Leakage from the turbine main shaft seal can result in significant losses in efficiency and revenue at a hydroelectric facility. By understanding the problem and the solutions available, hydro plant owners/operators can recoup some of these losses.
By Stephen N. Wilson
Owners of hydroelectric facilities who simply accept efficiency losses resulting from turbine shaft seal leakage are shortchanging themselves and the entire industry. Many operators realize these losses are occurring but accept them as an inherent shortcoming of the turbine. And even if they do wish to rectify the problem, they struggle to justify the investment required. The investment required varies depending on the shaft size, speed, and pressure of the application. However, the return on investment in terms of reduced unit downtime and increased unit availability and generation can be measured in months.
Despite this age of striving for increased efficiency and decreased costs, little attention is paid to the turbine main shaft seal. Selecting the wrong seal type for a particular application can be extremely costly in terms of increased downtime and high leakage.
This situation raises three key questions: What are the reasons for this seeming lack of attention to this issue? Who selects the seal or seal type? Is this decision made at an early enough stage in the turbine design process, or is it left to the end?
The cost of leakage
Excessive leakage from a hydro turbine creates unnecessary damage and cost. This damage can come in the form of flooding of the turbine guide bearing or corrosion of the surrounding structure and degradation of the civil works. All seals must leak to survive, and the amount of leakage may range anywhere from imperceptible (less than 1 L/min) to extreme (defined as a flow rate approaching or exceeding the capacity of the headcover drain). In most cases, excessive leakage from the main shaft seal is due to selection of an incorrect seal type or a badly designed seal.
Many people can be involved in the sealing of a turbine shaft. All turbine manufacturers have their own seal designs, and there are specialist companies whose sole purpose is to provide a sealing solution for a particular application. The specialist companies typically will design a system for a particular hydro turbine, thus avoiding the pitfalls of a “one size fits all” solution.
Unfortunately, some turbine operators believe that leakage of 400 L/min, or even more, is acceptable. In fact, acceptable leakage is only the amount required to sufficiently cool and lubricate the sealing faces. So where did this perception come from, and how do we overcome it? Education is part of the answer.
Leakage at a pumped-storage facility that had been operating for several decades averaged 400 L/min for each of the two turbine-generating units. To put this into perspective, leakage of 400 L/min equates to 210,240 m3/year. This is enough water to fill 84 Olympic-size swimming pools every year or enough water to supply the average family household for about 800 years.
The turbine manufacturer told the facility owner that this amount of leakage was to be expected because of the arduous conditions under which the seal had to operate. These conditions consisted of a circumferential velocity of up to 15 m/sec and 8 bar pressure in both water and air. While this may have been the case when the turbine was originally designed, this amount of leakage should not be considered acceptable today in light of the materials and technology available.
Seals at this facility consisted of three-tier radial carbon segment seals. The water that leaked while the units were operating had to be pumped from the turbine pit into the common drain and then passed through separators before being discharged into the lower reservoir. The irony is that, because this is a pumped-storage facility, the leaked water made no contribution to generation and then had to be pumped back to the upper reservoir, only to appear as leakage again. Not the most efficient use of power or water!
Another negative effect resulting from seals that allow water to leak profusely is that these seals will allow air to pass with even greater ease. Thus, when the unit is running in the “blown down” condition (i.e., the water level is depressed and the turbine is spinning in air), the air escapes past the shaft seal. This requires all the facility’s 3 MVA compressors to run at full capacity just to keep the water level depressed. This provides yet another example of inefficient use of available power at this facility.
A secondary effect of seal leakage is corrosion. This is not easy to quantify, but excessive leakage causes corrosion that leads to bacterial and algae growth, which create a significant health risk for personnel as well as equipment blockages and malfunction.
|Excessive leakage from the turbine main shaft seal wastes water that could be used for generation and can cause damage to the plant, including corrosion of the surrounding equipment.|
At this facility, the cumulative losses involved equate to about a 1% reduction in unit efficiency. The owners of this facility are now evaluating the situation, with a view to replacing the seals during the next major unit outage.
All of the effects described above are as a direct result of the wrong seal type being selected for this application. This was determined through calculations, based on pressure and flow, which proved that the existing seal was totally ineffective. But, most people in the hydro industry may not even realize they have a choice with regard to design of the main turbine shaft seal.
Therefore, education must take place for several people involved in the process:
- - Owners: Most owners do not routinely count the cost of leakage, and some may not realize they have such losses. Generally, because hydro plant operators may not be able to quantify the losses from leakage, owners cannot justify the expenditure needed to reduce the problem.
- - Operators: Most operators, particularly well-established ones, accept high leakage as “normal,” and many have installed extra pumps to deal with the problem. Whole life costs related to the leakage have not been fully evaluated.
- - Buyers: Most buyers, their agents, or consulting agents do not address levels of efficiency for the shaft seal, apart from longevity of the wearing parts.
- - Turbine manufacturers: Most turbine manufacturers have a “best fit” type seal that may or may not be correct for the application. The type used depends on the space available and is, in the main, based on 1960s (or earlier) technology and the budget available. Turbine manufacturers also may not have the expertise or resources within their organizations to dedicate to the shaft seal system.
The purpose of this article is not to provide specific details about the various types of seals available. Suffice it to say that, in the long term, a fully balanced mechanical seal allows more control and can be easily instrumented for remote operation.
In extreme situations, turbine main shaft seal leakage results in flooding of the turbine pit and costly unit downtime.
More power plants are becoming remotely operated, meaning an on-site maintenance team may not be available. This makes it even more important to install a sealing system that can be relied upon. A seal that allows for installation of the necessary instrumentation and controls lends itself to remote operation. Data such as seal face temperature, wear, and flow can be transmitted to the station control system to provide alarms, history, and trending. When correlated with other turbine data, such as mode of operation and running hours, this data becomes a useful tool for condition monitoring and predictive (scheduled) maintenance.
Turbine shaft sealing is a niche market, and there are companies that specialize in it and can provide the expertise and resources needed to provide a successful solution for any particular turbine application. In many cases, particularly during construction of a new hydro facility, such specialists may not become involved until very late in the process, which may then present design difficulties that could have been addressed and overcome at an earlier stage.
For example, during recent development of a hydroelectric project, S M Seals was consulted very late in the process. This resulted in a change in design for the turbine main shaft seal that affected the design of the turbine, after the manufacturing process had begun. Earlier involvement of the company, such as during the turbine design process, could have helped avoid this situation.
Obviously, the greatest incentive to make a change is money, and very few hydro plant owners/operators fully evaluate lifetime costs of their sealing systems.
A correctly designed sealing system can reduce leakage by 90% or more, and there are systems available that are maintenance-free throughout the life of the wearing components, which can be as much as 10 years.
Steve Wilson is technical manager for S M Seals Ltd., which specializes in designing, testing, manufacturing, and supplying mechanical sealing solutions for the power generation industry.