Today, especially in the United States where compressors are aging, it is becoming increasingly common to retrofit older compressors with dry gas seals. While the end result may be improved reliability (eliminating all the additional OIL SEAL system components from the circuit always improves reliability), there are a few things the end user should consider before making a decision.
Removing the oil seal from the compressor also eliminates the significant damping effect of the oil on the rotor. Therefore, we need to conduct a rotor dynamics study to ensure that the critical speed is minimally affected when the seal is removed from the machine. This study was conducted before any changes were made to the dry gas seal.
Most suppliers today recommend performing a rotor dynamics study before upgrading an older compressor with a dry gas seal. However, following this step will help you avoid unexpected problems during startup.
In recent years, we have seen this problem with customers who have had poor ATS reliability due to migration of unfiltered process gas through process labyrinth seals or leakage of process gas through the intermediate laboratory to the atmosphere (through secondary vents).
In Fig. Figure 1 shows a typical seal gas system diagram. When gas is applied to the primary seal, only a very small amount of gas (less than 1%) leaks through the seal surface, with the remainder passing through the process labyrinth seal (indicated in red).
The higher the gas velocity through the labyrinth seal, the more it separates unfiltered process gas from the main seal. If this occurs, end users may experience problems with deposits in the seal grooves, resulting in failure or even dynamic seal ring sticking.
Likewise, if the flow rate of the intermediate gas (usually nitrogen) through the intermediate lab (shown in green) is too low, the compressor will not have a nitrogen-rich secondary seal, so the end user selects that seal first. place to release nitrogen into the secondary exhaust system only!
We recommend a minimum of 30 ft/sec for both labyrinth seals at twice the maximum clearance (to allow for labyrinth seal wear). This will ensure proper isolation of unwanted process gases on the other side of the labyrinth seal.
Another common problem recently discovered in compressors equipped with dry gas seals is oil migration through the breakaway seal. If the oil is not drained from the cavity, it will eventually fill the groove and cause catastrophic failure of the secondary seal (another topic for another time). .
The main reason is that the axial space between the old oil seal and the bearing is very small, and the old rotor usually does not have a step on the shaft between the oil seal and the bearing. This will provide a path for oil to pass through the rupture seal and into the secondary drain chamber.
Therefore, we strongly recommend installing an oil deflector on the (rotating) seal bushing on the outside of the rupture seal, which will direct oil away from the rupture seal bore. If these three conditions are met, along with a well-equipped sealing gas panel, the end user will find that dry gas sealing can survive several repairs. Dry gas oil seal is a non-contact mechanical seal developed on the basis of gas dynamic pressure bearings, which is lubricated with gas film during dry operation. This seal utilizes the principle of fluid dynamics and achieves non-contact operation of the sealing end face by opening a dynamic pressure groove on the sealing end face. Initially, dry gas sealing was mainly used to improve the shaft sealing problem of high-speed centrifugal compressors. Due to the non-contact operation of the sealing, dry gas sealing has the characteristics of being unaffected by PV value, low leakage rate, wear free operation, low energy consumption, long service life, high efficiency, simple and reliable operation, and being free from oil pollution of the sealed fluid. It has a good prospect for application in high-pressure equipment, high-speed equipment, and various types of compressor equipment.
Post time: Nov-24-2023