Principle and Leakage Cause Analysis of Dry Gas Seal System for Centrifugal Compressors
The sealing system adopts a tandem double-face dry gas seal, composed of two sets of single-face dry gas seals arranged in series. Filtered clean syngas serves as primary seal gas and enters the primary seal chamber. Its pressure is 0.2–0.3 MPa higher than the process gas pressure to form a barrier. A small amount of seal gas flows into the cylinder, yet no contamination occurs as it is pure syngas.
Part of the gas passes through the labyrinth seal between the two stages of dry gas seals and splits into two streams. One stream is primary leakage gas, directly vented to the flare system; the other flows into the secondary seal chamber as secondary seal gas. Afterwards, it passes through another labyrinth seal and is discharged together with separation gas via the secondary leakage pipeline to the flare system. Separation gas (nitrogen) acts as the final barrier seal, with pressure slightly higher than that of the secondary seal gas, preventing secondary seal gas from leaking to the atmosphere side.
Based on leakage cases of syngas in centrifugal compressors, this paper analyzes gas leakage failures of centrifugal compressors equipped with dry gas seal systems, such as primary leakage gas and primary seal gas of the dry gas seal leaking into the bearing housing through the casing horizontal joint.
1. Introduction
With the rapid development of the petroleum and chemical industries, low energy consumption, high efficiency, zero pollution and long-cycle operation have become the development trends of the petrochemical sector. Large compressor units are critical equipment in the petrochemical industry, and their sealing performance determines stable and safe operation of the plant. Dry gas seals stand out featuring low leakage, good economy, long service life and reliable operation.
A dry gas seal is a new type of non-contact seal for rotating shafts. It is evolved from contact liquid film mechanical seals on the basis of gas-lubricated bearing technology. At the end of the 1960s, John Crane developed the first set of dry gas seals and applied them to centrifugal compressors. With rapid advances in sealing technology and fluid dynamics, various types of dry gas seals have been developed. At present, dry gas seals are widely used in petroleum, chemical, metallurgical, aviation and other industries.
Therefore, this paper comprehensively analyzes and studies the working principle and leakage causes of dry gas seal systems for centrifugal compressors. Relevant conclusions are put forward on this basis, hoping to provide valuable references for practitioners in the same industry.
2. Working Principle of Dry Gas Seals
A dry gas seal is a new non-contact seal. Utilizing hydrodynamic principles, hydrodynamic grooves machined on seal faces enable non-contact operation between sealing surfaces. It consists of coil springs, rotating ring, stationary ring, auxiliary O-rings, spring seat and shaft sleeve. The sealing surface of the rotating ring is ground and polished, and machined with functional hydrodynamic grooves.
When the rotating ring of the dry gas seal spins, seal gas is drawn into the hydrodynamic grooves and flows along the seal dam. Under throttling action of the seal dam, gas is compressed and pressure rises, pushing the seal faces apart and forming an ultra-thin gas film between the two surfaces.
Gas dynamics research shows that stable gas flow can be achieved when the gas film thickness between dry gas seal faces ranges from 2 μm to 3 μm. Therefore, the designed gas film thickness of dry gas seals is set at 2–3 μm. When the closing force generated by static gas pressure and spring force balances the gas film opening force, the gas film thickness remains constant and the dry gas seal operates steadily.
In case of external disturbance that reduces gas film thickness, the gas film reaction force increases, making the opening force greater than the closing force. The seal face clearance expands under opening force. As the clearance increases, the opening force decreases accordingly until the opening force equals the closing force, and the seal clearance returns to normal.
If external disturbance increases the gas film thickness, the gas film reaction force declines, and the closing force exceeds the opening force. The seal clearance shrinks under closing force; the closing force reduces with decreasing clearance until equilibrium is restored between opening force and closing force, and the seal faces recover to normal clearance.
Accordingly, as long as the seal clearance is within the designed range during installation, the sealing system will return to stable operation after external disturbance disappears.
3. Causes of Seal Failure
Dry gas seal failure
Operating data of the dry gas seal at the compressor front end (coupling side) for nearly half a year were retrieved. The data shows that the primary leakage pressure of the front-end dry gas seal of a certain compressor rose from the original 0.15 MPa to 0.2 MPa starting from December 2011 and maintained abnormal fluctuation for three months, indicating dry gas seal malfunction.
Rubber auxiliary seal failure
The seal gas of the dry gas seal is syngas, and the process medium is circulating gas (including syngas, propylene and aldehyde gas). The process medium corrodes rubber seals of the dry gas seal system, which may lead to seal leakage.
Failure of casing horizontal joint sealing
If the sealant on the compressor casing horizontal joint ages or fails, process medium will enter the bearing housing through the horizontal joint. Since the pipeline connection point for dry gas seal primary leakage is near this horizontal joint, syngas may also enter the primary leakage pipeline via the horizontal joint and raise the primary leakage pressure of the dry gas seal. This issue requires sufficient attention.
4. Analysis of Leakage Paths
Primary leakage gas of the dry gas seal enters the bearing housing through the horizontal joint connection. On-site conditions confirmed syngas exists inside the bearing housing, which proves this leakage path is feasible.
Primary seal gas (syngas) leaks into the bearing housing through the horizontal joint, indicating failure of horizontal joint sealant. Actual maintenance revealed peeling of sealing rubber at the contact horizontal joint. Syngas was detected in the bearing housing during leakage, verifying this path.
Leakage routes
After passing through the primary seal of the dry gas seal, primary seal gas (syngas) enters the primary leakage seal chamber. Part of the gas is vented to the flare, while another part of primary leakage gas leaks into the bearing housing through the casing upper cover horizontal joint connected to the primary leakage seal chamber.
The other leakage route: primary seal gas directly leaks into the bearing housing through the sealing interface between the primary seal chamber and the compressor casing upper cover horizontal joint.
Root cause of failure
The casing horizontal joint is a rigid sealing surface. 704 sealant is applied during maintenance. Disassembly inspection found no pits, scratches or other defects on the horizontal joint. Thus, the failure is determined to be caused by ineffective sealant.
Root causes summary:
① Unqualified sealant quality, improper storage, use beyond validity period, or mixed use of sealants from multiple manufacturers.
② Loose maintenance quality control: uneven sealant coating, insufficient curing time (required 12–24 h), and uneven pre-tightening force of upper cover bolts.
③ Unstable operation conditions: frequent production switching, adjustment of process parameters, load variation or improper operation by operators.
5. Treatment Measures
Replace both front and rear dry gas seal sets.
Clean the surface of the casing horizontal joint and remove residual rubber, rust, dust, oil stains and other impurities.
Select the same specification of Model 704 sealant from a single manufacturer and check the expiry date.
Evenly apply sealant along the horizontal joint with a putty knife at a coating thickness of approximately 1 mm. Reserve a 3 mm blank zone on the inner side of the horizontal joint to prevent extruded sealant from entering the cylinder.
Refit the upper cover, and tighten bolts symmetrically and evenly to specified pre-tightening torque using a torque wrench.
After compressor casing reassembly, wait 24 hours for full curing of horizontal joint sealant.
After commissioning and normal operation of the oil system, carry out dry gas seal testing, system pressurization and air tightness tests on all sealing points.
6. Experience and Lessons
Implement predictive maintenance for equipment. Monitor, evaluate, overhaul and replace dry gas seals in advance according to their service life, to avoid passive breakdown maintenance after seal failure. The plant originally planned a major overhaul of the compressor during unit shutdown. If mandatory inspection had been implemented during the previous turnaround, this leakage incident could have been prevented.
Strengthen maintenance quality control. Comply with the specification for Model 704 sealant used on the compressor casing upper cover. Strictly control sealant model, performance, validity period, coating thickness and curing time. In accordance with compressor installation requirements, use a torque wrench to tighten bolts symmetrically and sequentially to specified pre-tightening force to ensure uniform and adequate fastening.
Improve the dedicated unit protection system. Refine the dedicated protection system for large compressor units and condition monitoring system for key equipment, and enhance capabilities for fault diagnosis and emergency repair. When fluctuation of front seal leakage pressure was detected, insufficient attention was paid. Disassembly and inspection were delayed until the scheduled turnaround, and reasonable and accurate operation assessment reports were not prepared.
Although only syngas leakage occurred without process medium leakage in this incident, lessons shall be learned. In-depth reflection shall be conducted on management mechanisms of large units and maintenance quality control. All potential gas leakage scenarios of the dry gas seal system shall be fully understood to effectively improve the safety management level of compressors.
Part of the gas passes through the labyrinth seal between the two stages of dry gas seals and splits into two streams. One stream is primary leakage gas, directly vented to the flare system; the other flows into the secondary seal chamber as secondary seal gas. Afterwards, it passes through another labyrinth seal and is discharged together with separation gas via the secondary leakage pipeline to the flare system. Separation gas (nitrogen) acts as the final barrier seal, with pressure slightly higher than that of the secondary seal gas, preventing secondary seal gas from leaking to the atmosphere side.
Based on leakage cases of syngas in centrifugal compressors, this paper analyzes gas leakage failures of centrifugal compressors equipped with dry gas seal systems, such as primary leakage gas and primary seal gas of the dry gas seal leaking into the bearing housing through the casing horizontal joint.
1. Introduction
With the rapid development of the petroleum and chemical industries, low energy consumption, high efficiency, zero pollution and long-cycle operation have become the development trends of the petrochemical sector. Large compressor units are critical equipment in the petrochemical industry, and their sealing performance determines stable and safe operation of the plant. Dry gas seals stand out featuring low leakage, good economy, long service life and reliable operation.
A dry gas seal is a new type of non-contact seal for rotating shafts. It is evolved from contact liquid film mechanical seals on the basis of gas-lubricated bearing technology. At the end of the 1960s, John Crane developed the first set of dry gas seals and applied them to centrifugal compressors. With rapid advances in sealing technology and fluid dynamics, various types of dry gas seals have been developed. At present, dry gas seals are widely used in petroleum, chemical, metallurgical, aviation and other industries.
Therefore, this paper comprehensively analyzes and studies the working principle and leakage causes of dry gas seal systems for centrifugal compressors. Relevant conclusions are put forward on this basis, hoping to provide valuable references for practitioners in the same industry.
2. Working Principle of Dry Gas Seals
A dry gas seal is a new non-contact seal. Utilizing hydrodynamic principles, hydrodynamic grooves machined on seal faces enable non-contact operation between sealing surfaces. It consists of coil springs, rotating ring, stationary ring, auxiliary O-rings, spring seat and shaft sleeve. The sealing surface of the rotating ring is ground and polished, and machined with functional hydrodynamic grooves.
When the rotating ring of the dry gas seal spins, seal gas is drawn into the hydrodynamic grooves and flows along the seal dam. Under throttling action of the seal dam, gas is compressed and pressure rises, pushing the seal faces apart and forming an ultra-thin gas film between the two surfaces.
Gas dynamics research shows that stable gas flow can be achieved when the gas film thickness between dry gas seal faces ranges from 2 μm to 3 μm. Therefore, the designed gas film thickness of dry gas seals is set at 2–3 μm. When the closing force generated by static gas pressure and spring force balances the gas film opening force, the gas film thickness remains constant and the dry gas seal operates steadily.
In case of external disturbance that reduces gas film thickness, the gas film reaction force increases, making the opening force greater than the closing force. The seal face clearance expands under opening force. As the clearance increases, the opening force decreases accordingly until the opening force equals the closing force, and the seal clearance returns to normal.
If external disturbance increases the gas film thickness, the gas film reaction force declines, and the closing force exceeds the opening force. The seal clearance shrinks under closing force; the closing force reduces with decreasing clearance until equilibrium is restored between opening force and closing force, and the seal faces recover to normal clearance.
Accordingly, as long as the seal clearance is within the designed range during installation, the sealing system will return to stable operation after external disturbance disappears.
3. Causes of Seal Failure
Dry gas seal failure
Operating data of the dry gas seal at the compressor front end (coupling side) for nearly half a year were retrieved. The data shows that the primary leakage pressure of the front-end dry gas seal of a certain compressor rose from the original 0.15 MPa to 0.2 MPa starting from December 2011 and maintained abnormal fluctuation for three months, indicating dry gas seal malfunction.
Rubber auxiliary seal failure
The seal gas of the dry gas seal is syngas, and the process medium is circulating gas (including syngas, propylene and aldehyde gas). The process medium corrodes rubber seals of the dry gas seal system, which may lead to seal leakage.
Failure of casing horizontal joint sealing
If the sealant on the compressor casing horizontal joint ages or fails, process medium will enter the bearing housing through the horizontal joint. Since the pipeline connection point for dry gas seal primary leakage is near this horizontal joint, syngas may also enter the primary leakage pipeline via the horizontal joint and raise the primary leakage pressure of the dry gas seal. This issue requires sufficient attention.
4. Analysis of Leakage Paths
Primary leakage gas of the dry gas seal enters the bearing housing through the horizontal joint connection. On-site conditions confirmed syngas exists inside the bearing housing, which proves this leakage path is feasible.
Primary seal gas (syngas) leaks into the bearing housing through the horizontal joint, indicating failure of horizontal joint sealant. Actual maintenance revealed peeling of sealing rubber at the contact horizontal joint. Syngas was detected in the bearing housing during leakage, verifying this path.
Leakage routes
After passing through the primary seal of the dry gas seal, primary seal gas (syngas) enters the primary leakage seal chamber. Part of the gas is vented to the flare, while another part of primary leakage gas leaks into the bearing housing through the casing upper cover horizontal joint connected to the primary leakage seal chamber.
The other leakage route: primary seal gas directly leaks into the bearing housing through the sealing interface between the primary seal chamber and the compressor casing upper cover horizontal joint.
Root cause of failure
The casing horizontal joint is a rigid sealing surface. 704 sealant is applied during maintenance. Disassembly inspection found no pits, scratches or other defects on the horizontal joint. Thus, the failure is determined to be caused by ineffective sealant.
Root causes summary:
① Unqualified sealant quality, improper storage, use beyond validity period, or mixed use of sealants from multiple manufacturers.
② Loose maintenance quality control: uneven sealant coating, insufficient curing time (required 12–24 h), and uneven pre-tightening force of upper cover bolts.
③ Unstable operation conditions: frequent production switching, adjustment of process parameters, load variation or improper operation by operators.
5. Treatment Measures
Replace both front and rear dry gas seal sets.
Clean the surface of the casing horizontal joint and remove residual rubber, rust, dust, oil stains and other impurities.
Select the same specification of Model 704 sealant from a single manufacturer and check the expiry date.
Evenly apply sealant along the horizontal joint with a putty knife at a coating thickness of approximately 1 mm. Reserve a 3 mm blank zone on the inner side of the horizontal joint to prevent extruded sealant from entering the cylinder.
Refit the upper cover, and tighten bolts symmetrically and evenly to specified pre-tightening torque using a torque wrench.
After compressor casing reassembly, wait 24 hours for full curing of horizontal joint sealant.
After commissioning and normal operation of the oil system, carry out dry gas seal testing, system pressurization and air tightness tests on all sealing points.
6. Experience and Lessons
Implement predictive maintenance for equipment. Monitor, evaluate, overhaul and replace dry gas seals in advance according to their service life, to avoid passive breakdown maintenance after seal failure. The plant originally planned a major overhaul of the compressor during unit shutdown. If mandatory inspection had been implemented during the previous turnaround, this leakage incident could have been prevented.
Strengthen maintenance quality control. Comply with the specification for Model 704 sealant used on the compressor casing upper cover. Strictly control sealant model, performance, validity period, coating thickness and curing time. In accordance with compressor installation requirements, use a torque wrench to tighten bolts symmetrically and sequentially to specified pre-tightening force to ensure uniform and adequate fastening.
Improve the dedicated unit protection system. Refine the dedicated protection system for large compressor units and condition monitoring system for key equipment, and enhance capabilities for fault diagnosis and emergency repair. When fluctuation of front seal leakage pressure was detected, insufficient attention was paid. Disassembly and inspection were delayed until the scheduled turnaround, and reasonable and accurate operation assessment reports were not prepared.
Although only syngas leakage occurred without process medium leakage in this incident, lessons shall be learned. In-depth reflection shall be conducted on management mechanisms of large units and maintenance quality control. All potential gas leakage scenarios of the dry gas seal system shall be fully understood to effectively improve the safety management level of compressors.
