Depending on the application, one or two pairs of faces may be used in various arrangements, usually in conjunction with labyrinth seals, to achieve the desired process gas containment level. One pair of faces (a single seal) may be used for moderate pressure applications that are neither flammable, toxic nor environmentally harmful (air, nitrogen), since the normal seal leakage will be to atmosphere. However, low pressure services suitable for a single seal are also suitable for a labyrinth seals, which offer greater simplicity and reliability, as well as significantly lower initial cost. A single seal arrangement is shown in Figure 200-50.
More typical applications require a dual seal arrangement to further limit or prevent leakage to atmosphere, as well as to provide a back-up (or secondary) seal in the event of a failure of the inner (or primary) seal faces. Dual seals can be provided in either a double seal arrangement or a tandem seal arrangement. Double seals are oriented in an opposed fashion to contain seal gas (sometimes called barrier gas in double seals) supplied between the two seals from an external source (see Figure 200-51).
The seal gas must be available at all times at a pressure higher than the process gas pressure at the seals (or the sealing pressure). Although the sealing pressure is usually very close to suction pressure during operation, a compressor trip can cause sealing pressure to rise to a settle-out pressure in some compressor circuits. The double arrangement is most desirable when nitrogen can be used as the seal gas, especially when emissions containment is of primary concern. The double arrangement is also desirable when there is a high potential for primary seal reverse pressurization in a tandem arrangement (see the Seal Gas Supply and Venting Systems section). The double arrangement allows a small amount of seal gas leakage both into the compressor across the primary seal, and also to atmosphere across the secondary seal. When using properly filtered nitrogen as the seal gas, it provides both dry and clean conditions for both seals, prevents harmful emissions to atmosphere and requires a relatively simple auxiliary system. In services where the process gas is either wet or dirty, it may still be necessary to use a purge gas to keep liquids and solids away from the primary seal. It is important to consider that a reduction of nitrogen pressure below the sealing pressure will result in process gas emission and possible damage to the primary seal faces, so some back-up or safety provisions may be needed to avoid these consequences (see the Seal Gas Supply and Venting Systems and Shutdown Protection sections). Furthermore, if the nitrogen supply is known to have poor reliability, a tandem seal arrangement may be the best choice. Since nitrogen is not always available at high enough pressures, double seal arrangements are usually limited to lower pressure services such as FCC or coker wet gas.
The tandem seal arrangement is the most commonly used on compressors, especially in moderate to high pressure services. The seals are oriented in tandem to restrict outward leakage (see Figure 200-52), although the primary seal normally provides essentially all of the sealing duty. In addition to acting as the back-up seal, the secondary seal provides emission containment under normal conditions. The cavity between the two seals is typically vented to flare (or safe location) through porting in both the seal housing and compressor. If the seal gas is environmentally harmful, toxic or has the potential to be toxic, a tandem seal with an intermediate (or interstage) labyrinth should be selected, provided there is an inert gas available for buffering. The intermediate labyrinth is located between the primary and secondary seals, so pressure in this cavity is normally very low. A port between the labyrinth and the secondary seal allows the buffer gas (typically nitrogen) to flow across the labyrinth, preventing seal gas from reaching the secondary seal. Most of the buffer gas exits the seal through the primary seal vent, which is piped to the flare, while a smaller amount leaks across the secondary seal. The tandem seal arrangement generally requires the most extensive auxiliary system, which must deliver seal gas, deliver buffer gas (if needed), and monitor seal venting conditions. The tandem arrangement allows for seal gas to be supplied from either the compressor discharge or an external source, provided the external source pressure exceeds the sealing pressure. As a result, tandem arrangements are currently the only choice for moderate to high pressure services.
A labyrinth seal just inboard of the primary seal is often included in the design of any of the above three arrangements (see Figure 200-53). This inner or primary seal labyrinth:
• limits leakage to atmosphere in the event a primary seal failure (this function is mostly for single seals)
• prevents large amounts of seal gas from flowing into the compressor, and
• minimizes the chance of solids and liquids from getting close to the primary seal faces.
The inner labyrinth seal can either be integral to the seal assembly or provided as a separate compressor component. Similarly, labyrinths can be used on the outboard side of the seal assembly to prevent bearing lube oil from contaminating the seal faces (this and other options are described in better detail in the Separation Seal section). For either application, the use of abradable seals (rotating labyrinth teeth running within a soft, non-metallic, close-clearance stationary ring) should be avoided, as users have experienced failures due to excessive heat generation and particulates generated from the abradable material. Properly engineered abradable seals continue to be acceptable for interstage and balance piston sealing.