Mounting Stationary Pump Wearing Rings
In small single-suction pumps with suction heads, a stationary wearing ring is usually pressed into a bore in the head and may or may not be further locked by several set screws located half in the head and half in the ring (refer to Figure 41). Larger pumps often use an L-type ring with the flange held against a face on the head. In axially split casing pumps, the cylindrical casing bore (in which the casing ring will be mounted) should be slightly larger than the outside diameter of the ring. Unless some clearance is provided, distortion of the ring may occur when the two casing halves are assembled. However, the joint between the casing ring and the casing must be tight enough to prevent leakage. This is usually provided by a radial metal to metal joint (refer to Figure 43) arranged so that the discharge pressure presses the ring against the casing surface.
As it is not desirable for the casing ring of an axially split casing pump to be pinched by the casing, the ring will not be held tightly enough to prevent its rotation unless special provisions are made to keep it in place. One means of accomplishing this is to place a pin in the casing that will project into a hole bored in the ring or, conversely, to provide a pin in the ring that will fit into a hole bored in the casing or into a recess at the casing split joint.
Another method is to have a tongue on the casing ring that extends around 180° and engages a corresponding groove in one-half of the casing. This method can be used with casing rings having a central flange by making the diameter of the flange larger for 180° and cutting a deeper groove in that half of the casing.
Many methods are used for holding impeller rings on the impeller. Probably the simplest is to rely on a press fit of the ring on the impeller or, if the ring is of proper material, on a shrink fit. Designers do not usually feel that a press fit is sufficient and often add several machine screws or set screws located half in the ring and half in the impeller, as in Figure 41.
An alternative to axial machine screws being located half in the ring and half in the impeller is the radial pin, which goes through the center of the ring into the impeller (or from the inside of the impeller eye outward into the ring). This pinning method avoids having to drill and tap holes half in a hardened wearing ring and half in a softer impeller hub. For higher speeds, installing the radial pins from the inside of the impeller eye outward into the wearing ring captures the pins and protects against pin loss, especially at higher operating speeds. Generally, some additional locking method is used rather than relying solely on friction between the ring and the impeller.
In the design of impeller rings, consideration has to be given to the stretch of the ring caused by centrifugal force, especially if the pump is of a high-speed design for the capacity involved. For example, some pumps operate at speeds that would cause the rings to become loose if only a press fit is used. For such pumps, shrink fits should be used or, preferably, impeller rings should be eliminated.