Mark Savage is a product group manager at John Crane, responsible for the design, development and application of metal bellows seals for pumps, compressors and rotating machinery. He has worked in the sealing industry for 24 years and has been involved with development of best practices for shaft seals and their support systems. Savage holds a Bachelor of Engineering from the University of Sydney, Australia. For more information, visit www.johncrane.com
Thermal expansion (or in the case of cold services, contraction) is also a consideration. The cavities that elastomers or engineered polymer seals are installed in will change with decreasing temperatures as will the dimensions of sealing elements installed in these cavities. Additionally, clearances between dissimilar materials, such as bushings, need reviewing. Mechanical seal manufacturers take these factors into consideration during the design of the mechanical seal.
Another effect of cold temperatures is that metals become increasingly brittle as the temperature is reduced. As a general rule of thumb, martensitic and ferritic stainless steels should be avoided in favor of austenitic stainless steels.
Pumping Equipment for Liquefied Atmospheric Gases
Similar to pumping equipment for low temperature hydrocarbons, a mixture of vertical multistage and horizontal single stage pumps are used. These machines generally do not follow API pump design standards. However, since the temperatures are much colder than those at which hydrocarbons are pumped, cofferdams are not employed on these machines. A mixture of vertical and horizontal pumps are commonly employed at air liquefaction plants, but mobile trailer-truck unloading pumps are almost exclusively overhung single stage pumps, either with direct drive or speed increasing gearboxes.
Seal Selection for Liquefied Atmospheric Gases
Unlike hydrocarbons, emissions to atmosphere of atmospheric gases pose relatively minor hazards, thus shaft seal reliability takes precedence when selecting a shaft sealing system. There are two commonly employed shaft sealing technologies used in these pumps:
- Single Mechanical Seals
Single mechanical seals are the most common solution to pumps used in air liquefaction plant and mobile transportation unloading pumps. The major difference between the two is that the mobile unloading pumps tend to be smaller and often use noncartridge seals. Cartridge seals are commonly found in larger machinery at an air liquefaction plant.
These seals fall into two subcategories: contacting wet seals and vaporizing liquid gas seals.
Single mechanical seals use a metal bellows to provide elastomer-free axial flexibility. Seal face materials typically include filled TFE running against a tungsten carbide or hard-coated stainless-steel mating ring.
Similar to contacting wet seals, vaporizing liquid gas seals feature seal face topography that allows the controlled vaporization of the pumped atmospheric gas to produce a highly reliable seal that exhibits controlled
low-level leakage rates.
- Segmented Bushings
This sealing configuration is most often found in vertical multistage pumps at air liquefaction plants. The design provides a controlled leakage by breaking down the sealed pressure over a series of tightly controlled bushing clearances. Leakage rates are higher than that of mechanical seals, however, these leakage rates are often considered acceptable in this industry.
Material Considerations for Liquefied Atmospheric Gases
Sealing liquefied atmospheric gases presents some unique challenges to the selection of materials as the pumping equipment is often used interchangeably between the different atmospheric gases. Liquefied oxygen presents some special challenges since it is a very strong oxidizer and can cause certain materials to spontaneously combust. Additionally, any organic contaminates on the seal can also lead to spontaneous combustion.
Aluminum alloys should be avoided as they can become hazardous when their protective oxide film is stripped from the material such as when abrasion occurs. Lubricants used in the assembly and operation of the mechanical seal must be free of hydrocarbons and compatible for use in oxygen. Packaging of the seal should also be suitable to preserve the cleanliness of the seal prior to installation into the pumping equipment that is performed in a suitably clean environment.
Next Month: Cryogenic applications for static seals
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How Cold is Cold?
Typically, low temperature fluids can be broadly classified into two groups:
Low temperature hydrocarbons are typically pumped at subcryogenic temperatures between -20 degrees C and -140 C (-5 F to -220 F), although lower temperatures can be sometimes encountered. They have high vapor pressures at ambient temperatures and are pumped at low temperatures to reduce pressure. Typical fluids include ethylene, liquefied natural gas (LNG), liquefied petroleum gas (LPG), methane, butane and propylene.
Liquefied atmospheric gases include oxygen, nitrogen, argon and noble gases. They are typically pumped at cryogenic temperatures ranging from -175 C to -198 C (-285 F to -325 F) and are often pumped with a low vapor pressure margin at the pump suction.