Seeking advice! Difficulties encountered during the operation of centrifugal pumps!

Could it be the cause of bearing pitting? Could some expert give me some advice!

I personally think it might be due to pit corrosion

Excerpted from Baidu Baike; cave erosion, for reference only. 【Formation of pitting】 1. Diesel engine pitting The surface of the outer wall of the wet cylinder liner in a diesel engine that comes into contact with the coolant is damaged, forming pin-like holes; these holes gradually expand and deepen, eventually resulting in deep holes or cracks ； The holes are generally clean, with no corrosion products present. The wet cylinder liner is in direct contact with the coolant, and it is inevitably subject to gradual pitting. It is generally believed that pitting is caused by the high-frequency vibrations of the cylinder liner. Mechanical vibration causes changes in coolant pressure, leading to the formation and collapse of bubbles within the coolant; this process is what causes cavitation. Due to the presence of gases in the coolant, when high-frequency vibrations of the cylinder cause the local pressure of the coolant to drop to a certain critical value, the gases dissolved in the coolant separate out in the form of bubbles. These bubbles move toward areas of higher pressure, and they burst when the pressure there exceeds the pressure inside the bubbles. The gas in bubble form re-liquids or dissolves into the coolant, causing its volume to decrease sharply. The coolant moves at high speed toward the center of the bubbles, resulting in water hammer effects that generate immense force and high temperatures. These forces are transmitted at supersonic speeds in the form of pressure waves; when they impact the outer surface of the cylinder liner, they cause significant shock, compression, and high temperatures. Under the repeated action of such forces, fatigue occurs on the outer surface of the cylinder liner, leading to its gradual detachment; this results in pitted and pinhole-like defects that expand as cavitation progresses. 2. Bearing Cavitation: Under the repeated impact loads from cylinder pressure, the surface layer of sliding bearings undergoes plastic deformation and cold working hardening, losing its ability to deform in certain areas. Patterns are formed and these patterns continue to expand; subsequently, as wear debris falls off, cavities are created in the stressed surface layer. When pitting occurs in bearing bushes, pits appear first; these pits then gradually enlarge, causing cracks to form at the interface of the alloy layers. The cracks extend parallel to this interface until the material peels off. The main cause of pitting in sliding bearings is the intense disruption of oil flow resulting from sudden changes in the cross-sections of structural elements such as oil grooves and oil holes. Bubbles form in the vacuum areas created by this disrupted oil flow, and these bubbles burst due to rising pressure, thereby causing pitting. Pitting generally occurs in the high-load areas of bearings, such as the lower bearing shell of the crankshaft main bearings. 【Harm of pitting】 Hydraulic components in construction machinery have high precision, with small clearance between moving parts. Pitting can cause the mating surfaces to darken or even develop small pits, leading to valve stem jamming and pressure imbalances; in severe cases, the machinery may fail to function properly. When pitting on the cylinder liner is mild, its surface is locally relatively clean or appears to be polished ； When pitting is severe, the damaged areas of the cylinder liner develop deep, irregularly shaped dents, as if the surface of the cylinder liner has been corroded by a strong acid ； If pitting becomes severe enough, the pits can penetrate the cylinder liner wall, allowing the cooling fluid inside the engine to enter the cylinders, which can lead to serious damage to the engine. 【Factors Affecting Cavitation】 (1) Oil quality The quality of the oil has a direct impact on the operation of various systems; if the oil has poor antifoaming properties and tends to vaporize and form foam, it is more likely to cause cavitation. 　　(2) Oil level too high or too low 　When the oil level is too high, the oil is subject to mechanical agitation, which can cause foaming ； When the oil level is too low, the working pump is prone to drawing in air, which results in an insufficient flow rate of the circulating oil and increases the likelihood of air bubbles or water vapor forming within the oil. For example, if the transmission oil level is too low, the pressure at the inlet of the transmission pump will be lower than the pressure required for oil-gas separation; as a result, the liquid vaporizes or evaporates, forming bubbles. When these bubbles reach high-pressure areas, they burst rapidly, thereby causing cavitation. 　　(3) Oil overheating 　When the oil temperature is too high, the oil vaporizes, water evaporates, foam and air bubbles increase, and the rate of cavitation erosion rises. 　　(4) High frequency of oil pressure changes The frequency of changes in the oil level directly affects the speed at which air bubbles form and burst; in areas where pressure changes frequently, cavitation occurs at a faster rate. In construction machinery, the boom and bucket need to be operated continuously during operation; as a result, at the positions in the control valve where the spools correspond to the oil passages, corrosion occurs most severely due to the high frequency of pressure changes. This leads to blackening caused by carbon buildup, along with small pits and scratches. 　　(5) Poor quality of cooling water When cooling water contains corrosive substances and anionic ions, it leads to a combined effect of corrosion and pitting, accelerating the rate of damage caused by pitting. 　　(6) Air and moisture entering the oil The more air and moisture that enter the hydraulic system, the greater the area affected by cavitation. The main way air gets in is due to a poor seal in the low-pressure area at the pump inlet, which allows the hydraulic pump to draw in air. The main pathway for moisture intrusion is leakage inside the oil cooler. 　　(7) Influence of the fit clearance at the mating surfaces The greater the clearance between the piston and the cylinder liner, as well as between various hydraulic valves, the greater the vibration intensity, which in turn exacerbates pitting. 　　(8) Influence of the structure 　　Since the root cause of pitting is vibration, and the intensity of vibration is directly related to the structural stiffness. Pitting is likely to occur when the structural stiffness is low. For example, the structure of the steel cylinder, the structure of the piston, and the cross-sectional shape of the water chamber are all related to pitting corrosion. 　　(9) Poor maintenance of the cooling system The radiator pressure cap and thermostat have a significant impact on cylinder liner pitting; a good radiator pressure cap can maintain the coolant pressure at a level higher than the vapor pressure, thereby reducing the occurrence of pitting. The thermostat keeps the coolant within an appropriate temperature range, which reduces the energy released when bubbles burst; therefore, abnormal temperatures and pressures resulting from poor maintenance will accelerate the rate of pitting. 　　(10) Operating conditions The working environment for construction machinery is harsh; especially in the case of diesel engines, the high maximum explosion pressure leads to increased lateral knocking forces, which in turn causes pitting. 【Prevention of pitting】 1. Prevention by category of factors: 1) Usage factors a. The coolant temperature at which pitting is most likely to occur is 40°C–60°C; therefore, it is necessary to keep the coolant at its normal operating temperature (80°C–85°C) during use. b. The coolant specified by the manufacturer must be used; hard water or coolant containing many impurities should not be used. Experiments show that hard water containing salts and alkalis has a pitting rate dozens of times higher than that of clean soft water. c. Minimize the time spent idling, under heavy load, or operating beyond capacity. d. Add fuel as specified to reduce the rough operation of the diesel engine caused by combustion. 　　(2) Maintenance factors: a. Replace the coolant as specified and clean the cooling system promptly to keep it in good technical condition at all times. b. Adjust the fuel injection timing and various components of the fuel system as specified, to maintain optimal performance of the fuel system and reduce the damage caused by abnormal combustion. c. Maintain proper cylinder clearance, crankshaft bearing clearance, and connecting rod bearing clearance to reduce cylinder knocking and vibration. d. When installing components such as cylinder liners, pistons, and connecting rods, it is necessary to ensure proper verticality and parallelism as much as possible, in order to reduce abnormal impacts of the piston on the cylinder liner. 　　(3) Manufacturing factors a. Coat the side walls of the water jacket with pitting-resistant materials and vibration-damping substances. b. Increase the hardness of the outer surface of the cylinder liner ； Minimize the roughness of the outer surface ； Choose materials with strong resistance to pit erosion. c. Choose an appropriate cylinder liner wall thickness to increase its stiffness and avoid resonance. 2. Prevention of cavitation in construction machinery: (l) Select oil that ensures good quality and appropriate viscosity. 　　Care must be taken to keep it clean when refueling; do not allow water or impurities to get in ； When adding cooling water, it must not contain corrosive substances. 　　(2) Fill the oil strictly according to the dipstick specifications. The dipsticks for various oils in construction machinery are marked with upper and lower limits; maintaining the proper oil level helps to reduce pitting. 　　(3) Prevent excessive oil temperature: If the machine operates for too long, it should be shut down to cool down; if it is due to other reasons, an analysis should be conducted to resolve the issue. Issues such as blocked oil passages, insufficient cooling water, or internal/external leaks should be resolved promptly. 　　(4) Reduce hydraulic shock: Operate the various hydraulic control valves and distribution valves slowly and gently; also, do not increase the throttle too frequently in order to reduce hydraulic shock. 　　(5) Maintain the normal clearance at each joint surface. During manufacturing or repair, assembling according to the lower limit of the assembly tolerances can reduce the impact of pitting. If pitting has occurred, carbon buildup can only be removed by polishing with metallographic paper; it is absolutely not advisable to use ordinary fine-grit sandpaper for this purpose. 　　(6) Keep the hydraulic system clean to prevent air and moisture from entering. Clean the magnetic filter regularly. Replace various filter elements to keep the oil clean. Pay attention to checking the oil level, quality, and color. If water droplets are found in the hydraulic system, if the oil turns milky white, or if the oil is foamy, it is necessary to carefully analyze the source of water and air, and check the sealing at the oil cooler and the inlet of the hydraulic pump. 　　(7) Reducing cylinder barrel vibration 　　① Increase the stiffness of the cylinder barrel; when purchasing a cylinder barrel with a wall thickness of 0.08D or more (where D is the diameter of the cylinder), pitting rarely occurs. 　　②To reduce the impact force of the piston on the cylinder wall, use reinforced pistons or those with a uniform weight distribution, thereby reducing vibrations and minimizing cavitation. 　　③A one-way chamfered oil ring is used, with the upper ring scraping oil downward and the lower ring scraping oil upward, thereby facilitating the formation of an oil film between the cylinder liner and the piston to provide vibration damping and reduce pitting. 　　(8) Proper maintenance of the cooling system Ensure that the radiator pressure cap is in good condition and functioning properly, thereby reducing the occurrence of pitting ； Keep the temperature of the cooling system within an appropriate range to reduce the energy required for bubble rupture, and use an appropriate amount of coolant additives to prevent rusting.

It was clearly stated upstairs; think it through by yourself.

Air and moisture penetrate into the oil. The more air and moisture that enter the hydraulic system, the greater the area affected by cavitation. The main way air gets in is due to a poor seal in the low-pressure area at the pump inlet, which allows the hydraulic pump to draw in air. The main pathway for moisture intrusion is leakage inside the oil cooler. 4 [5 Q9 p' u5 M' B! I% L I’ve encountered this problem too – the lubricant turned milky! ~

As the original poster said, in our area, there are no bubbles in the oil of smaller equipment, but a lot of foam appears in the fuel tanks of larger equipment. Please provide clarification on this

Attention, all sailors: it’s a centrifugal pump! It’s normal for the lubricating oil to have some foam in a pump that has just been started! It will be fine in a bit!

Temperature changes, or operational agitation

It is normal to have foam in the oil test of vertical centrifugal pumps. In the oil inspection of horizontal centrifugal pumps, there should be no foam or only minimal foam; the high-speed rotation of the pump shaft can also cause the oil to form bubbles. If the amount of foam is large, it may indicate that the oil has deteriorated.

Has water gotten in? Check to see if the oil has been emulsified.

I thank all of you for your guidance and explanations; they have helped me gain new insights. The pump in my unit is a standard horizontal centrifugal pump, and bubbles start to appear as soon as it is turned on. Even after replacing the oil, bubbles still appear immediately! Will it affect normal production? Is there any way to get rid of the bubbles?

Bubbles are still forming even after changing the oil!

The high-speed rotation of the pump shaft causes the oil to foam, and the pressure inside the bearing housing can easily exceed normal levels; this is why there is a small vent device on the bearing housing

Since it is already indicated as “Solved” in the title, I hope the original poster will clearly state the answer to everyone.

What was said upstairs is correct; since it’s already resolved, could you please tell us the answer?