Materials to be processed at cryogenic temperatures pose unique conditions for mechanical seals. In order to engineer the ideal mechanical seal for cryogenic applications, there are several challenges to overcome.
First, what is cryogenics?
Cryogenics is the scientific study of how matter behaves at very low temperatures, sometimes approaching absolute zero, and how such temperatures can be achieved and maintained. This field typically encompasses liquefied gasses, low temperature hydrocarbons, refrigerants, and coolants. For our purposes, it includes all applications where the temperature falls below zero degrees Celsius (32 degrees Fahrenheit).
Cryogenic liquids such as oxygen, nitrogen, and argon slow down and stabilize chemical reactions in many different processes. This ensures accurate measurement or transportation of volatile materials. Cryogenic liquids are typically delivered via trucks or railroad cars which require specific sealing to protect the liquids.
Here are some examples of industries where low-temperature applications are often found:
- Food and beverage production
- Oil and gas
- Pharmaceutical and chemical processing
- Healthcare and medical research
What are the common concerns when sealing low temperature applications?
Conventional mechanical seals are not typically designed for use in extremely cold processes. Low temperature (below 0° C) conditions require specific modifications and adjustments to a seal design. These challenges include:
- Health, Safety, and Environmental Concerns: Cryogenic liquids can be dangerous. They present potentially life-threatening hazards to those who come into contact with them, including hazards of extreme cold, asphyxiation, toxicity, flammability, and explosion due to rapid expansion. Every precaution must be taken to ensure that personnel and the surrounding environment are safe when cryogenic liquids are involved. Therefore, the seal design must be specifically address these critical concerns.
- Elastomer Selection: In many cryo situations, application temperatures may be far below the tolerances of customary O-rings. In these cases, consideration must be given to the operating conditions and selecting a compatible O-ring.
- Expansion and Contraction: Temperature differentials will cause expansion and contraction of metallurgy. When this affects a metal retainer, the inserted face can become dislodged. This issue must be addressed when designing an effective cryogenic seal.
- Lubrication Issues: Many cryogenic fluids are poor lubricators. Since successful mechanical seal operation depends on a film of fluid across the faces, this particular aspect of sealing cryogenic product can create problems for conventional mechanical seal face materials. Again, it is crucial for the seal design to address this issue.
Designing a mechanical seal specifically for cryogenics
Mechanical seals in cryogenic applications must be specifically engineered to address each of the above issues. Flex-A-Seal has successfully applied our exemplary welded metal bellows engineering capabilities to these rigorous demands. All our manufacturing processes are done in-house with tightly controlled tolerances on all seal components, resulting in proven successful seal life.
We extensively evaluated and addressed this list of concerns, enabling us to offer ideal answers for a wide variety of cryogenic applications. Our solutions for cryogenic process pumps incorporate the following design features:
- Edge welded metal bellows allow for higher pressure capabilities while maintaining lower, more accurate spring rates than inconsistent formed bellows.
- The “rolled” section of the common formed bellows has inherent thin areas on the O.D. and I.D. surfaces created in the forming process of the metal. Edge welded bellows eliminate these weak stress points. As a result, edge welded bellows are a more reliable compression unit because of the ability to handle repeated flexing on every revolution.
- Our cryogenic bellows core is 347 Stainless Steel which maintains its strength and does not become brittle in extremely cold environments.
- Our combination of tungsten carbide and special carbon graphite faces maximizes the PV values and optimizes the face balance.
- The tungsten carbide rotating face provides better thermal conductivity properties and increased ruggedness, improving performance over the industry standard of simple hardened stainless steel.
- Flex-A-Seal uses a special cryogenic grade of carbon graphite with excellent film forming properties. This material enables a state of “self-lubrication” operation in the extreme low temperatures of cryogenic operation.
- More accurate spring rates for more precise face loads, providing a minimum amount of heat generation which is critical for sealing liquid close to its vapor point.
Flex-A-Seal has provided successful cryo sealing options for an extensive range of equipment manufacturers including ACD, Cosmodyne, Airco Paul, Cryostar, Grofe, and JC Carter. Contact us today to discuss your cryogenic sealing challenges!