Axial shuttling is one of the most prevalent failure mechanisms in the single-stage, double suction pump design. When present, it has a disastrous effect on seal and thrust bearing life.
Theoretically, the double suction impeller allows for equal wear ring diameters on either side of the impeller, resulting in zero net axial thrust. This axial balance is one of the stated advantages of the double-suction impeller design. However, in reality axial thrust is rarely completely balanced; in fact, one of the greatest concerns in the single-stage, double suction pump design is axial shuttling.
Axial shuttling results from a disruption of the pressure regions on either side of the impeller. As the pressure regions intermittently grow weaker and stronger, an axial thrust is created. The figure below shows how intermittent disruption of the pressure regions biases the axial thrust in the direction where the pressure region is destroyed. This increases the propensity for thrust reversals.
As can be observed in the figure, discharge recirculation is the primary engine for As can be observed in the figure, discharge recirculation is the primary engine for axial shuttling. This is because the fluid swirl causes localized pressure drops that affect the net axial thrust of the rotor. The root cause for discharge recirculation is a combination of large areas that allow the formation of fluid swirl and operation removed from the pump’s best efficiency flow, causing a mismatch of the fluid and volute cutwater inlet vane angles.
There are two approaches that help to mitigate axial shuttling:
- Provide design changes that will reduce discharge recirculation
- Provide a constant, unidirectional thrust
Gaps A & C
The fluid swirl that affects the pressure regions on either side of the impeller can be straightened by forcing the fluid through a tight-fitting orifice of sufficient length. This orifice is formed through a redesign of the casing and impeller.
Gap A is the term used to refer to the orifice clearance and is designed as the radial clearance between the impeller exit shroud and the volute inlet shroud.
D3’ = Volute/diffuser inlet shroud
D2’ = Impeller exit shroud
Gap C, or Overlap, is the term used to refer to the orifice length and is the amount that the impeller exit shroud is overshadowed by the volute inlet shroud. The proper Gap A is approximately three (3) times the wear ring clearance. The optimum Gap C, which completes the flow straightening orifice, is four (4) to six (6) times the Gap A value. In the figure below, the picture on the left depicts the existing design on most single-stage double suction pumps where there is no Gap A or Overlap. The picture on the right represents the recommended design.
Bias Ring Design
The retrofitting of an old pump design to contain the proper Gap A and C can be a costly modification and may not be financially feasible for non-critical or low-energy applications. An alternative solution is to live with the fluid swirl but to redesign the pump to obtain uni-directional thrust for any operating mode by creating an axial pre-load on the pump rotor.
This preload can be achieved by designing the wear ring bores at different diameters. For a double suction impeller design, the impeller is in complete hydraulic balance when the wear ring bores have the same diameter. By arranging the bores at different diameters, a continuous unidirectional thrust is created:
Taxial = axial thrust (lb)
Dring,2 = larger wear ring bore (in)
Dring,1 = smaller wear ring bore (in)
Pd = discharge pressure (psi)
Ps = suction pressure (psi)
Although the bore sizes are different, the design clearance must be maintained on both sides. If there is sufficient material on the impeller hub, this can be achieved by machining the impeller hub on one side of the impeller and providing a ring with an undersized bore. Alternatively, an impeller ring can be added on one side and a wear ring with an oversized bore can be installed.
The larger wear ring bore diameter should be designed on the inboard end of the pump to place the shaft in continuous tension. The design usually requires a larger thrust bearing to accept the continous unidirectional thrust.