offer protection by forming a barrier between the metal and the electrolyte and preventing the
Erosion resistant coatings, in order to be effective, must be able to withstand the cutting action of the
suspended abrasive. A coating of neoprene has been proven successful for sand erosion protection. There
are other coatings available that have also been proven to be resistant to erosion, but many of these
coatings can be difficult to apply and maintain and may, because of coating thickness, restrict water
passages somewhat. Erosion resistant coatings should be chosen based on the design of the turbine or
pump and the severity of erosion.
2.4 Wearing Rings
The purpose of wearing rings, or seal rings as they are also called, is to provide a renewable seal or leakage
joint between a pump impeller or a turbine runner and its casing. As the name implies, these rings can
wear over time; and as the clearance increases, efficiency can decrease. As a general rule, when the
wearing ring clearance exceeds 200 percent of the design clearance, the wearing rings should be replaced or
renewed. If a design does not include replaceable wearing rings, it may be necessary to build up the
wearing ring area by welding or another acceptable process and machining back to the original clearances;
remachine the wear ring area and impeller or runner to accept replaceable wearing rings or, on small
pumps, replace the impeller and casing.
The location of the wearing rings varies depending on the design of the pump or turbine. Francis turbines
and most closed impeller pumps have two wearing rings, although some pump impellers may only have
suction side wearing rings. Propeller turbines, open impeller, and many semi-open impeller pumps do not
have wearing rings, relying instead on a close fit between the runner or impeller vanes and the casing to
The most common method of controlling leakage past a pump, turbine, or wicket gate shaft is by
the use of compression packing. The standard packing or stuffing box will contain several rings of packing
with a packing gland to hold the packing in place and maintain the desired compression. Some leakage past
the packing is necessary to cool and lubricate the packing and shaft. If additional lubrication or cooling is
required, a lantern ring may be also be installed along with an external packing water source.
Over time, the packing gland will have to be tightened to control leakage. To prevent burning the packing
or scoring the shaft when these adjustments are made, most compression packings contain a lubricant. As
the packing is tightened, the lubricant is released to lubricate the shaft until leakage past the packing is
reestablished. Eventually, the packing will be compressed to a point where no lubricant remains, and
replacement is required. Continued operation with packing in this condition can severely damage the shaft.
When packing replacement is necessary, remove all of the old packing. If the packing box is equipped with
a lantern ring, this also must be removed along with all of the packing below it. With the packing removed,
special attention should be given to the cleaning and inspection of the packing box bore and the shaft or
shaft sleeve. To provide an adequate sealing surface for the new packing, a severely worn shaft or shaft
sleeve should be repaired or replaced. Likewise, severe pitting in the packing box bore should be repaired.