Upthrust Problems On Multistage Vertical Turbine Pumps

Abstract

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 operate 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 cause the pump shaft to buckle, resulting in failure of the mechanical seals and destruction of the shaft. Measure field data are presented for each case. Diagnostic techniques and instrumentation needed to obtain the field data required to solve these problems are discussed.