Unit 3

1. Condenser vacuum control:

The steam condenser, shown below, is a major component of the steam cycle in power

generation facilities.. It is a closed space into which the steam exits the turbine and is forced

to give up its latent heat of vaporization.

It is a necessary component of the steam cycle for two reasons.

1. It converts the used steam back into water for return to the steam generator or boiler as

feedwater. This lowers the operational cost of the plant by allowing the clean and treated

condensate to be reused, and it is far easier to pump a liquid than steam.

2. It increases the cycle's efficiency.

Because condensation is taking place, the term latent heat of condensation is used instead of

latent heat of vaporization. The steam's latent heat of condensation is passed to the water

flowing through the tubes of the condenser.

After the steam condenses, the saturated liquid continues to transfer heat to the cooling water

as it falls to the bottom of the condenser, or hotwell. This is called subcooling, and a certain

amount is desirable.

The difference between the saturation temperature for the existing condenser vacuum and the

temperature of the condensate is termed condensate depression. This is expressed as a

number of degrees condensate depression or degrees subcooled. Excessive condensate

depression decreases the operating efficiency of the plant because the subcooled condensate

must be reheated in the boiler, which in turn requires more heat from the reactor, fossil fuel,

or other heat source.

This condenser design provides cooling water flow through straight tubes from the inlet

water box on one end, to the outlet water box on the other end. The cooling water flows once

through the condenser and is termed a single pass. The separation between the water box

areas and the steam condensing area is accomplished by a tube sheet to which the cooling

water tubes are attached. The cooling water tubes are supported within the condenser by the

tube support sheets. Condensers normally have a series of baffles that redirect the steam to

minimize direct impingement on the cooling water tubes.

The bottom area of the condenser is the hotwell. This is where the condensate collects and the

condensate pump takes its suction. If non condensable gasses are allowed to build up in the

condenser, vacuum will decrease and the saturation temperature at which the steam will

condense increases.

Non-condensable gasses also blanket the tubes of the condenser, thus reducing the heat

transfer surface area of the condenser. This surface area can also be reduced if the condensate

level is allowed to rise over the lower tubes of the condenser.

A reduction in the heat transfer surface has the same effect as a reduction in cooling water

flow. If the condenser is operating near its design capacity, a reduction in the effective

surface area results in difficulty maintaining condenser vacuum.

The temperature and flow rate of the cooling water through the condenser controls the

temperature of the condensate. This in turn controls the saturation pressure (vacuum) of the

condenser. To prevent the condensate level from rising to the lower tubes of the condenser, a

hotwell level control system may be employed. Varying the flow of the condensate pumps is

one method used to accomplish hotwell level control.

A level sensing network controls the condensate pumps speed or pump discharge flow

control valve position. Another method employs an overflow system that spills water from

the hotwell when a high level is reached.

Condenser vacuum should be maintained as close to 29 inches Hg as practical. This allows

maximum expansion of the steam, and therefore, the maximum work.

If the condenser were perfectly air-tight (no air or non condensable gasses present in the

exhaust steam), it would be necessary only to condense the steam and remove the condensate

to create and maintain a vacuum.

The sudden reduction in steam volume, as it condenses, would maintain the vacuum.

Pumping the water from the condenser as fast as it is formed would maintain the vacuum. It

is, however, impossible to prevent the entrance of air and other non condensable gasses into

the condenser.

2. Speed control:

The control of a turbine with a governor is essential, as turbines need to be run up slowly, to

prevent damage while some applications (such as the generation of alternating current

electricity) require precise speed control.

Uncontrolled acceleration of the turbine rotor can lead to an over speed trip, which causes

the nozzle valves that control the flow of steam to the turbine to close. If this fails then the

turbine may continue accelerating until it breaks apart, often spectacularly.

Turbines are expensive to make, requiring precision manufacture and special quality

materials. During normal operation in synchronization with the electricity net power plants

are governed with a five percent droop speed control .

This means the full load speed is 100% and the no load speed is 105%. This is required for

the stable operation of the net without hunting and dropouts of power plants. Normally the

changes in speed are minor. Adjustments in power output are made by slowly raising the

droop curve by increasing the spring pressure on a centrifugal governor.

Generally this is a basic system requirement for all power plants because the older and newer

plants have to be compatible in response to the instantaneous changes in frequency without

depending on outside communication.

Droop speed control is the primary instantaneous system using net frequency deviations to

distribute with stability load changes over generating plants.

For stable operation of the electrical grid of North America, power plants operate with a five

percent speed droop. This means the full-load speed is 100% and the no-load speed is 105%.

This is required for the stable operation of the net without hunting and dropouts of power

plants.

Normally the changes in speed are minor due to inertia of the total rotating mass of all

generators and motors running in the net. Adjustments in power output are made by slowly

raising the droop curve by increasing the spring pressure on a centrifugal governor or engine

control unit adjustment . Generally this is a basic system requirement for all power plants

because the older and newer plants have to be compatible in response to the instantaneous

changes in frequency without depending on outside communication. Voltage control of

several power sources is not practical because there would not be any independent feedback,

resulting in the total load being put on one power plant.

It can be mathematically shown that if all machines synchronized to a system have the same

droop speed control, they will share load proportionate to the machine ratings.

Generator Hydrogen Cooling System:

Hydrogen is used for cooling in most large generators rather than air for several reasons.

1. Inherently better heat transfer characteristic (14 times).

2. Better the heat transfer with higher hydrogen pressure.

3. Less windage and friction losses than air.

4. Suppression of partial discharge with increased hydrogen pressure.

5. Significant increase of the breakdown voltage of generator components.

In additional to the hydrogen, a separate supply system is required for CO2 to purge the

generator of hydrogen during filling and de-gassing. CO2 is used because it is inert and will

not react with the hydrogen. If hydrogen in the generator were to be purged with air, this

would encroach upon both the upper and lower explosive limit due to combustible nature of a

hydrogen/oxygen mixture. Hydrogen at high purity will not support combustion above 90%,

and at this level there is no danger of explosive since the explosive range of a

hydrogen/oxygen mixture is 4 to 75% hydrogen in air.

To prevent the possibility of an explosive mixture when filling the generator with hydrogen

for operation, air is first purged from the generator by CO2, and CO2 is then purged by

hydrogen. When degassing the generator for shutdown, hydrogen is first displaced by CO2

and then CO2 is purged by air. This way no explosive mixture of hydrogen and oxygen can

occur.

If hydrogen leaks occur, the pressure regulator admits additional hydrogen from the supply

system until predetermined pressure is restored. There is always a certain amount of expected

leakage into the seal oil, through minute leaks, permeation through the stator winding hoses

and so forth, but most generator should capable of continuous operation below 500 cubic feet

per day loss. If the loss increases to 1500 cubic feet per day, the source of leak should be

investigated immediately and corrected.

A hydrogen gas analyzer is usually present to monitor the hydrogen purity, which should be

maintained above 97%. Dew point monitoring is sometimes provided to control the level of

moisture inside the generator. The dew point is generally maintained below -10C and should

not be allowed to rise above 0C.

4. Gland steam exhaust pressure control: