SEAL RING DESIGN AND SPECIFICATIONS
The purpose of seal rings or wearing rings, as they are also called, is to provide a renewable seal
or leakage joint between a turbine runner and its casing. The small clearance between the
stationary and rotating rings limits the flow of water from the high pressure areas to low pressure
areas of the turbine. As the clearance increases over time, the amount of water flowing through
the seal rings increases, decreasing the efficiency of the turbine. As seal ring clearances increase
to 200 percent of the manufacturer's design clearance, the efficiency can decrease as much as 2
percent. Prior to a scheduled overhaul, the seal ring clearances and the design of the seal rings
should be analyzed. If the seal ring clearance has increased significantly or if the seal ring design
is questionable, replacement should be considered.
Seal ring design
Ideally, seal rings should last 20 years or more. This type of longevity is not always possible, but
to achieve the longest life possible, the seal rings must be constructed of materials that are
resistant to wear and corrosion. Wear can occur as a result of cavitation or abrasive erosion.
Abrasive erosion caused by silt or other suspended particles in the water is usually not a concern
in turbines that receive their water from deep reservoirs, but it can be a problem in pump-turbines
or turbines that receive their water from a canal or other shallow waterway. Cavitation erosion is
dependent on the amount of head of the unit, the seal ring clearance, and the hydraulic
characteristics of the unit. The occurrence of corrosion will depend on the seal ring material and
the quality of the water.
The galling resistance between the rotating and stationary ring materials must also be considered.
Under normal operating conditions, the rings should never touch. However, in some
circumstances such as the failure of the turbine guide bearing, failure of a seal ring, or during unit
alignment, they can come in contact. If contact is made between materials that are prone to
galling, the rings can be severely damaged, and in extreme cases, the rings may seize.
Over the years, seal ring designs have varied. In some plants, cast steel rings that were integral
parts of the turbine runner and casing were used. While cast steel is not prone to galling, it is
susceptible to cavitation erosion, abrasive erosion, and corrosion. The cast steel performed
acceptably in most circumstances, but as material technology improved, other materials were tried
in an attempt to achieve longer life.
In the 1950's and 1960's, the Bureau of Reclamation commonly specified hardened 400 series
martensitic stainless steel for both the rotating and stationary seal rings. These rings were much
more resistant to corrosion and cavitation erosion than the cast steel rings. The rings were heat
treated to different hardness values, apparently with the idea that it would prevent galling in the
event of contact between the rings. The rotating ring was a shrink fit on the runner, and the
stationary rings consisted of dovetail segments held in place with soft copper caulking in a
dovetail groove. This design was not successful as the copper would erode over time allowing
the segments to come loose. The stainless steel against stainless steel, even with the different