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, hydrogen, 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 including 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 specifically address these critical concerns.
- Elastomer Selection: In many cryo situations, application temperatures may be far below the tolerances of customary O-rings. Operating conditions must always be considered when selecting a compatible O-ring which will remain pliable in extreme temperatures.
- 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: Successful mechanical seal operation relies on a film of fluid or gas across the seal faces. Many cryogenic fluids are poor lubrication agents so it is crucial that an effective seal design includes a solution to minimize the friction from insufficient lubrication.
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 which allows us to monitor and maintain tightly controlled tolerances on all seal components.
We extensively evaluated and addressed this list of concerns and have designed solutions for cryogenic process pumps incorporating the following design features:
- Edge welded metal bellows allow for higher pressure capabilities while maintaining lower, more accurate spring rates than formed bellows. Accurate spring rates in conjunction with precise face-loading minimize the amount of heat generation at the seal faces which is critical when sealing liquid close to its vapor point.
- The “rolled” section of the common formed bellows has inherent thin areas on the O.D. and I.D. surfaces which are created during the bellow forming process. Edge welded bellows do not have these weak stress points. As a result, edge welded bellows are a more reliable compression unit capable of withstanding repeated flexing in severe operating conditions.
- Our cryogenic bellows core is 347 Stainless Steel which maintains its strength and does not become brittle in extremely cold environments.
- Flex-A-Seal’s design provides an optimized face combination of tungsten carbide and cryogenic-grade Carbon Graphite to maximize PV values. This hydraulic seal face balance is specifically engineered for rigorous cryogenic services by minimizing the face closing load, lowering any generated heat near volatile fluids and gases.
- The tungsten carbide rotating face provides superior thermal conductivity properties and increased ruggedness – a definitive performance improvement over the industry standard of simple hardened stainless steel.
- Flex-A-Seal’s cryogenic grade of carbon graphite has excellent film-forming properties. This material enables a state of “self-lubricated” operation in extreme low temperature environments.
- Flex-A-Seal has also incorporated additional design elements per CGA G-4.7-2014: Reciprocating Cryogenic Pumps and Pump Installation which include (1) no metal-to-metal contact between the stationary seal ring retainer and the rotating seat over the entire axial range of the bellows and (2) a protective, non-sparking sleeve used between the bellows unit and rotating shaft to prevent potential contact of the bellows with the rotating shaft which may result in ignition.
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!