Basic knowledge requirements for centrifugal pumps that operators must possess before taking up their duties

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Working principle of centrifugal pumps: 1. The electric motor drives the impeller to rotate at high speed, generating centrifugal force. Due to this 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 causes the liquid to be continuously drawn in or discharged from the pump, as the impeller keeps rotating. Functions of lubricating oil (grease): lubrication and cooling, flushing, sealing, vibration damping, protection, and load relief. What are the three stages of filtration that lubricant must go through before use? Stage 1: Between the original container of the lubricant and the fixed container ； Level 2: Fixing between the oil barrel and the oil jug ； Level 3: Between the fuel tank and the fuel filling point. The “five fixed principles” for equipment lubrication: 4. Fixed location: Fill oil at the designated locations as specified ； Timing: Apply lubricant to the lubricated parts at specified times, and change the oil regularly ； Quantitative: Refuel in accordance with the specified quantity ； Quality control: Select different lubricants based on the type of equipment, and ensure that the oil quality remains satisfactory ； Designated person: Each refueling station must have a designated person in charge. What are the hazards of water contamination in lubricating oil for pumps and motors? 5. Water can reduce the viscosity of the lubricating oil, weaken the strength of the oil film, and diminish its lubricating effect. 2. Water freezes when the temperature is below 0°C, which severely affects the low-temperature fluidity of lubricating oil. 3. Moisture can accelerate the oxidation of lubricating oil and promote the corrosion of metals by low-molecular-weight organic acids. 4. Moisture increases the foaming tendency of lubricating oil, making it prone to forming foam. 5. Moisture can cause metal parts to rust. What are the tasks involved in the maintenance of mechanical pumps? 6 1. Strictly abide by the post responsibility system as well as various rules and regulations regarding equipment maintenance. 2. Equipment lubrication shall follow the \"five fixed principles\" and \"three-stage filtration\"; lubrication equipment must be complete and clean. 3. Maintenance tools, safety facilities, fire-fighting equipment, etc. are complete and in good condition, and are neatly arranged. Standard values for common shaft seal leaks: Packing seals: less than 20 drops per minute for light oils, and less than 10 drops per minute for heavy oils. Mechanical seals: less than 10 drops per minute for light oils, and less than 5 drops per minute for heavy oils. Why can’t a centrifugal pump be started when its impeller doesn’t rotate? 8 If the impeller of a centrifugal pump doesn’t rotate, it indicates that there is a fault inside the pump; this fault could be caused by something getting stuck in the impeller, excessive bending of the pump shaft, rusting of the moving and stationary parts of the pump, or high pressure inside the pump. If the pump is attempted to start without the pump disc rotating, the powerful force of the motor will cause the pump shaft to rotate forcefully, leading to damage to the internal components such as a broken or twisted pump shaft, a damaged impeller, burned-out motor coils, and it may also cause the motor to trip, resulting in a failed start. What is the function of seal oil? 9 1. Cooling the sealed components ； 2. Lubricated friction pair ； 3. Prevent vacuum-induced damage. Why is it necessary to regularly turn the standby pump by hand? There are three purposes to regular manual rotation: 1. To prevent scaling and jamming inside the pump ； 2. Prevent pump shaft deformation ； 3. Turning the shaft also delivers lubricating oil to various lubrication points, preventing the shaft from rusting; lubricated bearings facilitate immediate operation in emergency situations. Reasons for abnormal heating in operating centrifugal pumps 11. Heating is a manifestation of mechanical energy being converted into thermal energy. Common causes of abnormal heating in pumps include: 1. Heating accompanied by noise, usually due to damage to the bearing ball retainers ； 2. The bearing retainers in the bearing housing are loose, as are the front and rear pressure caps; heating occurs due to friction ； 3. The bearing hole is too large, causing the outer ring of the bearing to become loose ； 4. There are foreign objects inside the pump ； 5. Excessive rotor vibration causes wear of the sealing ring ； 6. The pump is evacuated or the load on the pump is too high ； 7. Rotor imbalance ； 8. Excessive or insufficient lubricating oil, as well as substandard oil quality. Reasons for vibration in centrifugal pumps 12 1. Unbalanced rotor ； 2. The pump shaft is out of alignment with the motor, and the rubber gaskets on the coupling are worn out ； 3. Excessive wear of the bearings or sealing rings, resulting in rotor eccentricity ； 4. The pump is evacuated or there is gas inside the pump ； 5. The suction pressure is too low, causing the liquid to vaporize or nearly vaporize ； 6. The axial thrust increases, resulting in shaft misalignment ； 7. Improper lubrication of bearings and packing, resulting in excessive wear ； 8. Bearing wear or damage ； 9. Local blockage of the impeller or vibration of external connecting pipelines ； 10. Excessive or insufficient lubricating oil (grease) ； 11. The stiffness of the pump foundation is insufficient, and the bolts are loose. Standards for vibration and bearing temperature of centrifugal pumps 13: The vibration standard for centrifugal pumps is that the rotation speed should be less than 1500 vpm, with vibration levels below 0.09 mm. Rotation speed: 1500–3000 vpm, vibration: less than 0.06 mm. The standard bearing temperature is: less than 65°C for sliding bearings, and less than 70°C for rolling bearings. When the pump is operating normally, what should be the appropriate flow rate of cooling water? For a pump operating under normal conditions, it is most suitable to keep the discharge water temperature at 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. What are the symptoms when a centrifugal pump starts to evacuate? 15 When a pump in operation begins to evacuate, it suddenly generates noise and vibration, accompanied by a decrease in pressure and flow rate as well as a reduction in current. When the suction condition is severe, the pump experiences intense vibration, the pressure drops to zero, and no liquid is discharged from the pump. Reasons for bearing housing overheating 16 1. The pump shaft is not aligned with the motor shaft. 2. Excessive or insufficient lubricating oil (grease), or degraded oil (grease). 3. The oil ring is broken or has shifted out of its fixed position. 4. Axial thrust increases. 5. Insufficient or interrupted cooling water. 6. Bearing wear. 7. The flow rate is too low, causing the pump to be nearly in a vacuum state. 8. Vibration of the pump caused by reasons such as a weak foundation, unbalanced rotor, and loose bolts. Why is it necessary to protect pumps from freezing in winter? 17 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 volume expansion at low temperatures can cause the pump to crack, resulting in unnecessary damage. The main methods for preventing freezing are as follows: 1. Drain the water remaining in the idle pump. 2. Keep the cooling water flowing continuously. 3. Insulate the pump or use steam or hot water for heating. 4. Keep the inlet and outlet of the standby pump open for flow. What to do if a pump freezes 18 1. Once a pump has frozen, it must never be blown on with steam, to prevent the pump from cracking due to uneven heating. 2. After the pump freezes, first pour cold water on it; once it starts moving, it can be treated with steam or hot water. Why is it necessary to cool the pump body? During operation, fluid flow induces friction between the liquid and the pump body. Friction also occurs between rotating parts and fixed components, 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 the high temperature of the medium further contributes to heating the pump body. 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. Under what circumstances should a motor be stopped urgently? 20 1. When it endangers human safety ； 2. When the motor is smoking or emitting an odor, or is on fire ； 3. When there is significant vibration or axial movement ； 4. When the aircraft body or bearings heat up to their limit ； 5. The motor runs at a low speed and produces abnormal noises. Explanation of the models of domestic centrifugal oil pumps: 21 100 – Pump suction inlet diameter; Y – Single-stage centrifugal pump; II – Code for the material used in the pump; the second type is carbon steel, which is not resistant to corrosion at temperatures of 200–400°C; A – Outer diameter of the impeller after the first machining. How to identify cavitation: 22 1. Intense noises occurring inside the pump ； 2. The outlet pressure is unstable; in severe cases, no flow can be achieved ； 3. Pump body vibration ； 4. Increase in medium temperature.