Seal face materials and designs vary between different suppliers. Since the seal faces are the components that have the greatest influence on the operating envelope, reliability and leakage rate, they are the focus of ongoing design improvements.
Face designs must be optimized to address numerous issues, including:
• Hydrostatic lift (slight separation of the faces caused by pressure while rotor is static)
• Dry running tolerance
• Lift-off properties
• Gas film stiffness
• Seal gas properties and their variability
• Stresses and deflections due to sealing conditions (pressure and temperature)
• Stresses and deflections due mounting/driving forces and dynamic forces
• Tolerance to reverse rotation
Some of these issues are addressed with the face materials. Earlier designs of dry gas seals typically used tungsten carbide for the rotating seal face and carbon for the stationary seal face. Current designs are making greater use silicon carbide, silicon nitride or in some cases, a coated, ductile steel for rotating faces. Silicon carbide has also become the popular alternate material for the stationary face, especially when high pressures raise deformation to unacceptable levels in carbon materials. At present, and depending on the supplier, low to moderate duty services use either tungsten carbide/carbon or silicon carbide/carbon face combinations, while high pressure, high speed services use silicon carbide/silicon carbide or silicon nitride/silicon carbide combinations. Other material combinations have been used, especially at extreme conditions.
It is important to note that except for coated ductile steel faces, rotating face materials are very brittle, making them vulnerable to excessive stress with the potential to break up very quickly. Although silicon carbide and silicon nitride tend to disintegrate in to very small pieces, these pieces can still upset or damage the secondary seal. In contrast, once tungsten carbide is broken, sizable fragments can cause significant secondary damage to the entire seal assembly and even the compressor. In order to mitigate damage or unsafe conditions in the event of a failure, a
shrouded face design (see Figure 200-53) should be specified for a tungsten carbide rotating face, if not provided as the standard. In addition to providing burst containment, the shroud also offers the ability to drive the rotating face at its outer diameter, which results in reduced face stresses. For silicon-based faces, burst containment is of less value, and reduced heat transfer is a trade-off. However, the stress reduction provided by outer diameter drive methods may be desirable or necessary for some applications.
Seal face groove geometry also varies between suppliers, and has evolved over the years. Most suppliers offer both unidirectional and bi-directional face designs. Unidirectional faces typically have a spiral groove geometry, although L-shaped grooves have also been used (see Figure 200-54). Unidirectional seals generally offer the best performance with regard to lift-off, gas film stiffness and stability, making them the best choice for many of the more difficult applications. Unidirectional seals have the disadvantage of being intolerant of reverse rotation, which can cause dry running damage to the faces. Because of this vulnerability, it is important to incorporate assembly features (both labeling and geometry differences) which can help prevent the installation of the wrong seal parts or assemblies (inboard vs. outboard) on a between bearings compressor design.
Bi-directional face designs have a greater variety of groove geometries among suppliers, including U-shapes, “spruce tree”-shapes and T-shapes, (see Figure 200-55). Bi-directional designs offer equal performance in both directions of rotation, but this performance is generally less than that of a unidirectional seal. They are acceptable for services where gas film stiffness and face separation is not marginal (best evaluated by the supplier). They may be most attractive in services where compressor flow reversal potential exists (i.e., back pressure services rather than recycle services), especially if there is a history of compressor discharge check valve problems. One other advantage of bi-directional designs is that one seal assembly can be used to spare both sides of the machine. However, this should play little or no role in selecting a bi-directional design, especially considering that seal assemblies are usually changed out in pairs, and critical services warrant having a full set of spares.