In most small machines there is a need to limit both contaminant ingress and oil leakage. Inexpensive lip seals are sometimes used for sealing at the bearing housing, but lip seals typically last only about 2,000 operating hoursthree months. When lip seals are too tight, they cause shaft wear and, in some cases, lubricant discoloration known as black oil. Once lip seals have worn and no longer seal tightly, oil is lost through leakage. This fact is recognized by the API-610 standard for process pumps, which disallows lip seals and calls for either rotating labyrinth-style or contacting face seals.
Small steam turbines often suffer from steam leakage at both drive- and governor-end sealing glands. Each bearing housing (Fig. 1) is located adjacent to one of these two glands, which contain carbon rings. It is a well-known fact that, as soon as the internally split carbon rings start to wear, high-pressure and high-velocity leakage steam finds its way into the bearing housings. Traditional labyrinth seals have proven ineffective in many such cases and only solidly engineered bearing protector seals now manage to block leakage steam passage.
|Fig. 1. Small steam turbine cross-section view|
(Source: Worthington-Turbodyne S.A.).
The bearing housing protector seal in Fig. 2 was designed for steam turbines. It incorporates a small- and a large-diameter dynamic O-ring. This bearing protector seal is highly stable and not likely to wobble on the shaft; it is also field-repairable. With sufficient shaft rotational speed, one of the rotating (dynamic) O-rings is flung outward and away from the larger O-ring. The larger cross-section O-ring is then free to move axially and a micro-gap opens up.
When the turbine is stopped, the outer of the two dynamic O-rings will move back to its stand-still position. At stand-still, the outer O-ring contracts and touches the larger cross-section O-ring. In this highly purposeful design, the larger cross-section O-ring touches a relatively large contoured area. Because Contact Pressure = Force/Area, a good design aims for low pressure. Good designs differ greatly from technologically outdated configurations wherein contact with the sharp edges of an O-ring groove will cause O-ring damage.
Fortunately, concerns as to the time it might take to upgrade to advanced bearing protector seals have been alleviated. In June 2009 Total Raffinaderij Nederland (TRN) asked for the installation of the bearing protector seal shown in Fig. 2 in one of its 350 kW/3,000-rpm steam turbines. No modifications were allowed on the existing equipment and installation of three LabTecta-STAX seals on the first machine had to take place during a scheduled plant shutdown in June 2009.
With no detailed drawings of the bearing housings available, the exact installation geometry could only be finalized after dismantling the Turbodyne turbine. One of the main problems was the short outboard length: less than 0.25 in. (6.35 mm) was available due to the presence of steam deflectors and oil flingers. But the manufacturers engineers were able to modify the advanced design in Fig. 2 to fit into the existing OEM labyrinth seal groove. Delivery was made within one week of taking steam turbine and bearing housing measurements and the turbine has been running flawlessly since June 2009.
|Fig. 2. Cross-sectioned half-view of a bearing housing protector seal for small steam turbines (Source: LabTecta-STAX, AESSEAL Inc., Rotherham, UK and Rockford, TN). |
Our point is that highly cost-effective equipment upgrades are possible at hundreds of refineries. However, superior bearing protector products for use in steam turbines must be purposefully developed. The type described here has important advantages compared with standard products typically used in pumps:
It is suitable for high temperatures.
It incorporates Aflas O-rings as the standard elastomer.
Extra axial clearance is provided to accommodate thermal expansion.
High-temperature graphite gaskets are incorporated in this design.
There should no longer be any reason for water intrusion into the bearing housings of small steam turbines at reliability-focused HPI facilities. HP
|The authors |
Heinz P. Bloch is HPs Equipment/Reliability Editor. A practicing consulting engineer with close to 50 years of applicable experience, he advises process plants worldwide on failure analysis, reliability improvement and maintenance cost-avoidance topics. Mr. Bloch has authored or coauthored 17 textbooks on machinery reliability improvement and over 470 papers or articles dealing with related subjects.