The pump shaft of the multi-stage centrifugal pump is equipped with two or more impellers in series, which can achieve a higher head than a general single-stage centrifugal pump; compared with a reciprocating pump such as a piston pump or a diaphragm pump, it can be pumped. Large traffic. The multi-stage centrifugal pump has high efficiency and can meet the needs of high lift and high flow conditions, and has been widely used in petrochemical, chemical, electric power, construction, fire protection and other industries.
Due to its particularity, multistage centrifugal pumps have different and higher technical requirements in terms of design, use and maintenance than single-stage centrifugal pumps. It is often the negligence or inconsistency of people in some details, which makes the abnormal wear, vibration, and axle hung frequently occur after the multi-stage centrifugal pump is put into use, which also causes downtime.
1 design aspects
1.1 The underlying structure
The basic structure of the commonly used multi-stage centrifugal pump has two types: horizontal open type and segment type or multi-stage series type. The structural feature of the horizontal open type is 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 channel 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 also fixed in series by the fixed rod. The advantage is high pressure resistance. It is not easy to leak, but it is necessary to disassemble the imported pipe during maintenance, which is difficult to disassemble and assemble. It is generally believed that the horizontal open multistage pump has better stiffness than the segmental multistage pump and the pump vibration value is low.
In the suction chamber structure, the horizontal open 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 open multistage pump generally adopts a volute structure; however, due to the asymmetrical shape of the volute, the shaft is easy to be used. Bending, in the segmented multistage pump is limited to the first and the tail section can also use the volute, while in the middle section the guide wheel device is used to carry out the energy conversion between the primary impeller and the secondary impeller.
The first stage impeller of the multistage pump is generally designed as a double suction impeller, and the other impellers are designed as a single suction impeller, especially for mediums with high temperature, large flow and easy to generate cavitation.
For pumps with very high pressures, it is difficult to withstand the pressure of a single-layer pump housing. A double-layer pump housing is often used to make the pump body into a cylinder type. The cylinder type pump body is subjected to high pressure, and the horizontally open or segmented rotor is installed in the cylinder body.
China's relevant standard rules, high-pressure boiler feed pumps use single-shell segmental or double-shell tubular structure, 300MW and its generators for generator sets should generally adopt a double-shell tubular structure. The inner casing of the double casing adopts a segmental or horizontal open structure.
1.2 axial force balance
1.2.1 Common axial force balance measures
The balancing measures of the axial force of the multi-stage centrifugal pump generally include: the symmetrical arrangement of the impeller, the use of the balance drum device, the balance disc device, the balance drum, the balance disc combination device and the like. There are also double balance drum balancing mechanisms, 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. Axial force.
When designing the balance plate and balance drum of a multi-stage pump, a suitable balance line must be configured to meet the design requirements of the axial force balance device. In the multi-stage 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 balance ability cannot meet the requirements. The literature [1] also balances the drum device as an example, and proposes a calculation method for the balance pipe diameter.
For the multi-stage centrifugal pump, the balance plate and the balance plate seat are easy to be attached, which causes the balance plate and the pump to be damaged. The multi-stage centrifugal pump power wedge anti-wear balance plate is designed. The structure is similar to the principle of dry gas sealing of a centrifugal compressor: when the balance disc approaches the balance disc seat, the power wedge can generate a large opening force, thereby preventing the balance disc from adhering to the balance disc seat. After running the test, the balance plate works normally, and the working surface has no wear and scratches. It can be seen that the new power wedge anti-wear balance plate can effectively prevent the balance plate and the balance plate seat from being fitted. The power wedge balancing disc not only prolongs the life of the balance disc, but also reduces the leakage of the balance disc gap, saving energy and reducing consumption.
According to some people, the axial force of the multi-stage pump is caused by the fact that 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 make the impellers enter the water on both sides. There is no need to set up a balance disc, a balance drum, etc., and there is no need to consider the amount of axial yaw.
1.2.2 Limitations of balance disc and balance drum mechanism
a) Variable working condition: When the pump starts and stops, the instantaneous axial force is supported by the direct contact between the balance disc and the balance disc seat. The friction may cause the balance disc, the seat to bite, dry and even the pump shaft is broken. The accident occurs; when the load is abrupt, the axial force changes, and the rotor also pulsates axially, resulting in a sudden change in the gap between the balance plate and the seat, which is prone to cavitation and vibration.
b) Liquid-solid two-phase flow medium: the medium pressure entering the balance plate, balance drum and other balancing mechanism is the output pressure of the pump. The pressure after the throttling is the inlet pressure of the pump, and the medium flows from the high pressure zone to the low pressure zone. Jet scouring, the solid particles in the liquid-solid two-phase flow medium will quickly abrade the balance plate, seat and other moving and static parts of the bad balance mechanism, and finally the pump can not operate normally.
1.3 Axis deflection
Multi-stage centrifugal pump shaft deflection is too large, it is easy to cause abnormal vibration, holding shaft, mechanical seal sealing surface uneven force and failure, etc., should design the radial force generation, minimize the pump shaft in operation Deflection value. The measures considered in the design are generally:
a) Multi-stage 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.
b) The number of stages of the pump impeller should not be too much. If necessary, increase the head 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.
c) When selecting a multi-stage centrifugal pump shaft material, it is preferable to select materials with good mechanical properties such as strength and stiffness, taking into account the needs of the type of medium and temperature.
d) When designing and calculating the diameter of the pump shaft, consider the factors such as transmission power, starting method, radial force, shaft deflection and related inertia load; consider the radial force that may be generated during non-design flow operation to resist bending deformation of the pump shaft Need.
e) Reasonably select the support point of the pump shaft.
1.4 Anti-vibration and vibration reduction considerations
The measures for anti-vibration and vibration reduction of multi-stage pumps that can also be considered in design are:
a) Control pump shaft deflection within the regular range.
b) Clearly require the pump shaft, impeller, etc. to perform dynamic and static balance tests.
c) The pump shaft of the multi-stage pump should be designed according to the rigid axis, and the working speed should be less than or equal to 0.75 times the first-order critical speed.
d) The impeller and the pump shaft are independently positioned in a single stage, and the impeller and the pump shaft are assembled by heating with an interference fit, which also improves the rigidity and critical speed of the rotor assembly.
e) When selecting the material such as the pump shaft and the impeller, select the material with good uniformity of the material itself, and select the material supply state and processing method that can ensure the uniform quality of the cross-section of the material.
f) Design suitable shaft and radial clearance to avoid vibration caused by abnormal friction of the rotor and stator and axial turbulence.
g) Multi-stage pump with balance disc to balance axial force, reasonable and correct design of the balance disc mechanism.
1.5 vertical multistage pump
For the vertical multistage centrifugal pump, the general axial force is considered downward in normal operation, but in the initial stage of driving, since the outlet pressure has not risen, the pressure difference between the front and the rear of the impeller has not been established, and there is an upward axial direction. Force, and some cause the axial up, accompanied by organic seals, bearing parts overheating, motor over current phenomenon, in case of serious jumps. Structurally, the bearing bushing and the shaft should be relatively fixed so that the upward axial force is also balanced by the thrust bearing.
The balance plate device with automatic adjustment of axial force is too large in structure and requires a pressure relief pipe to be installed in the deep well submersible pump limited by the caliper. The axial force balance problem has always been the design of high lift deep well submersible pump. A problem in the middle. An axial force balancing method was introduced to expand the diameter of the impeller front cover of the deep well submersible pump to the edge of the inner wall of the pump so that the diameter of the impeller reaches a maximum value under the same diameter and the diameter of the rear cover of the impeller is appropriately reduced. The axial force on the impeller is perfectly balanced.
Another new type of axial force balance device is introduced, which is equipped with a pair of dynamic and static friction pairs after the final stage impeller, the moving ring rotates with the impeller, the static ring does not rotate, and the front end of the end seal pair is the high pressure liquid at the exit of the final impeller. After the end face sealing pair is connected with the atmospheric pressure or the low pressure zone of the pump inlet, the axial force of the high and low pressure difference is formed by the seal. The new balanced shaft sealing device not only balances the axial force but also has no leakage at all. It is mainly suitable for deep well submersible pumps and segmental multistage pumps. After using this device, the total efficiency of the pump can be increased by 3%-6%.
1.6 Multistage centrifugal pump for conveying liquid-solid two-phase flow
1.6.1 Axial force balance
The segmental multi-stage slurry centrifugal pump conveying the medium such as mortar and slurry, 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 Close together, 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 the friction causes cracking, which has caused many malignant accidents. In order to extend the overhaul life of this pump and slow down the wear rate of the seal gap, the following measures have been taken in design:
1 Improve the balance mechanism of the pump to make a balanced 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 is also prolonged.
2 Impeller, seal ring, bushing, guide bushing, balance disc, balance disc seat, etc. are spray-welded.
In the first phase of the Hualu Hengsheng domestically produced large nitrogen fertilizer project, the high-pressure ash water pump adopts a segment-type multi-stage centrifugal pump, and the axial force balance device adopts the method of “balance drum + thrust bearing bush†due to axial force. The balance is not good, the strength of the pump shaft is not designed enough. In the course of use, there are many accidents such as balance drum damage, bearing bush burnout, axle hung, and broken shaft. The horizontal open-type multi-stage centrifugal pump is adopted. The symmetrical arrangement of the impeller automatically balances most of the axial force. The residual axial force is absorbed by the thrust bearing. There is no balance mechanism such as balance disc and balance drum. The field is in good condition. Performance indicators fully meet the requirements of use.
1.6.2 Interstage and shaft end seals
In order to overcome and avoid the abrasion of the hard particles in the liquid-solid two-phase flow medium between the rotating part and the stationary part, all the pump body sealing rings, the throttle sleeve and the sealing sleeve of the multi-stage pump adopt a reverse spiral groove sealing structure. Reduced particle abrasion.
A combined seal structure with a contactless labyrinth spiral seal and a mechanical seal is also used at the shaft end, which is particularly suitable for liquid-solid two-phase flow media.
1.6.3 The flow rate should be reduced from the pump speed, pump structure and other aspects to reduce the medium flow rate, and also reduce the erosion of the hard particles in the liquid-solid two-phase flow medium to the overcurrent parts of the multi-stage pump. The speed of the pump should be as low as possible, and it is not suitable to choose the speed of 1450 rpm.
2 use and maintenance aspects
2.1 Before pumping
When the transported high-temperature liquid suddenly enters the multi-stage pump cold pump body, the temperature of the pump body will change greatly, and the pump body and the rotor component will be deformed due to uneven heating and thermal deformation, and the wear-resistant parts will be There are only a small gap in itself that leads to abnormal contact. If the device is started in this situation, it will cause vibration, bite, and shaft hung due to overheating. It is also said that when the pump is used to transport high-temperature liquid, it must be fully warmed before starting. The pump can only be started when the pump temperature is consistent. Emergency start of the multistage pump in the cold state is not acceptable.
The high-pressure differential multi-stage centrifugal pump used to pump gray water on the coal-water slurry gasification device has many failures of bearing bushing and machine seal damage after being put into operation, that is, the preparation work is insufficient before each pump is turned on, and the pump and exhaust are performed. The method is incorrect [. After the improvement of the pump, exhaust and other work, there was no problem.
2.2 in operation
The multi-stage centrifugal pump balances the axial force by the balance mechanism such as the balance disc and the balance drum. The balance liquid flows out in the balance device, and the balance liquid is connected to the inlet end of the pump through the balance tube to ensure the normal operation of the pump:
a) The balance tube must not be blocked.
b) If fouling occurs in the balance pipe, it should be cleared in time.
c) Install a pressure gauge on the high pressure side of the balance tube to monitor the outlet pressure of the balance tube.
The multi-stage centrifugal pump for conveying slurry adopts the balance plate. When operating, it needs to inject high-pressure sealed water to make the balance plate and balance plate seat work in clean water to prevent the wear of slurry and hard particles against the balance plate and balance plate.
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Tag: Multistage centrifugal pump Use Repair
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