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Analysis of the causes of over-current faults for multistage
Source:未知|Popularity:0|release date:2020-04-07 10:28:21
Among the many faults of multistage centrifugal pumps, the matching over-current is considered to be a typical one, and its investigation involves many problems, and it’s difficult and complicated to check. First summarize some common faults as follows
1. The over-current of the supporting motor caused by improper selection
Multistage centrifugal pumps as fluid machinery, the two most important parameters are flow rate and head. During the operation of the centrifugal pump, the flow rate and head parameters are not constant, that is: when the actual head does not reach the rated head, the head margin of the centrifugal pump will be automatically converted to flow.
Often designers such as design institutes will consider factors such as increased pipe loss and reduced pump wear efficiency when selecting a centrifugal pump, and amplify the head margin. In the case of a particularly large head margin, it’s easy to cause the centrifugal pump to run far beyond the rated flow. If the motor power is insufficiently matched, it’s easy to cause the motor to over-current. The pump may be accompanied by problems such as cavitation and vibration.
2. Multistage centrifugal pump over-current caused by mechanical friction
The resistance generated by mechanical friction makes the supporting motor over-current exist in many aspects
(1) The pump shaft is bent or the radial run-out of each part of the rotor is too large. During the rotation of the rotor and the casing in contact with the pump shaft, the pump shaft or its sleeve and the guide vane sleeve are rubbed, the impeller and the seal ring are rubbed, which causes excessive operating load.
(2) Ordinary multistage centrifugal pumps have too large tilting of the balance plate. This is mainly because the axial run-out of the balance plate is too large. When the water pump is running, the balance plate often rubs against the balance plate, thus increasing the power of the motor.
(3) Impellers with uneven spacing have poor assembly quality. Multistage centrifugal pumps have uneven spacing between impellers. As a result, individual impellers rub against the middle section or seal ring, increasing the load on the motor.
(4) The packing gland is pressed too tightly. When filling the packing, if the packing gland is pressed too tight, the friction between the packing and the pump bushing will increase, increasing the motor load.
(5) The balance chamber and the suction section of the ordinary multistage centrifugal pump are connected with a balance pipe. If the balance water pipe is blocked, the balance plate can’t be opened, the axial thrust can’t be balanced, the balance plate and the balance plate are directly rubbed, and the motor load is very large. Can’t start.
(6) When the ordinary multistage centrifugal pump balance plate and balance plate wear too much, the pump moves axially on the water suction side. As a result, the impeller and the mouth ring bear against the phase friction, which increases the load and makes the impeller damaged quickly and the efficiency of the pump will be reduced (because the impeller and guide vane are not centered). In severe cases, the pump will not start.
(7) The gap of the coupling is too small. When the balance plate and the balance plate are seriously worn, the water pump moves axially on the water suction side. As a result, the water pump coupling and the motor coupling collide and increase the motor load.
(8) The bearing of the centrifugal pump is stuck due to lack of oil or other reasons, which causes a large load on the motor.
3. Over-current due to under-voltage
The voltage is too low due to the power grid. When the output power is unchanged, the current increases, resulting in over-current. In addition, in some cases where the inverter is used for operation, because some inverters are frequency-modulated by adjusting the voltage, resulting in the current exceeds the standard when running at low frequency.
1. The over-current of the supporting motor caused by improper selection
Multistage centrifugal pumps as fluid machinery, the two most important parameters are flow rate and head. During the operation of the centrifugal pump, the flow rate and head parameters are not constant, that is: when the actual head does not reach the rated head, the head margin of the centrifugal pump will be automatically converted to flow.
Often designers such as design institutes will consider factors such as increased pipe loss and reduced pump wear efficiency when selecting a centrifugal pump, and amplify the head margin. In the case of a particularly large head margin, it’s easy to cause the centrifugal pump to run far beyond the rated flow. If the motor power is insufficiently matched, it’s easy to cause the motor to over-current. The pump may be accompanied by problems such as cavitation and vibration.
2. Multistage centrifugal pump over-current caused by mechanical friction
The resistance generated by mechanical friction makes the supporting motor over-current exist in many aspects
(1) The pump shaft is bent or the radial run-out of each part of the rotor is too large. During the rotation of the rotor and the casing in contact with the pump shaft, the pump shaft or its sleeve and the guide vane sleeve are rubbed, the impeller and the seal ring are rubbed, which causes excessive operating load.
(2) Ordinary multistage centrifugal pumps have too large tilting of the balance plate. This is mainly because the axial run-out of the balance plate is too large. When the water pump is running, the balance plate often rubs against the balance plate, thus increasing the power of the motor.
(3) Impellers with uneven spacing have poor assembly quality. Multistage centrifugal pumps have uneven spacing between impellers. As a result, individual impellers rub against the middle section or seal ring, increasing the load on the motor.
(4) The packing gland is pressed too tightly. When filling the packing, if the packing gland is pressed too tight, the friction between the packing and the pump bushing will increase, increasing the motor load.
(5) The balance chamber and the suction section of the ordinary multistage centrifugal pump are connected with a balance pipe. If the balance water pipe is blocked, the balance plate can’t be opened, the axial thrust can’t be balanced, the balance plate and the balance plate are directly rubbed, and the motor load is very large. Can’t start.
(6) When the ordinary multistage centrifugal pump balance plate and balance plate wear too much, the pump moves axially on the water suction side. As a result, the impeller and the mouth ring bear against the phase friction, which increases the load and makes the impeller damaged quickly and the efficiency of the pump will be reduced (because the impeller and guide vane are not centered). In severe cases, the pump will not start.
(7) The gap of the coupling is too small. When the balance plate and the balance plate are seriously worn, the water pump moves axially on the water suction side. As a result, the water pump coupling and the motor coupling collide and increase the motor load.
(8) The bearing of the centrifugal pump is stuck due to lack of oil or other reasons, which causes a large load on the motor.
3. Over-current due to under-voltage
The voltage is too low due to the power grid. When the output power is unchanged, the current increases, resulting in over-current. In addition, in some cases where the inverter is used for operation, because some inverters are frequency-modulated by adjusting the voltage, resulting in the current exceeds the standard when running at low frequency.