Some common questions and answers about centrifugal pump maintenance – essential for beginners!

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In the context of after-sales customer service, Richter has summarized his insights on the maintenance of centrifugal pumps and presents an analysis as follows: What are the vulnerable components of centrifugal pumps? Answer: Bearings, shaft sleeves, impeller mouth rings, housing mouth rings, dynamic and static rings of mechanical seals, sealing rings, coupling rubber rings, spring washers for coupling removal, etc. Why should the outlet valve be closed when starting a centrifugal pump? Answer: Because the power is smallest when the flow rate of the centrifugal pump is zero. This helps to reduce the starting current of the motor, thereby protecting it. How are the rated flow rate, head, and power of centrifugal pumps specified? Answer: The flow rate, head, and power at which the centrifugal pump operates with the highest efficiency are defined as the pump’s rated flow rate, head, and power. What are the consequences of cavitation in centrifugal pumps? Answer: (1) Generate noise and vibration ； (2) Accompanied by a decrease in flow rate, head, and efficiency, to the point where it becomes impossible to operate ； (3) The impeller casing and blades exhibit pitting and honeycombing damage, which can even lead to penetration of the casing ； (4) Cause damage to the pump shaft. Does a change in flow rate during the normal operation of a centrifugal pump generate a radial force? Answer: It can generate a radial force. As changes in flow cause the pressure distribution around the impeller to become uneven, radial forces act on the impeller. What are the hazards caused by the radial force in centrifugal pumps? Answer: Since the radial force of a centrifugal pump acts perpendicularly on the pump shaft, the shaft experiences significant deflection. A rotating shaft is subjected to alternating loads, and the presence of such radial forces can cause the pump shaft to fatigue and fail. What are the hazards caused by the axial force in centrifugal pumps? Answer: The presence of axial force in centrifugal pumps causes wear on the mouth ring, balance disk, and impeller ； Damage to mechanical seals, bearings, and motors. What measures are taken in the design of centrifugal pumps to eliminate axial forces? (1) Use balance holes, balance discs, balance pipes, and balance drums ； (2) Double-suction impeller ； (3) Utilize thrust bearings ； (4) Utilizing the symmetric arrangement of impellers, etc. What are the main reasons why a centrifugal pump fails to draw water? (1) Gas present at the pump inlet and in the inlet pipe ； (2) The increase in medium viscosity is not in line with the design ； (3) Leakage in the inlet pipeline or mechanical seal ； (4) The inlet pipeline is blocked ； (5) The pump’s rotational speed has not reached the rated speed ； (6) Pump reversal ； (7) The gap of the mouth ring is too large. What are the main reasons for vibration in centrifugal pumps? (1) The couplings are not aligned ； (2) Axial bending degree exceeds the limit ； (3) Excessive bearing clearance or damage ； (4) Rotor imbalance ； (5) Friction between the rotor and stator occurs ； (6) The medium carries gas, and cavitation occurs ； (7) Pump evacuation ； (8) There are foreign objects in the leaf rotation path ； (9) The improper installation of the process pipelines subjects the pump to external forces. (10) Operating at low load for a long time ； (11) Impact of vibration in auxiliary pipelines ； (12) The anchor bolts are loose. What are the causes of overheating in centrifugal pump bearings? (1) Poor quality of rolling bearings or wear during operation ； (2) The clearance of the sliding bearing is too small ； (3) Poor cooling of the bearing housing ； (4) Insufficient, excessive, or contaminated amount of lubricating oil and grease. What are the reasons for excessive current in a centrifugal pump motor? (1) The specific gravity or viscosity of the medium is too high, which does not conform to the design ； (2) Severe friction between the rotor and the housing ； (3) The coupling is out of alignment or the bearings are worn. For which types of centrifugal pump rotors is it sufficient to only check for static balance during installation? (1) Single-stage pump ； (2) Rotor of multi-stage pumps with smaller diameter ； (3) Rotor with a lower rotational speed. For which types of centrifugal pump rotors is dynamic balancing necessary? (1) Many impellers ； (2) Larger diameter ； (3) Rotor with high rotational speed. What are the methods for aligning couplings? (1) Single-table alignment method ； (2) Double-table alignment method ； (3) The three-table alignment method. Analyze the reasons for the unstable or significantly fluctuating pointer of the outlet pressure gauge on a centrifugal pump (1) The pressure gauge is damaged and does not function properly ； (2) Pump evacuation or flow rate fluctuations ； (3) Impurities or blockages in the pipeline ； (4) Pump vibration ； (5) Gas present in the pipeline or pump casing ； (6) Cavitation. What is the impact of pump cavitation on mechanical seals? (1) It will cause severe leakage from the mechanical seal ； (2) It damages the sealing surfaces of the mechanical seal, or even causes damage to them. What is the general maximum allowable value for the shaft bending of a centrifugal pump? At the journal: not more than 0.02 mm ； Middle part of the shaft: (1) 0.08 mm or less at 1500 revolutions per minute ； (2) 3,000 revolutions per minute: not more than 0.06 mm. What is the typical required clearance for the impeller collar of a centrifugal pump (expressed as a fraction)? Answer: For pumps used at normal temperatures, when the diameter of the mouth ring is less than 100 mm, a clearance of 0.5 mm is appropriate; if it is greater than 100 mm, the clearance is 0.5 + (D–100) × 0.002 (where D is the diameter). An additional 0.1 mm should be added at high temperatures. What are the consequences of an excessively large or small gap in the impeller ring of a centrifugal pump? (1) Excessive clearance in the mouth ring results in reduced pumping efficiency ； (2) An excessively small gap in the mouth ring can cause friction and even lead to shaft seizure. What tasks must be carried out before testing a centrifugal pump after maintenance? (1) Consult the maintenance procedure steps ； (2) Perform turning of the shaft ； (3) Open the inlet valve, slightly open the outlet valve of the high-pressure pump ； (4) Open the vent valve ； (5) Start the pump and pay attention to the direction of rotation ； (6) It can be put into use after checking that its operating conditions meet the requirements. What is the purpose of turning a centrifugal pump shaft? Answer: Mainly to prevent the shaft from bending or deforming due to its own weight or friction.