14.4 Axial thrust
The balance drum and external thrust bearing has proven to be a more reliable device than the balance disc, and this has led to its widespread adoption on large boiler feed pumps.
Normal practice is to react around 95% of the impeller axial load by the balance drum, leaving the remaining 5% to be carried by the double-acting tilting pad type oil-lubricated thrust bearing. This arrangement gives the essential security against vaporisation or foreign matter compared with the balance disc device. The preferred practice is to ensure that the residual thrust is unidirectional to avoid any possibility of axial shuttling. This is essential for pumps with mechanical seals to prevent possible over/under loading of the seal faces.
Theoretical formulae have been derived to calculate the range of the individual components of the axial thrust over the complete flow range. Although there may not be any large errors in the calculation of these individual thrusts, care is required by the designer to ensure that there is not an exaggerated overall error which could lead to an overloading of the thrust assembly. For all new designs, the CEGB requires axial thrust tests to confirm that the thrust bearing catalogue rating is never exceeded over the complete range of the pump operating envelope, taking into account both the effects of pump overspeed and internal wear. Measurements are taken using a simple load cell attached to the pad carrier within the thrust assembly.
Thrust bearings (Fig 4.59) can be of the 'flooded' or 'directed' lubrication type. For high speed applications, the directed arrangement offers considerable power savings by significantly reducing parasitic churning losses. In addition, the directed arrangement provides an increased oil wedge thickness, which leads to greater operational margins against bearing failure.