Multi-stage centrifugal pumps are widely used in refining and chemical enterprises due to their high lift, medium pressure, large flow and good stability.
1, the phenomenon of failure
Since the pump was put into production, the maintenance difficulty is high. The failure phenomenon is characterized by large vibration of the pump body, leakage of the mechanical seal, multiple vibration speeds of 13mm/s or more, and motor current exceeding the standard. The first two inspections only removed the front and rear end caps to check the balance plate of the bearing, mechanical seal and liquid discharge end, and replaced the damaged parts of the bearing and mechanical seal. Every time, the balance disc and the balance seat end face were found to have different degrees of friction damage. The fault phenomenon remains after the light knife is loaded back. After the entire pump body was overhauled, all parts were inspected, and it was found that all levels of impeller, guide impeller, bushing, balance disc, balance sleeve and seal ring have different degrees of wear, the pump shaft is bent, and the bearing housing is coaxial with the pump shaft. The degree of deviation is large.
2, the cause analysis
2.1 Multi-stage centrifugal pump has large axial thrust
Every time the inspection and unloading check the balance plate, it was found that the surface was scratched, mostly caused by excessive axial thrust. The axial thrust of a multistage centrifugal pump is much larger than that of a single-stage centrifugal pump. If the axial thrust of a single-stage impeller is FA, for a multistage centrifugal pump impeller of the same size, the number of stages is i, the total axial direction. The thrust is iFA, and the axial thrust of the multistage centrifugal pump can be several tens of kN or even hundreds of kN. Its axial thrust balancing method is to use a balance disc, the structure of which is shown in Figure 1. When the centrifugal pump is working normally, after the pressure P2 at the outlet of the final impeller passes through the radial gap b, the liquid pressure leaking to the intermediate chamber of the balance disc drops to the pressure P1 before the balance disc, and the liquid passes through the axial gap, and the pressure drops to P0. On both sides of the balance disc, due to the presence of the pressure difference P1-P0, acting on the corresponding effective area, a balance force -FA opposite to the axial force direction is generated. If the axial thrust is increased due to the change in load, when the balance acting on the balance plate has not changed, the axial thrust will be greater than the balance force, and the rotor will be displaced a small distance toward the suction side. At this point, the axial gap is reduced and the amount of liquid leaking will decrease. While the radial gap b is constant, when the leakage amount is reduced, the resistance loss is reduced, and the pressure P1 before the balance disc is increased. At the same time, the reduction in leakage will also reduce the pressure P0 in the balance chamber. Thus, the pressure difference on both sides of the balance plate increases, and the balance force increases. Until the axial gap b0 is reduced to equalize the balance force and the axial thrust. vice versa.
2.2 Influence of impeller seal ring clearance
During the inspection, it was found that the gap of the seal ring of the impeller was more serious. The maintenance procedure required to be controlled at 0.3~0.44mm, while the actual majority had reached more than 1mm, and some gaps even had 2mm. When the gap of the seal ring becomes larger, radial flow occurs in the front cover plate of the impeller and the pump chamber. When there is radial flow, the pressure distribution in the pump chamber is changed, so that the liquid pressure in the front pump chamber is reduced. This is because the impeller outlet pressure is constant and the liquid inevitably creates additional pressure in the flow. This increases the axial force. The seal ring gap of the eight impellers is greatly worn, and the axial thrust of the single impeller is also increased, and the axial thrust of the entire pump is the superposition of the axial thrust of the eight impellers. Moreover, the gap between the impeller and the impeller is also increased, and the axial thrust is further increased. After the entire axial thrust is increased, the structure of the previous balance disc cannot completely offset the axial thrust.
2.3 Interaction of parts
The gap between the seal ring and the increased wear of the impeller and the impeller lead eventually lead to the wear of the balance disc. This is not a one-way problem. The fault effects between the many shaft parts of the pump body are mutual. The pump shaft is bent, the coaxiality of the bearing seat and the pump shaft is deviated, causing the wear of the impeller, the guide impeller, the pump casing, the seal ring and the bushing, so that the vibration of the pump body is increased, the axial thrust is also increased, and the balance disc is The balance seat also rubs. On the contrary, the vibration of the pump body is also increased by the friction of the balance disc, so that the wear of the impeller, the guide impeller, the seal ring, the bushing, the bearing and the like is intensified, and the fault is further deteriorated.
For the specific problems, the following aspects have been dealt with: First, the previous pump shaft material is 316 austenitic stainless steel, the rigidity is not enough in actual operation, and later changed to 00Cr25Ni22Mo2 austenitic stainless steel with higher strength and hardness to achieve better. The stiffness, machining a new shaft, to ensure that its radial circular runout is not more than 0.03mm based on the journal at the bearing. At the same time, the shaft sleeves of each stage are processed and replaced Secondly, the inner hole of the bearing housing is re-inlaid with a light knife to ensure the coaxiality between the bearing center and the pump shaft after the bearing is installed, to reduce vibration and avoid internal parts friction. Thirdly, the material of the balance disc and the balance sleeve is also 316 stainless steel. In order to enhance the wear resistance of the balance disc, the surface is welded by arc welding, and the welding rod is made of 00Cr25Ni22Mo2 stainless steel with high hardness.
4,problems should be paid attention to in the process operation
For the operation of multistage centrifugal pumps, there are some issues to be aware of. First of all, it should be avoided to cut the pump frequently. In the process of starting the pump and cutting the pump, care should be taken to prevent the vaporization of the pump. Before the pump is started, in order to prevent the leakage of the medium in the pump body, the medium is vaporized or cavitation in the pump body, causing frictional damage between the balance plate and the balance seat and the moving ring and the stationary ring of the machine seal. Therefore, after introducing the medium before starting the pump, it should be fully exhausted, and then the pump will start to move after the temperature of the pump is lowered. If there is no pressure at the outlet after the pump is started, stop the pump immediately and start it after the exhaust is sufficient. Process operation requirements are stable.