by Hydraulic Institute
December 17, 2011

Q.  How is the axial thrust for an end suction 
impeller calculated?

A.  In a horizontal pump, the axial thrust is a net force acting through the rotor on the thrust bearing, including dynamic loading from pressure and momentum acting on the impeller and other rotor components.

The net axial thrust is the sum of all effects. The appropriate equations for the effects of pressure on axial thrust, as shown in Figure 1.3.5.2.1a, must be used with the equations for the effects of fluid momentum change on axial thrust, shown later.

The forces creating thrust on an enclosed impeller are due to the difference in pressure distribution on the front and back shrouds and a force from the momentum due to the flow's change of direction through the impeller. A standard calculation method for a single-stage casing to determine the axial thrust force (developed and acts on the impeller in a direction parallel to the shaft) can help answer the question. This specification is for single-stage single-suction pumps only (specific speed range of 10 to 67 [500 to 3,500]) with enclosed impellers having no back vanes with a plain horizontal ring(s) and with a diametral clearance of 0.25 to 0.5 millimeter (0.010 to 0.020 inch). The complexity of the interaction of inter-stage pressures, casting tolerances and machining tolerances does not make having an overall method of axial thrust calculation for multistage pumps practical.

For vertical pumps, the impeller's and shaft's weights should be added to the axial thrust. Depending on construction, the coupling's, driver motor's or balancing device's weight may also be added.

The values of axial thrust are for 25 to 125 percent of best efficiency point (BEP) flow. Within this range, maximum axial thrust is developed and determined by:

 

The axial thrust factor (Figure 1.3.5.2.1b) is derived from an assumption of a free vortex between the casing wall and impeller shroud.

The liquid velocity is one-half the impeller's peripheral velocity. Values were adjusted using empirical data.

In single-suction impellers, the axial force related to the incoming fluid's momentum should be considered, particularly at points beyond BEP. This only applies to single-suction impellers with radial or mixed flow discharges. The incoming liquid has mass and travels at a relatively high velocity. The momentum change in the impeller creates an axial force that should be taken into account in the overall axial thrust calculation.

For information on other impeller types see HI Standard ANSI/HI 1.3 Rotodynamic (Centrifugal) Pumps for Design and Application available at estore.pumps.org/.

 

Q. When is a regenerative turbine pump used, and how does it work?

A.  Regenerative turbine pumps are characterized by a low rate of flow and high head. They develop three to four times the pressure of a rotodynamic pump for the same impeller diameter and speed. The rotodynamic pump develops its pressure as a result of the difference of discharge and suction velocities. The impeller of the regenerative pump depends on the specific drag of the impeller and its peripheral velocity.

This design uses 20 to 40 peripheral or side channel vanes or machined buckets that are manufactured integral, and the impeller imparts energy to the pumped liquid. The liquid travels in a helical pattern through the impeller vanes and corresponding flow passages. The liquid pressure increases uniformly through the passages from inlet to discharge (Figure 1.1.5aa).

 

The vanes allow the liquid to re-enter the impeller as it rotates. Clearance between the impeller and the casing wall is important, and efficiency decreases with wear . However, the pump is compact for a low rate of flow and high pressure.

 

Q.  What is a balancing drum, and how is it used?

A.  A balancing drum is often included in a multistage pump to offset or balance the high axial force when all impellers face in the same direction. See Figure 1.3.3.5.2.

The drum or cylinder is made with the same diameter as the impeller wearing ring and is vented to a lower pressure. This offsets the higher pressure behind each of the impellers.

 

Pumps & Systems, December 2011 

 

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