Some Q&A on screw pumps

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1. What are the precautions for testing a screw pump? Before testing, it is necessary to check whether the maintenance quality meets the quality standards, and whether the records are complete and accurate ； The cranking is smooth and effortless, with no sticking ； The packing gland is not skewed, and the leakage is within acceptable limits; the inlet valve is opened to allow self-pressurized filling of the pump and for emptying. When operating under load, the machine should run smoothly without any noise or vibration; the motor should not experience overcurrents. The flow rate and pressure of the medium should remain stable, meeting the specifications indicated on the nameplate or fulfilling the requirements of production. Sealing leaks should also be within acceptable limits. 2. There may be several reasons why the screw pump does not deliver fluid: 1. The suction line is blocked or leaking; the suction line should be inspected ； 2. Wear of the screw and pump casing; repair or replace components ； 3. If the safety valve is not functioning properly, adjust the spring, or grind and repair the valve disc and seat ； 4. The motor is reversing or not rotating fast enough; repair or replace the motor. 3. What are the requirements for disassembling, assembling, and adjusting screw pumps? Remove the shaft seal and bearing lock nut gently to avoid damaging the threads ； Remove the connecting bolts between the bearing housing and the pump body, take off the inner end cover, and then the screws can be pulled out. Special tools are required to pull out the screws, with three or two screws being pulled out at the same time. During assembly and adjustment, a little lubricant should be applied to the areas where the end face of the screw comes into contact with the end cover, as well as between the screw and the bearing sleeve. This is done to prevent dry friction in these areas when the screw is turned by hand during the assembly process ； Tighten the bolts that connect the outer end cover, bearing housing, and pump body. Perform the operation symmetrically and apply even force; turn the screw while tightening it. If it is difficult to rotate the screw after tightening, loosen the bolt and tighten it again. 4. What are the characteristics of the oil seal and sealing structure of screw pumps? The shaft seal of a screw pump is generally designed as a rubber-reinforced oil seal, which should be replaced during each major overhaul. It consists of a lip seal made from materials such as nylon or polyethylene, and it exhibits excellent wear and pressure resistance. It can operate in environments where the surface finish of the shaft is not high and the lubrication conditions are somewhat poor. Its suitable operating temperature range is from –70 to +100°C, and it can be used with oils, water, and other fluids at pressures below 0.01 MPa. The seal’s resilience is relatively low, so wire rings or similar elements are typically used to enhance its sealing performance. 5. What are the technical standards for the maintenance of screw pumps? The technical standards for the maintenance of screw pumps are as follows: 1. The clearance between various components of the screw pump is shown in Table 1–1 ； 2. The alignment error is 0.05~0.10 mm; 3. The sealing leakage should not exceed 10 drops per second. 6. The main reasons for vibration in screw pumps are as follows: 1. The pump and the motor are not aligned; it is necessary to realign them to correct the alignment issue. 2. If the screw is not aligned with the pump casing or the gap between them is too large, it needs to be repaired and adjusted. 3. If air enters the pump, the suction pipeline should be inspected to locate and fix the leaks. 4. If the installation height is too high, cavitation occurs inside the pump; the installation height should be reduced or the rotation speed lowered. 7. Table of common faults and troubleshooting methods for screw pumps. This article details the symptoms of faults in screw pumps (including twin-screw pumps and triple-screw pumps), the causes of these faults, as well as the corresponding troubleshooting methods. 8. The tasks involved in the disassembly and major repair of screw pumps are as follows: 1. Inspect the babbitt in the cylinder liner; if the gap between the cylinder liner and the screw is beyond the specified limits, the babbitt in the cylinder liner should be recast. 2. Inspect the screw to ensure its surface is smooth and free of burrs. 3. Inspection and maintenance of the copper sleeve: The copper sleeve should fit well with the screw shaft, and turning the shaft should be smooth and effortless; otherwise, the copper sleeve should be ground, or the inner wall of the cylinder sleeve should be polished to meet the required standards. 4. For the maintenance of the shaft seal, an appropriate type of packing should be selected and it should be installed in the correct manner. If the shaft seal is a mechanical seal, it must be maintained in accordance with the relevant requirements. 5. Perform centering and alignment: fix the dial indicator on the alignment frame, then secure the alignment frame to the rear wheel; the pointer of the indicator should point at the outer diameter of the other rear wheel, with the error remaining within the specified range, typically 0.05~0.10 mm. 6. During testing, if the operation is smooth, there are no abnormal noises or vibrations, current levels are within normal limits, and flow rate and pressure remain stable to meet production requirements, then it is considered qualified. 9. Types of screw pumps and a comparison table. This article analyzes in detail the differences among single-screw pumps, twin-screw pumps, and triple-screw pumps from four aspects: structure, characteristics, performance parameters, and application areas, as well as their respective advantages, disadvantages, and suitable applications.