The axial thrust of centrifugal pump and its balance

Teacher Guan Xingfan's "Modern Pump Theory and Design Manual" pointed out that when the pump is running, an axial force acts on the rotor, which will pull the rotor to move axially. Therefore, we must try to eliminate or balance this axial force in order to make the pump work properly. The axial force acting on the pump rotor consists of the following components:

1) The axial force generated by the asymmetry of the front and rear cover plates of the centrifugal pump impeller points to the direction of the suction port of the impeller; 2) The direction of the axial force caused by structural factors such as the pillow block and shaft end depends on the specific situation; 3) The axial force caused by the weight of the rotor (such as a vertical pump) is related to the arrangement of the rotor; 4) Other factors affecting the axial force; 5) Dynamic reaction force, this force points behind the impeller. The main content of this article comes from the KSB website to see how Europeans understand axial thrust. picture The composition of axial thrust Axial thrust is the resultant of all axial forces (F) acting on the pump rotor, see Figure 1. picture Figure 1: Axial thrust of a single-stage centrifugal pump For a single-stage centrifugal pump, the axial thrust acting on the rotor includes: 1) Impeller axial force (F1): is the difference between the axial pressures on the discharge side impeller cover plate (Fd) and the suction side impeller cover plate (Fs), namely F1 = Fd-Fs 2) Momentum (FJ): is a force that continuously acts on the fluid in a specific space (see the principle of conservation of momentum in fluid mechanics), which is calculated as follows: FJ = ρ·Q·ΔVax In the formula, ρ is the density of the pumped medium Q is the pumping flow ΔVax is the difference between the axial components of the absolute velocity at the inlet and outlet of the impeller 3) The resultant pressure generated by the static pressure upstream and downstream of the shaft seal on the cross-section Ass of the shaft at the shaft seal, namely FWd = AWd·ΔpWd 4) Special axial forces, such as those generated when the eddy current conditions in the gap (backlash) between the impeller and the casing change during pump start-up.

5) Other axial forces, such as rotor weight (FW) on a non-horizontal centrifugal pump or magnetic pull force (Fmech) in a motor, etc. For the axial thrust composition of a closed impeller that is not hydraulically balanced (see Figure 2 for the calculation of the axial thrust of the impeller): picture In the formula, α is the axial thrust coefficient (based on experience) ρ is the density of the pumping medium g is the gravitational constant (acceleration of gravity) H is the lift D2m is the average impeller diameter, picture The axial thrust coefficient basically depends on the specific speed (ns, Pump Salon Note: here is the specific speed used in the EU). For radial and mixed flow impellers, the following calculation formula applies for the range 6 rpm < ns < 130 rpm: α=0.5 ×(Dsp/D2m)3 + 0.09 ≈ 0.1 ～ 0.3 where Dsp is the diameter of the controlled gap at the suction side impeller cover. picture Figure 2: Mixed flow unbalanced impeller This formula applies to a flow rate (Q) of 0.8·Qopt to 1.0·Qopt, and a gap width S=0.1 mm. If the gap width is doubled, α increases by about 8 %. pump salon note The standard unit of specific speed in European countries such as Germany and the United Kingdom is r/min, and its calculation formula is as follows: picture In the formula, Qopt is the flow rate at the best efficiency, in m3/s Hopt is the head at the best efficiency, in m n is the speed of the pump, in r/min ns is the specific speed, in r/min If it is converted into dimensionless specific speed, its calculation formula is as follows: picture In the formula, g is the acceleration of gravity, 9.81 m/s2. For multi-stage pumps with guide vanes (eg boiler feed pumps), the impeller axial force (F1) depends to a large extent on the axial position of the impeller relative to the guide vanes. In the case of an open radial impeller without a cover plate on the suction side, the axial force (Fs) is much lower than that of a closed impeller, which means that the impeller axial force (F1) is higher. An open impeller with a cutout in the impeller cover between adjacent impeller blades produces lower pressure (Fd) and therefore has a lower axial force (F1) than an impeller with a full discharge side cover Also lower, see Figure 13 for the impeller. For an axial propeller (axial impeller, see Figure 14), the axial thrust coefficient (α) is almost equal to the reaction force (rth). Axial thrust can be roughly calculated using the outer diameter (OD) of the axial impeller: picture The following ratios apply to the axial thrust F1 component of a geometrically similar pump at the specified rotational speed (n) and maximum impeller diameter (D2)

(see Figure 1: Axial thrust): picture In the discharge side and suction side gaps ofcentrifugal pump between the impeller and the casing, the rotation of the treated fluid has a great influence on the axial pressures (Fd) and (Fs). The average angular velocity of the rotating fluid being processed is about half the rotational speed of the impeller. Additionally, the inward clearance flow in the suction side (ie outer) clearance (side clearance) between the impeller and casing further increases side clearance turbulence due to Coriolis acceleration (compound centripetal acceleration). In the discharge side (i.e. internal) side clearance of the pump, there is an outward clearance flow due to the impeller not being hydraulically balanced (the process is the reverse of the above). The vortex motion is decelerated, resulting in an increase in the axial force Fd and therefore F1. The impeller axial force during start-up is higher than that during steady state operation because during start-up, the fluid being treated begins to rub due to disc friction caused by the braking action of the impeller cover or stationary housing surface. Rotate slowly. Various forms of axial thrust balance 1) Mechanical balance: Axial thrust is completely absorbed through thrust bearings (such as rolling bearings, tilting pad bearings). 2) Based on design: The impellers are arranged back-to-back and absorb the remaining axial thrust through thrust bearings. 3) Balance or reduce the axial thrust on a single impeller through the balance hole, see Figure 7 and Figure 9. 4) Balance the entire rotating assembly by a balancing device with automatic balancing function (such as a balance disc), or partially balance by a balance drum and double balance drum. 5) The axial thrust on a single impeller is reduced by the back vane, see Figure 8 for details. Mechanical axial thrust balance Rolling bearings to absorb axial thrust are the most efficient and economical solution.

However, in the absence of special balancing equipment of centrifugal pump, the use of particularly complex thrust bearings may be required, and the benefits in terms of efficiency and cost may be negated. Design-Based Axial Thrust Balancing For example, for an inline pump with a 4-stage impeller, there are two sets of impellers arranged back-to-back (two in series) to balance the axial thrust. If system conditions cause cavitation in both stages, the maximum axial thrust for each set may be twice the normal axial thrust. See Figure 5. However, if a more complex (impeller inlet and outlet) parallel connected back-to-back impeller arrangement is chosen, only normal axial thrust is generated per stage. See Figure 6. Elimination of axial thrust: 1) Adopt double suction impeller, see Figure 3. 2) Two-stage impellers are arranged back-to-back, see Figure 4. 3) Multistage, with impellers arranged back to back, see Figure 5. 4) Arrange impellers in parallel connection back to back (such as multi-stage pipeline pump), see Figure 6.