What to do about vibration during startup of the frequency converter in a vertical shielded pump?
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Vertical shielded pump with a 55KW motor; it starts using frequency conversion. The operator increases the frequency in steps of 5HZ, with long intervals between each step; when the frequency reaches 20HZ, the pump begins to vibrate and make abnormal noises. The pump was tested for two days, and it’s the same every time. Two new pumps for liquefied gas, with an inlet pressure head of 3 kilograms and a rated outlet pressure of 10 kilograms. I believe vertical shielded pumps should not be designed with variable-frequency starting. Based on the structure and working principle of shielded pumps, during the initial start-up phase, the lower thrust disk exerts the entire weight of the rotor on the lower graphite bearings; as a result, the medium inside the pump does not have enough pressure to lift the rotor. This leads to prolonged friction between the lower thrust disk and the bearings, causing wear. The radial clearance of the bearings as well as the play of the rotor increase, which in turn results in vibration and abnormal noises. The manufacturer has been contacted to send spare parts for replacement. Now the questions are: 1, can a vertical shielded pump be started using an inverter-driven starter? 2, if so, how should the pump be started and stopped? 3. By changing the parameters of the frequency converter, the motor’s acceleration time can be reduced, allowing the pump’s speed to reach a higher level in a short period of time. This results in an increase in pressure inside the pump within a short time, enabling the thrust disc and bearings to separate after brief frictional contact – is this feasible?
Is the abnormal vibration caused by the rotational speed passing through a certain critical frequency multiple (reciprocal of a frequency multiple)?
Is it true that just exceeding the critical speed will do? I’ve only heard that steam turbines have a critical speed. This pump can reach up to 2950 revolutions per minute; such a critical speed shouldn’t exist. It’s already operating at 20 Hz, which is only 40% of that speed. There was noise and vibration from the very beginning, and the more the speed increased, the greater the vibration became
Was the problem solved after the manufacturer replaced the parts? Has the final issue been determined? Why is the 55kW shielded pump designed to start with variable frequency? I want to learn* it too.
When the manufacturer came to test the pump, it was functioning normally, with flow rate, head, vibration, and temperature all within the designed ranges. The manufacturer also answered my questions: after the canned pump starts up, the flow rate must reach 70% within a short time and remain within the range of 70% to 120%. Otherwise, the pump does not have enough thrust to keep the rotor in axial balance. The manufacturer has not tested flow rates above 120%, so they are not sure what might happen in such cases. When starting a canned motor pump using variable frequency drive, a frequency of 35 Hz is set, which corresponds to 70% of the motor’s speed. During the tuning of the variable frequency drive, electrical technicians set the motor’s acceleration time to 30 seconds in order to reach 50 Hz; in other words, it takes 21 seconds to reach 70% of the speed. This represents the minimum required startup time. It is possible to reduce the motor’s acceleration time to reach 70% of the speed more quickly, but this may cause the motor to experience overcurrent or damage the variable frequency drive. Therefore, electrical technicians do not allow reducing the acceleration time when testing the pump. During the design phase by the design institute, the shielded pumps of our company were intended for loading liquefied gas and transferring liquid from spherical tanks; the frequency converter was installed so that the outlet flow rate and outlet pressure of the pumps could be controlled from the control room via DCS. Choosing a canned motor pump is also for safety reasons. But they must have overlooked the working principle of the shielded pump – even under control, its operation rate should not be below 70%. Frequent starting and stopping of the pump will **reduce the service life of the graphite bearings and thrust discs; a purchase plan for these bearings needs to be prepared next month.
To stop the pump, simply reduce the frequency drive to 0.
In principle, shielded pumps cannot operate below 30 Hz, so it was correct of you to increase the frequency quickly. As for the flow rate range: theoretically, 70–120% is the range that must be maintained to ensure stable pump operation. In practice, it is possible to negotiate with the manufacturer to expand this range, or to reduce the axial force at lower flow rates; in the case of vertical pumps, the axial force is generally directed upward to counteract the weight of the rotor.