Eliminating moisture damage in steam turbines
01 December 2017
Moisture contamination is one of the most common and costly causes of premature bearing failures on steam turbines.
Chris Rehmann, Business development manager at AESSEAL, looks at the issue and how installing turbine-specific labyrinth bearing protectors can eliminate moisture contamination completely – achieving sustainability and significant long-term operational savings.
Steam turbine bearings operate in some of the most challenging conditions – extreme high temperatures, high transient thrust loads, long states of ‘idle readiness’ and an environment which is literally a cloud of steam.
It is, therefore, little surprise that moisture contamination of the lubrication oil is one of the most common and costly causes of premature bearing failures and a serious hindrance to achieving L10 life cycle.
Of all the operating environments, the state of ‘idle readiness’ presents the highest level of contamination risk to a turbine bearing and lube oil system, because the rotor and, in most cases, lubrication system is in a static state while live steam leaks past the carbon shaft seals.
Because these carbon seals function properly only when the shaft is rotating, moisture condenses on the cooler inner surfaces of the bearing housing and collects in the sump. The result is moisture contamination and, ultimately, bearing failures once the turbine is put into service. If the turbine has been idle for a very long period in these conditions, the bearings might already have reached stage five failure on the PF curve, a type of deterioration model used by reliability professionals that utilises a graph to represent the relationship between potential failure (P) and functional failure (F) of a piece of equipment.
Moisture contamination had presented a costly and disruptive challenge to US phosphate fertiliser producer Mosaic for over 40 years before it finally found a solution in AESSEAL, a specialist in the design and manufacture of mechanical seals, bearing protectors, seal support systems and gland packing.
Mosaic operates Terry GAF4 steam turbines as primary drivers at its ammonia plant, using steam produced by waste heat generators. Its turbines were fitted with Original Equipment Manufacturer (OEM) standard non-contacting labyrinth seals, which were failing to prevent the steam leaking past the carbon rings into the bearing housings.
By 2008 water contamination had presented the company with years of operational problems and repair costs. Mean Time Between Repairs (MTBR) for the turbines could be just four months, with bearing replacement on the Terry GAF4 costing $14,000.
Mosaic had trialled a series of redesigns and adaptations to try to improve the performance of the OEM seals, without success.
The solution to its long-standing problem came in the form of a labyrinth-style bearing protector which AESSEAL adapted specifically to the Terry GAF4 steam turbine.
Labyrinth bearing protectors have been gaining popularity and replacing lip seals in bearing housings at reliability-focused plants for more than 30 years.
The basic elements of any rotating labyrinth isolator are a stationary portion which fixes to the machine housing; a rotary portion which fixes to the shaft and a ‘shut-off’ mechanism which seals the oil chamber when the machine is stopped but allows the air in the oil chamber to expand outward during operation.
AESSEAL has developed LabTecta bearing protectors, featuring five key improvements to make the modern rotating labyrinth bearing protector more effective at containing lubricating oil and excluding moisture and other contaminants:
• Removable/replaceable ring for easy field-refurbishment
• Two shut-off o-rings that land on a smoothly-contoured surface
• Two drive o-rings on the shaft provide better drive and more stability
• Two water expulsion ports
• Two oil-retention mechanisms.
It took this improved design and adapted it to create an engineered solution specific to the Terry GAF4 steam turbine. This solution fits the narrow space envelope and special operating conditions of a steam turbine and featured two unique adaptations:
• A ring of graphite packing is placed on the side nearest the steam to help protect the Aflas o-rings from the intense heat
• The design of the rotating unit was changed to create a ‘steam deflector’ which repels the impinging steam.
In 2008, Mosaic installed three LabTecta bearing isolators on the Terry GAF4 turbine at a cost of $2,400. The results were unequivocal.
In the first two years alone Mosaic avoided at least three turbine repairs, saving a total $100,000 and achieving a payback period of less than three weeks.
From a MTBF as low as four months, the specifically adapted labyrinth bearing protectors operated successfully for the entire three-year duty cycle of the steam turbines, with weekly sample checks showing zero moisture in the oil.
This success led Mosaic to fit all steam turbines affected by moisture contamination with LabTecta bearing isolators. Ongoing savings are estimated at $50,000 per year, per turbine.
In addition to its steam turbines, Mosaic also standardised the modern bearing isolator design on all rotating shafts up to 3” diameter on pumps and trunnions. Shafts over 3” 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-energised type of bearing isolator.
The effectiveness – and in particular the cost-effectiveness - of using adapted labyrinth bearing protectors is evident year on year for Mosaic. It’s also worth noting that since 2008 the company has taken some modified turbines out of service for reasons unrelated to functionality – none has shown any sign of corrosion and conditions inside the bearing housings have remained pristine.
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