The multistage centrifugal pump is equipped with two or more impellers in series, which can achieve a higher head than a single-stage centrifugal pump; it can pump a larger flow rate than a piston pump or a diaphragm pump. The multistage centrifugal pump has high efficiency and can meet the needs of high lift and high flow conditions, and has been widely used in petrochemical, electric power, construction, fire protection and other industries.
In terms of design, multistage centrifugal pumps are different from other pumps, mainly considering the following aspects:
1.Structure
There are two kinds of multistage centrifugal pump which are horizontal split casing and segmented (or multi-stage series). The horizontal open type structure is characterized in that the upper and lower pump bodies are butted through the horizontal split surface of the shaft center, and the inlet and outlet pipes, part of the volute and the flow passage are cast on the lower pump casing, which is convenient for maintenance and repair, and does not need to be disassembled during maintenance. The pump line can be removed directly from the upper housing of the pump. The segmental structural feature is that each stage consists of an impeller located in the diffuser housing. The diffuser is connected by bolts and connecting rods, and the stages are fixed together by a fixed rod in series. The advantage is high pressure resistance. It is not easy to leak, but it is necessary to disassemble the imported pipeline during maintenance, which is difficult to disassemble.
In the structure of the suction chamber, the horizontal split casing multistage pumps generally adopt a semi-spiral shape, and the segment type multistage pumps mostly adopt a circular ring shape. The extrusion chamber of each stage of the impeller is easy to manufacture, and the efficiency of converting liquid kinetic energy into pressure energy is high. The horizontal split casing multistage pump generally adopts a volute structure; however, due to the asymmetrical shape of the volute, the shaft is easy to be used. In the segmented multistage pump, the volute can be used only for the first and the tail segments, and the guide wheel device is used for the energy conversion between the primary and secondary impellers in the middle section.
For pumps with very high pressures, the housing of a single-layer pump is difficult to withstand the pressure. A double-layer pump housing is often used to make the pump body into a cylinder. The cylinder type pump body is subjected to a relatively high pressure, and a horizontally open or segmented rotor is installed in the cylinder body.
According to the relevant standards, the high-pressure boiler feed water pump adopts single-shell segment type or double-shell tubular structure, and the pump for generator sets of 300 MW and above should generally adopt double-shell tubular structure. The inner casing of the double casing adopts a segmental or horizontal split casing structure.
2.Balance of axial force
The method for balancing the axial force of a multistage centrifugal pump generally includes: a symmetric arrangement of the impeller, a balance drum device, a balance disc device, a balance drum, a balance disc combination device, and etc. There are also double balance drum balancer structures, such as high pressure boiler feed pumps. The impeller is symmetrically arranged or the balance drum is used. The axial force cannot be completely balanced. The thrust bearing is still required to bear the residual axial force. The multistage centrifugal pump is more balanced by a balance disc with automatic adjustment of the axial force. In a large nitrogen fertilizer project, the high-pressure ash water pump adopts a segment type multistage centrifugal pump, and the axial force balance device adopts the method of “balance drum + thrust bearing bush”. Because the axial force balance is not good, the strength of the pump shaft The design is not enough. In the course of use, there are many accidents such as bearing burnout, axle hung, and broken shaft. In the second phase of the company's large nitrogen fertilizer project, the high-pressure ash water pump adopts a horizontal split casing type multistage centrifugal pump. The impeller is symmetrically arranged, which can automatically balance the axial force, and has no balance mechanism such as balance disc and balance drum. Since it has been in use for more than 10 months, it has been in good working condition and all performance indicators have fully met the requirements for use. The segmental multistage slurry centrifugal pump for sending mortar, slurry and other medium, the flushing and abrasion of the slurry increases the annular sealing gap between the rotor and the stator of the pump, and the axial force of the balance disc and the balance disc seat Leaning together with sharp wear. The entire rotor component is axially turbulent, and the impeller collides with the middle partition plate and the sealing ring at high speed, and causes friction and breakage, which has caused many vicious accidents.
In order to extend the overhaul time of this pump and slow down the wear rate of the seal gap, it can be improved by:
(1) Improve the balance mechanism of the pump to make a balance disc holder (balance board) and two balance discs, as shown in Figure 1. This can reduce the leakage loss of the balance mechanism at the beginning of the pump operation, and ensure the safety and reliability of the pump in the later stage of operation, and the overhaul life of the pump can be extended.
(2) The impeller, seal ring, bushing, guide bushing, balance disc, balance disc seat, etc. are spray-welded. A multistage centrifugal pump is prone to the phenomenon that the balance disc and the balance disc seat are attached to cause the balance disc and the pump to be damaged. The multistage centrifugal pump dynamic mold anti-wear balance disc is designed, as shown in Fig. 2.
The structure is similar to the dry gas sealing principle of the centrifugal compressor: when the balance disc approaches the balance disc seat, the power mold can generate a huge opening force, thereby preventing the balance disc from adhering to the balance disc seat. After 9 months of running test, the balance disc works normally, and the working surface has no wear and scratches. It can be seen that this new dynamic mold anti-wear balance disc can effectively prevent the balance disc and the balance disc seat. The dynamic mode balance plate can not only prolong the service life of the balance disc, but also reduce the leakage of the balance disc gap, saving energy and reducing consumption. When designing the balance plate and balance drum of a multistage pump, a suitable balance line must be configured to meet the design requirements of the axial force balance device. In the case of multistage pump bearing temperature rise and bearing burnout accidents, many of them are caused by the small cross-flow area of the balance pipe, the excessive loss of pipeline resistance, and the unbalanced capacity. According to the multistage pump, the axial force is generated because all the impellers are water-absorbing on one side. It is proposed to improve the axial force balance by improving the pump body, the impeller and the stepped plate structure to achieve water balance on both sides of the impeller. It is necessary to set up a balance disc, a balance drum, and the like, and it is not necessary to consider the amount of axial yaw.
3.Axis deflection
The multistage centrifugal pump shaft deflection is too large, which may cause abnormal vibration, holding shaft, mechanical seal sealing surface uneven force and failure, etc. The radial force should be controlled from the design to minimize the operation of the pump shaft. Deflection value. In design considerations:
(1) Multistage pump with volute structure for diversion and energy conversion. The asymmetry of the shape of the volute tends to bend the shaft during operation. The adjacent two-stage volute should be offset by 180° to reduce the radial force.
(2) The number of stages of the pump impeller should not be too much. If necessary, increase the lift of each stage of the impeller to ensure the total lift, so as to minimize the length of the pump shaft by reducing the number of pump impellers.
(3) When selecting a multistage centrifugal pump shaft material, it is preferable to select materials with good mechanical properties of strength and rigidity while considering the needs of the type of medium and temperature.
4) When designing and calculating the diameter of the pump shaft, consider the factors of transmission power, starting method, radial force, shaft deflection and related inertia load: consider the radial force that may be generated when the non-design flow is working, and resist the bending deformation of the pump shaft. need.
(5) Reasonably select the support point of the pump shaft.
4.Anti-vibration damping
The measures for anti-vibration and vibration reduction of multistage pumps that can be considered in design are as follows:
(1) Control the deflection of the pump shaft within the specified range.
(2) Clearly require the pump shaft, impeller, etc. to perform dynamic and static balance tests
(3) Try to design the pump shaft of the multistage pump as the rigid shaft, and the working speed should be less than or equal to 0.75 times the first-order critical speed.
(4) When selecting the material such as pump shaft and impeller, the quality uniformity should be selected.
The material is selected to ensure a uniform cross-section quality.
(5) Design reasonable axial and radial clearance to avoid vibration caused by abnormal friction of the rotor and stator and the shaft.
(6) A reasonable and correct balance disc mechanism is designed for a multistage pump that uses a balance disc to balance the axial force.
