Multistage centrifugal pumps are commonly-used pieces of turbomachinery for applications with high head and flow duty requirements. So, the improvement of their technological indexes is greatly significant for saving energy, ensuring production and raising economic benefit.
Since the axial force in high-pressure multistage pumps reaches tens of tons, it is essential to balance hydraulic axial thrust. Even then, an axial bearing is required; and its size depends largely on the prediction accuracy of axial thrust and on the type of the balancing device. Therefore, the size of bearings and the axial thrust balance systems of multistage pumps are frequently determined on the basis of axial force measurements carried out on each particular type and hydraulic pressure of pumps.
In general, balancing devices are used for balancing the axial thrust acting on the rotors of multistage centrifugal pumps. Balancing devices create forces that oppose the axial thrust generated by the impellers. When dimensioned correctly, balancing devices are able to completely balance the axial thrust, so the required pressure automatically builds up in the radial gap due to an axial displacement of the rotor. Therefore, the axial clearance at the device depends on the axial force acting on the rotor, i.e. on the operation point of the pump.
Balancing devices have two gaps: a radial gap and an axial clearance. The mutual coupling of these two gaps allows automatic balancing of the axial force. According to the variation of operation point of a centrifugal pump, the axial thrust changes. The rotor moves left and right in the axial direction to be in a new equilibrium state, when the axial clearance changes with the movement.
If the axial displacement of the rotor is too large, the balancing device may contact with the casing to cause wear. Thus, the rotor vibrates and the stability of operation is destroyed. Therefore, when balancing device is designed, even small axial displacement must yield large changes in the balancing force. In previous studies, this performance was estimated by sensitivity coefficient.
Sensitivity factor represents the variation of balancing force according to the pressure distribution in front and rear shrouds of impeller, but does not represent variation of balancing force when a rotor moves. Therefore, it can be more correct to estimate the automatic balancing ability of a balancing device by the parameter representing the variation of balancing force when a rotor moves a unit length.
Han Pok Nam, an institute head at the Faculty of Mechanical Science and Technology, has decided leakage through radial and axial clearance, pressure difference and balancing force based on the theoretical analysis of the gap flow of a balancing device. Then, he has defined a new concept of stiffness coefficient and analyzed the relationship between stiffness coefficient and geometrical parameters.
Thus, he has found that the stiffness coefficient decreases with too large or too small sensitivity coefficient, and a reasonable sensitivity coefficient should be chosen around 0.5.
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