The earliest form of shaft seal was the so-called “packing box.” The main disadvantage of these boxes was that the packing material became hardened after years of use and often scored the liner and shaft.

Another type of seal was the Labyrinth Seal. This was a very simple seal, which mainly relied on the pressure above the crown of the runner to be very low. The seal is used on units with shafts of 6" dia or less, or on mini-turbines with a few KWs (e.g. Gilbert Gilkes turbines). The seal uses a pressed-in bush, which has to be replaced when the leakage gets too high.

The Carbon Segment type seal was invented by a Swede, Gustav Hunn. It consists of a series of carbon segments around the perimeter of the shaft. This seal type is very good in clean water areas such as Quebec, Norway, Sweden, where there is almost no silt in the supply water. The carbon segments do not rotate, and are pressed in radially by springs. On new segments, allowance has to be made for later interlocking of segments as radial wear occurs so initial leakage will be high. Carbon segments can also be disturbed by cavitation and pieces can break off due to brittleness. Then the seal will completely fail.


Design of Sterling’s seals and type to be used.
Seal design is influenced by the speed and pressure and by the available space to fit a seal. The seal is also influenced by the design of the machine (e.g. Kaplan - such a turbine could have springs on top pushing down on the seal). Normal flow rate for cooling water to the seal face chamber is 10 liters/min (40 galls/min) for every 100mm (4") of shaft dia, or as agreed with the customer. Hydrostatic feed requires much less cooling water than this. Because the wearing element is never in contact with the shaft or liner surface, Sterling does not ever need these replacing. Sterling offers a well-proven fiber reinforced resin type material which has been used successfully for over 40 years and has considerable advantages over carbon segment seals.

Sterling Advantages	Carbon Ring Disadvantages
Sterling fiber/resin seal faces are extremely robust and can even be dropped without damage.	Carbon faces are very fragile and are easily damaged during handling and installation as well as in service.
Seals are easy to install and dismantle and thereby offer reduced maintenance downtime.	Rings are difficult to re-place and require exact positioning to avoid future leakages.
Can operate with unfiltered water if necessary.	Very poor performance where water conditions are less than perfect.
Seals operate with low leakage even in the worst water supply conditions.	Rings will almost certainly have higher leakage than Sterling design.
Proven cases of seals which have never been changed in many years operation (see ref lists).	Carbon rings invariably require regular replacement involving expensive downtime.
Sterling can and do offer guarantee on the wearing face of at least 3 years of life.	Carbon manufacturers rarely offer guarantees in the same way that Sterling can do.

Return to main page