Dry Gas Seal Failure Diagnosis: Common Failures and Diagnostic Methods
Centrifugal compressors are critical equipment in process industries such as petrochemical processing, natural gas transportation, and coal chemical production. The reliable operation of these compressors is essential to the safe and stable operation of an entire process plant. Dry gas seals are widely used as shaft-end sealing solutions for centrifugal compressors because they operate without contact, have very low friction power consumption, and provide excellent sealing performance.
However, a dry gas seal failure can result in an unplanned compressor shutdown and may also cause process gas leakage and potential safety risks. For this reason, understanding common dry gas seal failure modes and effective diagnostic methods is important for maintaining the reliable and long-term operation of centrifugal compressors.
How Does a Dry Gas Seal Work?
The basic operating principle of a dry gas seal is based on a thin gas film formed between the rotating and stationary sealing faces. Spiral grooves machined into the rotating ring introduce sealing gas between the seal faces as the rotating ring operates at high speed. This process generates a gas film approximately 3 to 5 micrometers thick, allowing the rotating ring and stationary ring to operate without direct contact.
According to the American Petroleum Institute (API) 692 standard, a dry gas seal system should be designed to maintain reliable sealing performance under both normal and transient operating conditions. The standard specifies technical requirements for sealing gas quality, supply pressure, leakage rate limits, and monitoring system configuration.
Understanding the operating principle of a dry gas seal is the basis for identifying and diagnosing potential seal failures.
Common Dry Gas Seal Failure Modes
Analysis of sealing face damage is an important part of dry gas seal failure diagnosis. During operation, common types of dry gas seal damage include thermal cracking, mechanical wear, particle embedment, and chemical corrosion.
Thermal Cracking
Thermal cracks generally occur when the temperature distribution across the sealing faces becomes uneven. During frequent compressor starts and stops or significant operating fluctuations, excessive local temperature gradients can result in thermal stress concentration. This may cause crack-like damage to the sealing faces.
Mechanical Wear
Mechanical wear is mainly caused by abnormal contact between the sealing faces. When the gas film becomes insufficient or temporarily loses stability, the rotating ring and stationary ring may come into contact. Dry friction can then occur between the sealing faces, resulting in rapid wear and failure of materials such as silicon carbide or carbon graphite.
Particle Embedment
Particle embedment occurs when solid particles carried by the sealing gas enter the gap between the sealing faces. These particles may come from pipeline welding slag, corrosion products, or deposits released from inside the compressor. Once particles enter the sealing faces, they can damage the surfaces and affect the normal operation of the dry gas seal.
Chemical Corrosion
Chemical corrosion is more commonly found in compressor units handling corrosive media such as hydrogen sulfide and carbon dioxide. Under specific temperature and pressure conditions, chemical reactions involving the sealing materials can occur and gradually cause performance degradation.
Identifying the specific damage mode is an important step in dry gas seal troubleshooting, because different damage mechanisms may require different corrective measures.
Dry Gas Seal Vibration Monitoring
Vibration analysis is playing an increasingly important role in dry gas seal failure diagnosis. Traditional vibration monitoring mainly uses accelerometers installed on compressor bearing housings. Frequency spectrum analysis can then be used to identify characteristic frequencies associated with potential seal problems.
In recent years, direct sealing face vibration monitoring technologies based on fiber-optic sensors and eddy-current sensors have gradually been adopted. These sensors can directly measure the axial runout and radial deflection of the sealing faces, providing more accurate information about the operating condition of the dry gas seal.
According to engineering experience, when the axial vibration amplitude of a sealing face exceeds 0.05 mm, it generally indicates a tendency toward abnormal contact or gas film instability. When frequency spectrum analysis shows significant second-order or sub-synchronous frequency components, this may indicate problems such as poor alignment or sealing face deformation.
Some advanced companies have also introduced machine-learning-based diagnostic algorithms. By analyzing large amounts of historical operating data, these systems can establish seal condition assessment models and provide early warnings of potential dry gas seal failures.
Dry Gas Seal Leakage Rate Monitoring
Leakage rate is one of the most direct indicators for evaluating the operating condition of a dry gas seal. According to API 692, under normal operating conditions, the leakage rate of the primary seal should remain within the range specified by the manufacturer, typically around 10 to 50 standard liters per minute.
A continuous increase in leakage rate generally indicates damage or deformation of the sealing faces, while a sudden increase may indicate a serious dry gas seal failure.
It is important to note that the leakage rate trend can be more valuable for failure diagnosis than the absolute leakage value. During operation and maintenance, long-term leakage rate records should be established and the change in leakage rate over operating time should be tracked.
If the rate of leakage increase begins to accelerate significantly, further diagnostic analysis should be carried out even when the absolute leakage rate has not yet exceeded the alarm limit. Early identification of abnormal leakage trends can help identify potential dry gas seal problems before a serious failure occurs.
Changes in ambient temperature and atmospheric pressure can also affect measured leakage rates. Therefore, appropriate corrections should be applied when analyzing leakage data.
Early Diagnosis of Dry Gas Seal Failures
Effective dry gas seal failure diagnosis should not rely on a single operating parameter. Sealing face damage, vibration characteristics, leakage rate, and changes in operating data should be considered together when evaluating the condition of a dry gas seal.
Vibration abnormalities may indicate gas film instability, abnormal contact, poor alignment, or sealing face deformation. At the same time, an increasing leakage rate may indicate sealing face damage or deformation. By monitoring these parameters continuously and analyzing their trends, potential seal problems can be identified at an earlier stage.
This approach can help reduce the risk of unexpected dry gas seal failure and unplanned compressor shutdowns, while improving the reliability of centrifugal compressor operation.
Conclusion
Dry gas seal failure diagnosis is a comprehensive technical task involving sealing technology, rotor dynamics, materials science, and other related disciplines. Common diagnostic methods include sealing face damage analysis, vibration monitoring, frequency spectrum analysis, and leakage rate monitoring.
With continuous monitoring of dry gas seal operating conditions and analysis of changes in key parameters, potential failures can be identified at an early stage. Establishing a systematic approach to dry gas seal failure diagnosis can help improve compressor reliability, reduce unplanned shutdowns, and support the safe and stable operation of centrifugal compressor units.