Analysis and Countermeasures of Compressor Failures for Long-Distance Natural Gas Transmission Pipelines
Compressors serve as the core equipment for natural gas pipeline transportation in China. Generally, natural gas pipelines impose stringent requirements on compressors, demanding high standards in technology, materials and sealing performance. In particular, fault judgment shall be conducted according to gas properties during daily maintenance. With the rapid development of China’s natural gas pipeline industry, compressors have been widely adopted by numerous natural gas pipeline enterprises due to their convenience and high efficiency, which greatly promotes the advancement of natural gas pipeline processes. Nevertheless, owing to high technical requirements for compressors, many natural gas pipeline enterprises lack in-depth technical research on compressors in practical operation, resulting in various problems.
1 Overview of Compressors
Equipment technology has achieved remarkable progress in recent years, especially centrifugal compressors. Featuring outstanding reliability and relatively small footprint, they are extensively applied in natural gas pipelines, coal chemical industry, metallurgy and other sectors.
Structurally, centrifugal compressors are divided into single-shaft and multi-shaft types. In the early stage, single-shaft compressors adopted multi-stage impellers connected in series on one shaft, yet their operating efficiency was relatively low. Continuous technological advances have optimized relevant structural designs. Modern single-shaft centrifugal compressors can operate stably under high pressure ratio conditions. For instance, coolers are integrated on both sides of the casing, enabling compressed gas to flow into coolers rapidly after compression and improving the overall unit operating efficiency.
Multi-shaft centrifugal compressors fall into two categories: H-type and M-type.
The structure of the H-type multi-shaft centrifugal compressor resembles the letter H. A large gear drives two small gears fitted with impellers. Different impellers are connected by pipelines and coolers to realize isothermal compression.
The M-type multi-shaft centrifugal compressor is equipped with multiple impellers. A large gear drives multiple small gear shafts rotating at different speeds. As gas is continuously compressed, pressure rises gradually, and the outlet pressure increases accordingly.
2.1.1 Fixed Limit Flow Method
This method applies to operating conditions with constant compressor speed. Its basic principle is to set a limit value for the compressor inlet flow. When the inlet flow falls below this value, the anti-surge valve opens to compensate the pressure difference between inlet flow and pipeline network so as to avoid surge. The limit value is normally set as the surge flow at maximum speed. However, when the compressor runs at low speed, excessive control margin will be generated, causing frequent opening of the anti-surge valve. For safety purposes, a safety margin of 5%~10% is reserved outside the limit value to form the anti-surge control line, which is perpendicular to the horizontal axis. The area on the right side of this line represents the compressor operating range.
2.1.2 Variable Limit Flow Method
When the compressor speed changes, the corresponding limit flow value varies at different rotational speeds. Connecting the limit flow values at various speeds forms the surge line. The anti-surge control line runs parallel to the surge line with a reserved safety margin of 5%~10%. This method expands the effective operating range of the compressor.
2.1.3 Constant Pressure Approaching Control Method
The above two methods belong to passive control strategies. When the composition of transported gas changes, the compressor performance curve will shift, and passive control cannot achieve precise regulation or prevent surge. Therefore, the constant pressure approaching control method is proposed to further expand the compressor operating window.
2.2.1 Excessively High Temperature of Compressor Support Bearings
Multiple factors may lead to overheating of support bearings, which require sequential inspection.
Check the bearing shell clearance. If the clearance is smaller than the normal value, temperature will rise, and the clearance shall be adjusted to the standard range. Inspect whether the Babbitt metal on the bearing pad surface is damaged; re-casting is required if damage occurs. Verify whether the structural design and operating load of bearing shells are reasonable. Poor design requires structural improvement to enhance bearing capacity. Continuous vibration during compressor operation accelerates bearing shell wear; therefore, vibration issues must be eliminated to maintain stable shaft system operation.
In addition, excess moisture or impurities in the lubricating oil for support bearings will deteriorate friction conditions between friction pairs. The quality of lubricating oil shall be inspected regularly during operation. Replace the lubricant if its quality fails to meet standards, disassemble damaged bearing shells for inspection, and clean bearing housings and bearing shells.
If overheating occurs at the oil inlet area of the support bearing assembly, enlarge the cooling water opening. If insufficient oil supply results from lower-than-normal oil inlet pressure, raise the oil inlet pressure, inspect and clean the oil inlet passages of bearing housings. Options include enlarging the oil inlet bore of bearing shells or scraping the oil wedge on the oil inlet side of bearing pads to increase the depth and width of oil wedges, so as to effectively boost oil supply to bearing pads.
2.2.2 Excessive Operating Temperature of Compressor Thrust Bearings
When the operating temperature of thrust bearings exceeds the standard value, inspect the oil inlet pressure and flow rate. If either parameter is abnormal, reasonably adjust oil inlet pressure and clean the oil inlet passages of bearing housings. Alternative solutions include enlarging the oil inlet bore of bearing shells or scraping the oil wedge on the oil inlet side of bearing pads to ensure sufficient oil supply. If the temperature at the oil supply area is high, increase cooling water flow and guarantee lubricating oil complies with quality specifications.
Focus on the installation status of thrust bearings. Improper installation or inadequate fitting of the thrust plate will trigger sharp temperature rise. Disassemble the thrust bearing, check whether jamming exists between bearing pads, which restricts pad swing. Adjust horizontal blocks to ensure flexible swing of bearing pads, and inspect and rectify the installation of the thrust plate.
Check the axial force of the thrust bearing assembly. If axial force is excessive, reasonably adjust process parameters to reduce axial load. Verify and calculate design deviations of axial force. According to calculation results, expand the balance disc or replace the sealing structure of the balance disc.
Inspect the operating status of support bearing sealing assemblies and disassemble the seals. Replace sealing components with excessive clearance or severe wear and failure. Prioritize inspection of sealing gaskets at the horizontal split surfaces between adjacent upper and lower diaphragms; replace gaskets with unsatisfactory sealing performance. Furthermore, check the smooth flow of the balance pipe. Blockage will hinder rapid pressure relief in the negative pressure cavity of the balance disc and impair its balancing function. Blockages shall be cleared timely.
2.2.3 Severe Axial Shaft Displacement
Large axial shaft displacement is generally caused by compressor surge. Surge triggers dramatic pressure fluctuations at oil inlet and outlet ports, destabilizing axial movement of the rotor. Hence, surge prevention and control must be implemented during compressor operation. Measure the load of the shaft assembly. If the load exceeds the standard value, it indicates that the pressure of each component fails to stay within specifications. Reasonably regulate unit operating processes and strengthen structural stability. Inspect sealing structures and replace defective seals promptly. If excessive runout of the displacement disc occurs, carry out mechanical processing and adjustment on the displacement disc.
2.3.1 Cause Analysis of Centrifugal Compressor Vibration
Vibration of centrifugal compressors stems from various factors, mainly categorized into mechanical, instrument and process operation factors. Typical vibration faults encountered in practice are summarized as follows for reference:
(1) Rotor dynamic unbalance of centrifugal compressor. Unbalance arises from misalignment between the geometric center and mass center of the rotor, such as damage to rotor components or impellers. Dynamic unbalance is divided into initial unbalance, progressive unbalance and sudden unbalance.
(2) Misalignment between driving shaft and driven shaft. Misalignment of centrifugal compressor units leads to severe vibration, caused by foundation settlement, coupling defects or low original equipment installation precision. Misalignment includes vertical misalignment and horizontal misalignment.
(3) Permanent bending of centrifugal compressor rotor. Permanent rotor bending induces intense vibration during operation, usually manifested as obvious increase in 1× vibration frequency. In most cases, failure to implement regular barring operation for long-term shutdown compressors will result in permanent rotor bending.
(4) Excessive radial bearing shell clearance at the front and rear of the compressor, damaged bearing pads, or foreign matter entering bearing shells (see Figure 1).
(5) False signals transmitted by vibration probes lead to excessive vibration indication of centrifugal compressors.
2.3.2 Solutions for Centrifugal Compressor Vibration Faults
Vibration faults of centrifugal compressors include sudden vibration and gradually increasing vibration during operation, which shall be treated discriminately.
(1) For rotor dynamic unbalance: first remove scale from the rotor, then perform dynamic balancing to eliminate unbalance magnitude. In case of sudden rotor unbalance during operation, shut down the compressor, replace the rotor, inspect and clear foreign objects in flow partitions. Directly replace damaged rotors.
(2) For misalignment between driving and driven shafts, realign the shafts. Refer to the original alignment curve in drawings to restore alignment values within qualified range.
(3) Once permanent bending occurs to the centrifugal compressor rotor, repair is generally infeasible, and the rotor shall be replaced directly.
(4) For excessive radial bearing shell clearance or damaged bearing pads, adjust bearing clearance to specified values and replace damaged pads. If foreign objects enter bearing shells, thoroughly clean the entire lubrication system, with emphasis on inspection and maintenance of pipelines from oil filters to bearing shells.
(5) If false signals from vibration probes cause abnormal vibration readings, inspect and troubleshoot all related links including vibration probes, preamplifiers and signal cables.
2.4 Treatment Measures for Bearing Box Faults
Potential fault causes require repeated inspection. Repeated survey shall be conducted on high-speed shaft couplings of bearing boxes; bearing clearances and distances shall be adjusted timely. Anchor bolts at the bottom of bearing boxes shall be reinforced, and spacing between related equipment adjusted continuously. Regularly replace shims and couplings to ensure base bolts fully penetrate the base. If no obvious defects are found during inspection, continuous monitoring shall be maintained during operation. For example, if vibration increases gradually during operation, the maximum axial vibration shall not exceed 11.5 mm/s.
2.5 Strengthen Equipment Maintenance
Enterprises shall integrate actual production demands and reinforce compressor maintenance.
First, fully understand the characteristics of various compressor components and implement proper maintenance. Real-time monitoring can be conducted by technical means or manual inspection to eliminate faults in a timely manner.
Second, implement regular monitoring of compressors in daily work. Adopt the three diagnosis methods mentioned above to predict potential fault causes, accurately locate fault positions and formulate targeted solutions. This reduces consumption of manpower and materials and improves overall work efficiency.
Finally, strengthen safety management of measuring instruments, recruit professional technicians, arrange regular equipment inspections, and verify the adequacy of equipment maintenance and compliance of repair quality. Optimize management efficiency to maximize equipment performance.
2.6 Causes and Treatments of Abnormal Noise
Abnormal noise is one of the most frequent faults of natural gas pipeline compressors. Maintenance personnel can conduct troubleshooting according to root causes.
Pistons and cylinder heads are the most common sources of abnormal noise. Excessive internal clearances cause collision between pistons and cylinder heads during unit operation. Maintenance staff shall adjust excessive clearances timely to avoid repeated collision and noise.
Secondly, loose piston rods constitute another major cause of abnormal noise. Regular inspection of piston rods is required; fasteners of loose piston rods shall be tightened to prevent reoccurrence of looseness.
In addition, foreign objects entering the compressor will collide with internal structures during operation and generate abnormal noise. Therefore, thorough inspection of the compressor must be carried out during troubleshooting. Foreign objects shall be removed immediately once detected to eliminate abnormal noise.
3 Conclusion
Compressors occupy a vital position in natural gas transportation. During practical management, fault handling shall be prioritized to reduce failure frequency, improve unit availability, guarantee efficient and stable gas transmission and satisfy market demand.
1 Overview of Compressors
Equipment technology has achieved remarkable progress in recent years, especially centrifugal compressors. Featuring outstanding reliability and relatively small footprint, they are extensively applied in natural gas pipelines, coal chemical industry, metallurgy and other sectors.
Structurally, centrifugal compressors are divided into single-shaft and multi-shaft types. In the early stage, single-shaft compressors adopted multi-stage impellers connected in series on one shaft, yet their operating efficiency was relatively low. Continuous technological advances have optimized relevant structural designs. Modern single-shaft centrifugal compressors can operate stably under high pressure ratio conditions. For instance, coolers are integrated on both sides of the casing, enabling compressed gas to flow into coolers rapidly after compression and improving the overall unit operating efficiency.
Multi-shaft centrifugal compressors fall into two categories: H-type and M-type.
The structure of the H-type multi-shaft centrifugal compressor resembles the letter H. A large gear drives two small gears fitted with impellers. Different impellers are connected by pipelines and coolers to realize isothermal compression.
The M-type multi-shaft centrifugal compressor is equipped with multiple impellers. A large gear drives multiple small gear shafts rotating at different speeds. As gas is continuously compressed, pressure rises gradually, and the outlet pressure increases accordingly.
2.1.1 Fixed Limit Flow Method
This method applies to operating conditions with constant compressor speed. Its basic principle is to set a limit value for the compressor inlet flow. When the inlet flow falls below this value, the anti-surge valve opens to compensate the pressure difference between inlet flow and pipeline network so as to avoid surge. The limit value is normally set as the surge flow at maximum speed. However, when the compressor runs at low speed, excessive control margin will be generated, causing frequent opening of the anti-surge valve. For safety purposes, a safety margin of 5%~10% is reserved outside the limit value to form the anti-surge control line, which is perpendicular to the horizontal axis. The area on the right side of this line represents the compressor operating range.
2.1.2 Variable Limit Flow Method
When the compressor speed changes, the corresponding limit flow value varies at different rotational speeds. Connecting the limit flow values at various speeds forms the surge line. The anti-surge control line runs parallel to the surge line with a reserved safety margin of 5%~10%. This method expands the effective operating range of the compressor.
2.1.3 Constant Pressure Approaching Control Method
The above two methods belong to passive control strategies. When the composition of transported gas changes, the compressor performance curve will shift, and passive control cannot achieve precise regulation or prevent surge. Therefore, the constant pressure approaching control method is proposed to further expand the compressor operating window.
2.2.1 Excessively High Temperature of Compressor Support Bearings
Multiple factors may lead to overheating of support bearings, which require sequential inspection.
Check the bearing shell clearance. If the clearance is smaller than the normal value, temperature will rise, and the clearance shall be adjusted to the standard range. Inspect whether the Babbitt metal on the bearing pad surface is damaged; re-casting is required if damage occurs. Verify whether the structural design and operating load of bearing shells are reasonable. Poor design requires structural improvement to enhance bearing capacity. Continuous vibration during compressor operation accelerates bearing shell wear; therefore, vibration issues must be eliminated to maintain stable shaft system operation.
In addition, excess moisture or impurities in the lubricating oil for support bearings will deteriorate friction conditions between friction pairs. The quality of lubricating oil shall be inspected regularly during operation. Replace the lubricant if its quality fails to meet standards, disassemble damaged bearing shells for inspection, and clean bearing housings and bearing shells.
If overheating occurs at the oil inlet area of the support bearing assembly, enlarge the cooling water opening. If insufficient oil supply results from lower-than-normal oil inlet pressure, raise the oil inlet pressure, inspect and clean the oil inlet passages of bearing housings. Options include enlarging the oil inlet bore of bearing shells or scraping the oil wedge on the oil inlet side of bearing pads to increase the depth and width of oil wedges, so as to effectively boost oil supply to bearing pads.
2.2.2 Excessive Operating Temperature of Compressor Thrust Bearings
When the operating temperature of thrust bearings exceeds the standard value, inspect the oil inlet pressure and flow rate. If either parameter is abnormal, reasonably adjust oil inlet pressure and clean the oil inlet passages of bearing housings. Alternative solutions include enlarging the oil inlet bore of bearing shells or scraping the oil wedge on the oil inlet side of bearing pads to ensure sufficient oil supply. If the temperature at the oil supply area is high, increase cooling water flow and guarantee lubricating oil complies with quality specifications.
Focus on the installation status of thrust bearings. Improper installation or inadequate fitting of the thrust plate will trigger sharp temperature rise. Disassemble the thrust bearing, check whether jamming exists between bearing pads, which restricts pad swing. Adjust horizontal blocks to ensure flexible swing of bearing pads, and inspect and rectify the installation of the thrust plate.
Check the axial force of the thrust bearing assembly. If axial force is excessive, reasonably adjust process parameters to reduce axial load. Verify and calculate design deviations of axial force. According to calculation results, expand the balance disc or replace the sealing structure of the balance disc.
Inspect the operating status of support bearing sealing assemblies and disassemble the seals. Replace sealing components with excessive clearance or severe wear and failure. Prioritize inspection of sealing gaskets at the horizontal split surfaces between adjacent upper and lower diaphragms; replace gaskets with unsatisfactory sealing performance. Furthermore, check the smooth flow of the balance pipe. Blockage will hinder rapid pressure relief in the negative pressure cavity of the balance disc and impair its balancing function. Blockages shall be cleared timely.
2.2.3 Severe Axial Shaft Displacement
Large axial shaft displacement is generally caused by compressor surge. Surge triggers dramatic pressure fluctuations at oil inlet and outlet ports, destabilizing axial movement of the rotor. Hence, surge prevention and control must be implemented during compressor operation. Measure the load of the shaft assembly. If the load exceeds the standard value, it indicates that the pressure of each component fails to stay within specifications. Reasonably regulate unit operating processes and strengthen structural stability. Inspect sealing structures and replace defective seals promptly. If excessive runout of the displacement disc occurs, carry out mechanical processing and adjustment on the displacement disc.
2.3.1 Cause Analysis of Centrifugal Compressor Vibration
Vibration of centrifugal compressors stems from various factors, mainly categorized into mechanical, instrument and process operation factors. Typical vibration faults encountered in practice are summarized as follows for reference:
(1) Rotor dynamic unbalance of centrifugal compressor. Unbalance arises from misalignment between the geometric center and mass center of the rotor, such as damage to rotor components or impellers. Dynamic unbalance is divided into initial unbalance, progressive unbalance and sudden unbalance.
(2) Misalignment between driving shaft and driven shaft. Misalignment of centrifugal compressor units leads to severe vibration, caused by foundation settlement, coupling defects or low original equipment installation precision. Misalignment includes vertical misalignment and horizontal misalignment.
(3) Permanent bending of centrifugal compressor rotor. Permanent rotor bending induces intense vibration during operation, usually manifested as obvious increase in 1× vibration frequency. In most cases, failure to implement regular barring operation for long-term shutdown compressors will result in permanent rotor bending.
(4) Excessive radial bearing shell clearance at the front and rear of the compressor, damaged bearing pads, or foreign matter entering bearing shells (see Figure 1).
(5) False signals transmitted by vibration probes lead to excessive vibration indication of centrifugal compressors.
2.3.2 Solutions for Centrifugal Compressor Vibration Faults
Vibration faults of centrifugal compressors include sudden vibration and gradually increasing vibration during operation, which shall be treated discriminately.
(1) For rotor dynamic unbalance: first remove scale from the rotor, then perform dynamic balancing to eliminate unbalance magnitude. In case of sudden rotor unbalance during operation, shut down the compressor, replace the rotor, inspect and clear foreign objects in flow partitions. Directly replace damaged rotors.
(2) For misalignment between driving and driven shafts, realign the shafts. Refer to the original alignment curve in drawings to restore alignment values within qualified range.
(3) Once permanent bending occurs to the centrifugal compressor rotor, repair is generally infeasible, and the rotor shall be replaced directly.
(4) For excessive radial bearing shell clearance or damaged bearing pads, adjust bearing clearance to specified values and replace damaged pads. If foreign objects enter bearing shells, thoroughly clean the entire lubrication system, with emphasis on inspection and maintenance of pipelines from oil filters to bearing shells.
(5) If false signals from vibration probes cause abnormal vibration readings, inspect and troubleshoot all related links including vibration probes, preamplifiers and signal cables.
2.4 Treatment Measures for Bearing Box Faults
Potential fault causes require repeated inspection. Repeated survey shall be conducted on high-speed shaft couplings of bearing boxes; bearing clearances and distances shall be adjusted timely. Anchor bolts at the bottom of bearing boxes shall be reinforced, and spacing between related equipment adjusted continuously. Regularly replace shims and couplings to ensure base bolts fully penetrate the base. If no obvious defects are found during inspection, continuous monitoring shall be maintained during operation. For example, if vibration increases gradually during operation, the maximum axial vibration shall not exceed 11.5 mm/s.
2.5 Strengthen Equipment Maintenance
Enterprises shall integrate actual production demands and reinforce compressor maintenance.
First, fully understand the characteristics of various compressor components and implement proper maintenance. Real-time monitoring can be conducted by technical means or manual inspection to eliminate faults in a timely manner.
Second, implement regular monitoring of compressors in daily work. Adopt the three diagnosis methods mentioned above to predict potential fault causes, accurately locate fault positions and formulate targeted solutions. This reduces consumption of manpower and materials and improves overall work efficiency.
Finally, strengthen safety management of measuring instruments, recruit professional technicians, arrange regular equipment inspections, and verify the adequacy of equipment maintenance and compliance of repair quality. Optimize management efficiency to maximize equipment performance.
2.6 Causes and Treatments of Abnormal Noise
Abnormal noise is one of the most frequent faults of natural gas pipeline compressors. Maintenance personnel can conduct troubleshooting according to root causes.
Pistons and cylinder heads are the most common sources of abnormal noise. Excessive internal clearances cause collision between pistons and cylinder heads during unit operation. Maintenance staff shall adjust excessive clearances timely to avoid repeated collision and noise.
Secondly, loose piston rods constitute another major cause of abnormal noise. Regular inspection of piston rods is required; fasteners of loose piston rods shall be tightened to prevent reoccurrence of looseness.
In addition, foreign objects entering the compressor will collide with internal structures during operation and generate abnormal noise. Therefore, thorough inspection of the compressor must be carried out during troubleshooting. Foreign objects shall be removed immediately once detected to eliminate abnormal noise.
3 Conclusion
Compressors occupy a vital position in natural gas transportation. During practical management, fault handling shall be prioritized to reduce failure frequency, improve unit availability, guarantee efficient and stable gas transmission and satisfy market demand.
