Turbine vacuum system

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Turbine Vacuum
System

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Presentation outline
 Why Vacuum system is required ?
Parts of Vacuum system?
Steam Ejectors
Vacuum Pumps

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Why is it required?
• The steam turbine itself is a device to convert the
heat in steam to mechanical power.
• Enthalpy drop across the turbine decides the
work output of the turbine. For increasing this
enthalpy drop across the turbine we need
effective condenser vacuum system.

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• By condensing the exhaust steam of turbine, the
exhaust pressure is brought down below
atmospheric pressure, increasing the steam
pressure drop between inlet and exhaust of steam
turbine. This further reduction in exhaust pressure
gives out more heat per unit weight of steam input
to the steam turbine, for conversion to mechanical
power.

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Parts of Vacuum System
• Condenser
• CW system
• Ejectors/Vacuum pumps
• Gland Sealing System

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Condenser
Steam from last stage
of LPT Exhausts on
condenser tube
 condensation of
steam takes place
Water collected in hot
well

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Shell of the Condenser
• The shell is the outer most body of the condenser
• shell is fabricated from fairly thick carbon steel plates.
• Due to its large size the shell is sufficiently strengthened or
stiffened internally with carbon steel plates to give sufficient
rigidity for the shell proper.
• The shell also gives support to number of intermediate support
plates for the long tubes, depending on the size of the condenser.
• At the same time the intermediate tube support plates allow for the
free movements of tubes in all directions particularly lengthwise
due to expansion and contraction occurring during operation.

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• The whole condenser is supported on heavy springs, mounted on
steel sole plates at suitable places on the concrete foundation
• At the bottom of the shell where the condensate is allowed to
collect, a sump (often referred to as the hotwell) is provided.
• This sump is common to both the halves but separated by a
partition wall in the middle up to the height of the bottom row of
tubes.
• The inside of shell and outside the tubes as a whole remains
under vacuum under normal operating conditions. Inside the
tubes the cooling or circulating water passes through.

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Air Zone
• Inside the shell, a central or side portion
longitudinally is separated by an outer shield
except at the bottom. This partition is called the
Air zone.
• All the gases released in the condenser due to
cooling are taken out via these air zone tubes.
• From a suitable portion of this air zone inside the
shell an air vent pipe is taken out and brought out
of the shell for connection to an air extraction
device.

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Tube Sheets
• At each end of the shell, tube sheet of sufficient
thickness is provided, with holes for the tubes to be
inserted and rolled.
• To take care of length wise expansion of tubes some
designs have expansion joint between the shell and the
tube sheet allowing the latter to move longitudinally.

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Water Boxes
• The tube sheet at each end with tube ends rolled, for each half
condenser is enclosed in a fabricated box known as water box.
• These water boxes on inlet side will also have big size flanged
connections for cooling water inlet at lower level for butterfly
valves.
• small vent pipe with hand valve for air venting at higher level, and
hand operated drain valve at bottom to drain the water box for
maintenance.
• Similarly thermometer pockets are located at inlet and outlet pipes
for local measurements cooling water temperature.

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Tubes
• Generally the tubes are made of brass, aluminum brass,
cupro nickel, stainless steel or titanium depending on
the cooling water chemistry.
• The lengths are fixed at about 20 ft (6 m) (for the 200
MW device mentioned above), depending on the size of
the condenser.
• The outer diameter is limited to a maximum of one inch
for ease of handling and ease of insertion through the
shell tube holes and for rolling at both ends.

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CW system
CW pumps supply cooling
water to condensers
CW maintains vacuum in
condensers
CW flows through
condensers tubes

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STEAM EJECTORS
• Air and water vapor are removed from the main
steam condenser, enter the 1st stage ejector and are
compressed to the interstage pressure by means of
the high pressure motive steam.
• The load and motive steam are discharged to the
inter condenser and a portion of the water vapor
load and motive steam are condensed by
condensate from the main condenser.
• Non-condensibles and associated water vapor are
removed from the inter condenser by the 2nd stage
ejector.

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STEAM EJECTORS
Motive
steam
Non condensibile gases and
water vapour from condenser
Convergent
divergent
diffuser
Nozzle
Discharge
to
condenser

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STEAM EJECTORS
• Multistage condensing ejector systems can be designed to
operate at any condenser pressure and designs are not limited
by the available cooling water temperature to the
intercondenser (condensate cooled systems are common).
• These systems have no moving parts, are the most reliable,
require the least maintenance of all venting systems, and are
the least expensive in initial cost.
• Once equipment is built for a given motive steam pressure that
pressure must be maintained or the ejector will become
unstable and lose vacuum.

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STEAM EJECTORS

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EJECTORS
• MAIN AIR EJECTOR
• STARTING AIR EJECTORS

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Ejectors

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VACUUM PUMPS
• The liquid-ring vacuum pump is a specific form of rotary
positive-displacement pump utilizing liquid as the principal
element in gas compression.
• The working parts of the liquid ring vacuum pump consist
of a multi-bladed impeller mounted eccentrically in a round
casing which is partly filled with liquid. As the impeller
rotates, the liquid is thrown by centrifugal force to form a
liquid ring which is concentric with the periphery of the
casing.

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LRVP
Suction port
Impeller
Gas vapour
mixture Liquid ring
Casing
Discharge port

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LRVP
• In addition to being the compressing medium, the
liquid ring absorbs the heat generated by
compression and friction, absorbs any liquid slugs
or vapor entering with the gas stream, and
condenses water vapor entering with the gas.
• A closed loop (or total recirculation) seal system is
commonly used. The seal water temperature warmer
than the cooling water to the pump heat exchanger,
which is normally taken from the same source as the
condenser cooling water (CW or ARCW).

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LRVP
Separator
vent
Non condensible gases and
water vapour from condenser
Cooling
water
Liquid ring vacuum pump system
Seal water
LRVP
Seal cooler
Makeup

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LRVP
• The vacuum attainable by a liquid ring vacuum pump is limited by
the vapor pressure of the seal fluid.
• As the operating vacuum approaches the vapor pressure of the
seal, more and more of the seal fluid will “flash” into vapor.
• The capacity of the liquid ring vacuum pump is reduced as more of
the impeller space is occupied by vapor from the seal fluid, leaving
less space available to accept the incoming load.
• If allowed to continue, cavitation will occur inside the pump,
resulting in damage to internal surfaces, and preventing the pump
from achieving greater vacuum levels.

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THANK YOU