fire rows and generating tubes running at right angle to the boiler tubes. • The super-heater tubes are supported by special alloy heat resisting steel plates. • Super-heater tubes are connected through headers which carry internal baffle plates to provide longer flow path to the wet steam to absorb required amount of heat. • Baffle plates also regulate flow velocity of the steam in the super-heater tubes.

Superheater Tubes (Mitsubishi NE-1S)

Contd • Furnace floor and front wall are refractory lined and oil burners are fitted in front wall. • Economiser and air heaters are located high up in the gas uptake. • Adequate number of soot blowers are provided.



Comparison of D and ESD types Boiler D ESDI ESD II ESD III Steam Kg/hr 34000 34000 34000 45000 Tsup ( C) 450 450 450 510 Desgn Pr (bar) 50 47.5 47.5 73 Working Pr(bar) 41 41 41 63.5 Feed Temp © 115 115 115 140 Funnel Temp© 154 154 154 172

Heating Surface According to ASME code Heating surface of water tube boilers comprise of following • 2/3 of shell area • Plus all tube outer surface area • Plus 2/3 area of both heads • Minus area of the tube holes

ESD Design • Since superheat temperature required is only around 515 C therefore it is not necessary to place the super-heater in side the furnace where temperature is 1200-1400 deg C. • Effect of slow boiler response could be addressed by providing super-heat temperature controller. • This concept led to development of boilers with superheater tubes located outside the furnace, in low temperature region of gas uptake.







•Super Htr located in ESD I Boiler low temp region exhaust gas path •Both Primary and Second have contra flow flow heating •Metal temp of secondary high •Air attemperator less efficient •Burner front fired •Flame impingement reduced not eliminated •Response to sudden load is slow.

ESD-II • Poor response of ESD I was mainly due to air attemperation. • This was eliminated by installing damper operated attemperator which could regulate the flow of hot gases over the super-heater tubes. • A feed water temperature control element was also incorporated to absorb heat from the exhaust gas when the damper was in the by- pass mode at low loads. • Feed water control element is basically an extension economiser .



contd • The problem of superheater metal temperature continued to exist resulting in slagging. • Burner remained front fired so all problems of flame impingement and non-uniform heat flux in the furnace remained. • Frequent failure of dampers due to corrosion and poor lubrication. • Heat transfer between gas and steam poor due poor thermal conductivity of hot gases. This affected boiler response to sudden load demands.

ESD III • Following changes were made to eliminate drawback of ESD II. – Water Attemperation, more effective because water is better coolant. – Secondary Super Heater banks receive heat through parallel flow, hence metal temperature low reducing slag problem. – Roof firing to eliminate flame impingement and uniform heat flux distribution. – Mono/membrane wall construction, fully water cooled furnace resulting in less heat loss. – Due to monowall construction, refractory use has been reduced almost negligible levels.

Water Drum size reduced considerably in comparison to ESD I/II

Disadvantages • Two additional opening in steam drum, weakens the drum and also danger of water leakage. • High risk of furnace explosion due to roof firing which forms gas pocket in the upper part of furnace. • Poor purging of furnace due to gas pocket formation. • Explosion more severe due to gas tight construction.