Generator Gas Cooling System

GENERATOR  COMPONENTS , AUXILIARIES AUXILIARIES , AND EXCITATION ( COOLING SYSTEM )

LEARNING OBJECTIVES STATE THE FUNCTION OF THE GENERATOR GAS COOLING SYSTEM STATE THE FUNCTION AND OPERATION OF THE FOLLOWING COMPONENTS : A) HYDROGEN STORAGE AND DELIVERY SYSTEM B) HYDROGEN GAS COOLERS C) CARBON DIOXIDE STORAGE AND DELIVERY SYSTEM D) GAS ANALYZING AND MONITORING SYSTEM DISCUSS THE OPERATION OF THE GENERATOR GAS COOLING SYSTEM CONTROLS , INDICATIONS , AND ALARMS DESCRIBE THE START  UP , NORMAL OPERATION , AND SHUTDOWN OF THE GENERATOR GAS COOLING SYSTEM DESCRIBE THE INTERRELATIONSHIPS BETWEEN THE GENERATOR GAS COOLING SYSTEM

STATE THE FUNCTION OF THE GENERATOR GAS SYSTEM THE FUNCTION OF THE GENERATOR GAS SYSTEM IS TO REMOVE THE HEAT GENERATED INSIDE THE GENERATOR AS THE GENERATOR PRODUCES ELECTRICITY.

PRIMARY FLOW PATH THE GENERATOR GAS COOLING SYSTEM IS A CLOSED LOOP SYSTEM THAT UTILIZES HYDROGEN GAS AS A COOLING MEDIUM AND CARBON DIOXIDE GAS AS AN INTERMEDIATE GAS DURING GENERATOR PURGES

HYDROGEN HAS FOUR MAJOR ADVANTAGES OVER AIR FOR HEAT REMOVAL FROM THE GENERATOR 1.0 The density of hydrogen is one  fourteenth that of air . The windage losses (caused by churning the gas around the rotor are much less with hydrogen. 2.0 The heat removal capability of hydrogen (at operating pressure ) is about 10 times higher than that for air . 3.0 The degradation by oxidation process cannot occur , because hydrogen is free from oxygen. 4.0 Hydrogen does not support fire which can start by arcing .

The major disadvantage of hydrogen is that it forms an explosive mixture when it combines with air within a volumetric concentration in the range from 4 to 76 % . Sophisticated sealing arrangement are required to ensured leak -tight casing .

It is essential to prevent air & hydrogen mixture inside the generator . Carbon dioxide is used as a buffer gas between air and hydrogen . The processed is called scavenging or gassing  up & degassing carbon dioxide is normally stored as a liquid . It is expanded to a low pressure above atmospheric . It is also heated to prevent it from freezing due to expansion process . CO2 is fed into the bottom of the casing through a long perforated pipe .It displaces air from the top via the hydrogen inlet distribution pipe to atmosphere outside the plant station. The proportion of CO2 in the gas passing to atmosphere is being monitored by a gas analyzer . When the CO2 concentration becomes sufficiently high , flow of CO2 is interrupted .

The Generator Gas Cooling System has four primary flow paths that include the hydrogen storage and delivery system , instrument air supply , the circulation of hydrogen gas through the generator , and carbon dioxide storage and delivery system .Hydrogen gas is delivered from storage bottles to the hydrogen gas header through isolation valves . The header directs the gas through a series of pressure reducing valves to gradually reduce the hydrogen gas pressure to the proper generator supply pressure . Hydrogen gas at the proper pressure is then delivered to the top of the generator through the use of a three  way valve .

Three  way valve

Fig. 1

Fig. 2

STATE THE FUNCTION & OPERATION OF THE FOLLOWING COMPONENTS Hydrogen storage and delivery system Hydrogen gas coolers Carbon dioxide storage and delivery system Gas analyzing & monitoring system

Hydrogen Storage and Delivery System

The function of the hydrogen storage and delivery system is to provide storage for the hydrogen gas and to supply the generator with hydrogen gas at the proper pressure. The hydrogen storage and delivery system consists of hydrogen bottles, a first expansion assembly, and second expansion assembly. The volume of hydrogen gas necessary to fill the generator is 360 m³. The rated daily hydrogen leakage is 11 m³.

Hydrogen Gas Coolers The function of the hydrogen gas coolers is to remove the heat from the hydrogen gas circulating inside the generator. The hydrogen gas coolers are located inside the generator casing . Two hydrogen gas coolers are located on the north end, and two hydrogen gas coolers are located on the south end.

Hydrogen Gas Coolers

Pulled out Hydrogen Gas Coolers

The generator is equipped with four shell and tube type hydrogen gas coolers (heat exchangers). The hydrogen gas coolers have bearing cooling water flowing through the tube side and hydrogen gas flowing through the shell side. As the hydrogen gas is circulated through the generator by two single stage axial flow fans mounted on the rotor, it picks up heat. The heated gas is then ducted through four hydrogen gas coolers to remove heat from the hydrogen gas. The hydrogen gas is ducted to flow across the cooler tubes at a right angle. The hydrogen gas transfers its heat through the tube metal to the bearing cooling water flowing through them

Single Stage Axial Flow Fan Connected to rotor

Carbon Dioxide Storage and Delivery System The function of the carbon dioxide storage and delivery system is to provide storage for the liquid carbon dioxide, heat the liquid carbon to a gas, and supply the generator with carbon dioxide gas at the proper pressure. The carbon dioxide storage and delivery system consists of  carbon dioxide bottles, a carbon dioxide anti-freeze unit and an expansion assembly.

Carbon dioxide anti-freeze unit

Expansion assembly

Gas Analyzing and Monitoring System The function of the gas analyzing and monitoring system is to measure and report the concentrations of air, carbon dioxide gas, and hydrogen gas within the generator.

System Operation Initial Condition

Electrical, valve, and instrument valve line-ups are performed and signed off. The Generator Seal Oil System is in operation. All katharometers and gas monitors are adjusted and calibrated. All alarms are operational. The carbon dioxide anti-freeze unit is filled with Gilotherm Ald. All gas circuits are closed. The generator is on turning gear or is at rest.

Start-up Air is removed from the generator by filling the generator with carbon dioxide gas during start-up. Carbon dioxide gas is admitted to the generator through a supply line on the bottom of the generator casing. Because carbon dioxide is heavier (or more dense) than air, the entering carbon dioxide gas pushes the air upwards through an outlet connection. The air is then vented to the atmosphere. Carbon dioxide is admitted to the generator until its percentage reaches 95 %. After purging the air from the generator with carbon dioxide, hydrogen gas is admitted to the generator through a top supply line. Because hydrogen is lighter than carbon dioxide (less dense), the entering hydrogen gas pushes the carbon dioxide downwards through a bottom outlet connection. The carbon dioxide is then vented to the atmosphere. The hydrogen gas is admitted to the generator until its purity reaches 97%.

Normal

Operation

Certain checks are performed after the generator hydrogen filling and pressurizing operations are completed, and the generator has reached its normal operating speed. The hydrogen gas pressure is maintained 3.5kg/cm², and the purity is maintained 98%. Hydrogen is vented from the bottom of the generator if the hydrogen purity falls below 95%.

Shutdown The Generator Gas Cooling System is shutdown after the generator is shutdown. Hydrogen gas is removed from the generator by admitting carbon dioxide gas through a bottom supply line during plant long outages. The carbon dioxide gas fills the generator and pushes the hydrogen gas upwards though a top outlet connection because carbon dioxide is heavier (more dense) than hydrogen. Instrument air is admitted to the generator through a top supply line after the hydrogen is purged from the generator. The air fills the generator and pushes the carbon dioxide gas downwards through a bottom outlet connection because air is lighter (or less dense) than carbon dioxide. The carbon dioxide is then vented to the atmosphere. No one should enter the generator when carbon dioxide is present in the generator. It is important to note that after the carbon dioxide gas is purged from the generator with air, the air inside the generator must be allowed to drop to atmospheric pressure before any maintenance is performed on the generator casing. The Generator Seal Oil System must remain in service until the generator casing is filled with air at atmospheric pressure.

The End  THANK YOU Prepared by: Leonel A. Alolor (Electrical Engr.)