Radial force of double-suction split pump and its balance calculation
1. Radial force generation of double-suction split pump
The double-suction, split-open pump with a scroll-shaped press-out chamber under the optimal working conditions, the pressure in each section of the scroll chamber is uniform. When the pump flow rate is less than the optimal working condition flow rate, the liquid flow rate in the volute chamber slows down, and the absolute velocity of the liquid at the outlet of the impeller, which can be seen from the outlet velocity triangle, is greater than the absolute speed in the optimal working condition, and also greater than the volute Speed, the liquid flowing out of the impeller continuously hits the liquid in the volute chamber, so that the liquid in the volute chamber receives energy, and the pressure of the liquid in the volute chamber increases from the tongue to the inlet of the diffuser, as shown in Figure 2-42(a ) As shown;
When the flow rate of the pump is greater than the flow rate in the optimal working condition, contrary to the above situation, the absolute velocity of the liquid flowing out of the impeller is less than the absolute velocity in the optimal working condition, and also less than the liquid velocity in the volute chamber. The two liquids are in the volute chamber. As a result of the impact, the liquid in the volute chamber must continue to pay energy to increase the speed of the liquid flowing out of the impeller. In this way, the pressure of the liquid in the volute chamber gradually decreases from the tongue to the inlet of the diffuser, as shown in Figure 2— Shown in 42(b).
Radial force generation of double-suction split pump
Because the pressure in each end face of the scroll chamber is not equal, a radial force is generated on the impeller.
Because the pressure distribution of the liquid around the impeller is not uniform, it destroys the axisymmetric flow of the liquid in the impeller. The liquid flows out of the impeller less in places with high pressure, and more flows out from the impeller in places with low pressure. The amount of liquid flowing out along the circumference of the impeller is different, and the reaction force of the liquid acting on the circumference of the impeller is also different, which generates a radial force.
The radial force acting on the impeller is the vector sum of the above two radial forces.
In a segmented multistage centrifugal pump, when the pump's working condition leaves the optimal working condition, if the impeller is eccentric, a radial force acts on the impeller, and the magnitude of this force depends on the pump's working condition. And it increases with the increase of eccentricity.
Figure 2-42 Radial force distribution in the volute. When the flow rate is very small, the radial force will change unsteadily and rotate at a frequency much lower than the pump speed, which will cause the rotor to vibrate.
Practice has proved that for the volute pump under the same working conditions, the radial force also changes significantly when the rotor deviates from the center of the volute base circle and does not deviate from the center. This change depends on the size and direction of the eccentricity.
2. Calculation of radial force of volute double-suction split pump
The pressure outlet chamber is a volute-shaped pump. When it deviates from the design conditions, the radial force is calculated as follows:
Calculation of Radial Force of Volute Double Suction Split Pump
Among them, K—radial force coefficient can be obtained by the following formula:
Calculation formula of radial force of volute double-suction split pump
When the pump is running under design conditions. According to the above formula, the radial force coefficient is equal to zero, and the radial force coefficient is the largest at zero flow, that is, K=0.36. The size of the coefficient K is also related to the type of pump. In some cases, the actual value of K is larger than that obtained by the above formula. K may reach 06 at zero flow.
3. The harm of radial force
The radial force is proportional to the diameter of the outlet of the impeller and the width of the outlet of the impeller, and its influence will increase with the increase of the pump size.
When the radial force causes a large deflection of the shaft, it will cause rapid wear of the seal ring and the shaft sleeve.
At the same time for the rotating shaft, the radial force is an alternating load, and a larger radial force will damage the shaft due to fatigue.
Therefore, the balance of radial force is very important, especially for high-energy pumps with larger sizes and higher heads.
4, single-stage volute double-suction split pump radial force balance
The radial force balance of the single-stage volute pump can be realized by using double volutes or adding a guide vane, as shown in Figure 2-43(a) and Figure 2-43(b).
Radial force balance of single-stage volute double-suction split pump
In the double volute, although each volute does not eliminate the radial force, the two volutes are separated by 180. Symmetrically arranged, the radial forces acting on the impeller are balanced with each other.
If the guide vane is used, although the radial force can be balanced, the structure of the pump is complicated.
5, the balance of the radial force of the split multistage pump
The balance of the radial force of the volute type multi-stage pump (that is, the split multi-stage pump) adopts the method of inverting the volute extruding chamber, that is, arranging the respective volute extruding chamber in each two adjacent stages The difference is 180°, as shown in Figure 2-44.
Radial force balance of split multistage pump
The radial force balance structure diagram of the split multistage pump
In this way, the radial forces acting on the two adjacent impellers differ by 180° and cancel each other out.
Because these two forces are not in the same plane perpendicular to the axis, they form a force couple whose force arm is equal to the distance between the two impellers. This force couple needs to be balanced by a force couple composed of the radial forces of the other two impellers, or a force couple composed of the bearing support reaction force.
This kind of radial force balancing method is suitable for split-open multistage pumps with an even number of stages and a single-suction impeller. For an odd number of stages, the first stage impeller is a double-suction, split, multi-stage pump. The method of radial force balance is that the first stage volute is made into a double volute. Stagger by 180°.
For larger-sized split-type multi-stage pumps, the balance of radial force can also consider the method of using double volutes, as shown in Figure 2-45.
For segmented multi-stage centrifugal pumps, the eccentricity of the impeller to the guide vanes must be minimized to reduce the radial force.
