Upthrust Problems on Multistage Vertical Turbine Pumps

Vertical turbine pumps are designed to operate with the pump shafts in tension (downthrust). Most pump and motor manufacturers caution users to not operate the vertical turbine pumps at conditions that result in “upthrust.” Upthrust conditions are usually encountered for a few seconds each time the pump is started; however, the pump should not be operated for periods longer than a few seconds in this condition. Due to lack of instrumentation on the pumps, it is often difficult to determine if the pump is operating in an upthrust condition. Pump impellers are thrust balanced to maintain a downthrust without excessive loads on the thrust bearings. Changing the pump operating conditions and the pumped fluid can significantly change the thrust balance on the pump, resulting in excessive downthrust that can overload the thrust bearings, or excessive upthrust conditions that can cause undesirable shaft behaviors. Therefore, it is very important to calculate the thrust loads when changing the pump operating conditions. Two case histories are presented that involved two different abnormal operating conditions that produced an upward thrust on the pump shaft. In both cases, the upthrust caused the pump shaft to buckle, resulting in failures of the mechanical seals and destruction of the shaft. Measured field data are presented for each case. Diagnostic techniques and instrumentation needed to obtain the field data required to solve these problems are discussed. AXIAL THRUST FORCES ON VERTICAL PUMPS In centrifugal pumps, axial hydraulic thrust forces result from internal pressures acting on the exposed areas of the rotating elements. These axial forces are the sum of the unbalanced forces acting in the axial direction and are affected by many variables including: impeller type, impeller shroud geometry, and the location of the impeller relative to the stationary walls (Lobanoff and Ross, 1992). These pressure forces acting on various impeller types are illustrated in many textbooks. Theoretically, the axial forces are balanced on the double-suction impeller; however, due to many factors, such as nonuniform flows and nonsymmetrical parts, the axial forces are rarely balanced in actual service. The pressure forces are not balanced on the single-suction impeller and thus substantial axial thrust forces are developed. Thrust bearings are normally required to restrain these axial forces. To reduce the axial thrust on a single-suction impeller, the thrust areas are equalized by drilling holes through the back shroud of the impeller and maintaining suction pressure in a chamber behind the impeller. These holes through the impeller shroud are referred to as balance holes. The arrangement aids in reducing the thrust because the pressure behind the balance holes will be less than the pressure above the wear ring, but greater than the suction pressure. Balancing holes are often considered undesirable due to leakage back to the impeller suction, which opposes the main flow. This leakage reduces the efficiency and sometimes can result in recirculation problems. Vertical turbine pumps were originally developed for pumping water from wells and have been called deep-well pumps, turbine well pumps, and borehole pumps. Although these pumps have been used in other applications, the name vertical turbine pump has been generally adapted by pump manufacturers (Karassik, et al., 2001). A typical multistage vertical turbine pump is shown in Figure 1. 46 UPTHRUST PROBLEMS ON MULTISTAGE VERTICAL TURBINE PUMPS