The gas-rich centrifugal compressor unit is a key unit required for production facilities. However, problems such as seal damage and increased dry gas seal leakage occur from time to time, affecting the long-term operation of the unit and seriously restricting the long-term operation of the facility. How to solve the problem of shaft end seal damage has become a difficult issue. Next, let's discuss it together based on the equipment problems of a certain oil refinery.

The structure and medium of the compressor unit in the oil refinery

The 2MCL606 centrifugal compressor unit of this oil refinery plays a decisive role in controlling the pressure of the reaction and fractionation systems, as well as regulating the absorption stabilization system. The stable operation of this unit is an important guarantee for the safe and stable production of the facility.

The 2MCL606 centrifugal compressor features a horizontally split, two-stage compression structure with a total of six stages of impellers, three stages each for the high and low pressure sections. The impellers are installed back-to-back. The compressor is mainly composed of the stator, the rotor and the base. The stator consists of: the casing, the partition, the bearings, the shaft end seal, the dry gas seal, etc. The rotor consists of the shaft, impeller, shaft sleeve, balance disc and thrust disc.

The rich gas undergoes two stages of compression in the compressor flow channel. After the first stage of compression, the medium is led out through pipelines, passes through the intercooler and gas-liquid separator, and returns to the inlet of the second stage for further compression. The compressor seal adopts labyrinth seal at the shaft end and double-end face dry gas seal. The seals between each stage and the balance disc are all labyrinth seals. The shaft end sealing body is made of LD10 forged aluminum or magnesium alloy when leaving the factory.

As can be seen from the composition of the gas-rich medium in the following table, the working medium of the gas-rich compressor contains components such as water vapor, hydrogen sulfide, and sulfur dioxide.

Basic operation conditions of the compressor

The 2MCL606 compressor of this oil refinery was put into operation in 2015. During the five years, it underwent four maintenance operations. Each time the unit was started up and ran for a period of time, the dry gas flow at the high and low pressure ends of the dry gas seal gradually increased. The flow at the high pressure end rose from a maximum of 0.03m3/h to 3.5m3/h. The increase in leakage not only increased energy consumption It also affected the normal operation of the unit.

Over the years, when disassembling and inspecting the stator assembly of the air compressor, it is always found that the shaft end seal has been damaged to varying degrees. During the maintenance and disassembly inspection in 2016, it was found that the middle surface of the compressor shaft end seal was severely corroded and basically lost its sealing effect.

The alloy shaft end seal with damaged middle parting surface

In 2017, the shell was replaced with aluminum and magnesium alloys, and the shaft end seal was embedded with PEEK material that was corrosion-resistant and wear-resistant. However, the same situation still occurred again, but the problem began to appear at a later time. After disassembly and inspection, it was found that the PEEK material inner sleeve was basically undamaged, while the outer aluminum and magnesium alloy shell was severely corroded at the fixing screw holes.

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The damage condition of the shaft end seal embedded with PEEK material

In 2018, the shaft end seal with PEEK material embedded in the alloy was still used. Only a copper gasket was added inside the fixed countersunk nail of the air seal body. At the same time, 704 sealant was used to fill the fixing screws and the middle parting surface of the shaft end seal during installation. The problem of increased leakage of the dry air seal did not occur again.

Analysis of the Cause of Damage

one

Based on the actual damage situation of the shaft end seal, the factors of corrosion are analyzed. When disassembling the shaft end seal, white powder-like substances were found. After laboratory analysis, it was found that the main components of this substance were compounds of aluminum and magnesium metals. Combined with the H₂S, SO₂ and water vapor contained in the gas-rich components, it had the material conditions to form acids. H₂S and SO₂ combined with condensate water generated corrosive acids, and magnesium and aluminum metals reacted with acids to form unstable salts. Then, double hydrolysis and other processes occur, and eventually white metal compounds such as Al(OH)₃ and Mg(OH)₂ are produced.

The process is as follows:

Considering the actual damage of the shaft end seal, corrosion is not the only cause of the damage.

In 2017, the overall material of the front shaft end seal was all aluminum and magnesium alloy, and the corrosion location was mainly concentrated at the fixing screw hole of the middle part surface of the shaft end seal, and spread from the hole to the surrounding area. At this time, it was determined that the aluminum and magnesium alloy were corroded. In 2017, to prevent the medium from corroding the aluminum and magnesium alloy, they were replaced, but the price was relatively high. The new type of shaft end seal with PEEK material embedded and the shell still made of aluminum and magnesium alloy, however, this improvement only delayed the time when the problem began to occur, and the problem was not fundamentally solved.

When the shaft end seal was installed in 2018, a purple copper gasket was added to the fixing screw for sealing, and sealant was filled after the fixing screw was installed for further sealing, as shown in the figure

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All-peek material shaft end seal with copper gasket

The problems of erosion and corrosion have been basically controlled. During disassembly, only minor erosion exists at the contact surface between the shaft end seal and the rich gas. In 2019, the replacement cost was higher. The overall PEEK material shaft end seal was replaced, and the dry gas seal leakage problem did not occur again, and the problem was fundamentally solved.

Based on the above-mentioned maintenance and disassembly inspection situations over the years and the measures taken, the causes of the problems are analyzed as follows:

one

The shaft end seals of aluminum and magnesium alloys are not resistant to corrosion and erosion.

2.

After installing the screws through the sealing and fixing screw holes at the shaft end, a relatively small hollow cavity is left. During the installation of the screws, no sealing treatment was carried out, resulting in slight leakage and providing an environment and space for the formation of acid media.

3.

When the steam in the rich gas flows through the middle plane, it forms an expansion cavity at the screw hole, increasing the residence time of the leaked rich gas. The steam condenses into water droplets and absorbs H2S and SO2 in the rich gas, forming a small amount of acid. The corrosion of aluminum and magnesium alloys begins to occur, the damaged metal surface continues to expand, the amount of accumulated acid increases, the corrosion accelerates, and the leakage of the medium keeps increasing.

4.

The leakage increases while the erosion keeps intensifying. The increase in the medium leakage volume leads to the continuous formation of new acids. The gas flow intensifies, and the areas where the gas flows increase the erosion and corrosion rates.

Conclusion

The main cause of seal damage is the corrosion of the middle surface of the shaft end seal of the gas-rich compressor by acidic substances and the erosion of the leaked medium. When the rich gas contains media such as H2S and SO2, aluminum-magnesium alloy materials are not suitable for the end seal of the compressor shaft. For devices with long maintenance cycles and units that need to operate continuously for more than two years, the shaft end seals should be made of high-molecular materials that are corrosion-resistant and erosion-resistant. If you have different opinions, please feel free to leave a comment or send us a private message for discussion.

Source: Internet

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