GENERATOR SEAL OIL SYSTEM

SYSTEM DESCRIPTION

• Shaft seals are supplied with pressurized seal oil to prevent hydrogen losses at the shaft and air entry into the hydrogen-cooled generator.
• As long as the seal oil pressure in the annular gap exceeds, the gas pressure in the generator, no hydrogen escapes from the generator housing.
• The oil in the seal oil system is the same as turbine lubricating system oil.
• The seal oil system consist the seal oil unit and the connecting pipe work to and from the generator.
• During normal operation, the main seal oil pump 1 draws the seal oil from the seal oil tank (vacuum tank) and feeds it to the shaft seals via coolers and filters.
• The seal oil supplied to the shaft seals is drained towards the hydrogen side and the air side through the annular gaps between the shaft and the seal rings.
• Air side drain returned directly to the seal oil tank via a float valve.
• Hydrogen side drain is collected in the generator prechambers and then passed to the intermediate oil tank.
• Three seal oil pumps (2 nos AC & 1 nos DC) are provided for oil circulation.
• The seal oil pumps are three-screw pumps.
• One double-thread driving rotor and two driven idler screws are closely, meshed and run with a close clearance in the casing insert.

COMPONENTS IN SEAL OIL SYSTEM

• AC seal oil pump (2 nos)
• DC seal oil pump
• Oil filters (duplex filter)
• Oil coolers
• Pressure regulator
• Differential pressure regulator
• Vacuum pumps(2 nos)
• Vacuum tank
• Intermediate oil tank (IOT)
• Seal oil storage tank (SOST)

SEAL OIL PRESSURE REGULATION

• Seal oil pressure is controlled in two stages.

Stage-I

 Seal Oil Pressure Regulation Downstream of the Seal Oil Pumps

• The seal Oil pressure downstream of each pump is separately maintained constant by a self-operated pressure regulating valve working on the relief-valve principle.
• Depending on the valve setting and the pressure signal prevailing (seal oil pressure downstream of the pump), a large or smaller quantity of seal oil is returned to the seal oil tank, so that an approximately constant seal oil pressure is established downstream of the seal oil pumps.

Stage-II

 Seal Oil Pressure Regulation Upstream of the Shaft Seals

• The required seal oil pressure (0.8 to 1.5 bar above hydrogen casing pressure) is controlled by a pressure regulating valve.
• Two pressure regulating valves with graded settings are provided.
• Depending on the differential pressure setting, a larger or smaller quantity of oil flows to the shaft seals, so that the required seal oil pressure is established at the shaft seals.
• The function of the pressure regulating valves can be seen in the attached drawing.
• Since the gas and signal oil pressures act in opposite directions, the valve stem is moved upwards or downwards when the pressures become unbalanced. The valve cone is arranged so that the valve opens further for a downward movement of the valve stem (occurs at rising gas pressure or falling seal oil pressure).
• The increased oil flow results in a rise of the seal oil pressure at the shaft seals.
• Setting of the desired differential pressure (set value) to be maintained by the valve is done by a corresponding preloading of the working diaphragm.
• The preloading is adjusted with a compression spring, the upper end of which is rigidly connected to the valve yoke, while its lower end is linked to the valve stem by means of an adjusting nut.

SEAL OIL DRAINS

• Hydrogen side drain discharged into the generator prechambers. The prechambers reduce the oil flow and permit the escape of entrained gas bubbles and defaming of the oil.
• Downstream of the prechambers the oil flows are combined (turbine end & exciter end) and returned into the intermediate seal oil tank.
• A float valve keeps the oil level in the intermediate seal oil tank at a predetermined level and prevents the ingress of hydrogen into the vacuum.
• The oil from the intermediate oil tank is returned into the vacuum tank & air side seal oil drain is also drained in to vacuum tank via a float valve.
• It excessive amount of oil is supplied to the vacuum tank an over-flow upstream of the regulating valve allows some oil to return to the seal oil storage tank (SOST).
• The small amount of hydrogen escaping from the generator together with the oil does not present a danger to the generator surroundings since the seal oil storage tank is connected to the bearing vapour exhausters to ensure continuous venting.

SEAL RING RELIEF

• To ensure free movement of the seal ring even at higher casing pressures the shaft seals are provided with pressure oil for seal ring relief.
• This system must be in operation when the generator is operated at gas pressures of more than 1 bar.
• The oil supply for ring relief is obtained upstream of the differential pressure regulating valve.
•  The required pressure setting for each shaft seal is done separately by means of a flow control valve.

VACUUM TREATMENT SYSTEM

• A vacuum pump establishes a vacuum in the vacuum-tank. This removes entrained air bubbles from the seal oil.
• The seal oil supplied to the shaft seals by the seal oil pumps is thus largely free of gas which prevents a deterioration of the hydrogen purity in the generator.

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