Executive Summary
Mosaic Fertilizer, Louisiana Operations, produces phosphate fertilizer. Steam turbines are used as primary drivers (in place of electric motors) to utilize steam produced by waste heat generators. 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.
Mosaic installed labyrinth-style bearing protectors designed specifically for steam turbines on a Terry GAF4 turbine. The results have been excellent, with zero water in the oil, and no repairs required over the last 24 months (and counting). Mosaic 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 Protector Design
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 is shown in Figure 1.
5 important design features:
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 Figure 1 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, as shown in Figure 2. 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 as shown in Figure 2. 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. The location of the escaping steam can be seen in the turbine cross-section diagram of Figure 3, which shows the rather simple, stationary labyrinth isolators (in circles) that are provided by some OEM steam turbine manufacturers. Figure 4 is a photo of the actual environment a steam turbine operates in, which is literally a "cloud of steam."
Mosaic Fertilizer, Louisiana 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 (see Figure 5).
Mosaic Faustina Ammonia Plant's Terry GAF4 steam turbines were suffering from water contamination of the bearings, leading to an MTBR of as little as 4 months. This had been a chronic problem for the last 40 years. Just replacing the bearings on this turbine costs $14,000. A full turbine rebuild costs about $35,000. An emergency rebuild in August 2010 of an Elliott EPG-4 turbine & 90P single-stage compressor due to water-related failure cost over $300,000. A solution was needed to stop this frequent and expensive damage.
To reduce the ingress of moisture, and improve the MTBR of the
turbine bearings, Mosaic 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 qualities.
None of the above modifications were completely successful.
Several well-known after-market bearing isolators were then investigated and tested. Most labyrinth isolators showed some improvement over the OEM seal, but none fulfilled all of Mosaic's requirements (i.e. no oil leakage out, and no water entry in).
The Solution to the Problem
In July 2008, a set of 3 Labtecta bearing isolators (Figure 2) were installed on a Terry GAF4 turbine. The cost for these isolators was $2,400. This turbine is still running fine after 24+ months, and Mosaic estimates that for this initial investment in state-of-the-art bearing protection, it has avoided at least three turbine repairs over this time period, for a total savings of $100,000. This results in a payback period on the initial investment of less than THREE WEEKS, with ongoing savings of $50,000 per year thereafter. Weekly oil samples taken from this steam turbine with modern labyrinth isolators continue to show zero moisture in the oil. Turbines with Labtecta isolators that are taken out of service for other reasons have no corrosion and pristine conditions inside the bearing housings (Figure 6).
Using an asset criticality matrix and tribology study, Mosaic's reliability manager identified turbine seal upgrades with the most profitable ROIC. In addition to steam turbines, Mosaic has also standardized on this bearing isolator design for all rotating shafts up to 3.00" diameter on pumps (Ash, Wilfley, Denver-Orion, GIW), gearboxes (Falk, Marley, Philly, Lightnin'), and trunions. Shafts over 3.00" diameter are evaluated by the Rotating Equipment Engineer on a case-by-case basis. Bearings that are lubricated with oil mist are sealed with a contacting face-seal, magnetically-energized type of bearing isolator (see Figure 7).
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.
The payback period for a set of three labyrinth seals for one steam turbine was less than three weeks, with documented savings of $50,000 per year per turbine afterwards.
Labyrinth seals installed in July 2008 are still running fine as of the time of this writing, 24+ months later. Turbines that contain labyrinth seals and that were torn down for other reasons show absolutely pristine condition of the bearings and bearing chambers.
Oil samples are taken from the turbines and evaluated weekly and show ZERO water in the oil on steam turbines that have modern labyrinth isolators installed.
Chris Rehmann is Business Development Manager for AESSEAL in Knoxville, Tennessee. He earned his engineering degree from Notre Dame and worked for 15 years in various management positions with an oilfield engineering services company in Alaska, Egypt, Indonesia, Australia, and Saudi Arabia, before coming to AESSEAL in 2002. Chris has authored a number of technical papers on bearing protection and seal support systems, in Uptime Magazine and other publications. www.aesseal.com
John Harms is Reliability Engineer at Mosaic Fertilizer, Louisiana Operations,Faustina, LA. He has 2 years of oil & gas industry field maintenance experience and 34 years of maintenance experience.
