The vibration affected by the mechanical seal performance of centrifugal pump is analyzed from the direction of centrifugal pump itself and external operation environment. The influence of vibration source on the performance of the centrifugal pump and the related countermeasures are summarized into four aspects.
Vibration caused by the basic structural components of a centrifugal pump
The basic structural components of centrifugal pump are made up of five parts: impeller, pump body, pump shaft, bearing, and mechanical seal, and are driven by external power, such as motors, steam turbines or gas turbines. Any combination of rotating parts, imbalance between impeller and rotating shaft, bearing defects, rough manufacturing of rotor, loose assembly of blades and other components may cause vibration of centrifugal pump shaft. The mechanical seals mounted on the pump shaft undergo different vibration frequencies during operation. At low speeds, they may be below the rotational speed of the centrifugal pump, and the maximum may reach thousands of times per second. The single vibration frequency of the above vibration sources can be measured by vibration monitoring device. If the vibration frequency coincides with the natural frequency of the spring or bellows in the mechanical seal elastic element, the mechanical seal will have abnormal wear or fatigue damage. Of course, there are other factors that cause this damage, and there is not enough data to be fully supported as a result of the vibration of a mechanical seal. Alcohol pump
To avoid the resonance, in the design of mechanical seal must ensure predictable resonance frequency and excitation frequency do not coincide, and should take measures to reduce vibration and reliable; can not with the critical speed of the design is too close and causes resonance; in normal operation state, the installation error should be reduced to a minimum, because these errors will lead to displacement of mechanical seal spring device; the rotor is required to achieve the full balance. On the vertical axis, the load on the rolling bearing on the shaft should be enough to minimize the possibility of roller skidding.
Two. Vibration caused by the manufacture and installation of the mechanical seal of the centrifugal pump
How does the sealing face fit between two components of mechanical seal affect not only the sealing effect, but also the operation stability and service life of the seal. If the sealing face of mechanical seal of centrifugal pump can not guarantee good cohesion, the stability of mechanical seal will be affected. The degree of its influence depends not only on the specific structure of mechanical seal, but also on the operation parameters of mechanical seal. Every rotating part of the mechanical seal is rotated for a week, and the compensation ring should be moved accordingly to ensure the sealing end sealing. And in effect on the auxiliary seal is similar, especially for the sliding auxiliary seal ring, such as O, should be load on the seal will be evenly distributed along the circumference in the manufacturing and installation process, to prevent the load or load side of the lighter liquid leakage, while the load side is excessive wear.
When an internal mechanical seal is used, the other force acting on the mechanical seal is often caused by the self circulation of the pump medium. In order to ensure the sealing performance, should be designed as self circulating fluid from the exterior through the seal face two. Once the flow is too large, the pressure on the sealing ring may also make the seal face misplaced and affect the sealing effect. In order to reduce the effect of vibration, the self circulating fluid can be used from the tangential inlet, or the self circulating fluid is flowing along the seal. If the actual situation is allowed, the circulating flow can be controlled by the fit clearance of the bushing in the sealing cavity, or the reverse connection between the suction pipe and the discharge pipe in the sealing cavity will be reversed, so that the reverse flow will be formed.
Three. Vibration produced by a centrifugal pump when operating under normal operation and under disturbance.
When the centrifugal pump is running, the rotor and the mechanical seal will be affected by the hydraulic vibration. When the flow of the centrifugal pump changes, the fluid flow at the inlet of the impeller, including the two aspects of the axial and radial directions, will fluctuate. If the centrifugal pump is not running at the best efficiency, the pressure head at the impeller inlet and diffuser pipe will increase. When the flow rate is lower or higher than the best efficiency point, it may cause cavitation. When the centrifugal pump operates at a non full load condition, the radial force on the impeller will increase and will be transferred to the mechanical seal. The amplitude and frequency of vibration caused by the mechanical vibration of the rotor system and the pressure change on the seal will be random. Therefore, when designing the system and selecting the centrifugal pump, we should try to avoid running under the condition of non full load, so that it can run near the highest efficiency and ensure the stability of the centrifugal pump under the condition of negative pressure. When the diameter of the impeller of a centrifugal pump is large, the effect of the stress of the blade must be considered.
Four. Vibration caused by deformation of the centrifugal pump due to the change of temperature and pressure in the operating environment
In the operating condition of centrifugal pump, when the temperature change of centrifugal pump because of temperature rise and thermal deformation, thus affecting the centrifugal pump to drive the shifting of each component of the centrifugal pump will deform, resulting in deformation of mechanical seal face. It is found that if no corresponding measures are taken, the thermal expansion of the connecting pipe will apply a large load to the centrifugal pump. The influence of temperature on the shell and rotor of centrifugal pump is also different. In general, the expansion and contraction of the rotor is faster than that of the shell, which affects the change of the clearance of the sealing surface. When the operating temperature fluctuates, the mechanical seal is required to bear the rotation of the parts caused by the misplacement of the components.
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