UPTHRUST PROBLEMS ON MULTISTAGE VERTICAL TURBINE PUMPS

摘要

Vertical turbine pumps are designed to operate with the pumpshafts in tension (downthrust). Most pump and motor manufacturerscaution users to not operate the vertical turbine pumps atconditions that result in “upthrust.” Upthrust conditions are usuallyencountered for a few seconds each time the pump is started;however, the pump should not be operated for periods longer thana few seconds in this condition. Due to lack of instrumentation onthe pumps, it is often difficult to determine if the pump is operatingin an upthrust condition.Pump impellers are thrust balanced to maintain a downthrustwithout excessive loads on the thrust bearings. Changing the pumpoperating conditions and the pumped fluid can significantly changethe thrust balance on the pump, resulting in excessive downthrustthat can overload the thrust bearings, or excessive upthrust conditionsthat can cause undesirable shaft behaviors. Therefore, it isvery important to calculate the thrust loads when changing thepump operating conditions.Two case histories are presented that involved two differentabnormal operating conditions that produced an upward thrust onthe pump shaft. In both cases, the upthrust caused the pump shaftto buckle, resulting in failures of the mechanical seals and destructionof the shaft. Measured field data are presented for each case.Diagnostic techniques and instrumentation needed to obtain thefield data required to solve these problems are discussed.