Auxiliary Systems Data

Auxiliary systems addressed in this part of the overall assessment are (1) the overall steam seal system, (2) the generator hydrogen seal oil system, and (3) the generator stator cooling water system. This portion of the condition assessment is meant to review the mechanical integrity of the two principal sealing systems found on the turbine and generator and to determine that the generator stator is being cooled effectively.

Data Sheet #13 organizes the audit of the steam seals, hydrogen seal oil system, and stator cooling systems into three parts.

Steam Seals: Leaks in the shaft steam sealing system may result in steam entering the lubricating oil system, thereby increasing its water content and resulting in the problems identified in

Section 1.5.7.

Steam leakage past the steam seals is typically due to either (a) improper adjustment of the steam seal pressure and/or vacuum at the regulator or (b) the steam seals have experienced a rub, thus increasing the shaft-to-seal clearances.

The best way to initially determine if a problem exists with the steam seal system is to walk the turbine deck during a startup of the machine and observe if steam leakage by the packing cases or gland is occurring on a frequent basis. This can also be determined through interviews with operators who have also observed unit startups in this manner. To confirm the possibility of a steam seal leakage problem, inspect the lubricating oil at the oil tank to determine if water is in the oil. The oil will be yellow to orange in color if excessive water is present.

In addition to contamination, a large steam seal leak can also result in unexpected heating of the

Turbine-Generator Condition Assessment – In Service

the turbine bearings. In the vibration signature, this may appear as symptoms of misalignment, but it is indirectly related back to the leakage and overheating that is occurring. If steam leakage is seen at the packing cases or glands, a recommendation should be made to take corrective action during the next outage or a weekend shutdown, depending on the degree of the steam leak.

Corrective action while the unit is operating involves adjusting steam seal pressure at the steam seal regulator (steam feed or unloading valves) and adjusting the vacuum at the steam packing or gland exhauster. Reliance on the plant instruction book is recommended. Corrective action with the unit off gear is to take feeler checks between the shaft and the steam seals using a feeler gage to be sure that excess clearances are not present that result in steam leakage even if seal pressure and vacuum are properly set. If this is found, it may be necessary to replace the labyrinth seals during a unit outage. In the interim, frequent lubricating oil checks should be performed to ensure that less than 1,000-ppm water is maintained in the lubricating oil. This may require connecting a vacuum dehydration unit to the lubricating oil system to achieve the required water content in the lubricating oil.

Hydrogen seal oil system: The purpose of the system is to maintain the hydrogen pressure inside the generator. The primary issue of concern to the assessor is contamination of the lubrication and the reliability issues this raises that are associated with the generator.

Leakage can be due to:

• Problems in maintaining seal oil pressure differential

• Excessively worn hydrogen seals or excessive inner oil seal problems

• Hydrogen seal casing and outer end shield contact problems on vertical and horizontal joints

• Failure of a float valve in the closed position or hydrogen seal oil cooler tube leaks (depending on whether water is seen in the seal oil)

Water may also be a result of excessive steam seal leakage or lubricating oil cooler leaks if the hydrogen seal oil coolers have no problems, or it may be from failure of the vacuum pump. From a long-term perspective, oil ingress into the generator is detrimental to stator coils because oil causes the coils and wedges to become loose and thus vibrate and wear at a faster rate than normal, causing a potential re-wedge/rewind or more than normal damage to the end winding system during overhauls.

Maintenance of hydrogen purity is important in order to prevent a possible explosion and fire. If purity is less than 90%, a recommendation for immediate shutdown should be made to correct the problem. A review of instruction book recommendations regarding hydrogen purity problems should be thoroughly reviewed. Excess hydrogen leakage (twice the design leakage rate) should be checked and corrected during a weekend shutdown or during a unit outage.

A vibration problem during startup/shutdown could be related to unbalance or generator thermal problems causing bowing of the rotor at high load and field current. The extent of this problem

Turbine-Generator Condition Assessment – In Service

problems. If problems are found, a review of the vibration data should be performed to determine if the generator has a thermal vibration problem or a mechanical unbalance problem as it passes through its critical speeds.

The best method to determine the health of the hydrogen seal oil system is to question plant operators about the system and maintenance personnel about PM and PdM checks that are performed.

Intermediate recommendations should be made if no such programs exist or if problems found during such inspections have not been corrected.

Stator cooling water system: The purpose of the assessment is to ensure that the water system will maintain an adequate supply of cooling to the stator conductors.

Condition is assessed based on an audit of water conductivity, pressures, and temperatures and stator bar temperatures. Criteria for each are noted on the audit portion of the data sheets. It should be noted these values may vary with the unit and should therefore be checked against the OEM specifications.

The basic condition of the stator cooling water system can be determined by interviewing the operators and plant maintenance personnel who have responsibility for stator cooling system PM/PdM programs.

Any instrument that does not function properly requires immediate actions to correct the problem because these protect the stator from failure of the stator coils.

Intermediate action is generally necessary if a reasonable PM/PdM program is not being performed on the equipment. No short-term recommendations are necessary if the system

appears to be in good condition, if operators have performed tests as recommended by the OEM, and if maintenance programs are adequate for the system.

There may be long-term recommendations by the OEM that improve the stator cooling system’s reliability and performance. If such an OEM recommendation exists, it should be implemented during the next major overhaul if it is technically and economically reasonable.

A partial listing of EPRI published research on auxiliary systems (steam seals, hydrogen oil seals, and the generator stator cooling system) is as follows (by year of publication):

Primer on Maintaining the Integrity of Water-Cooled Generator Stator Windings, EPRI, Palo Alto, CA: 1995. TR-105504.

Preventing Leakage in Water Cooled Stator Windings (Phase 2), EPRI, Palo Alto, CA: 1998.

TR-111180.

Prevention of Flow Restrictions in Water Cooled Generators, EPRI, Palo Alto CA: 2001.

1006684.

Turbine-Generator Condition Assessment – In Service