What are the vulnerable parts of a centrifugal pump?
Bearings, shaft sleeves, impeller throat bushings, casing throat bushings, wearing rings, sealing rings, coupling gaskets, etc.
Why should the outlet valve be closed when starting a centrifugal pump?
Because when the flow rate of the centrifugal pump is zero, the power is minimized. This reduces the starting current of the motor, protecting the motor.
How are the rated flow rate, head, and power of a centrifugal pump determined?
The flow rate, head, and power of a centrifugal pump at the highest efficiency point are defined as the rated flow rate, head, and power.
What are the consequences of cavitation in a centrifugal pump?
(1) It generates noise and vibration. (2) It leads to a decrease in flow rate, head, and efficiency, and may even render the pump inoperable. (3) Cavitation can cause pitting and honeycomb-like damage to the impeller and casing, and even result in penetration of the casing. (4) It causes the pump shaft to cavitate.
Does flow rate variation during normal operation of a centrifugal pump generate radial forces?
Yes, it can generate radial forces. Flow rate variation causes an uneven pressure distribution around the impeller, resulting in a radial force acting on the impeller.
What are the hazards of the existence of radial forces in a centrifugal pump?
Since the radial force of a centrifugal pump acts vertically on the pump shaft, the shaft experiences significant deflection. For a rotating shaft, this results in fatigue and potential damage.
What are the hazards of the existence of axial forces in a centrifugal pump?
The existence of axial forces in a centrifugal pump can lead to wear on the wear ring, balance disk, and impeller. It can also cause damage to mechanical seals, bearings, and the motor.
What methods are used in the design of centrifugal pumps to eliminate axial forces?
(1) Use of balance holes, balance disks, balance pipes, and balance drums; (2) Double-suction impeller; (3) Use of thrust bearings; (4) Symmetrical arrangement of impellers, among others.
What are the main reasons for a centrifugal pump not delivering fluid?
(1) Presence of air in the pump inlet and inlet pipe; (2) Increase in medium viscosity beyond design parameters; (3) Leakage in the inlet pipeline or mechanical seal; (4) Blockage in the inlet pipeline; (5) Pump speed not reaching rated speed; (6) Pump running in reverse; (7) Excessive clearance in the wear ring.
What are the main reasons for excessive current in the motor of a centrifugal pump?
(1) Medium density or viscosity exceeds design parameters; (2) Severe friction between the rotor and the casing; (3) Coupling eccentricity or bearing wear.
For which types of centrifugal pump rotors is only static balancing required during installation?
(1) Single-stage pumps; (2) Multi-stage pump rotors with smaller diameters; (3) Rotors with low speeds.
For which types of centrifugal pump rotors is dynamic balancing required during installation?
(1) Rotors with multiple impellers; (2) Rotors with large diameters; (3) Rotors with high speeds.
What are the methods for finding the correct alignment of a coupling?
(1) Single dial indicator method; (2) Double dial indicator method; (3) Three dial indicator method.
What are the possible reasons for unstable or fluctuating pressure gauge readings at the outlet of a centrifugal pump?
(1) Pressure gauge is damaged and its indication is not accurate; (2) Cavitation or fluctuation in flow rate of the pump; (3) Foreign matter or blockage in the pipeline; (4) Vibration of the pump; (5) Air in the pipeline or pump casing; (6) Cavitation.
What impact does cavitation have on the mechanical seal of a centrifugal pump?
(1) It can cause significant leakage of the mechanical seal; (2) It can damage the sealing faces of the mechanical seal, or even cause it to fail.
What is the maximum allowable value for shaft bending in a centrifugal pump?
At the journal bearing: Not more than 0.02mm; At the middle of the shaft: (1) Not more than 0.08mm at 1500 RPM; (2) Not more than 0.06mm at 3000 RPM.
What is the recommended clearance for the wearing ring of a centrifugal pump (expressed as a fraction)?
For pumps operating at normal temperatures, when the wearing ring diameter is less than 100mm, a clearance of 0.5mm is appropriate. For diameters greater than 100mm, a clearance of 0.5 + (D – 100) × 0.002 (D represents the diameter) should be used. An additional 0.1mm should be added for high temperatures.
What are the consequences of excessive or insufficient clearance in the wearing ring of a centrifugal pump?
(1) Excessive clearance leads to reduced pump efficiency and inability to deliver fluid; (2) Insufficient clearance can cause friction and even result in seizing of the impeller.
What must be done before conducting a trial run after overhauling a centrifugal pump?
(1) Review the overhaul work order; (2) Conduct a turning test; (3) Open the inlet valve and slightly open the outlet valve for high-pressure pumps; (4) Open the air release valve; (5) Start the pump and pay attention to the rotation direction; (6) After confirming that the operation meets requirements, the pump can be put into use.
What is the allowable operating temperature for a centrifugal pump with rolling bearings?
Less than 70°C.
What is the allowable operating temperature for a centrifugal pump with sliding bearings?
Less than 60°C.
What is the purpose of conducting a turning test for a centrifugal pump?
The primary purpose is to prevent shaft bending and deformation due to its own weight.