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Centrifugal pumps in downstream: operational safety increase

1. Introduction Modern centrifugal pump units are commonly used in downstream industry. In order to maintain the technological process, refineries demand tens of thousands centrifugal pump units of various types. One of the criteria for pump units condition is the unit’s vibration. When a pump unit is in operation, its assemblies and details are degrading and the unit’s vibration level changes .
   The current research is aimed at operational safety increase due to early detection of strength degradation of pump units’ details.

According to the classical theory of failure [3], the wear-out curve is usually divided into three areas (Figure 1): ОА – early life. This stage features a decreasing failure rate, which is gradually decreasing and turning into a constant wear-out rate (Figure 2, area 3); АВ – pump unit operation with a constant degradation rate – constant wear-out of details and assemblies until reaching their limiting state (Figure 2, area 1, 4); ВС – fast wear-out period of details with a high degradation rate causing unit failure (Figure 2, area 2). Wide application of stationary condition monitoring and diagnostic systems at the refineries has allowed to ensure constant automatic machinery control, monitor development of defects and warn personnel about scheduling repair and maintenance works to be done with units in “Actions Required” state and decommissioning of units with “Unacceptable” state [4]. After a long-time monitoring of pump units by the stationary monitoring and diagnostic systems, it was discovered, that degradation pattern of several units (Figure 3) was different from the traditional wear-out pattern. During operation of units, sudden changes of vibrational parameters amplitude may appear, testifying process of details degradation [5]. Often, personnel does not pay attention to vibration jumps when the machinery state is “Acceptable” (Figure 3, area 1-38), and starts making actions when vibration exceed thresholds of “Actions required” state (Figure 3, area 39-43) or “Unacceptable” state (Figure 3, area 46). Vibration jump while pump operating shows that strength degradation of pump units’ assemblies and details is taking place. Amplitude of such jumps is An , where n is a number from 1 to 46.

It is known, that crack development in the material, its appearance on the surface and flaking of small parts from the friction/running/sliding metal surface is going stage-by-stage [6]. Operation of centrifugal pump components is connected with cyclic change of applied load, which, consequently, leads to changes of their strain-stress state. Crack development in details is fatigue, but when loads and unloads are constant, it may lead to elastic-plastic deformation loops - hysteresis, which provide the details with a certain level of fracture tolerance [7]. Since a centrifugal pump unit is not a “perfect machine”, its assembly may have various gaps, misalignment, unbalance and all this imperfections influence the operation of details of centrifugal pump units, and, particularly, rolling bearings. Rolling bearings are operating in constant cyclic change of loads. There are fatigue processes of nucleation and development of defects in the points of load transferring at inner and outer bearing rings, rolling bodies and local unevenness of metal. If bearing material does not retain properties of elastic-plastic deformation, dislocation energy accumulation may occur randomly in case of cylindrical loads in bearing elements. That may cause weakening and further disruption of interatomic bonds. Such situation leads to nucleation, development and growth of cracks with their following appearance on the surface of details in points of contact with flaking and pitting of metal. And that leads to intermittent growth of centrifugal pump vibration, and after the alignment of details and roll-in of the crack edges, the vibration level usually lowers to the previous values. To provide diagnostics of centrifugal pump details breakage and monitoring of their degradation it is required to ensure fixed vibration jump rate, starting from the early stages of defects development, and an automatic building of jump trend (Figure 3, trend III). Also, the monitoring system provides visualization of degradation process and early warning of the personnel about such process in a pump unit. As such, by controlling and trending all changes in vibration level of a centrifugal pump, during the whole operation period, we ensure long-time diagnostics of the unit condition. After setting a threshold value (Figure 3, trend II), which separates smooth and intermittent growth of vibration, we are recording jump amplitudes An and monitor jump amplitude changes during the pump operation (Figure 3). When amplitude of the first jump A1 is recorded (figure 3, trend 1, area 1), we are setting the initial level of the pump details damage. Then, as soon as the degradation is developing and new vibration jumps are appearing, we are calculating ratio of amplitude of further jumps to amplitude of the first jump
          Вn=An/A1                                              (1)
and trending relative amplitude of vibration jumps Вn. (Figure 4). In order to evaluate the condition of centrifugal pump units using vibration of its case, according to the common approach one can use the following criteria [8]: - criterion 1 is related with determining threshold values of “Actions Required” (ARQ) ad “Unacceptable” (UAC) states of machinery in order to increase the absolute vibration level (Figure 3); -criterion 2 is used for evaluation of vibration level change. Considering the second criteria, 25% change of vibration level is critical, especially, if it is sudden. Thus, relative vibration amplitude trend will be having the following values: if the first vibration jump amplitude exceeds the relative amplitude of vibration jumps by 25%, the state is “Actions Required” (ARQ), and if the first vibration jump amplitude exceeds the relative amplitude of vibration jumps by 50%, the state is “Unacceptable” (UAC) [9].

     As you can see on the figure 3 – during the 5th jump (Figure 2, trend I, section 5), the relative amplitude В5=A5/A1 exceeds “Actions required” threshold (Figure 4, section 5). More than 56 hours before the pump unit shutdown, the unit personnel were warned that the details degradation process is increased by more than 25% in comparison to the set initial level of details damage, and that repair works have to be scheduled. Then, right after the 7th jump, the relative amplitude В7=A7/A1 exceeds “Unacceptable” threshold (Figure 4, section 7), which means that the damage rate of the machinery details was increased more than by 50% in comparison to the set initial level of details damage. Therefore, 51 hour before the unit was shut down due to established regulations [10], when the 7th jump appeared (Figures 3, 4, section 7), the personnel were informed that the relative amplitude В7 has exceeded the “Unacceptable” threshold and the pump unit must be shut down.
2. Conclusion 1. Monitoring vibration jumps during a centrifugal pump unit operation and trending the changes in amplitude of jumps allow visualizing the degradation process of the unit’s details, watching the defects growth, degradation stages and degree of their danger during the whole operation life of the unit. 2. Using relative amplitude of vibration jumps allows to make long-term diagnostics of pump units details degradation since the first jump for centrifugal pump units of various types, and to eliminate the errors due to the different initial condition of the operating units’ details. Also, it ensures early - in comparison to the criterion 1 approach – warning about the dangerous degradation process and the details degradation level reaching “Actions required” or “Unacceptable” state.        3. Control over the stadial degradation of centrifugal pumps during operation provides timely prevention of machinery accidents and increases operational safety of an oil-refining production.
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