High-temperature, noncontacting, edge-welded metal bellows seals are an alternative to conventional wet seal designs and their associated support systems. These noncontacting seals can help reduce costs and meet operational, environmental and rotating equipment requirements.
Noncontacting, gas-lubricated technology is operated with a pressurized barrier gas that can be either nitrogen, argon or steam and reliably contains process fluids up to 800 F (425 C). The seal is ideal for sealing hot hydrocarbons, cold fluids or applications requiring inert secondary seals for reliable operation.
These seals can extend the mean time between repair (MTBR) as compared to conventional contacting seals operating in extreme temperature environments and utilize maximum achievable control technology (MACT).
Green Technology
High-temperature, noncontacting seals with a pressurized gas barrier ensure zero fugitive emissions when sealing hazardous fluids, which can help plant operators meet their environmental obligations. These seals can also eliminate the need for cooling water and barrier fluids.
They help reduce the costs of complying with environmental regulations for a particular sealing solution that could be subject to penalties and fines for fugitive emissions. Noncontacting, gas-lubricated seals can also minimize unplanned seal-related shutdowns such as from leakage or spill of barrier fluids and the associated costs of cleanup.
The seal operates friction-free with significantly lower power (energy) requirements, as well as reduced energy consumption from the elimination of cooling requirements often used with contacting wet seals. The pump and seal generally operate with greater efficiency and minimal power losses, which results in operations with less energy and lower horsepower requirements. The performance of a conventional centrifugal pump can be made to operate with zero emissions and increased safety while increasing pump efficiency and increased mean time between preventative maintenance (MTBPM) while reducing a user’s carbon footprint when compared with dual wet seals and support systems.
Applications
The hydrocarbon processing, chemical and petrochemical industries may benefit from using noncontacting seal technology. The majority of the seal designs are American Petroleum Institute (API) Type C, Arrangements 2 and 3 Category 3 seals that meet API 682 specifications and are useful in industries that specify and use API 610 7th and 8th edition pumps. They are used on rotating equipment such as pumps, fans and blowers. Because the seals utilize a nonelastomeric inert flexible carbon graphite secondary seal, they eliminate the cost associated with expensive perfluoroelastomers (FFKM). Nonelastomeric secondary seals also offer a solution to the proposed per- and polyfluoroalkyl substance (PFAS) compliance requirements or in applications where an O-ring is subject to attack by the process fluid.
For refineries, petrochemical, chemical and similar plants handling volatile organic compounds (VOC) or other hazardous fluids, the noncontacting, nonelastomeric, gas-lubricated sealing systems can offer emissions control and increased safety.
Typical high-temperature hydrocarbon applications include fluids found in refinery distillation units, crude furnaces, transfer lines, feedstock heaters, feed and reflux sections of columns and atmospheric and vacuum columns. Other fluids include carcinogenic fluids and heat-dependent fluids (thermosensitive). Typically, fluids with temperatures greater than 500 F to 800 F (260 C to 425 C) or cold fluids as low as -100 F (-73 C)—e.g., ethane, methanol or methylene chloride—are ideal candidates for a noncontacting metal bellows seal design.
Technology
Noncontacting seals are self-contained, preassembled metal bellows cartridge seals that do not require centering or measuring during seal installation. The seal designs can comply with API 682 design requirements for reliable process containment. Dual gas-lubricated seals differ from other pressurized multiple seal arrangements in that they do not have circulation of a fluid between the inner and outer seals but rely on an external source of an inert barrier gas to pressurize the barrier cavity. The pressurized gas barrier functions to prevent leakage of the process fluid to the atmosphere as well as to lubricate and cool the seal faces.
The minimum seal support system required for proper operation of these seals is an API Plan 74. Typically, the external source of barrier gas is a pressurized nitrogen line within the plant and applied to the seal with a pressure greater than the process pressure being sealed. An inert gas is recommended.
The principles of operation are based on a seal face that has a topology applied to it, such as a spiral groove, and are designed in such a way to generate both hydrostatic and hydrodynamic “lift.” The seals rely on a thin, stable film of gas between the sealing faces. When the pressurized barrier gas is applied to the seal and the shaft is rotating, a band of high-pressure barrier gas is created between the seal faces. This separates the seal faces and creates a noncontacting dynamic seal with no friction, no wear and no cooling requirements.
Metal bellows primary seals are required since they utilize static flexible carbon graphite secondary seals, which are inert and can handle a wide range of high- and low-temperature applications effectively. The metal bellows assembly also allows for modest axial displacements while maintaining sufficient face load to provide a high level of performance in sealing high-temperature applications.
Because these seals are often used in corrosive, high-temperature services, the materials of construction are all Inconel metal bellows with either silicon carbide, silicon carbide/graphite composites or tungsten carbide seal faces. Seals’ faces can be designed to be pressure balanced so they can handle full reverse pressure.
The gas barrier support system is a critical component in the operation of the seal. The appropriate gas panel supporting a nitrogen/argon barrier gas should always be recommended to the user and supplied with the seal. Steam may also be used as a barrier gas, and an alternative steam support system is required for these applications.
The demands for extending seal performance and meeting environmental obligations are a driving force for applying noncontacting, gas-lubricated seals in both hot and cold fluids. Rotating equipment performance and reliability can be increased through the proper application of these seals while reducing life cycle costs when compared to conventional wet contacting seals and support systems.
