Analysis and Improvement of Broken Shaft of Main Oil Pump of Centrifugal Compressor

Analysis and Improvement of Broken Shaft of Main Oil Pump of Centrifugal Compressor I. Introduction

2. CNOOC Dongfang Petrochemical's air compressor station installation provides non-purified air and clean air for instruments to all production installations and auxiliary facilities of public works throughout the plant. The unit is manufactured by a foreign company, C1000 series CENTAC C75M; 3 is a multi-stage centrifugal compressor. The unit is a vertically split structure and adopts three-stage compression. The compression end (head part) is driven by a large gear with two impeller shafts. The first stage impeller is arranged separately to optimize efficiency. The second and third stages are arranged back to back coaxially. The compression end is arranged on the cooler at the bottom of the unit, the motor is supported on the oil tank by the steel base, and the cooler and the motor support structure are connected to each other.

3. The main part of the compressor includes the following main components: gearbox, rotor assembly, impeller, bearing, etc.

Four, 1. Gear box

5. The gear box is designed for vertical split, the first and second stage impellers are on the same side, and the third stage impeller is on the other side. The gearbox consists of two parts: the box body and the cover plate, which are connected as a whole by bolts. The cover is on the motor side, the box is on the air side of the compression end, and the gear system is arranged in it. The volute is connected to the gear box by bolts, and the intake ring and the volute together form a compression chamber. The gear shaft is directly driven by the elastic body

Six, screw main oil pump VII. 2. Rotor assemblyC75M; 3 adopts the design that the first-stage rotor is independently driven by the big gear, and the second and third-stage rotors are arranged back-to-back coaxially. All three-stage impellers adopt high-efficiency backward-curved impellers, which are connected to the impeller shaft through a unique triangular arc cone P3 connector. From the perspective of the motor, the impeller turns counterclockwise. Each rotor is composed of high-quality 15-5PH stainless steel impeller, thrust ring and impeller shaft. The function of the thrust ring is to absorb the axial thrust generated by the aerodynamic force during the operation of the unit, so as to avoid transferring this load to the large gear, thereby prolonging the life of the gear. The first-stage thrust ring is connected to the other end of the impeller shaft through a triangular arc connecting piece. The second and third-stage impellers are coaxial, so the thrust ring and the impeller shaft are integrated and are located between the two impellers. All rotating parts have passed the dynamic balance test. A radial vibration probe is installed at the rotor shaft behind each stage of the impeller, which is wired to the control cabinet to monitor the vibration of each stage.

8. C75M;3 uses a low-pressure lubricating oil system (oil pressure 1.7～2.2 barG, oil temperature 43～46C) to lubricate the bearings. The main components are as follows: a lubricating oil tank built into the compressor chassis, and the oil tank is coated with an anti-corrosion layer. The main oil pump installed on the large gear shaft is 120% oversized to protect the compressor from full lubrication in the event of a sudden power failure. The pre-lubricating oil pump is driven by a 380V, 3-phase, 50Hz motor and has a built-in pressure reducing valve. The pre-lubricating oil pump lubricates the unit before and after the compressor is started: when the compressor is running, the main oil pump supplies oil, and the pre-lubricating pump detects the pressure signal and automatically stops working; when the compressor is stopped, the pre-lubricating oil pump will automatically stop working. The lubricating oil pump starts automatically. The seal gas pressure sensor is connected to the control cabinet to provide self-locking protection. When the seal gas pressure does not reach the set value, the pre-lubricating oil pump will not work. The pre-lubricating oil pump automatically starts when the oil pressure is low. The inlet of the lubricating oil path is equipped with a screen, a 10-micron double oil filter, and the plate oil cooler is designed according to 35℃ cooling water, and the material is stainless steel plate and navy copper fins. Safety in the main oil circuit

9. To control the oil pressure in the compressor bearing, check valves are installed at the inlet of the main oil pump and the outlet of the pre-lubricating oil pump. A pressure sensor is installed behind the oil filter, which is wired to the control cabinet. In order to realize the low oil pressure display, alarm and shutdown. A resistance thermometer is installed behind the oil filter, which is wired to the control cabinet. Display, alarm and shut down the high/low oil temperature. There is an oil pressure difference gauge on the oil filter to display the pressure difference before and after the lubricating oil filter

2. Trouble phenomenon Since the unit was installed and operated, there have been fluctuations in lubricating oil pressure during startup, and the lubricating oil has been replaced recently. Check the bearing sealing parts of the cleaning oil system and the main oil pump. After replacing the lubricating oil, the machine will be cut off routinely. After 30 seconds of start-up, the lubricating oil pressure is low and interlocked to stop. Check the main oil pump, pre-lubricating oil pump, and lubricating oil pipeline system. It was found that the main oil pump's active shaft was broken and the secondary thrust surface of the driven screw was worn out. No other abnormalities. 3. Analysis of the reasons for the broken shaft of the main oil pump 1. From the section of the main oil pump's active screw shaft at the coupling keyway, the section is flat and there is no obvious undulation and deformation. It is judged that the main oil pump's active screw is subjected to excessive torque and exceeds the yield strength value. , Torsion break occurs at the stress concentration of the screw shaft shoulder. 2. From the wear traces of the main oil pump's main and follower screws, the analysis is due to the accumulation of air in the main oil pump inlet pipeline, resulting in lack of oil dry friction on the thrust surfaces of the main and auxiliary screws, wear on the contact surfaces, and changes in the gap between the main and driven screws. The oil accelerates wear and seizes and locks, causing the driving screw to be strongly twisted at the shaft shoulder. 3. Check the oil circuit components of the unit and the design of the oil circuit. The pre-lubricating oil pump is intact, and the pipeline connection is intact and there is no sign of leakage. Disassemble and inspect the pre-lubricating oil pump and the main oil pump inlet check valve. The check valve is a copper check valve. The disc and the main sealing seat form a metal hard seal, and the sealing surface is intact; the check valve is filled with oil and stored for 12 hours, no obvious Leakage situation. Check that the oil circuit pressure regulating valve is in good condition. Check the lubricating oil temperature control valve, the test is normal, and the double oil filter is normal. Ultimately, the problem focused on the design of the oil circuit. The main oil pump is directly driven by the big gear through the elastic body, and its location is the highest position of the entire oil circuit, which is about 1.1 meters higher than the pre-lubricating oil pump. And before starting the machine, start the pre-lubricating oil pump first, and the oil circuit will establish oil pressure, but the lubricating oil will reverse the main oil pump inlet pipe, that is, the lubricating oil enters the pump body through the main oil pump outlet, and passes through the triple screw pump and the cavity. The gap enters the main oil pump inlet, then enters the main oil pump inlet pipeline, and finally flows to the main oil pump inlet check valve to stop. But because the main oil pump is the highest position in the entire oil circuit, and the main oil pump inlet and outlet pipes are vertically parallel under the pump body, forming a type, and the main oil pump is at the top of this type. In the end, the air in the pipeline will be compressed and collected in the inlet section and pump cavity of the main oil pump, and cannot be discharged, because the main oil pump is at the highest position without an exhaust valve, in fact, the entire oil circuit is not equipped with an exhaust valve.

Inspection and verification: After the emergency purchase of the new pump arrives, remove the elbow of the inlet section of the old pump to drill the hole and install an exhaust valve. After 30 minutes of pre-lubricating oil pump operation, compressed gas can be discharged from the exhaust valve of the main oil pump. The exhaust time is about 5 seconds, which verifies the fact that the pipeline cannot be vented by itself. Recalling the previous startup process that caused the oil pressure to drop instantaneously due to the air in the pipeline, it was due to the maintenance personnel who slowly filled the pump by manually refueling the main oil pump inlet pipeline when reinstalling the main oil pump until the air in the inlet section was complete. Exhaust, eliminating hidden dangers, but the system itself cannot automatically exhaust air

4. Summary of reasons and measures

In summary, the reason for the fault of the main oil pump of the compressor to determine the broken shaft is: the design defect of the lubricating oil pipeline, the position of the main oil pump is the highest position of the entire oil circuit, the main oil pump is overhauled or the oil circuit is replaced with lubricating oil. The vacuum of the system is broken, the pipeline enters air, and the gas accumulated after the pre-lubricating oil pump is started due to the operation of the lubricating oil pump, the lubricating oil is reversed to the main oil pump. And after the air in the inlet section of the main oil pump is compressed, due to the check valve of the pipeline, the lubricating oil does not flow and the air is difficult to remove. As a result, the screw pair of the main oil pump sucks in compressed air when the unit is started. Locked, the active screw shaft is broken at the weak point of the stress concentration on the shaft shoulder

. It is finally determined at the top of the inlet elbow of the main oil pump body. Drill a 10mm hole and install an exhaust valve. Revise the operating rules to synchronize the exhaust when the lubricating oil pump is turned on before starting the machine. When the main engine is started, it is ensured that the compressed air in the main oil pump cavity and the inlet pipe has been reliably discharged. Effectively solve the defects of the lubricating oil circuit design of the unit.

5. Operation effect after the transformation After the transformation, before the compressor is started, the operating personnel shall discharge the air according to the revised operating procedures, and the compressor shall be cut off and started. There is no problem of lubricating oil pressure fluctuation and pressure drop.