Multistage centrifugal pump shaft is also equipped with a series of two also on the impeller, it is relative to the general single-stage centrifugal pump, but also to achieve a higher head; relative to the piston pump , diaphragm pump reciprocating pump, but also Pump larger flow. Multistage centrifugal pumps have higher efficiency and can meet the needs of high-lift and high-flow conditions and are widely used in petrochemical, chemical, electric power, construction and fire protection industries.
Due to its particularity, multi-stage centrifugal pumps have different and higher technical requirements in terms of design, use, maintenance and repair compared with single-stage centrifugal pumps. Often overlooked or neglected by people in some details, frequent failures such as abnormal wear, vibration, and axle holding after multistage centrifugal pumps are put into use have also caused the shutdown.
1 design
1.1 The basic structure
Commonly used multi-stage centrifugal pump has the level of the basic structure of open and multi-level or multi-stage type in two forms. Horizontal open structure is characterized by upper and lower pump body through the axis of the horizontal split surface docking, import and export pipe, part of the volute and runner casting in the lower part of the pump housing, maintenance and repair more convenient, maintenance without disassembly Pump line can be removed directly on the pump housing. The segmental structure features each stage consisting of an impeller located in the diffuser housing. The diffuser is bolted to the connecting rod, and the stages are also connected in series by the mounting rod. The benefits are high pressure resistance , Not easy to leak, but must be removed in the maintenance of imported pipe, demolition assembly difficult. Generally believed that the level of open multi-stage pump multi-stage multi-stage pump stiffness is good, low pump vibration.
Inhalation chamber structure, the level of open multi-stage pumps are generally used semi-spiral, multi-stage pump mostly circular ring. The impeller of each stage of the pressure chamber, due to the volute manufacturing convenience, the liquid kinetic energy into pressure energy is high, the level of open multi-stage pump generally adopts the volute structure; but because of volute shape asymmetry, easy axis Curved, in the multi-stage pump segment is limited to the first and last sections can also be used volute, while in the middle section is used guide wheel device for the first stage impeller and secondary impeller energy conversion.
The multistage pump's first-stage impeller is generally designed as a double-suction impeller. The other impellers at all levels are designed as single-suction impellers. This is especially true for media with higher temperatures, larger flow rates and cavitation.
For the very high pressure pump, with a single pump difficult to withstand the pressure of the shell, often using double-pump casing, the pump cylinder made into a barrel. Cylinder body to withstand high pressure pump, the cylinder installed horizontal open or segmented rotor.
China's standard rules, high-pressure boiler feed pump with single-shell or double-shell structure of the cylinder type, 300MW and its generating units should be generally used on the pump double-shell cylinder structure. The inner shell of the double shell adopts segmented or horizontal open structure.
1.2 axial force balance
1.2.1 commonly used axial force balance measures
Multi-stage centrifugal pump axial force balance measures are generally: impeller symmetrical layout, the balance drum device, balance disc device also balance drum, balance disc combination device and so on. There are also double balanced drum balancing mechanism, such as some high-pressure boiler feed pump. Impeller symmetrical arrangement or the use of balance drum device, the axial force can not be completely balanced, still need to install thrust bearing to withstand the residual axial force, multi-stage centrifugal pump is more balanced with an automatic adjustment of axial force to balance the disc Axial force.
In the design of multi-stage pump balance disc, balance drum and other devices, you must configure the appropriate balance of pipeline in order to make the axial force balance device to meet the design requirements. In the multi-stage pump bearing temperature is too high, the bearing burned accident, many are due to the balance of the flow area is too small, the pipeline resistance loss is too large, the balance of capacity failed to meet the requirements. Literature [1] also balancing drum device, for example, proposed a method of calculating the diameter of the balance pipe.
In view of the multistage centrifugal pump, the balance disc and the pump are likely to be damaged due to the fitting of the balance disc and the balance disc seat. A multi-stage centrifugal wedge anti-friction balance disc [2] is designed as shown in FIG. This structure is similar to the dry gas seal principle of centrifugal compressors: when the balance disc is close to the balance disc seat, the power wedge can generate a huge opening force, thus preventing the balance disc from being fitted with the balance disc seat. After nine months of operation test, the balance disc is working normally with no wear and scratches on the working surface. Thus, the new type of dynamic wedge wear balance disc can effectively prevent the balance disc from being fitted with the balance disc seat. The dynamic wedge balancing disc can not only prolong the service life of the balance disc, but also reduce the amount of leakage of the balance disc gap and save energy and reduce consumption.
Some people also based on multistage pump axial force is due to the impeller at all levels are one side of the water absorption, proposed by improving the pump body, impeller and interstage plate structure so that both sides of the impeller water, to achieve axial force balance, so Do not need to set the balance plate, balance drum and other agencies, do not need to consider the amount of axial play.
1.2.2 Balance plate, balance the limitations of the drum mechanism
a) Variable working conditions: When the pump starts and stops, the instantaneous axial force is borne by the direct contact between the balance disc and the balance disc seat. The friction may cause the balance disc, the seat to die, dry, or even the shaft is broken Of the accident; load mutation, the axial force changes, the rotor also axial movement, resulting in balance plate, the gap between the seat mutation, prone to cavitation and vibration.
b) Liquid-solid two-phase flow medium: The pressure of the medium entering the balance disc, the balance drum and other balance mechanisms is the output pressure of the pump. The pressure after the throttling is the inlet pressure of the pump, and the medium forms when flowing from the high pressure zone to the low pressure zone Jet erosion, liquid-solid two-phase flow of solid particles in the media will soon wear bad balance mechanism balance plate, seat and other dynamic and static parts, the final pump can not be normal operation.
1.3 axis deflection
Multistage centrifugal pump shaft deflection is too large, easy to cause abnormal vibration, shaft, mechanical seal sealing surface also caused by failure of failure, etc., should be designed to control the radial force generated to minimize the pump shaft is running The deflection value. The design considerations include:
a) Multi-stage pump with diversion and energy conversion using a volute structure. Asymmetric shape of the volute can easily bend the shaft during operation. The two adjacent volutes should be offset by 180 ° to reduce the radial force.
b) The pump impeller series not too much, if necessary by increasing the lift of each impeller to ensure that the total head, so by reducing the pump impeller series to minimize the length of the pump shaft.
c) When selecting multi-stage centrifugal pump shaft material, consider the material suitable for the type of medium, temperature and other needs at the same time, give priority to the selection of materials with good mechanical strength and stiffness.
d) When calculating the pump shaft diameter, consider the factors such as transmission power, starting method, radial force, shaft deflection and related inertial load. Consider the possible radial forces that may occur during non-designed flow operations to resist bending deformation of the shaft Need.
e) Reasonable choice of pump shaft support points.
1.4 vibration and vibration damping considerations
The design can also consider the multi-stage pump anti-vibration measures are:
a) Control pump shaft deflection within the regular range.
b) Clearly require the pump shaft, impeller and other dynamic and static balance test.
c) Multi-stage pump shaft should be designed according to the rigid shaft, the working speed should be less than or equal to 0.75 times the first-order critical speed.
d) Independent positioning of the impeller and the pump shaft in single-stage, the impeller and the pump shaft are fitted by interference fit and heating, and the rigidity and critical speed of the rotor assembly are also increased.
e) Selection of materials such as pump shafts and impellers, selection of material with good quality uniformity, selection of material supply status and processing method to ensure uniform cross-sectional quality of the material.
f) Design appropriate shaft and radial clearance to avoid vibration caused by abnormal friction and axial movement of rotor and stator.
g) Multistage pumps with balanced discs to balance axial forces to properly and correctly design the balance disc mechanism.
1.5 vertical multi-stage pump
For vertical multistage centrifugal pumps, the general axial force under normal operation is taken into consideration when designing the vertical multistage centrifugal pump. However, in the early stage of driving, due to the fact that the outlet pressure has not risen, the differential pressure before and after the impeller has not been established, and there is an upward axial Force, and some cause the shaft sprang up, and accompanied by mechanical seal, bearing overheating, motor over current phenomenon, serious jumping quickly. April 1999 Guangzhou ethylene Co., Ltd. Irrigation District 16 DL vertical multistage pumps have varying degrees of this situation. This is due to the structural design of the pump shaft assembly problems, should be structurally considered bearing shaft sleeve and the shaft is relatively fixed so that the upward axial force is also balanced by the thrust bearing [4].
Balance plate with automatic axial force adjustment Due to the size of the structure and the need for a pressure relief pipe In borehole-limited deep submersible pumps can not be installed The axial force balance problem has always been a high-lift deep submersible pump design In a problem. Literature [5] introduced an axial force balance method, the deep submersible pump impeller front cover diameter expanded to the pump body wall edge so that the impeller diameter in the same caliper conditions reach the maximum while the impeller rear cover diameter Appropriate to reduce the impeller axial force on the complete balance.
Introduced another new type of axial force balance device, which put a pair of static and dynamic friction mounted on the end of the impeller, the moving ring with the impeller rotation, the static ring does not rotate, front end seal for the last stage of the export of high-pressure liquid impeller , After the face seal with the atmospheric pressure or pump inlet low pressure area connected by the seal to form high and low pressure balance axial force. The new balance shaft seal assembly, which balances axial forces and is virtually leak-free, is primarily suitable for deep well submersible pumps and segmented multistage pumps. With this unit, total pump efficiency can be increased by 3% to 6%.
1.6 multi-stage delivery of liquid - solid two-phase centrifugal pump
1.6.1 Axial force balance
Segment mortar and slurry transmission slurry, slurry and other multi-stage slurry centrifugal pump, slurry erosion and erosion of the pump makes the rotor and the stator all the annular seal between the gap increases, the balance plate and the balance in the axial seat axial force Leaned together, drastic wear. The entire rotor components move axially, impeller and the middle partition, seal rings and other high-speed collision, friction, resulting in fragmentation, has led to the occurrence of many malignant accidents. In order to extend the overhaul life of this pump, slowing the wear rate of the seal gap, a unit in the design to take the following measures [3]:
â‘ improve the pump balance mechanism, create a balance plate base (balance plate), two balance plate, shown in Figure 1. This will not only reduce the leakage of the balancing mechanism during the initial operation of the pump, but also ensure the safety and reliability of the pump in the later stage of operation and prolong the life of the overhaul pump.
â‘¡ impeller, seal ring, bushings, guide wheel sets, balance plate, balance plate, etc. using spray welding.
In the first phase of the large-scale nitrogenous fertilizer project of Hualu Hengsheng, the high-pressure gray water pump adopts a segmented multistage centrifugal pump, and the axial force balance device adopts the method of "balance drum + thrust bearing bush". Because of the axial force The balance is not good, the strength of the pump shaft design is not enough, in the use of the balance drum damage occurred several times, bearing burn, axle, broken shaft and other accidents. In the company's large-scale nitrogen fertilizer project in the second phase, the high-pressure gray water pump uses a horizontal open multi-stage centrifugal pump impeller symmetrical arrangement automatically balance most of the axial force, the residual axial force from the thrust bearing, there is no balance plate , Balance drum and other balancing institutions, the site operating in good condition, the performance indicators fully meet the requirements of the use of more than 10 months have come, have not been to the problem.
1.6.2 interstage and shaft seal
In order to overcome and avoid the liquid-solid two-phase flow medium rigid particles between the rotating parts and static parts between the abrasion, Dalian Blue Pump Co., Ltd. for all pump multi-stage pump seal ring and throttling sleeve, gland used Anti-spiral groove seal structure reduces particle abrasion.
In the shaft end also uses a non-contact labyrinth seal combined with a mechanical seal seal structure, especially suitable for liquid - solid two-phase flow medium.
1.6.3 Flow rate Consideration should be given to reduce the medium flow rate from various aspects such as pump speed and pump structure, and to reduce the erosion and erosion of the hard particles in the liquid-solid two-phase flow medium to the overcurrent components around the multistage pump. Pump speed to be low, should not choose 1450rpm also on the speed.
2 use and maintenance aspects
2.1 Open the pump
When the transported high temperature liquid suddenly enters the multistage pump cold pump body, the temperature of the pump body will change greatly. Due to the uneven heating, the deformation of the pump body and the rotor part will not be uniform, There is only a small gap in itself resulting in abnormal contact. If the device is started in this condition, it will vibrate, bite and hold due to overheating. Also said that the pump used to transport high temperature liquid, before starting, to be fully warm pump. The pump can only be started if the temperature of the pump body is the same. Emergency start multi-stage pump in cold is not allowed.
High-pressure differential multistage centrifugal pump used to pump gray water in the coal-water slurry gasification device is damaged several times after it is put into operation, which means every time the pump is not fully prepared, the disk pump, exhaust gas Method is not correct [7]. Later to improve the pump, exhaust and other work, no problem also appeared.
2.2 in operation
Balanced by the balance plate, balanced drum balancing mechanism such as the balance of axial multi-stage centrifugal pump, the balance of the device has a balanced liquid outflow, the balance of liquid through the balance pipe connected to the pump inlet, in order to ensure the normal operation of the pump:
a) The balance tube never allows to block.
b) balance scaling occurred in the tube, should be timely Joe Pan, clear.
c) Install a pressure gauge on the high pressure side of the balance tube and monitor the outlet pressure of the balance tube.
Multistage centrifugal pump conveying slurry, the use of balance plate, the operation must be injected into the high-pressure seal water, the balance plate, balance plate seat work in water to prevent the slurry, hard particles on the balance plate seat, balance disc wear.