Decelerating centrifugal pump

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Currently, high-speed centrifugal pumps are primarily used in petrochemical production to transport substances such as liquid ammonia, methylamine, and liquefied gas. It consists of three parts: a motor, a speed increase gearbox, and a pump. Generally, high-speed centrifugal pumps are divided into horizontal high-pressure cylindrical pumps and partial-flow pumps. 1. The horizontal high-pressure cylindrical pump is a multi-stage centrifugal pump; under normal circumstances, a horizontal design is used when the driving power exceeds 160 kW. 2. Partial flow pumps are a type of open-radial-blade centrifugal pump, usually of vertical design. Their driving power ranges from 7.5 to 132 kW. The impeller of such pumps is mounted cantileveredly on the pump shaft, which is directly connected to the high-speed shaft of the speed increase gearbox; the structural diagram is shown in Figure 1. The fluid pressure chamber of some flow pumps is a circular space; a conical diffuser tube is connected to this circular pressure chamber via a nozzle, and it is arranged near the tangent direction, as shown in Figure 2. When the pump is in operation, the liquid enters the impeller axially through the suction pipe. Under the effect of centrifugal force, the liquid is thrown toward the circular pressure chamber; part of it flows out of the pump through the nozzle and the diffuser tube, while the remaining liquid continues to rotate along with the impeller. (1) Pump body, impeller: The pump body of this type of pump is different from that of ordinary centrifugal pumps; its liquid-pressure chamber is a circular space, rather than a volute shape. The impeller is an open-type impeller without front or rear cover plates; its blades extend in a straight radial pattern. In front of the impeller, there is usually an inducer wheel. There is no sealing ring between the impeller and the pump casing, and the clearance inside the pump is large. The gap between the impeller blades and the rear cover plate of the pump body as well as the diffuser cone is generally 2–3 mm; it can still be used if it reaches 3–4 mm without affecting efficiency. (2) Gearbox: The gearbox is mainly composed of gears, and there are two basic types: single-stage gear increase and two-stage gear increase. To avoid the generation of axial forces, gearboxes typically use spur gears with a small module. Due to the high rotational speed, high precision is required in gear manufacturing; the pitch error is generally 2-3 micrometers. Additionally, the gears are made of special steel that has been treated through nitriding or carburizing. The gearbox housing is divided into upper and lower halves, usually positioned using set screws. The gearbox housing is made of aluminum alloy with good heat dissipation properties. For low-power pumps, babbitt bearings are used for the bearings on the high-speed shaft, while for pumps with a power of 150 kW or more, a combination of segmented sliding bearings and face thrust bearings is employed. The lubrication of the speed increase gearbox is accomplished by an integrated oil pump, which sends oil through an oil filter and an oil cooler to various oil nozzles within the gearbox housing; these nozzles spray the oil in a mist form, using this oil mist to lubricate the gears and bearings. 2 Characteristics of high-speed centrifugal pumps 1. Advantages (1) The pump and the speed increase gearbox are generally of enclosed design, allowing them to be installed and used outdoors. (2) Compact structure, small size, light weight, and easy to maintain. (3) An open impeller is used, which generates no axial force during operation; therefore, there is no axial force balancing device in the pump. (4) A cyclone separator is installed inside the pump to purify the liquid pumped by it, directing it to the mechanical seal in order to extend its lifespan. (5) The gap between the impeller and the casing is large, allowing it to be used for transporting liquids containing solid particles and those with high viscosity. (6) An inducer wheel is used to increase the inlet pressure of the pump, preventing cavitation from occurring easily. 2. Disadvantage: High precision is required during processing, making it relatively difficult to manufacture. 3 Maintenance of High-Speed Centrifugal Pumps 3.1 Disassembly, Inspection, and Assembly of High-Speed Centrifugal Pumps 1. Disassembly Sequence 1) Disassembly of the drive unit (1) Disconnect the power cable of the motor or the inlet and outlet pipes of the turbine. (2) Remove the coupling. (3) Remove the connecting bolts between the drive motor and the gearbox, then lift out the drive motor. 2) Pump body disassembly (1) Disconnect the pipelines connected to the pump, such as the oil circuit, sealant system, and cooling system. (2) Remove the pump cover nuts and lift out components such as the gearbox, pump end cover, and impeller. (3) Measure the radial runout of the impeller using a dial indicator. (4) Remove the impeller locking nut, and sequentially remove the inducer wheel, impeller, mechanical seal rotating ring, and shaft sleeve. (5) Remove the mechanical seal stationary ring assembly. (6) Remove the end cap bolts and take out the end cap. 3) Removal of the speed increase gearbox: (1) First, measure the axial play of the high-speed shaft and record it. (2) Remove the connecting bolts between the upper and lower housings of the gearbox, take out the positioning pins, lift the upper housing using a lifting device, and gently strike the lower housing with a wooden hammer to separate the upper and lower housings, after which the upper housing can be removed. (3) Measure the thickness of the adjustment shims on the mid-plane of the upper and lower enclosures and record it. (4) Remove the low-speed shaft assembly, oil pump and limit spring, medium-speed shaft assembly, and high-speed shaft assembly in sequence. 2. Inspection of the clearance between components and assembly adjustment: During the disassembly of a high-speed centrifugal pump, or after the components have been removed and cleaned, inspections, measurements, and assembly should be carried out in accordance with the pump’s operation and maintenance instructions. In the absence of any specific requirements, the LMV/BMP-311, LMV-311, and LMV-322 high-speed pumps can be inspected, measured, and assembled in accordance with the standards specified in SHS 03044-2004, \"Maintenance and Repair Procedures for High-Speed Centrifugal Pumps\". Its maintenance mainly includes the following aspects. 1) Before assembly of the pump (1)’s impeller and inducer, it is necessary to ensure that they are free from damage or deformation; after assembly, the radial runout of the impeller must be less than or equal to 0.38 mm, and the end face runout must be less than or equal to 0.15 mm. (2) The drive key should be free from damage and deformation. (3) During assembly, the pump cover bolts, the bolts connecting the impeller to the inducer, and the bolts used to secure the sealing components should all be tightened using torque wrenches, with the torque values meeting the specified requirements. 2) Speed increase gearbox: (1) The bearing shells of the high-speed shaft should be free from defects such as ablation, adhesion, or wear; after assembly, the clearance value of the bearing shells must meet the requirements specified in the user manual. (2) Thrust bearing shells that are severely worn or have defects should be replaced. (3) The fit between the outer diameter of the bearing and the bushing is H7/h6; after wear, the inner diameter of the bushing should not exceed 0.068 mm ； Before assembling the rolling bearing, it is necessary to ensure that there are no defects such as pitting, scars, or burrs, and that it rotates smoothly; after assembly, its radial clearance should not exceed 0.06 mm. (4) The deep-groove ball bearings on the low-speed shaft or intermediate shaft must be installed in place. (5) When assembling the shaft, gears, and ball bearings, the gears and bearings should be installed while still warm; the shaft journals should be pre-cooled, with their temperature to be kept within the range specified in Table 1. Table 1: Bearing heating and journal cooling temperatures of high-speed centrifugal pumps – Part temperatures: Gear journal –18, rolling bearings 120. 3) High and low speed shafts: (1) The fit between the high-speed shaft and the impeller is of type K7/h6, with the clearance after wear not exceeding 0.03 mm. (2) The straightness of the high-speed and low-speed shaft axes shall not exceed 0.01 mm. (3) After assembly, the axial play of the high-speed shaft should be measured to ensure it is within the range of 0.33 – 0.43 mm. 4) Before assembling the thrust disc, its wear condition should be checked; the radial runout of the thrust disc’s end face should be less than or equal to 0.02 mm. 5) The meshing surfaces of the gears shall be free from defects such as pitting, scarring, or overheating; otherwise, they should be replaced ； The coverage of the gear meshing surface, as determined by coloring, should be greater than 80% in the direction of tooth width and 55% in the direction of tooth height. The clearance at the point where the two gears meet is to be checked using the lead compression method, and it should be within the range of (0.20–0.30)m, where m represents the gear module. The deviation in the parallelism of the two gear shafts should be less than 0.025mm. 6) When disassembling the speed gearbox of the oil system, the internal oil passages and oil nozzles should be carefully cleaned to ensure smooth flow of oil; the diameters of these passages and nozzles must not be enlarged. After assembly is complete, the quality of the oil pump’s assembly should be checked. To do this, press the low-speed shaft downward by hand; it should exhibit flexibility, otherwise the pin is not in place ； Additionally, it is also possible to turn it at the low-speed shaft coupling to check whether the oil pump is working. 7) The installation of the sealing mechanical seal follows the procedures for installing mechanical seals in centrifugal pumps ； When installing the motor shaft seal on the low-speed shaft of the gearbox, in order to prevent damage to the sealing surface during installation, grease should be applied to the seal lip. A special tool should be used for pressing it in; it is not allowed to use an iron rod directly. 8) Slightly worn spline shafts should be repaired, while those with severe wear should be replaced. During assembly, an appropriate amount of 1# extreme pressure potassium acetate mixed with molybdenum disulfide should be applied. 9) The shaft end distance and alignment of the coupling shall meet the requirements specified in the operating instructions. 3.2 Handling of Malfunctions in High-Speed Centrifugal Pump Stations 1. Commissioning of High-Speed Centrifugal Pumps 1) Preparatory work before commissioning (1) Check the maintenance records to ensure that the data is accurate, and prepare all the necessary record forms for the commissioning process ； (2) Fill the gearbox with qualified lubricant to the specified level ； (3) Check the power supply, static electricity connections, and pipeline connections ； (4) Open the water valve of the oil cooler and adjust the appropriate water volume ； (5) Open the pump inlet valve fully, turn on the seal flushing fluid; for those equipped with a series-connected sealing system, open the seal air valve as well ； (6) Open the vent valve on the pump outlet pipeline to exhaust the gas from the pump chamber and the sealing chamber ； (7) Check for leaks at the sealing points ； (8) Contact an electrician to check the motor resistance and supply power to it. 2) Trial operation: (1) Operate the motor in jog mode to confirm that the rotation direction is correct; if no abnormalities are observed, it can be started for operation ； (2) Immediately after startup, adjust the outlet valve to bring the pressure and flow rate to normal levels, and at the same time check whether the motor current is within the normal range ； (3) Control the cooling water flow rate of the oil cooler to maintain the oil temperature between 60 and 90 ； (4) During normal operation, the oil pressure of the oil pump built into the gearbox should be maintained between 0.2 and 0.3 Mpa; the gearbox must not be operated when the oil pressure is below 0.2 Mpa. (5) Normal operation can be resumed within 24 hours once the oil pressure, oil temperature, vibration, noise, and current are all within normal ranges. The vibration value shall not exceed 2.8 mm/s, and the leakage rate of the mechanical seal shall not be more than 1 drop per minute. 2. Fault handling of high-speed centrifugal pumps: The common fault symptoms, causes, and handling methods for high-speed centrifugal pumps are shown in Table 2. Table 2 Common Fault Causes and Solutions for High-Speed Centrifugal Pumps
Serial Number | Fault Symptoms | Fault Causes | Solutions
1 | No flow or insufficient flow, no pressure or low pressure | Pump not filled with liquid; vapor leakage in the inlet pipe; low suction head; motor running in reverse; improper impeller installation; blocked suction line; excessive back clearance of the impeller; gas not fully removed from the pump; liquid not properly cooled; leakage in the liquid handling pipeline, allowing volatile liquids to enter; low suction head | Reconnect wires properly; install the impeller correctly; clean the suction line; adjust the back clearance of the impeller
2 | Large pressure fluctuations, very low flow rate, low suction head | Failure of the flow control valve; two pumps operating simultaneously, causing competition for fluid; severe corrosion of the hydrocyclone; increase flow rate (open a bypass valve if necessary); increase suction head; repair the control valve; stop one of the pumps; repair or replace the hydrocyclone
3 | Leakage of liquid or oil from the mechanical seal | Damaged mechanical seal; solid particles or impurities in the sealing fluid or liquid; moisture introduced when transporting low-temperature liquids | Repair the mechanical seal; replace the filter element to prevent solid particles or impurities from entering; prevent moisture from entering, or fill the seal chamber with an anti-seepage agent (such as ethanol or propanol) before starting
4 | Oil leakage from the oil seal | Damaged oil seal | Replace the oil seal
5 | Excessive oil foam in the gearbox; degraded oil; too high oil level; too low oil temperature | Change the oil; reduce the amount of oil; adjust the cooling water volume
6 | Too high oil temperature in the gearbox; excessive oil amount; clogged oil filter; scaling in the oil cooler or insufficient water volume | Drain oil; replace the oil filter; remove scaling or adjust the water volume
7 | Severe vibration and noise | Serious wear of gears, bearing shells, bearings, etc.; serious wear or corrosion of the impeller and inducer; gases present in the liquid being pumped or the liquid being vaporized | Repair or replace damaged components; repair, replace, or balance the rotating parts; remove gases from the liquid and lower its temperature
8 | Lubricating oil in the gearbox becomes milky or turbid yellow | Lubricating oil contaminated by water or other substances | Check and eliminate the contamination
9 | Low oil pressure; low oil level; leakage in the heat exchanger; damaged oil nozzles or oil pumps; clogged oil filter | Add more oil; repair or replace damaged components; clean the oil filter