Axial thrust direction
Axial thrust direction
(OP)
When starting a vertical volute pump against a vertical rising main open ended with no valves from a wet sump will the hydraulic thrust ever act upwards? Conversely when pump stops could an upward thrust occur as the water will then case the pump to turbine?





RE: Axial thrust direction
For reverse flow turbining, it is difficult to say what the axial hydraulic thrust load direction will be. In a volute casing, the reverse flow won't follow the reverse of pumping direction through the casing spiral but will go directly into the nearest impeller channels. The shaft runaway speed, which is a function of specific speed, may influence the direction of axial thrust loading. A review of complete pump characteristic curves is needed to determine head and rotating speed for turbining operation which might enable an evalution of thrust magnitude and direction. Presence or absence of impeller balance holes might be a factor in transiently equalizing pressures between flow channels and the back (hubside) of a shrouded impeller.
RE: Axial thrust direction
1.Fang,K.S.,1978, "Research and Field Study Solve Problems of Upthrust in Vertical Turbine Pumps", Power, Sept. pp.118-119
2.Miyashiro,H.& Takada,K.,1972,"Axial Hydraulic Thrust Caused by Pump Starting", ASME J.Basic Engineering,Sept.pp.629-635 and Dec.pp.947-948
3.Tsukamoto,H.& Ohashi,H.,1982,"Transient Characteristics of a Centrifugal Pump During Starting Period",ASME J.Fluids Engineering, V.104,Mar.pp.6-14
Fang says momentary upthrusts require a shrouded impeller, an empty discharge column allowing the startup to track a low resistance(high flowrate) path to rated flow and a short suction line so weight of rotor assembly can't suppress the upthrust movement.
Miyashiro's tests showed a significant effect of impeller balance hole diameter and location on the flowrate at which axial thrust goes from positive (downthrust) to negative (upthrust)for a 2-stage vertical pump. They agreed with Fang that axial thrust behavior depends on pump specific speed.
Tsukamoto found that head coefficient is very much higher on initial startups than steady-state head providing an impulsive pressure difference across the impeller. A parameter controlling transient chacteristics of the tested pump was Nf*Tna where Nf is final running speed and Tna is nominal acceleration time from standstill to 63.2 percent of final speed.
For centrifugal pumps experiencing many startup cycles during their lifetime, reversed axial hydraulic thrust loading would be or should be a concern for thrust bearing damage that likely applies to horizontal as well as vertically mounted centrifugal pumps. Rolling element thrust bearings would seem to be even more vulnerable to reversed axial thrust than fluid film bearings. What do centrifugal pump manufacturers and or pump users do to evaluate thrust bearing damage potential for reversed thrust loading during startups???
RE: Axial thrust direction