Tham khảo tài liệu 'intro predictive maintenance 2e episode 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Machine-Train Monitoring Parameters 97 Hydraulic Instability Vane Pass . Hydraulic or flow instability is common in centrifugal pumps. In addition to the restrictions of the suction and discharge discussed previously the piping configuration in many applications creates instability. Although flow through the pump should be laminar sharp turns or other restrictions in the inlet piping can create turbulent flow conditions. Forcing functions such as these results in hydraulic instability which displaces the rotating element within the pump. In a vibration analysis hydraulic instability is displayed at the vane-pass frequency of the pump s impeller. Vane-pass frequency is equal to the number of vanes in the impeller multiplied by the actual running speed of the shaft. Therefore a narrowband window should be established to monitor the vane-pass frequency of all centrifugal pumps. Running Speed. Most pumps are considered constant speed but the true speed changes with variations in suction pressure and back-pressure caused by restrictions in the discharge piping. The narrowband should have lower and upper limits sufficient to compensate for these speed variations. Generally the limits should be set at speeds equal to the full-load and no-load ratings of the driver. There is a potential for unstable flow through pumps which is created by both the design-flow pattern and the radial deflection caused by back-pressure in the discharge piping. Pumps tend to operate at their second-mode shape or deflection pattern. This operation mode generates a unique vibration frequency at the second harmonic 2X of running speed. In extreme cases the shaft may be deflected further and operate in its third 3X mode shape. Therefore both of these frequencies should be monitored. Positive Displacement A variety of positive-displacement pumps is commonly used in industrial applications. Each type has unique characteristics that must be understood and monitored however most of the major types have
