Axial-Thrust in Multistage Pumps
Most multistage pumps are built with single-suction impellers in order to simplify the design of the interstage connections. Two obvious arrangements are possible for the single-suction impellers:
1. Several single-suction impellers can be mounted on one shaft, each having its suction inlet facing in the same direction and its stages following one another in ascending order of pressure (see Figure 59). The axial thrust is then balanced by a hydraulic balancing device.
2. An even number of single-suction impellers can be used, one-half facing in one direction and the other half facing in the opposite direction.With this arrangement, an axial thrust on the first half is compensated by the thrust in the opposite direction on the other half (see Figure 60). This mounting of single-suction impellers back to back is frequently called opposed impellers.
An uneven number of single-suction impellers can be used with this arrangement, provided the correct shaft and interstage bushing diameters are used to give the effect of a hydraulic balancing device that will compensate for the hydraulic thrust on one of the stages.
It is important to note that the opposed impeller arrangement completely balances an axial thrust only under the following conditions:
• The pump must be provided with two seal chambers.
• The shaft must have a constant diameter.
• The impeller hubs must not extend through the interstage portion of the casing separating adjacent stages.
Except for some special pumps that have an internal and enclosed bearing at one end, and therefore only one seal chamber, most multistage pumps fulfill the first condition. Because of structural requirements, however, the last two conditions are not practical. A slight residual thrust is usually present in multistage opposed impeller pumps and is carried on the thrust bearing.