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Q&A on Basic Knowledge of Centrifugal Pumps 1. What is the main working principle of a centrifugal pump? The electric motor drives the impeller to rotate at high speed, generating centrifugal force. Due to this centrifugal force, the liquid is forced into the side channels and discharged from the pump, or it enters the next stage of the impeller. This reduces the pressure at the inlet of the impeller, creating a pressure difference with the pressure acting on the liquid being drawn in. This pressure difference drives the liquid into the pump, and as the centrifugal pump continues to rotate, liquid is continuously drawn in and discharged. 2. What are the functions of lubricating oils (greases)? Lubrication and cooling, flushing, sealing, vibration damping, protection, and load relief. 3. Through which three levels of filtration must lubricating oil pass before use? Level 1: Between the original lubricant container and the fixed container ； Level 2: Fixing between the oil barrel and the oil pot ； Level 3: Between the fuel tank and the fuel filling point. 4. What are the “five fixed principles” for equipment lubrication? Fixed location: Refuel at the designated location as specified ； Timing: Apply lubricant to the lubricated parts at specified times, and change the oil regularly ； Quantitative: Fill up according to the consumption amount ； Qualification: Select appropriate lubricants based on different machine models, and ensure that the quality of the lubricants is satisfactory ； Designated person: Each fueling station must have a designated person in charge. 5. What are the hazards of water in the lubricating oil of mechanical pumps? Moisture can reduce the viscosity of lubricating oil, weaken the strength of the oil film, and diminish the lubrication effect. Water freezes below 0°C, severely affecting the low-temperature fluidity of lubricating oil. Moisture can accelerate the oxidation of lubricating oil and promote the corrosion of metals by low-molecular-weight organic acids. Moisture increases the foaming tendency of lubricating oil, making it prone to forming foam. Moisture can cause metal parts to rust. 6. What are the maintenance tasks for pumps? Strictly implement the post responsibility system as well as rules and regulations regarding equipment maintenance. Equipment lubrication follows the \"five fixed principles\" and \"three-stage filtration\"; the lubrication equipment is complete and clean. Maintenance tools, safety facilities, fire-fighting equipment, etc. are complete and in good condition, and are neatly arranged. 7. What is the difference between a hot oil pump and a cold oil pump? 1) Classified by medium temperature: those below 200°C are cold oil pumps (20–200°C), while those above that temperature are hot oil pumps (200–400°C) ； 2) Differentiated by seal oil: ordinary hot oil pumps use seal oil, while cold oil pumps do not ； 3) In terms of material, hot oil pumps are made of carbon steel or alloy steel, with their pump supports cooled by circulating water; whereas cold oil pumps can be made of cast iron, and their pump supports do not require cooling ； 4) In the pump model, hot oil pumps are indicated by the letter R, while cold oil pumps are indicated by the letter J ； 5) In standby mode, the hot oil pump needs to be preheated, while the cold oil pump does not require preheating. 8. What preparations should be made before starting a centrifugal pump? Check whether the pump body, outlet pipelines, valves, and flanges are properly tightened, whether the anchor bolts are tight, whether the couplings are correctly connected, and whether the pressure gauges and thermometers are accurate and functional. Turn the crank 2–3 times to check whether it rotates smoothly and if there are any abnormal noises. Check whether the quality of the lubricating oil is satisfactory, and ensure that the oil level remains between 1/3 and 1/2 of the mark on the gauge. Open the inlet valve and close the outlet valve; open the pressure gauge handwheel as well as all the cooling water valves, flushing oil valves, etc. Before starting the pump used to transport hot oil, it must be preheated to a temperature that is 40–60°C lower than the operating temperature; the heating rate must not exceed 50°C per hour, and the maximum temperature must not be 40°C higher than the operating temperature. Contact the electrician to restore power. For non-explosion-proof motors, it is necessary to start the fan or supply explosion-proof hot air to disperse any flammable gases inside the pump. 9. How to switch centrifugal pumps? First, all the preparatory work before starting the pump must be carried out, such as preheating the pump. Based on the mutual switching of relevant parameters such as the pump’s outlet flow rate, current, pressure, and liquid level, the principle is to first start the backup pump. Once all systems are functioning properly and the pressure rises, the outlet valve is gradually opened, while the outlet valve of the pump being switched off is slowly closed until it is completely shut. The pump that is being switched off is then stopped, but efforts should be made to minimize fluctuations in parameters such as flow rate caused by this switching process. 10. Why can’t a centrifugal pump be started when its impeller doesn’t rotate? If the centrifugal pump will not rotate, it indicates that there is a fault inside the pump; this fault could be caused by the impeller being stuck, the pump shaft being bent too much, the moving and stationary parts of the pump having rusted together, or excessive pressure inside the pump. If the pump is attempted to start without the pump disc rotating, the strong torque generated by the motor will force the pump shaft to spin, which can cause damage to the internal components such as a broken or twisted pump shaft, a damaged impeller, burned-out motor coils, and it may also lead to the motor tripping and failure to start. 11. What is the purpose of sealing oil? Cooling sealing parts ； Lubricated friction pair ； Prevent vacuum-induced damage. 12. Why is it necessary to regularly turn the standby pump manually? Regular turning of the pump serves three purposes: to prevent scaling and jamming inside the pump ； Prevent pump shaft deformation ； The barring gear can also deliver lubricating oil to various lubrication points, preventing the shafts from rusting; lubricated bearings facilitate immediate operation in emergency situations. 13. Why is preheating necessary before starting a hot oil pump? If a hot oil pump is started without preheating, the hot oil will quickly enter the cold pump body, causing uneven heating of the pump. The upper part of the pump expands more due to heat while the lower part expands less, which can lead to the bending of the pump shaft or to the jamming of the sealing rings on the pump body and the rotor ； Forcing it to start can cause wear, shaft binding, and shaft breakage. If high-viscosity oils are not preheated, they will condense inside the pump, resulting in no flow after startup, or the motor may trip due to the high starting torque. Due to insufficient preheating, uneven thermal expansion occurs throughout the pump, leading to leaks at the static sealing points. Issues such as leaks at the outlet and inlet flanges, the flange on the pump’s cover, or the balance pipes can even lead to serious accidents like fires and explosions. 14. What should be noted when preheating a hot oil pump? The preheating process must be correct. The general flow is: pump outlet pipeline → inlet and outlet crossover pipe → preheating line → pump body → pump inlet. The preheating valve should not be opened too wide to prevent the pump from reversing. The preheating rate of the pump body should generally not be too fast, and it should be less than 50°C/h. In special cases, the preheating speed can be increased by methods such as introducing steam or hot water into the pump body. During preheating, the pump shaft should be rotated 180° every 30–40 minutes to prevent bending due to uneven heating on the upper and lower parts of the shaft. The cooling water systems for the bearing housings and pump bases must be turned on to protect the bearings and shaft seals. 15. What should be noted after the hot oil pump is taken out of use? The cooling water for each section cannot be stopped immediately; it must be halted only after the temperature of each section has dropped to normal levels. It is strictly prohibited to wash the pump body with cold water, as this may cause the pump body to cool down too quickly and deform. Close the pump’s outlet valve, inlet valve, and inlet/outlet connection valve. Turn the pump 180° every 15–30 minutes until the pump body temperature drops below 100°C. 16. What are the reasons for abnormal overheating in a centrifugal pump during operation? Heat generation is a manifestation of mechanical energy being converted into thermal energy. Common causes of abnormal heating in pumps include: Heat generation accompanied by noise is usually due to damage to the bearing ball retainers. The bearing retainers in the bearing housing are loose, as are the front and rear pressure caps; friction causes heating. The bearing hole is too large, causing the outer ring of the bearing to become loose. There is a foreign object inside the pump. Severe rotor vibration causes wear of the sealing ring. The pump is evacuated or the load on the pump is too high. The rotor is unbalanced. Too much or too little lubricating oil, as well as substandard oil quality. 17. What are the causes of vibration in centrifugal pumps? The rotor is unbalanced. The pump shaft is out of alignment with the motor, and the rubber gaskets on the coupling are worn out. Excessive wear of the bearings or sealing rings causes the rotor to become eccentric. The pump is evacuated or there is gas inside the pump. The suction pressure is too low, causing the liquid to vaporize or nearly vaporize. The axial thrust increases, causing shaft misalignment. The bearings and packing are not properly lubricated, resulting in excessive wear. The bearing is worn or damaged. Local blockage of the impeller or vibration of external connected pipelines. Too much or too little lubricating oil (grease). The stiffness of the pump foundation is insufficient, and the bolts are loose. 19. What should be the appropriate flow rate of cooling water when the pump is operating normally? When the pump is operating normally, the flow rate of the cooling water should be set so that the discharge temperature remains around 40°C. At this time, the water discharged is slightly warm to the touch, providing the best cooling effect. In particular, the cooling water flow for end-face sealing should not be set too high in winter, to prevent the sealing rings of the moving and stationary rings from hardening due to low water temperatures, thereby losing their elasticity and causing the seal to fail. 20. What are the symptoms when a centrifugal pump is evacuated? When a pump in operation starts to lose vacuum, it suddenly emits noise and vibration, accompanied by a decrease in pressure and flow rate as well as a reduction in current. When the vacuum level becomes severe, the pump experiences intense vibration, the pressure drops to zero, and no liquid is discharged from the pump. 21. What are the reasons for the heating of the bearing housing? The pump shaft is not aligned with the motor shaft. Too much or too little lubricating oil (grease), or the oil (grease) has deteriorated. The oil ring is broken or has shifted out of its fixed position. The axial thrust increases. Insufficient or interrupted cooling water. Bearing wear. The flow rate is too low, causing the pump to be nearly emptied. Vibration of the pump caused by factors such as a weak foundation, unbalanced rotor, and loose bolts. 22. Why is it necessary to protect pumps from freezing in winter? Water expands in volume when the temperature drops below zero degrees. If the water remaining inside the pump is not removed, the force generated by this expansion at low temperatures can cause the pump to crack, leading to unnecessary damage. The main methods for preventing freezing are as follows: Drain the water remaining in idle pumps. Keep the cooling water flowing continuously. Insulate the pump or use steam or hot water for heating. The standby pump maintains flow in and out. 23. What should be done if a pump freezes? Once the pump freezes, it must never be blown with steam directly, to prevent it from breaking due to uneven thermal expansion of the pump body. If the pump freezes, first pour cold water on it; once it starts moving, it can be treated with steam or hot water. 24. Why does the pump body need to be cooled? 1) Measures to balance the axial force in single-stage pumps include: ① Using a double-suction impeller ； ②Open balance hole ； ③Balance tube 2) Measures for balancing the axial force in multi-stage centrifugal pumps include: symmetrical arrangement of impellers ； ②A balance drum with balance pipes is used ； ③A balance disc and balance tube are used ； ④A combined balancing mechanism of a balance drum, a balance disc, and balance pipes is adopted. During operation, fluid flow friction occurs between the liquid and the pump body. Friction also arises between the rotating parts and the fixed parts, such as the balls in bearings along with their inner and outer rings, as well as between shaft sleeves and packing. Friction generates heat, and since the temperature of the medium is high, this heat is transferred to the pump, causing the pump body to heat up. The purpose of cooling is to reduce the temperature of the pump body, pump base, bearing housing, and shaft seals, thereby preventing these components from deforming, aging, or being damaged due to increased temperatures. 25. Under what circumstances should a motor be stopped urgently? When personal safety is at risk. When the motor is smoking or emitting an odor, or is on fire. When there is significant vibration or axial movement. When the fuselage or bearings heat up to their limit. The motor runs slowly and makes abnormal noises. 27. How to identify cavitation? There is a loud noise inside the pump. The outlet pressure is unstable; in severe cases, no flow can be achieved. Vibration of the pump body. The medium temperature rises. 28. How are pumps classified? Pumps are generally classified into two main categories based on the principle by which they act on liquids: vane pumps and positive displacement pumps. Vane pumps transport liquid using an impeller that rotates inside the pump. Depending on the structure of the vanes within the pump, vane pumps are classified into centrifugal pumps, axial flow pumps, and vortex pumps, among others. Positive displacement pumps, on the other hand, transport liquid by utilizing the periodic changes in the volume of the pump’s working chamber. They are divided into reciprocating pumps (piston pumps, plunger pumps, diaphragm pumps, etc.) and rotary pumps (gear pumps and screw pumps, etc.). Pumps are also often named according to their purpose, such as water pumps, oil pumps, ammonia pumps, liquid hydrocarbon pumps, slurry pumps, corrosion-resistant pumps, condensate pumps, etc. 29. How is the axial force of a centrifugal pump eliminated? 1) Measures to balance the axial force in single-stage pumps include: ① Using a double-suction impeller ； ②Open balance hole ； ③Balance tube 2) Measures for balancing the axial force in multi-stage centrifugal pumps include: symmetrical arrangement of impellers ； ②A balance drum with balance pipes is used ； ③A balance disc and balance tube are used ； ④A combined balancing mechanism of a balance drum, a balance disc, and balance pipes is adopted. 30. Under what circumstances does a pump need to be cooled? What is the purpose of cooling water? When the temperature of the medium pumped by the pump is above 100°C, the bearings need to be cooled; when it is above 150°C, the sealing chamber generally needs to be cooled; and when it is above 200°C, the pump’s supports usually need to be cooled. The function of cooling water is to lower the temperature of the bearings ； Removes the small amount of liquid that leaks from the shaft seal and dissipates the friction heat ； Reduce the temperature of the stuffing box, improve the operating conditions of the mechanical seal, and extend its service life ； Cooling pump support, to prevent misalignment of the pump and motor due to thermal expansion ； Cooling water should preferably be recycled water or fresh water; softened water used in a recycled manner is employed only when its total hardness exceeds 4.5 milliequivalents per liter. 31. What is cavitation? When the pressure at the inlet of the impeller is lower than the saturation pressure of the working medium, it causes a portion of the liquid to evaporate (that is, to vaporize). After evaporation, when bubbles enter areas with higher pressure, they condense suddenly due to the pressure, and the liquid surrounding them flows in to fill that space, resulting in a hydraulic shock. This phenomenon is known as cavitation. This continuous local impact load causes gradual fatigue damage to the material’s surface, leading to erosion of the metal surface and the formation of honeycomb-shaped cavities of various sizes. The instability associated with cavitation results in vibration and noise in the pump; moreover, as bubbles block the channels within the impeller during cavitation, both flow rate and head are reduced, and efficiency declines. Therefore, it is necessary to prevent cavitation from occurring.