Compressors are critical equipment in my country's natural gas pipeline industry. Generally, natural gas pipelines impose stringent requirements on compressors regarding technology, materials, and sealing integrity; in particular, daily maintenance necessitates fault diagnosis based on the specific characteristics of the gas being handled. With the rapid expansion of my country's natural gas pipeline sector, compressors have become the preferred choice for many enterprises due to their convenience and high efficiency. Their widespread adoption has significantly advanced pipeline processing technologies. However, given the high technical standards involved, many enterprises lack in-depth technical research on compressors, leading to various operational issues.
1. Overview of Compressors
In recent years, equipment technology has advanced significantly, particularly regarding centrifugal compressors, which are valued for their high reliability and compact footprint. They are widely utilized across industries such as natural gas pipelines, coal chemicals, and metallurgy.
Centrifugal compressors are primarily categorized into single-shaft and multi-shaft types. Early single-shaft models typically utilized multiple stages of impellers arranged in series on a single shaft; however, their operational efficiency was relatively low. Continuous technological advancements have led to structural improvements, enabling single-shaft centrifugal compressors to operate effectively even in high-pressure-ratio environments. For instance, integrated coolers—cast as a single unit with the casing—allow compressed gas to flow rapidly into the cooling system, thereby enhancing the overall operational efficiency of the unit. Multi-shaft centrifugal compressors are further divided into H-type and M-type configurations. The H-type compressor features a structure resembling the letter "H," where a large bull gear drives two pinions; each pinion is fitted with impellers, and the stages are interconnected via piping and coolers to facilitate isothermal compression. In contrast, the M-type multi-shaft centrifugal compressor is characterized by a configuration involving multiple impellers. It primarily consists of a large gear driving multiple small gear shafts; this arrangement allows the small shafts to achieve varying rotational speeds. As the gas undergoes continuous compression, the overall pressure rises, thereby increasing the discharge pressure.
2. Analysis of causes and countermeasures for long-distance natural gas pipeline compressor failures
2.1. Anti-surge control methods
2.1.1. Fixed limit flow method
This method is suitable for operating conditions where the compressor speed remains constant. The basic principle involves setting a limit value for the compressor inlet flow; when the inlet flow drops below this value, the anti-surge valve opens to compensate for the pressure difference between the inlet and the pipeline network, thereby preventing surge. The limit value is typically set based on the surge flow at maximum speed; however, when the compressor operates at low speeds, this can result in an excessive control margin and frequent opening of the anti-surge valve. For safety purposes, a 5% to 10% safety margin is usually added beyond the limit value to establish the anti-surge control line; this line is perpendicular to the horizontal axis, with the compressor's operating range located to its right.
2.1.2. Variable limit flow method
When the compressor speed changes, the corresponding limit flow value also changes. Connecting the limit flow values for different speeds forms a surge line. The anti-surge control line runs parallel to the surge line while maintaining a 5% to 10% safety margin; this method expands the compressor's operating range.
2.1.3. Constant-pressure approach control method
The two methods described above are passive control methods. When the composition of the natural gas changes, the compressor's performance curve also shifts; passive control methods cannot provide precise regulation or effectively prevent surge. The constant-pressure approach control method is proposed here to further expand the compressor's operating range.
2.2. Fault diagnosis and analysis for turbine-driven centrifugal compressors
2.2.1. High temperature of the compressor support shaft
There are various reasons why the temperature of the support shaft might exceed normal levels, requiring a systematic investigation. One step is to check the bearing shell clearance; if the clearance is smaller than the normal value, the temperature will rise, necessitating an adjustment of the clearance to the proper range. Inspect the Babbitt metal surface of the bearing pads for damage; if damage is found, re-casting is required. Evaluate the structural design and operational load of the bearing shells; if conditions are suboptimal, the design must be improved to enhance load-bearing capacity. During actual compressor operation, continuous vibration accelerates bearing shell wear; therefore, vibration issues must be effectively addressed to maintain the stability of the shaft system.
Additionally, if the lubricating oil contains excess moisture or impurities, the friction characteristics of the bearing assembly are adversely affected. Consequently, the oil quality must be regularly inspected; if the quality is poor, the oil should be replaced with a high-quality alternative. Damaged bearing shells should be removed for inspection, and components such as the bearing housing and shells should be cleaned. If excessive temperatures occur near the oil inlet, the cooling water flow should be increased. If low inlet pressure results in insufficient oil flow, the inlet pressure must be raised, and the bearing housing's oil inlet passages inspected and cleaned. Measures such as enlarging the oil inlet aperture or scraping the oil wedge on the inlet side of the pad—thereby increasing the wedge's depth and width—can effectively boost oil flow to the pad.
2.2.2 Excessive Operating Temperature of the Compressor Thrust Bearing
When the thrust bearing's operating temperature exceeds standard limits, inspect the oil inlet pressure and flow rate. If these are not within normal ranges, adjust the inlet pressure appropriately and clean the oil inlet passages in the bearing housing. Flow can also be increased by enlarging the oil inlet aperture or scraping the oil wedge on the inlet side of the pad. If temperatures in the inlet area are high, increase the cooling water flow and ensure the lubricating oil meets quality standards. Particular attention must be paid to inspecting the installation of the thrust bearing; incorrect installation methods or improper seating of the thrust collar can lead to a rapid rise in temperature. The thrust bearing assembly should be dismantled to check the leveling blocks for any signs of binding or sticking, as this inhibits the tilting action of the bearing pads; the leveling blocks should be adjusted to ensure the pads retain proper flexibility, while the installation of the thrust collar is simultaneously inspected and adjusted.
Assess the axial force acting on the thrust bearing structure; if the force is excessive, the unit's operating parameters must be reasonably adjusted to reduce it. Verify and calculate the design deviation of the axial force; based on the results, consider enlarging the balance disk or replacing its sealing structure with a new one. Inspect the operating condition of the support bearing's sealing structure by dismantling it; if wear or failure is detected—such as clearances exceeding tolerances—replace the components accordingly, installing new parts for any damaged sealing structures. Focus the inspection on the sealant at the horizontal joint between the upper and lower diaphragms; if the sealing performance is inadequate, replace the sealant. Additionally, check the balance pipe for unobstructed flow; blockages prevent the rapid and effective relief of pressure from the negative-pressure chamber during balance disk operation, thereby impairing the disk's functionality, so any such blockages must be resolved promptly.