Dry gas seal in cryogenic applications
The materials for the dry gas seal faces, secondary sealing elements and metallic components must be suitable for all operating conditions and the gas composition of the application. As an example, the following factors deserve consideration when selecting the dry gas seals for centrifugal compressors in liquefied natural gas (LNG) service:
• Use of a bidirectional silicon nitride rotating face against silicone carbide/ carbon stationary face (depending on the pressure, size and speed)
• Amorphous diamond-like carbon coating on silicon carbide stationary face to enhance its hardness, lubricity and improve the chemical and thermal stability
• Special Teflon and spring-energized Teflon gaskets and O-rings for cryogenic application
• Low-speed liftoff face technology for the tertiary seals.
• Bone-dry nitrogen rating for the seal (-94°F [-70°C]).
• Flow-control method for the seal gas.
• Flow meters on individual seal gas supply lines.
• A seal gas conditioning system including a booster.
Sulfur is present in chemical compositions such as hydrogen sulfide (H2S) gas. The melting point of elemental sulfur ranges between 235° and 246°F. Sulfur solidification takes place at lower or higher temperatures and pressure depending on its content in a saturated gas. Also in gas streams containing H2S and/ or sulfur vapor, deposition of solidified sulfur can occur because of the chemical reaction supported by catalysts and the changes in pressure and temperature of the gas. One example is the separation of sulfur when H2S reacts with trace amounts (3 to 5%) of oxygen present in nitrogen, which is used as the source of a separation gas. Some possible locations of sulfur deposition in a dry gas seal are shown in Figure 13. Figure 14 includes the images of seal components contaminated by Sulfur.
In centrifugal compressors handling H2S gas, the integration of gas conditioning devices with the overall dry gas seal control system should be considered. The use of a coalescing prefilter to clean up the seal gas and a seal heater to increase seal gas supply temperature to 250°F (121°C) are the two proven effective measures to prevent sulfur solidification and deposition. Similarly, the addition of a booster unit helps to avoid the migration of unclean gas to the seal areas during any transient conditions.
Storage and protection of dry gas seals
The discussion below applies to the seals, which are stored in suitable packaging as well as those which are installed in machines for longer periods of time but have not been put into operation. During the storage period, functionally of essential parts of the seal can be subjected to alteration like aging and distortion. These situations can undermine the seal’s performance. The suggested measures for protection and preservation are listed below.
• The storage place should be dry (relative humidity < 65%) and free of dust. The temperature at the location of the storage should be between 58° and 78°F (14° and 26°C).
• The influences of radiation, heat (direct sunlight), steam and ozone must be avoided. Special storage containers with nitrogen blanketing should be used to protect the seals.
• Corrosion protection agents should not be used as they can have a direct impact on the functionality of the seal.
• Re-qualification of the seal cartridge prior to installation should be considered for storage durations of 36 months or more. The elastomeric secondary sealing elements should be replaced and the seal faces should be checked for alterations, which could have a detrimental effect on the seal’s performance.
• Spare gaskets (gaskets/O-rings not in- stalled and separately packed) should be protected from heat sources such as direct sunlight, artificial light with ultraviolet spectrum and heat-generating fixtures/appliances.
• Gaskets/O-rings should not be stored at temperatures less than 50°F (10°C) and should be warmed to 50°F (10°C) before installation.
• Precautions should be taken to protect stored dry gas seals from all sources of ionizing radiation likely to cause damage to seals and its components.