Eliminating Moisture Damage to Bearings on Critical Steam Turbines.AESSEAL Inc,UK / By Aziz Zehachi turbine expert . Email : [email protected]

29 june 2016 By - Aziz Zehachi turbine expert Skikda Sonatrach refinery Naftec. Algeria email : [email protected] 

Subject : Eliminating Moisture Damage to Bearings on Critical Steam Turbines . 

Steam turbines are used as primary drivers many industry Oil & Gas , paper and pulps , power generation etc...

During normal operations, steam leaking past the carbon rings caused moisture ingress into the bearing housings that the OEM standard seals could not effectively stop.

Mean Time Between Repairs (MTBR) for the turbines was only 4-12 months with the OEM seals. Turbine rebuilds cost $35,000 and up.

Several seal designs were tried to exclude the moisture from the bearing housings, but none were successful.

Sonatrach Anadarko Association installed labyrinth-style bearing protectors designed specifically for steam turbines on a  TAV01A turbine. The results have been excellent, with zero water in the oil, and no repairs required over the last 24 months (and counting) SAA has selectively scheduled Labtecta retrofits for all steam turbines with chronic moisture lube oil contamination and is aggressively retrofitting critical acid service pumps and gearboxes that have root cause failures associated with lube oil contamination.

Labyrinth bearing protectors have been gaining popularity and replacing lip seals as the preferred form of sealing bearing housings at reliability-focused plants over the last 25 years. Recent design improvements have greatly improved the labyrinth seal's ability to contain valuable lubricating oil and to exclude moisture and other contaminants. The basic elements of any rotating labyrinth isolator are (a) a stationary portion that fixes to the machine housing and contains the lubricating oil, (b) a rotary portion that fixes to the shaft and excludes moisture and dirt, and (c) a "shut-off" mechanism that seals the oil chamber when the machine is stopped, but that allows the air in the oil chamber to expand outward during operation. The basic design of a rotating labyrinth bearing protector . 

5 important design features: ( AESSEAL Inc Company UK )

1. Removable/replaceable ring for easy field-refurbishment,
2. Two shut-off O-rings that land on a smoothly-contoured surface,
3. Two drive O-rings on the shaft providing better drive and more stability,
4. Two water expulsion ports, and
5. Two oil-retention mechanisms.

The seal in  is widely used on pumps, motors, gearboxes, fans, pillow-block bearings, etc. This design has been adapted to fit the narrow space envelope and special operating conditions of a steam turbine. You can see that this seal also utilizes two drive O-rings, two shut-off O-rings that seal on a smooth surface, and easy refurbishment capability. But the steam-turbine isolator has two unique adaptations . First, a ring of graphite packing has been placed on the side nearest the steam to help protect the Aflas O-rings from the intense heat. Second, the design of the rotating unit has been changed to create a "steam deflector" that repels the impinging steam.

THE PROBLEMS WITH STEAM 

Like many other types of rotating equipment, steam turbines have bearings that support the axial and radial loads. These bearings must be properly lubricated in order to achieve L10 life cycle. Turbine bearings are exposed to high temperatures, high transient thrust loads, long states of idle readiness, and a moisture-laden environment. These are severe operating conditions at best. Of all operating environments, the state of idle readiness introduces the highest level of contamination exposure to a turbine bearing and lube oil system because the rotor and, in most cases, lubrication system are in a static state while live steam leaks past the carbon shaft seals. As the steam makes its way past the OEM labyrinth bearing housing seals, which function properly only in the dynamic state (i.e. when the shaft is rotating), it condenses on the cooler inner surfaces of the bearing housing and collects in the sump. When the turbine is put in service, the bearings will fail prematurely due to moisture contamination. 

S.A.A Operation's reliability engineer has studied MTBRs and performed root-cause failure analysis on a wide range of rotating equipment failures (turbines, pumps, gearboxes, trunions, etc), and concluded that bearing failures were frequently caused by lube oil contamination that was the result of the steam in the operating environment entering the bearing housings .

To reduce the ingress of moisture, and improve the MTBR of the
bearings, S.A.A attempted the following modifications:
? Tighter running clearances of the OEM labyrinth seal;
? Air buffer porting;
? Low pressure reducing baffle;
? Instrument air purging the oil reservoir, to prevent ingress of moist air;
? Nitrogen purging the oil reservoir, to prevent ingress of moist air;
? Desiccant vent filters, to dry the air entering the housing;
? Vent check valves, to allow one-way flow of air from the housing; and
? Synthetic lubricants, with improved moisture separating qualitie

CONCLUSIONS

Steam/moisture entry into the bearing housings of steam turbines and other rotating equipment was a major source of costly bearing failures at this plant. The OEM bearing protectors were determined to be inadequate for the severe conditions surrounding steam turbines.

Mosaic River tried numerous housing design and lubricant changes to eliminate or at least minimize the effect of the water in the oil. Mosaic investigated and tested a number of different bearing housing seals and isolators and standardized on the modern labyrinth design shown in Figure 1 for its pumps and gearboxes, and Figure 2 for its steam turbines.