Axial thrust direction

[chash]

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?

 

[vanstoja]

I have seen experimental axial hydraulic thrust data for a number of end-suction, single stage centrifugal pumps of about 2500 specific speed (RPM,GPM.Ft.) where momentary upthrust occurred on startup that transitioned to downthrust when the pump reached full speed and produced normal head. I believe the transient upthrust was probably due to the force exerted by the incoming water turning 90 degrees through the near-radial impeller channels before full pump head was developed that developed normal downthrust to overcome the fluid turning force. The pipe inlet diameter was about 12 inches and the rotating assembly weighed about 1700 pounds. With a fluid film thrust bearing designed only for downthrust, the rotor lifted off the thrust shoes and approached or possibly hit an upthrust bumper ring of carbon-graphite material located above the thrust bearing. The bumper was primarily intended to handle upthrust loads from check valve slam pressure surges or mechanical shock loads.
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.

 

[vanstoja]

Information on transient axial hydraulic thrust loading of centrifugal pumps can be found in
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???

 

[checman]

When you say volute you are saying the equipment is a vertical pump not a turbine. When starting either something should supply sufficient back pressure to assure the pumps do not run off the curve even for a short period. On your vertical pump the high thrust loads and hydraulic unbalance with the corresponding vibration from such starts will shorten the service life of your pump. Back flow at shut down will most likely cause reverse thrust loads on your bearings. The aforementioned is why most pumps in similar applications have a check valve on the discharge piping. Regards Checman