vacuum system.ppt

April 20, 2023 PMI Revision 00 1

Turbine Vacuum System


Presentation outline

Why Vacuum system is required ?

Parts of Vacuum system?

Steam Ejectors

Vacuum Pumps


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.


• 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.



Parts of Vacuum System

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



Condenser

Steam from last stage of LPT Exhausts on condenser tube

condensation of steam takes place

Water collected in hot well


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.


• 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.


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.


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.


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.


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.



CW system

CW pumps supply cooling water to condensers

CW maintains vacuum in condensers

CW flows through condensers tubes


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.


STEAM EJECTORS

Motive steam

Non condensibile gases and water vapour from condenser

Convergent divergent diffuser

Nozzle

Dis

char

ge

to c

on

den

ser


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.


STEAM EJECTORS


EJECTORS• MAIN AIR EJECTOR

• STARTING AIR EJECTORS


Ejectors


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.


LRVPSuction port

Impeller

Gas vapour mixture Liquid ring

Casing

Discharge port


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).


LRVP

Separator

vent

Non condensible gases and water vapour from condenser

Cooling water

Liquid ring vacuum pump system

Seal water

LRVP

Seal cooler

Makeup


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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vacuum system.ppt