Unit 9 Steam Propulsion
The Main Steam Cycle
GENERATION SEGMENT Notes: Absorb latent heat to produce saturated steam (boiler) TSES V – saturation conditions in boiler: 600 psig and 490oF Increase steam temperature above saturation (superheater) TSES V – temperature raised to 885oF (approx 400 degrees of superheat) with slight pressure drop Supply steam to Main Engines, TG’s and Auxiliaries At typical TSES speeds approx 1/3 steam generated is used for propulsion, 1/3 to generate electricity (TG steam), and 1/3 for everything else (Auxiliary Steam: driving pumps, heating fuel, distilling sea water, blowing whistle, etc.) At full power a much greater percentage of steam generated would be used for propulsion
EXPANSION SEGMENT High pressure & temperature steam enters engine TSES VI – at full power steam enters HP turbine at approx. 585 psig and 880oF; lower pressure & temperature at slower speeds Steam pressure & temperature reduced in stages —delivering work TSES VI – engine consists of two turbines (HP & LP) side by side (cross-compounded) with each shaft connected to set of reduction gears driving ta single propeller An astern turbine is mounted forward of, and on the same shaft as, the LP turbine to turn the propeller in the opposite direction Steam exhausts at low pressure & temperature TSES VI – exhaust pressure and (saturation) temperature is 28.5 “Hg (VAC) and 92oF respectively PS 111. Professional Studies Unit 9: Steam Propulsion
CONDENSATE SEGMENT Notes: Remove latent heat to condense exhaust steam into a liquid (condensate) Sea water absorbs the heat given up by the exhaust steam and leaves the condenser warmer than it entered That is, the latent heat given up by the exhaust steam equals the sensible heat gained by the sea water The working fluid, now a (very) low pressure liquid must be returned to the boiler To be injected into the boiler (600 psig) it must be raised to pressure higher than boiler pressure To avoid thermal shock and increase overall plant efficiency, it must be raised in temperature to near the saturation temperature in the boiler Working fluid pressure is raise in steps by a series of pumps, and in temperature by a series of heat exchangers
FEEDWATER SEGMENT Condensate is delivered to the DC Heater by the condensate pumps, is mixed with auxiliary exhaust steam and momentarily stored as feedwater Feedwater drops 60 ft or more to Main Feed Pump suction Pressure is raised by feed pump to above boiler pressure Final heating of fluid prior to steam generation in boiler takes place in one or more high pressure heaters through which the feedwater passes On TSES VI, there is only one HP heat exchanger—the economizer—which is located in the boiler uptake casing.
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Main Steam Cycle diagram:
Generation economizer Segment Expansion Segment H.P. turbine steam drum X-over
L.P. turbine superheater
exhaust trunk
main condenser
Feedwater D.C. Segment heater main (DFT) condensate pump
Condensate main feed L.P. heaters Segment booster pump (if installed) H.P. feed main feed heaters pump (if installed)
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Drum Type Boiler (similar to TSES VI)
STACK
STEAM DRUM
ECONOMIZER
GENERATING TUBES WATERWALL TUBES
SUPERHEATER TUBES
FURNACE INNER CASING BURNERS
WATER DRUM OUTER CASING
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Axial Flow Turbine Three stage, IMPULSE Turbine
TSES Arrangement (Cross compounded)
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CHANGING ENGINE SPEED Sea Speed – long duration (e.g., port to port) as opposed to maneuvering Throttle at fixed position (usually wide open) Nozzle Control Valve opens a chamber to a set number of 1st stage nozzles Number of nozzles open corresponds to a particular engine RPM and hence sea speed On TSES VI Four Nozzle Control Valves employ 4, 9, 12, or 14 nozzles (in any combination—4+9 = 13; 4+14 = 18; 9+12 = 21; etc. Maneuvering – entering and leaving port Fix number of nozzles to allow FULL AHEAD with throttle wide open (on TSES = 9 nozzles) Open AHEAD THROTTLE to position corresponding to desired AHEAD speed (DEAD SLOW—10 RPM; SLOW—20 RPM; HALF AHD—40 RPM; FULL AHD—60 RPM) Open ASTERN THROTTLE to position corresponding to desired ASTERN speed (DEAD SLOW, SLOW HALF, and FULL ASTERN) Both THROTTLES closed at STOP Both THROTTLES are NEVER open simultaneously, but with AHD throttle closed and engine coasting AHD, ASTERN throttle may be opened to brake engine to a stop.
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MAIN CONDENSATE SYSTEM
MAIN CONDENSER—large, evacuated vessel in which turbine exhaust condenses into a low pressure liquid Sea water is the heat sink into which the heat extracted from the exhaust steam is absorbed Thus the working fluid must be at a temperature above seawater temperature in order for heat to be transferred. Although conditions vary with sea temperatures, design standards usually assume a sea temperature of 85oF, with condensing taking place at 92oF and a corresponding saturation pressure of 28.5”Hg (VAC). Seawater is circulated through main condenser tubes via a large pump (Main Circulating Pump) or via a scoop in bottom hull of the vessel (if the ship is moving fast enough) As the condenser shell is a very low pressure reservoir, it is convenient to drag fluids (liquid or vapor) from other higher pressure locations in the plant, retrieving auxiliary fluids into the cycle for regeneration in the boilers
Vacuum Drags: Vacuum Drags: L.P. Turbine ADT Aux. Exh. Dump Exhaust Trunk MUF Cond. Recirc 1st Heater Loop Seal Air Ejector Turb Casing Drains Suction 1st Stg Htr Vent
overboard Main Circ Water (HIgh) from Mn. Circ P.
from scoop
overboard (Low)
to L.O. coolers
Cond Pump Main Vent Condensate Pump(s)
MAIN CONDENSATE PUMP(S)—draw condensate from condenser drain well and raise pressure of fluid to “push” it through a series of heat exchangers.
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Main Cond. Pumps (there are usually two, one in service, the other in standby status) are fitted with a vent and located below condenser level Pressure is raised high enough to ensure delivery of fluid to the DC Heater located high in the machinery space
MAIN AIR EJECTOR—is both a pump and a heat exchanger It pumps air from the low-low pressure of the main condenser to the atmosphere in two stages. Each stage is accomplished via a jet pump (ejector) that utilizes 150 psig auxiliary steam as the motivating fluid The motivating steam, discharging from the jet pump with the air it pumped, heats the condensate while it itself condenses. The air is non-condensable and proceeds to the next stage while the condensed motivating steam is drained away back into the cycle.
JET PUMP AUX STEAM (2ndt Stage) JET PUMP (150 psig) (1st Stage) AIR VENT AIR (VACUUM) DRAG LINE
CONDENSATE from Main INTER- AFTER- Cond. Pump CONDENSER CONDENSER 10–15 “Hg (VAC)0 ~ 1 Atm CONDENSATE to 1st stage Return to CONDENSER via heater LOOP SEAL
Drain to ADT
1st STAGE HEATER—a shell-and-tube heat exchanger utilizing higher temperature fluids from various parts of the plant to heat condensate. On the TSES VI the 1st Stage Heater is comprised of three units the 1st Stage Heater is actually only one compartmentt of this heat exchanger and utilizes bleed steam from the LP turbine to heat condensate the Gland Exhaust Condenser collects leak off steam (and some air) from the turbine glands the Drain Cooler receives drainage from the upper compartment which is cooled by the circulating condensate
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AUX EXHAUST STEAM (12—15 psig)
Saturated VAPOR/LIQUID
FEED LP Turbine WATER Bleed
(actual) 1st STAGE HEATER
GLAND EXHAUST CONDENSATE To MAIN FEED from AIR COND EJECTOR PUMP SUCTION (1st STAGE) DRAIN COOLER
1st STAGE HEATER: 3 Heat Exchangers in one casing
DC Heater—a direct contact (DC—as opposed to shell-and- tube) heat exchanger were heating medium (auxiliary exhaust steam is mixed with condensate. Scrubbing action of steam removes dissolved O2 from condensate DC Heater also called Dearation Feed Tank (DFT) Acts as surge tank in system (accommodates surplus or deficit of working fluid)
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Located high in machinery space, provides positive head to Feed Pump
FW Reg. Valve Mn. Feed
Aux. Feed ECONOMIZER
To Port BLR STM DRUM STOP VALVES STOP-CHECK VALVES R T H
C D
M g o i T From FD t
R s S c
p T
r
PUMP No.2 i X H 0 d c d m
U 0 e e e o C r 6 A e e R F D F F
. n i x a u
A SUPPLY
M SUCT VALVE VALVE DISCH MN. FEED VALVES PUMP NO. 1 EXH. VALVE
MAIN FEED SYSTEM
DC HEATER (above)—stores feedwater delivered by gravity to feed pump suction MAIN FEED PUMP(S)—are driven by small turbines utilizing 600 psig auxiliary steam which exhausts as 15 psig (aux. exhaust) steam used in the DC Heatyer to heat condensate! Fluid pressure at the pump discharge is the highest in the cycle (650-700 psig), enough to “push” feedwater into the boiler steam drum Two routes from feed pump to boiler: main feed & auxiliary feed A small amount of feedwater is re-circulated to DC heater so that there is always some flow through feed pump when boiler demand for replenishment is low MAIN FEED—passes through (main feed) STOP-CHECK vale, FW REGULATION valve, and STOP valve before passing through economizer AUXILIARY FEED—passes through (Auxiliary Feed) STOP- CHECK and STOP valves
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Under normal operating conditions: main feed stop-check and stop valves are wide open and REGULATING valve adjusts automatically to control flow to boiler. Auxiliary valves are closed In an emergency (failure of regulating valve) auxiliary feed STOP valve is opened wide and STOP-CHECK valve is manually adjusted to control flow. Main STOP and STOP- CHECK valves are closed to isolate regulating valve for repair. ECONOMIZER—is a high pressure heat exchanger which utilizes combustion gas from boiler to pre-heat feedwater.
TSES Main Steam Cycle Details (full power values—pressures & temperatures at lower speeds differ):
Component Pressure Temperature Remarks BOILER: Steam Dry (100% quality) saturated steam Drum, Water Drum & 600 psig 490oF Generating Tubes leaves steam drum Superheated vapor (highest temp in SUPERHEATER 590 psig 885oF cycle)
HP TURBINE 585 psig Super heated vapor
LP TURBINE 40 psig Slightly superheated vapor
MAIN Saturated vapor-liquid mix; liquid slightly 28.5 “Hg VAC 92oF CONDENSER sub-cooled in drain well (lowest temp) MN. COND. 50 psig ≥ 92oF Sub-cooled liquid; PUMP(S) Sub-cooled liquid; non-condensable AIR EJECTOR < 50 psig 115-130oF gasses removed from condenser 1st STAGE < 50 psig 140-160oF Sub-cooled liquid; HEATER
DC HEATER 12-15 psig 240oF Saturated vapor-liquid mix;
MN. FEED Sub-cooled liquid; highest pressure in 650—700 psig ≥ 240oF PUMP(S) cycle
ECONOMIZER > 600 psig 350oF Sub-cooled liquid;
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Visit: http://www.engineersedge.com/thermodynamics/typical_steam_cycle.htm www.ku.edu/~kunrotc/academics/180/Lesson%2005%20Main%20Steam%20Cycle.ppt cache.ussmissouri.org/steamcycle.htm
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