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Screw pumps are a kind of sub-volume pump. According to the number of screws, they can be divided into single screw pumps, twin screw pumps, three screw pumps and five screw pumps. Their working principles are basically similar. The difference lies in the number of screws, the geometry of the screw and the transport medium. 1. The working principle and structure of the screw pump. The screw pump relies on changes in the volume of the closed space formed between the intermeshing screw and the pump shell to complete the suction and discharge of liquid. Figure 1 is the structural diagram of a three-screw pump. It consists of a driving screw and two driven screws. The driving screw is connected to the driver, and the two driven screws are symmetrically arranged on both sides of the driving screw. The three screws are assembled in the pump casing by meshing with each other to form several sealed chambers separated from each other, which separate the suction port and the discharge port of the pump. When the screw rotates, the volume of the suction cavity changes, sucking the conveyed medium into the cavity, and moving the medium continuously and evenly along the axial direction through each sealed cavity to the discharge port. (1) The shell part consists of a pump body, a pump sleeve, a sealing gland, and a pump cover to form a closed body to withstand the pressure of the pump. (2) The rotor part consists of a driving screw and two driven screws. Due to the high pressure at the outlet end of the pump, the screw generates axial force. This force pushes the screw toward the suction end and causes wear at the end of the screw. In order to eliminate the axial force, a deep hole is usually drilled in the pump casing to guide the high-pressure liquid from the discharge end to the main screw. The back of the piston is balanced from the end of the screw to balance the axial force on the screw. The radial force acting on the screw is generated by the liquid pressure. The driven screw is symmetrically arranged on both sides of the driving screw to balance the radial force of the driving screw. For the radial force on the driven screw, there is usually no hydraulic balance device. When considering the structural design of the pump, a suitable working length of the screw can be determined based on the inlet and outlet pressure values of the pump and the characteristics of the liquid being transported. (3) The bearing part is equipped with a rolling bearing at the extended end of the screw. The remaining axial force on the driving screw that has not been balanced is borne by it, and the remaining axial force of the driven screw is borne by the thrust pad. (4) Shaft seal part: The screw pump shaft seal usually uses a mechanical seal. The high-pressure liquid passes through the seal chamber and then returns to the low-pressure chamber, forming a backflow to maintain a certain pressure in the mechanical seal chamber and take away the friction heat between the moving ring and the static ring of the mechanical seal. Screw pump shaft seals sometimes also use sealing structures such as packing and leather cups. (5) Safety valve part In order to prevent the pump or motor from being damaged due to a sudden increase in the working pressure of the pump when the discharge pipeline fails, a safety valve is installed on the pump. When the outlet pressure exceeds the specified working pressure, the safety valve automatically opens, allowing the discharge port to communicate with the suction port, forming a self-circulation of the medium in the pump, that is, the full reflux and full reflux time should not be too long, otherwise the pump will be easily damaged by heat. The safety valve can only be used as a protection mechanism for short-term working sand and cannot be used as a flow plate regulator. The safety valve is mainly composed of a valve body, a valve front cover and a valve back cover. The valve seat and seat flap are sealed under the action of the spring, which separates the high-pressure chamber and the low-pressure chamber. The adjusting rod is used to adjust the spring compression amount to change the total return pressure. After the adjusting rod position is adjusted, it is locked with a nut and the protective cap is sealed. 6.4 2 Characteristics of screw pumps Compared with other pumps, screw pumps have many advantages.: (1) The pressure and flow are stable with minimal pulsation. The medium flows continuously and uniformly in a straight line in the pump. No stirring. (2) It has self-priming capability and does not require a bottom valve or vacuuming accessories. (3) Smooth work and low noise. (4) High efficiency and long life. (5) The structure is simple, compact, small in size, and easy to disassemble and assemble. Disadvantages of screw pumps: The screw tooth profile is complex and requires high processing accuracy. 6.4.3 Maintenance of screw pump 6.4.3.1 Inspection of parts and assembly and adjustment of the screw pump During the disassembly process or after the parts have been disassembled and cleaned, inspection, measurement, and assembly should be carried out according to the pump operation and maintenance instructions. Without requirements, single-screw pumps, twin-screw pumps, and three-screw pumps can be inspected, measured, and assembled according to the standards of SHS 010162004 "Screw Pump Maintenance and Inspection Regulations". Its maintenance mainly includes the following aspects. 1. Screw (1) There should be no scratches or burrs on the screw surface. The spiral surface roughness is R;1: 6, the tooth top surface roughness is Ra1.6, and the spiral outer circle surface roughness is Ra1.6. The burrs on the screw surface should be removed with a whetstone until the screw is polished smooth, otherwise the cylinder liner wall will be severely worn. (2) The straightness of the screw axis is 0.05mm. (3) The threaded part of the screw is enclosed in the hole of the pump casing, and there is a certain gap between the tooth top and the pump casing, generally 0.11-0.48mm in the cold state, to ensure the working performance and reliable rotation of the pump. The feeler gauge method can be used for inspection. (4) When the screw is engaged, the gap between the tooth top and the tooth root is 0.11-0.48mm in the cold state, the normal gap is 0.10-0.29mm, and it is in the middle of two adjacent teeth. 2． The surface roughness of the pump body is Ra3.2. The mating and sealing surfaces of the pump body, end cover and bearing should have no obvious scars, and the roughness is Ra3.2. 3. Bearings (1) The matching of rolling bearings and shafts adopts H7/k6. (2) The combination of rolling bearing and bearing box adopts H7/h6. (3) The axial clearance between the outer ring of the rolling bearing and the bearing gland is 0.02-0.06mm. (4) The matching clearance (empirical value) between the sliding bearing bushing and the shaft is shown in Table 1. (5) The fit between the sliding bearing bushing and the bearing seat hole is R7/h6. Table 1: Matching clearance speed between journal and sliding bearing/(r/min) 1500 or less 1500-3000 3000 or more clearance/min1.2/1000D1.5/1000D2/1000D 4. For sealing, refer to Chapter 5 "5.2.5 Maintenance, Inspection and Installation of Mechanical Seals". 5. Synchronous gear For screw pumps with synchronized gears, the synchronized gears should meet the following requirements: (1) The fit between the driving gear and the shaft is H7/h6, the fit between the driven gear and the tapered hub is H7/h6, and the fit between the tapered hub and the shaft is H7/h6. (2) The quality of the tapered hub should meet the technical requirements, and the inner surface roughness should be Ra0.8. If there are cracks or a set of tapered hubs are severely worn, they should be replaced when the f value is less than 0.5mm, as shown in Figure 2. (3) Gears must not have burrs, cracks, breaks and other defects. The contact area of the gear is not less than 40% along the tooth height and not less than 55% along the tooth width, and is evenly distributed around the pitch circle line. The gear meshing side clearance is 0.08-0.10mm. 2 Trial operation and troubleshooting of screw pump 1. Trial operation of screw pump 1) Preparations before trial operation (1) Check the maintenance records, confirm that the data is correct, and prepare various record forms for the trial operation ； (2) Clean the area around the pump ； (3) Oil sealing and cooling water pipes are not blocked or leaking ； (4) There is no jamming or abnormal noise during turning. ； (5) Inject the transport medium into the pump ； (6) Open the inlet and outlet valves to at least 30% opening ； (7) Contact an electrician to check the motor resistance and power it on. 2) Trial operation (1) Open the pump outlet valve, open the inlet valve, and let the liquid fill the pump body. ； (2) After turning without any problems, twist the start switch and apply electricity to start. ； (3) Check whether the outlet pressure indication is normal ； (4) Check whether the shaft seal leakage meets the requirements. The standards for sealing medium leakage and centrifugal pump shaft seal leakage are the same. ； (5) Check whether the vibration value and bearing temperature of the pump are within the allowable range. The allowable values of vibration value and bearing temperature can refer to the standards of centrifugal pumps. 3) Precautions (1) Before starting the pump, be sure to confirm that the pump outlet is open ； (2) Do not close the outlet valve first when stopping the pump ； (3) The backflow of the safety valve shall not exceed 3 minutes. 2 Troubleshooting of screw pumps Common fault phenomena, causes and troubleshooting methods of screw pumps are shown in Table 2. Table 2 Common fault causes and treatment methods of screw pumps No. Failure phenomenon Failure treatment method 1. The pump does not absorb oil. The suction line is clogged or leaks. The suction height exceeds the allowable suction vacuum height. The motor reverses the medium viscosity. The medium viscosity is too large. Repair the suction line. Lower the suction height. Change the motor direction to heat the medium. 2. The pressure fluctuation is large. The suction line leaks. The relief valve is not adjusted properly or the working pressure is too high, causing the relief valve to open and close. Check the suction line. Adjust the relief valve or reduce the working pressure. 3. Insufficient flow rate and insufficient suction head of the pump. There is air leakage in the body or inlet pipeline. The inlet pipeline or filter is clogged. The screw clearance is too large. The return speed of the overflow valve at the pump body outlet does not reach the peak value. Increase the liquid level to plug the leak and eliminate the air leakage. Clean the system debris and adjust or replace the screw to make the clearance meet the requirements. And check the relief valve, check the motor, adjust the speed, 4-axis power increases sharply, the discharge pipeline is clogged, the screw and bushing are severely rubbed, the medium viscosity is too high, stop the pump, clean the pipeline, overhaul or replace relevant parts, heat the medium, 5. The pump vibrates, the coupling is badly aligned, and the bearings Worn or damaged debris has entered the pump casing. The synchronous gear is worn or misaligned. The screw and the casing are rubbed against each other. The anchor bolts are loose or the pipeline is affected by resonance. Re-align and replace the bearings, and adjust the clearance. Clear the debris. Adjust, repair or replace the synchronous gear. Disassemble and repair. Open the outlet. Valve 6 does not turn when turning. There is debris stuck in the pump. The screw is bent or the screw is poorly positioned. The synchronization gear is improperly adjusted. The bearing is worn or damaged. The screw radial bearing clearance is too small. The screw bearing seat is not concentric, causing eccentric wear. The pressure in the pump is high. Disassemble and clean the debris. Straighten the screw or advance. Screw positioning adjustment, re-adjustment, replacement or adjustment of the bearing, adjustment clearance, disintegration, inspection and opening of the outlet valve 7. Serious friction in the pump heat pump. Mechanical seal oil return hole is blocked. Oil temperature is too high. Check and adjust the gap between the screw and bushing. Clear the oil return hole. Lower the oil temperature appropriately. 8. Mechanical seal leaks. Poor seal installation. Damage to seal parts. Journal seal is worn or defective. Poor coupling alignment. Bearing damage. Sealing oil pressure is too low. Reassemble as required. Replace damaged parts. Repair or replace. Re-align and replace bearings. Adjust oil sealing pressure.