Y. Yuehai, CTCI, Beijing, China
As one kind of fluid-conveying equipment that has the characteristics of high specific speed, large fluid flowrates and low pump head rise for conveying chemical fluid, axial flow pumps have always been considered a critical piece of equipment for chemical fluid transportation in the chemical industry. However, the vibration of axial flow pumps has emerged as a key problem to the long-term, reliable and stable operation of these pumps in the chemical industry.
With the continuous development of the global chemical industry, large-scale refining and chemical projects are constantly upgrading in scale and capacity. As chemical project capacities increase, this will inevitably lead to an increase in equipment capacity. For fluid transportation equipment (such as pumps and blowers), an increase in conveying capacity is unavoidable. Axial flow pumps—generally considered suitable for operations where the transport of large flowrates is required—must be scaled up to handle larger amounts of fluids. Therefore, maintaining a long-term, reliable and stable operation of fluid-transport equipment has become the focus of attention. Among the various influencing factors is how to control or eliminate pump vibration.
An axial flow pump is a type of centrifugal pump. There are two types of axial flow pumps: vertical and horizontal. In horizontal-type axial flow pumps, the main working principle and structure of the pump is the centrifugal force generated by the impeller driven by the motor to transport the fluid. Due to the large flowrate, the inlet and outlet nozzles of the pump are much larger than those of the ordinary centrifugal pump (FIG. 1).
By comparing the structure of axial flow pumps, it was found that the main causes of pump vibration can be summarized as：
The span between the thrust bearing at the drive-end of the axial flow pump shaft and the radial bearing at the impeller end is too large, and the unsupported shaft system is too long. This gap increases the risk of instability of the entire shaft system. As this problem originated from the self-structure of axial pumps, it is fundamentally difficult to eradicate and may be the main reason behind axial pump vibrations.
Because the shaft of an axial flow pump is longer than that of other normal centrifugal pumps, the radial force at the pump inlet piping is accordingly larger. This is another cause of instability of the shaft system, leading to vibration.
Due to the direct connection between the axial flow pump and the reactor through pipelines, changes in the reactor’s temperature can cause changes in pipeline expansion. If the vertical direction contraction and expansion of the axial flow pump are not synchronized with the reactor, they will increase pipeline stress and cause vibration of the axial flow pump.
For axial flow pumps, if certain factors are removed during normal production and operation (e.g., abnormal equipment operation caused by inadequate shaft alignment during installation and/or component damage caused by errors in operations by personnel), it was found that the optimization of the structural design can greatly reduce the vibration of axial flow pumps. The means of structural optimization include the following points:
The shorter the axis system, the greater the stability. Therefore, pump manufacturers should use radius volutes that are as small as possible in axial pumps to shorten the span between the thrust bearing at the pump’s drive-end and the radial bearing at the impeller end. This is the most direct solution.
Using low-density materials, such as aluminum/carbon fiber, can greatly reduce the radial force generated by high-speed rotation of the impeller.
It is impossible to balance rotating parts because centrifugal force is generated during the rotation process. With centrifugal force, the rotating parts will produce deflection deformation—the extent of this deflection deformation is proportional to the rotational speed. When the pump speed is close to critical speed, a resonance phenomenon will occur due to the increase in deflection. However, when the pump speed largely exceeds the critical speed, the deflection significantly decreases. To avoid severe resonance, the design should ensure that the critical speed of the pump is at least 20% higher than the maximum continuous speed.
Operators should abandon the most conventional pump-supporting method on a foundation. Normally, the pump body of the axial flow pump is fixed on a foundation by anchor bolts, followed by the application of secondary grouting. Instead of this process, add adjustable spring supports or soft connections at the linking area between the foundation and the pump body to ensure that the central line of the pump body can be lifted and lowered together with the reactor, and to keep the vertical contraction and expansion of the axial flow pump synchronized with the reactor as much as possible. In this way, reducing pipeline stress and vibration of the axial flow pump can be achieved. FIG. 2 shows a typical arrangement of installing spring supports on the pump’s body.
As a separate mechanical seal flushing system is required on the mechanical seal of the axial flow pump, a separate base is designed for the mechanical seal flushing system, which is arranged separately from the pump body. A soft connection can be adopted for the connection between the mechanical seal flushing system and the mechanical seal system. This arrangement will minimize the pipeline stress caused by the mechanical seal flushing system to the pump body.
In short, damage to the pump bearing caused by axial flow pump vibration can only be solved by replacing the damaged bearing with a new one during shutdown and/or overhaul of the plant. With continuous technological upgrades and structural optimization, the vibration value of the axial flow pump can be controlled within 3 mm/sec for long-term operation, playing a significant role in the long-term stable operation of the entire device and greatly reducing maintenance costs for end users. HP
Yu Yuehai is a Senior Engineer at CTCI Beijing, a CTCI company. He has contributed to the selection and design of compressors and pumps for polyolefin and coal chemical industries for 16 yr and possesses extensive experience in equipment installation and commissioning.