The shaft end seals of high-speed rotating centrifugal compressor units mainly include four categories: labyrinth seals, float ring seals, mechanical seals and dry air seals. Among them, the dry gas seal, also known as the dry operation gas seal, uses the clean gas compressed by itself for sealing. It is an advanced sealing form at the shaft end of the turbine compressor and belongs to non-contact sealing. It has good sealing performance, long service life, and low operation and maintenance costs. Currently, this sealing form is the preferred choice for large-scale turbine compressor units.

During the operation of the unit, the dry gas seal often gets damaged, which seriously affects the high-load stable operation of the ammonia synthesis plant. The annual production loss and maintenance costs caused by this amount to tens of millions. Now let's analyze the causes of the damage based on relevant case studies, so as to solve this problem.

Analysis of the causes of frequent damage

The damage of dry gas seals is caused by multiple factors. From the situation of dry gas seal damage during the operation of many domestic centrifugal compressor units, it can be seen that the following factors are no more than: liquid entering the dry gas seal, solid impurities entering the dry gas seal, friction between the dynamic and static rings of the seal, frequent start-up and shutdown, operational errors, and failure of the static ring spring of the seal, etc.

After each damage to the dry gas seal, we conducted an analysis from various aspects such as design, equipment manufacturing, process operation conditions, and operation. Through long-term tracking, we identified several factors that affect the service life of the dry gas seal in this set of compressor units.

There are flaws in the design of the compressor

This set of compressors is the first three-cylinder four-stage centrifugal compressor designed by the manufacturer. During the design stage, due to lax quality control, there were certain flaws in technical details. The medium-pressure cylinder is located between the steam turbine and the low-pressure cylinder, with poor dynamic stability. This problem was not well solved during the design. The rated speed of this set of units is 11,900 rpm. During actual operation, the vibration value of the medium-pressure cylinder has always been relatively high, and the speed has never been able to reach the rated speed, resulting in the load not reaching 100%. When the load is 80%, the vibration value reaches about 42 μm, and at this time, the speed is only about 10,600 rpm. The alarm and interlock values are 31 μm and 50 μm respectively. The fundamental cause of the design defect was also verified in the communication with the compressor manufacturer.

The technical level of dry gas sealing fails to meet the requirements

This unit is equipped with a total of 6 sets of dry gas seals, one set at the driving end and one at the non-driving end of each cylinder block. Among them, the low and medium pressure cylinders use dry gas seals from a domestic manufacturer, while the high pressure cylinders use imported dry gas seals. The manufacturing technology level of domestic dry gas sealing devices lags behind that of imported ones to some extent. The higher the pressure level, the more obvious the gap. Therefore, when the working pressure is relatively high, imported dry gas seals are generally used. This unit's high-pressure cylinder has chosen imported dry gas seals for this reason. The frequently damaged dry gas seal at the drive end of the medium-pressure cylinder is located at the high-pressure end of the cylinder block, with a designed working pressure of approximately 7MPa.

There are flaws in the process flow design

The hydrogen and nitrogen from the previous system are mixed at the inlet of one section of the compressor. Only the nitrogen pipeline is equipped with a regulating valve, which can only adjust the hydrogen-nitrogen ratio by controlling the amount of nitrogen entering the system. Moreover, this regulating valve is a butterfly valve. When operating at high load, this process can meet the regulation requirements. However, during the start-up process, since the ammonia synthesis reaction has not yet consumed process gas, the consumption of nitrogen in the pressurization and heating stages of the ammonia synthesis system is very low. The butterfly regulating valve is completely unable to control the amount of nitrogen entering because the pressure in the nitrogen pipeline network is higher than that in the hydrogen pipeline network. This will cause nitrogen to enter the hydrogen pipeline. The inlet gas of a section of the originally 3:1 hydrogen-nitrogen mixed gas will turn into a gas mainly composed of nitrogen. Due to the drastic change in the molecular weight of the inlet gas, the inlet flow rate and outlet pressure of a section will fluctuate significantly in a very short time, which in turn leads to fluctuations in the rotor displacement. The gap between the moving and stationary rings of the dry gas seal is approximately 3μm during operation, while the displacement fluctuation range can reach 10 μm. The significant fluctuation of displacement within a short period of time causes collision and friction between the moving and stationary rings of the dry gas seal at high speed, affecting the stable operation of the dry gas seal.

There are flaws in the method of turning the wheel

The design turning mode of this compressor is electric continuous turning, with a speed of 8 rpm. During the cooling stage after parking, to prevent the rotor from deforming due to uneven heating and cooling, it is necessary to rotate the rotor for a long time, usually for more than 12 hours. When the vehicle is parked, the dynamic and static rings of the dry gas seal are in a tightly closed state. Under such circumstances, when the vehicle is turned by hand, friction will occur between the dynamic and static rings. Long-term continuous turning of the vehicle will cause significant damage to the dynamic and static rings of the dry gas seal, which greatly shortens the service life of the dry gas seal.

Conclusion and solutions

Through careful analysis and demonstration, it is determined that all the above factors will affect the service life of the dry gas seal. The damage of the dry gas seal at the drive end of the medium-pressure cylinder is the result of the combined effect of these four factors. Due to the unreasonable selection of the unit's turning mode, the dry gas seal suffered abnormal wear during the turning process after shutdown. Coupled with the poor dynamic stability of the medium-pressure cylinder and the high working pressure of the dry gas seal at the drive end of the medium-pressure cylinder, this set of dry gas seals was damaged during the start-up process due to system fluctuations.

Among the above four factors that cause damage to the dry gas seal at the drive end of the medium-pressure cylinder, except for the manufacturing technology level of the dry gas seal which cannot be improved for the time being, the service life of the dry gas seal can be extended through technological transformation for the other three factors.

one

The radial tilting bearing shells of the pressure cylinder in the compressor have been changed from 5 to 4. After the transformation, the vibration value of the medium-pressure cylinder decreased by approximately 10μm, and the dynamic stability of the medium-pressure cylinder rotor was significantly improved. Under the same conditions, the rotational speed of the syngas compressor increased by approximately 300 r.

2.

With the continuous improvement of domestic dry gas seal manufacturing level, it is necessary to replace the dry gas seal device with a higher technical level at the right time to enhance the anti-damage ability of the dry gas seal. Currently, the dry gas seal with a design pressure of 15 MPa developed by this manufacturer has entered the experimental stage.

3.

Add a bypass for medium-pressure nitrogen intake, and install a cage-type regulating valve with a diameter of DN100 on the bypass as a fine-tuning valve. During the start-up process, the DN150 butterfly regulating valve is not put into use. Only the fine-tuning valve is used to control the amount of nitrogen entering, so as to precisely control the hydrogen-nitrogen ratio at the inlet of the first section and avoid significant fluctuations in the flow rate at the inlet of the first section. Meanwhile, during the pressure increase and temperature rise of ammonia synthesis, the flow rate at the inlet of the first section is assisted in being stabilized by controlling the venting volume after the tower. Practice has proved that through the improvement of the process flow and the enhancement of the degree of operation refinement, the stability of the inlet flow of the first section of the compressor during the start-up process has been ensured.

4.

Change the electric and continuous turning gear to manual and intermittent turning gear. When the motor for turning the wheel could not be modified for the time being, we abandoned it. Within 12 hours after parking, we arranged for a dedicated person to manually turn the wheel every 15 minutes, turning 1/4 circle each time, which minimized the wear of the dry gas seal during the turning process to the greatest extent possible.

Conclusion

Through improvements in several aspects such as the bearing bush, process, operation mode, and turning mode, the service life of the dry gas seal at the drive end of the medium pressure cylinder of the syngas compressor has been significantly prolonged. Before the renovation, this set of dry gas seals needed to be replaced 3 or 4 times a year, and each replacement took three days. During the maintenance period of the dry gas seals, the entire device was shut down, resulting in huge economic losses. During the one-year period after the transformation, this set of dry gas seals operated normally. Even after being removed during the major overhaul, no obvious damage was observed, providing a guarantee for the stable operation of the syngas compressor and creating good economic benefits.

The sharing of cases about the frequent damage of dry gas seals comes to an end here. If you have different opinions, please feel free to leave a comment or send us a private message for discussion.