Introduction to the Generator II

The Seal Oil System is typically comprised of the following components:

Shaft Sealing Mechanism

The shaft sealing mechanism is provided for the purpose of preventing hydrogen gas leakage from the generator along the rotor shaft.

Several types of propriety type sealing mechanism are available including:

Oil, supplied at a pressure greater than the pressure of the gas within the generator frame, forces the sealing surfaces together while at the same time providing lubricant to the seal faces.

In the case of the sealing ring a single oil supply provides both the hydraulic pressure to force the seal against the shaft journal and the coolant/lubricant flow to the seal face.

In the case of the collar type seal, two seal oil flows, through separate pressure regulating stations, provide the hydraulic annular pressure to force the ring against the collar (annulus oil) and the coolant/lubricant flow to the seal face (face seal oil).

 

Figure 9:    Radial Oil Film Type Seal

Radial Type Seal

A radial oil film type seal (see Figure 9) consists of a seal housing containing a pair of segmented bronze alloy or babbitted steel rings. The segments are positioned against the side walls of the seal casing and are held concentric with the shaft by hydraulic pressure from the seal oil.

The seal rings have a bore diameter of only a few thousandths of an inch greater than the shaft journal and are free to float radially but are prevented from rotating with the shaft by stops in the casing.  

Seal Oil, at a pressure of approximately 0.35 bar greater than hydrogen pressure is supplied to the seal casing forcing the seal against the shaft journal face. The oil then passes through a space between the sealing rings to the seal face; from here it flows axially along the shaft in both directions. It is the thin film of oil that actually provides the seal between the hydrogen and atmosphere.

The pressure across the seal face is not uniform, the alternator side of the seal is at frame pressure (200 – 400 kPa), while the bearing side of the seal is under a slightly negative pressure. It therefore follows that the flow path of the majority of the seal oil will be toward the low pressure side, typical flows are in the order of 80 l/min to the bearing side and only 5 l/min to the alternator side.

Due to the fine clearances between the seal and the shaft journal, failure to provide seal oil flow to the seals while the shaft is rotating will result in overheating, binding and seizure of the seals. A flow of seal oil is therefore required to lubricate and cool the seal faces whenever the shaft is rotating, even when the frame is de-pressurised to air at ambient pressure.

To prevent seal oil, which has migrated to the hydrogen side of the seal, from entering the alternator frame, the alternator shaft is also fitted with oil deflectors and labyrinth type oil seals. The hydrogen side seal oil drain is located before the shaft oil seals. 

Axial Collar Type Seal

In seal designs employing a seal ring and shaft collar two oil supplies are provided; annulus oil and face seal oil.

Annulus oil is directed into an annulus in the seal ring and acts as a hydraulic force to drive the seal against the shaft collar. It is important that the annulus oil is supplied first when setting up the seal oil system in order to ensure that the sealing faces mate around the entire seal circumference. If the ring is slightly skewed excessive oil flows will be seen at the seal face. Although the annulus oil is predominantly a hydraulic medium a small flow is allowed to be bled from the annulus to prevent overheating of the oil at the seal.

As the function of the annulus oil is to maintain a hydraulic force on the seal its pressure is regulated to maintain a set positive differential between itself and the hydrogen pressure within the alternator frame (Annulus Oil pressure > Hydrogen pressure). Annulus oil pressure will therefore vary considerably during periods when the gas pressure is allowed to increase or decrease.

The Face Seal Oil is used as a lubricating and heat dissipation medium. The pressure of the face seal oil is constant.  

Seal Oil Pumps

Due to the need to supply a constant seal oil supply while ever the generator frame is filled with hydrogen, redundant seal oil pumps are required. Two 100% duty seal oil pumps are therefore normally provided, (Duty (A) and Standby (B). These pumps may be ac motor driven or a combination of ac motor and turbine shaft driven.

The pumps take their suction from a convenient location in the lubricating oil system.

Each pump is provided with suction and discharge isolating valves and a non-return valve in the pump discharge.

One Emergency dc motor driven Seal Oil Pump is normally provided to ensure seal oil supply is maintained on loss of all ac supply.

The Standby Seal Oil Pump will automatically start on loss of the Main Seal Oil Pump.

The Emergency Seal Oil Pump will start on loss of both the and Standby Seal Oil Pumps and on Seal Oil Differential Pressure falling to a low limit (typically in the order of 30 kPa)

In some cases, should all pumps fail while the lubricating oil system is still in service, low pressure backup oil supply from the Turbine Bearing Oil System may be used to maintain supply to the seals.

In such a situation, however, the hydrogen gas pressure should be reduced to an appropriate value to ensure sufficient differential pressure exists between the seal oil and hydrogen.

 

 Figure 10:  Main Seal Oil Pump Showing Differential Pressure Regulator, Relief Valve and Associated Pipework

Differential Pressure Regulators

In order to prevent the hydrogen from escaping through the seal the pressure of the seal oil must be higher than that of the hydrogen within the alternator frame.

Seal oil differential pressure regulators are used to control the pressure of the seal oil being supplied to the seals. If the pressure is too high then excessive amounts of oil may pass into the alternator side of the seal; if the pressure is too low then it may not be sufficient to prevent an outflow of hydrogen past the seal.

In the case of collar type seal separate regulators are provided for the anulus and face seal oil supplies.

The Differential Pressure Regulator works on the principle of a diaphragm or metal bellows being subjected to the generator gas pressure on the upper side and the seal oil pressure to the seals on the lower side. The bellows or diaphragm is connected to the regulating valve spindle. An adjustable spring gives the required amount of bias to establish the required differential pressure (in the order of 30 to 50 kPa).

Should the gas pressure in the generator frame increase at any time the pressure on the top side of the bellows will increase, driving the spindle down slightly and reducing the amount of seal oil being returned to the pump suction header. This will cause a corresponding increase in the seal oil supply pressure.

Under normal operating conditions, once the pressure regulator has been adjusted to the required difference between hydrogen and seal oil pressures it will maintain a constant pressure differential through the complete range of hydrogen pressures.

Figure 11:  Side View of Seal Oil Pumps Showing Differential Pressure Regulators, Relief Valves Suction and Discharge Lines

Seal Oil Cooler

Prior to reaching the shaft seal the seal oil passes through a heat exchanger to dissipate heat from the system and to maintain a set oil supply temperature (changes in oil temperature and viscosity will effect the performance of the seal).

Coolers may be shell and tube type with the seal oil flowing through the shell and the auxiliary cooling water flowing through the tubes or they may be plate or air cooled fin type.

Typical outlet temperature from the cooler ranges between 38 and 43º C.  

Seal Oil Filter

Filters are provided to remove any entrained particulate from the system.

Duplex filters that provide for in service change-over from one basket to the other without loss of seal oil flow are the most common type of filter in use. A changeover handle allows in service operation of either strainer basket. Vent and drain valves are provided for venting and draining of the filter prior to removal and cleaning of the strainer basket.

A differential pressure switch is provided to initiate an alarm on a high filter DP (in the order of 50 kPa). Response to high filter DP and filter cleaning should receive a high priority.

Figure 12:Seal Oil Cooler showing Oil Inlet, Outlet and Bypass Valves and Cooling Water Outlet Flow and Temperature Indication.

Seal Oil Return Lines

The oil returning from the seals takes two independent paths, the majority returns through the bearing side drain while a minor amount returns from the hydrogen or generator frame side of the seal.

Oil returning from the bearing housing side of the seal passes to a loop seal oil tank before returning to the seal oil pump suction side (or main oil tank). A vapour extraction fan is connected to the loop seal to maintain the bearing housing under a negative pressure and to remove any oil vapour or hydrogen present within the bearing housing.

 The oil returning from the alternator side of the seal may have hydrogen gas entrained within it, therefore this oil is passed first to a defoam tank or hydrogen side drain regulator before it is allowed to mix with the oil returning from the bearing side of the seal.

The function of the defoam tank is to allow the small quantity of oil returning from the frame to settle for a period of time during which the hydrogen gas is detrained from the oil.

The flow of oil from the defoam tank is regulated to maintain a constant level of oil within the tank. This is done to form a seal between the alternator frame and the seal oil pump suction pipework. Should the seal within this tank be lost the frame can connected to atmosphere through the loop seal oil tank and associated vapour extraction fan resulting in a major loss of hydrogen to atmosphere.

A float valve regulates the oil level to the centreline of the gauge glass fitted to the tank. Excess oil is released to the Seal Oil Pump suction header.

Level switches initiate an alarm if the oil level rises or falls to a point outside the limits set either side of the gauge glass mid point.

Figure 13:  Hydrogen Side Drain Regulator showing Level Indication and Manual Regulating Valve. 

When the gas pressure Hydrogen, CO₂ or Air) in the alternator is reduced the differential pressure across the seal face is reduced and therefore the flow of seal oil to the hydrogen side of the seal may increase. In such cases the level of the hydrogen side drain regulator vessel should be carefully monitored and if necessary, the drain may be manually operated to prevent overflow from the seal drain into the generator frame. 

Seal Oil Drain Level Switches 

The Hydrogen Side Drain Lines from both the exciter and turbine end of the generator are normally fitted with high level switches to provide an alarm in the event of the oil level within the drain lines rising to the alarm setting.

Excessive oil level could result in oil ingress into the generator frame and fouling of the windings. High priority should be given to this alarm especially in conjunction with a high level alarm from the drain regulator.

Figure 14:  Seal Oil Drain Level Switch

Liquid Detectors

Should the shaft seal face be damaged, allowing excessive flow of oil to the hydrogen side of the seal a condition could exist in which oil overflows from the internal drainage system to the alternator frame.

Liquid detectors with alarm contacts are fitted to the lower sections of the alternator frame to initiate an alarm should a quantity of oil or water be detected.  

Seal Oil Vapour Extraction Fan

In order to remove any hydrogen gas released from the seal oil and to ensure oil does not migrate along the shaft out of the bearing housings, the two alternator shaft bearing housings and the bearing side seal oil return line are placed under a negative pressure from a Vapour Extraction Fan.

The fan suction line may be provided with a valve to allow the line vacuum pressure to be regulated to a set pressure.

The discharge of the fan is normally taken to atmosphere at a for safety.

A parallel circuit incorporating a non-return valve is also taken to atmosphere. Normally the check valve is kept closed by the positive discharge pressure of the fan and the associated negative suction pressure on the opposing side. In the event of seal failure or loss of the fan the non-return valve will open to allow hydrogen gas to vent to atmosphere.

Figure 15:  Seal Oil Vapour Extraction Fan and Associated Pressure Gauge and Check Valve

Seal Oil System Monitoring and Control

To assist with local inspection and monitoring of the system, local indication is normally provided at a monitoring station adjacent to the pumps and cooler.

This station typically includes the following:

Figure 16: Typical Seal Oil Local Monitoring Station

 Figure 17:   Seal Oil Local Control Panel

Seal Oil Local Control Panel

Local control panels typically provide control stations for the following:

The Local Seal Oil Control Panel is normally provided with an alarm fascia to enunciate the following alarms:

 

  Figure 18: Typical Seal Oil System