What should I know about axial thrust for single suction impellers?
Axial thrust is a very important aspect of pump reliability. The net axial thrust of an impeller is the difference between forces acting on the back and front shrouds. The large surfaces of an impeller allows pressure to act on the shroud areas which results in high loads on the bearings in the axial direction.
A number of hydraulic effects determine these forces. These can include the positioning of the impeller in an axial direction in relation to the volute and leakage past the ring fits. These pressure changes are significant due to the relatively large shroud areas.
Different styles of impellers have very different axial load characteristics. The open impeller has no shrouds for the differential pressure to build upon and therefore has very low axial thrust loading. However, semi-open impellers actually have the worst axial thrust characteristics. This is because their single shroud allows the discharge pressure to build up across the whole surface. Meanwhile, on the other side of the shroud the pressure ranges from the section suction pressure to the (much higher) discharge pressure as you move out radially. This differential pressure can often be in the thousands of pounds and is directly applied to the bearings.
Closed impellers are more balanced because the pressures act on both front and back shrouds. Thrust balancing of some type is still required unless very large axial thrust bearings are used. Balancing can be achieved by incorporating back vanes on the rear shroud and/or balance holes in the impeller. The back vanes pump the liquid from behind the impeller towards the outside diameter. This reduces the pressure near the shaft and increases the pressure towards the discharge diameter, resulting in a matching thrust profile on the front of the impeller. This method is often used on semi-open impeller designs. Balance holes through the rear shroud of the impeller reduce the pressure at the back of the impeller by bleeding to the low pressure at the suction side on the impeller.
For pumps in a vertical position, the weight of the impeller and shaft should be added to the axial thrust. Also, depending on the construction, the coupling weight, the driver motor, or balancing device weight may also be added to the total axial thrust.