Causes of cavitation and gas entrapment in centrifugal pumps, treatment methods, and safety operating procedures

Thread Content

 A detailed explanation of cavitation and gas entrapment in centrifugal pumps: The phenomenon of cavitation – As can be understood from the working principle of centrifugal pumps, when the liquid between the blades is ejected from the rapidly rotating impeller, a low-pressure area is formed near the inlet of the impeller. When the pressure at the inlet of the impeller is equal to or lower than the saturated vapor pressure pV of the liquid being transported at the operating temperature, the liquid there will vaporize and form bubbles. When the bubbles flow with the liquid to a high-pressure area, they rapidly condense due to the pressure. At the moment the bubble collapses, a local vacuum is created, and the surrounding liquid rushes into the space previously occupied by the bubble at high speed, resulting in shock and vibration as well as a significant force. Especially when the condensation point of the bubbles is near the blade surface, numerous liquid particles strike the blade at high frequency and pressure ； At the same time, the bubbles may also contain small amounts of oxygen and other substances that can cause chemical corrosion of metal materials. Under the combined effect of continuous impact and chemical corrosion, the surface of the blade is damaged, resulting in scars and cracks, which leads to premature failure of the blade; this phenomenon is known as cavitation in centrifugal pumps. Air locking is a phenomenon that occurs when starting a centrifugal pump. If there is air inside the pump, its low density results in a weak centrifugal force generated upon rotation; as a result, the low pressure created in the center of the impeller is not sufficient to draw in liquid. Consequently, even though the centrifugal pump starts up, it is unable to perform its pumping function. This phenomenon is known as air locking. This means that the centrifugal pump does not have self-priming capability; therefore, the pump must be filled with the liquid to be transported before it is started. Of course, if the suction inlet of the centrifugal pump is placed below the liquid level of the fluid to be pumped, the liquid will flow into the pump automatically; this is a special case. The suction line of the centrifugal pump is equipped with a foot valve to prevent the liquid filled in before startup from flowing out of the pump. A filter screen prevents solids in the liquid from being drawn in and causing blockages. The control valve installed in the discharge line of the pump is used for starting and stopping the pump as well as regulating flow rate. From the perspective of the different causes of cavitation and air locking: air locking occurs when there is air inside the pump, usually during the start-up of the pump, and is mainly manifested by the failure to remove all the air from within the pump ； Cavitation occurs when a liquid reaches its vaporization pressure at a certain temperature; it is thus closely related to the medium being transported and the operating conditions. The following methods can be used to prevent gas entrapment: 1. Fill the shell with liquid before starting up. Ensure proper sealing of the casing; the valves and faucets used for watering must not leak, and good sealing is essential. ⒉The suction line of a centrifugal pump is equipped with a foot valve to prevent the liquid filled prior to startup from flowing back out of the pump. The filter screen prevents solids in the liquid from being drawn in. The discharge pipeline is equipped with a control valve for starting and stopping the pump as well as for regulating flow rate. ⒊Place the suction inlet of the centrifugal pump below the level of the liquid to be transported, and the liquid will flow into the pump automatically. Reasons for cavitation and solutions The main causes of cavitation are: 1. Excessive resistance in the inlet pipeline or overly small diameter of the pipe; 2. Too high temperature of the medium being transported ； 3. Excessive flow rate, that is, the outlet valve is opened too wide ； 4. The installation height is too high, affecting the pump’s suction capacity ； 5. Selection issues, including the selection of the pump and the material used for the pump. Solutions: 1. Remove any debris from the inlet pipeline to ensure unobstructed flow, or increase the diameter of the pipeline ； 2. Lower the temperature of the conveying medium ；  3. Reduce installation height ； 4. Select a different pump, or improve certain components of the pump, such as by using materials resistant to cavitation. Calculation of the maximum installation height for centrifugal pumps: To avoid cavitation, the installation height of centrifugal pumps must be carefully calculated and verified. The schematic diagram of the installation configuration on the water inlet side of the pump is as follows: The allowable geometric installation height of the pump depends on various factors, and the formula is as follows: Where: — Allowable geometric installation height of the pump, in meters ； (The calculation results are for use in design; the actual installation height must be lower than the allowable installation height.) pe—pressure at the water absorption surface, Pa ； (The atmospheric pressure at the water surface for water absorption; the higher the altitude, the lower the atmospheric pressure.) pv—saturated vapor pressure, Pa ； (It is related to water temperature; the higher the water temperature, the higher the saturated vapor pressure.) ρ — density of the fluid, kg/m3 ； g—acceleration due to gravity, 9.81m/s2 ； —Allowable net positive suction head of the water pump, m ； (Related to the performance of the water pump, provided by the pump manufacturer) hw—head loss in the suction pipeline, m. (It is related to the design of the suction pipeline and is determined by the designer.) As can be seen from the above formula: the higher the elevation, the higher the water temperature, the greater the allowable cavitation head, the greater the head loss in the inlet pipeline, and the lower the allowable geometric installation height. The atmosphere at different altitudes and the corresponding head heights are shown in the table below. The saturated vapor pressure of water at different temperatures and their corresponding head heights are also listed in the table below. Example: For a VISO125X100-315-55/2 water pump manufactured by a certain brand, its net positive suction head is 3.29 m. This pump is intended to operate at an altitude of 500 m, where the maximum water temperature in summer is 40°C. Given that the head loss in the suction pipe is 1 m, the required geometric installation height of the pump at this location can be calculated as follows: Assume that the pressure at the water surface being drawn from equals the local atmospheric pressure; according to the table, the atmospheric head at an altitude of 500 m is 9.7 m ； At a water temperature of 40°C, the saturated vapor pressure head of water is 0.752 m ； The calculation shows: =9.7-0.752-3.29-1=4.658 m. Safety operating procedures for centrifugal pumps: I. Preparations before starting 1. Check the condition of the pump equipment to ensure it is in good working order. 2. The bearings are properly lubricated, the oil level is normal, and the oil quality is satisfactory. 3. Open all the inlet valves of the centrifugal pump. 4. Fill the pump with water or use a vacuum pump to draw in water (excluding backflow), then open the vent valve to release air. 5. Check for leaks at the shaft seal; a slight drip from the packing seal is acceptable. 6. The motor rotates in the correct direction. II. Starting the pump 1. Before operation, check whether the pump rotates smoothly ； Mechanical seals should exhibit no friction ； Is the oil level in the oil reservoir normal? ； Is the rotation direction of the motor correct? 2. Close the pump outlet valve ； 3. For mechanical seals that have external flushing, the flushing fluid should be turned on before startup to fill the seal chamber with the sealing fluid ； 4. Check whether the liquid intake is functioning properly (to fill the pump chamber with liquid) ； 5. Connect the power supply; once the motor reaches its rated speed, gradually open the outlet valve (the pump cannot operate continuously for more than 3 minutes with the pump’s outlet valve closed). III. Operation of the pump 1. Regularly check the heating condition of the pump and motor (bearing temperature ≤ 75°C) ； 2. Regularly check the sealing performance of the mechanical seal ； 3. The amount of lubricating oil for the bearings should be such that it is about 2 mm above the center line of the oil level gauge ； 4. Check whether parameters such as motor current and temperature are within the specified range ； 5. Do not use the valve on the suction pipe to adjust the pump flow rate, to avoid cavitation ； 6. The pump cannot operate for extended periods at a flow rate lower than 30% of its design flow rate. IV. Parking 1. Slowly close the valve on the outlet pipeline, and turn off all instrument switches ； 2. Cut off the power supply ； 3. Turn off the seal flushing fluid. V. Maintenance and Care 1. Keep the equipment surface clean ； 2. Regularly inspect the motor’s insulation performance ； 3. When the ambient temperature in winter is below the freezing point of the liquid, the liquid inside the pump should be drained after shutdown to prevent it from freezing and cracking ； 4. When the pump is out of use for an extended period, it is necessary to thoroughly clean the equipment. The pump body should be disassembled, and any corrosive substances on all components must be wiped away. In particular, the sealing area needs to be carefully rinsed and reassembled; rust preventive oil should be applied, and the pump’s inlet and outlet ports should be sealed before storing it properly. VI. Use of Mechanical Seals 1. During maintenance, take strict precautions to prevent solid impurities from entering the mechanical seal surface, which could cause the seal to fail ； 2. When using mechanical seals in media that tend to crystallize, it is important to flush them regularly; after shutting down the system and restarting it, the crystallizations accumulated on the mechanical seals must be cleaned off ； 3. When removing the mechanical seal, care must be taken; it is not allowed to use hammers or similar tools for striking, to avoid damaging the moving and stationary sealing surfaces ； 4. The relative friction sealing surfaces of the moving ring and the stationary ring must be carefully inspected; no minor scratches, damages, or other defects are allowed. All parts should be thoroughly rinsed, wiped clean with a soft cloth, and then coated with grease or oil ； 5. During assembly, care should be taken to eliminate deviations; when tightening screws, apply even force ； 6. Adjust the compression amount of the spring properly: After the pump is assembled, when turning the rotor by hand, it should be felt that the sealing spring has an appropriate level of compression, allowing the rotor to rotate smoothly and flexibly without any feeling of binding.