Over the years, features in dry gas seals have changed, improving the technology. Many small adaptations have taken place to correct the flaws or deficiencies of past dry gas seal systems. These changes range from adjusting the hardness/durometer of an o-ring for handling high-pressure dry gas seals to using polymer gaskets, which have no depressurization limits and will not take a set such as elastomers do. The question is, “Although these are small improvements, how do they affect the reliability of dry gas seals?” The next article, in the March 2012 issue, will focus on selecting the proper seal features—face materials, groove geometries, secondary sealing elements and gasket extrusion/explosive decompression—to provide a reliable dry gas seal system. An important section of a dry gas seal is the vent system. Special attention is required for the primary vent system (vent system of primary seal). In the primary vent system, a vent rupture disc is typically installed in parallel with the vent line to rupture at a set pressure and prevent excessive pressure to instruments on a primary seal failure. A pressure-relief valve (PRV) may be installed instead of rupture disc, particularly for large machines.

To prevent a dangerous condition, process gas cannot leak into the bearing cavity, which could cause an explosion or expose operators or the environment to hazardous or toxic gas. To eliminate this danger, a separation seal (Figure 2) is used on the atmospheric side of the dry gas seal. Since a number of separation seals are available, selecting the correct seal for the application is important. Typically, compressors with dry gas seals are used in applications in which they are critical parts of the entire process. A well-designed dry gas seal system will prevent extensive damage to the dry gas seal, compressor and process. Compared to pump operations, dry gas seals and compressors usually operate at higher speeds and pressures. Every component of a dry gas seal must be designed to manage exposure to extreme conditions. Dry seals offer an alternative for dusty or extremely viscous applications that guarantees complete sealing, minimum maintenance, protection of the other system components such as the wear of the shaft and less energy consumption due to motor friction. Because dry gas seals operate under extreme conditions, choosing the right seal features can substantially increase the reliability by ensuring that they maintain a gap between the rotating seat and the stationary face. To understand this, let’s start with the basic principles of a dry gas seal (Figure 1). John Crane Inc. issued a patent for dry gas seals in 1968 with field applications beginning in 1975, though the technology is now widely available among seal manufacturers. When the technology is aimed at correcting the problems with dry gas film environments by eliminating friction. Soon, the technology became a common replacement for other lubricated seals. The patented spiral-groove (constant depth / variable diameter) technology of the dry gas seal allows for easy lifting and separation of seal faces during operation. Also the dry gas seal lift geometry can be unidirectional or bidirectional, depending on the specific design of the lifting geometry.Lubrication oil is another important root-cause of seal failures. The first section that will be contaminated by lubrication oil is the barrier seal (the separation seal). For such cases, there will be oil ingress past the barrier seal. The second section that will be contaminated is the secondary seal. If the lubrication oil can pass through barrier seal and contaminate the secondary seal, the secondary seal may fail, leading to problems with the primary seal (oil ingestion and subsequent failure). Seal system improvements The rotating seat is fixed to the compressor rotor, and the stationary face is connected to the compressor case. The stationary face must adjust axially to maintain the gap as the rotating face/compressor rotor moves and as the compressor case expands and contracts from changes in heat and pressure. To manage axial movement, the stationary face must move freely when required, and as a result, the dynamic secondary sealing design is key to attaining a response to axial changes without resistance or delay. Therefore, a reliable dry gas seal will have a parallel gap of 150 to 200 micro-inches between the rotating seat and the stationary face even when axial movement occurs.

All these factors—seal design, process and separation seal, seal configurations, dry gas seal panels and auxiliary systems—must be reviewed, assessed and designed for the seal’s intended application. With strict adherence to these guidelines, a dry gas seal system can be designed that offers the highest efficiency and reliability, preventing unnecessary plant shutdowns and subsequent loss of production and catalysts. As the population increases more and more buildings are going to be built to accommodate a already over populated world and if the problems of today’s technologies are not changed when we have solutions to change them, then our planet will just spiral out of control!The inspection of the seal can help identify the root-cause of high flow of the primary vent. If the primary seal faces are damaged or deteriorated, it could be deduced that the leakage path is the primary seal itself. Meanwhile, if the primary seal faces are in good condition, it can be determined that high primary vent gas leakage is not caused by damage to the primary seal faces. In such a case, other root-causes and leakage paths should be examined. To create the right environment for the seal, contamination must be prevented from entering the seal. From the process side, a process seal is required to prevent process gas from contaminating the dry gas seal environment. On the atmospheric side, where lubrication oil is present, the oil must be stopped from entering the dry gas seal environment.