Cryogenic valves are used in air separation plants to produce gases such as nitrogen, oxygen, argon, etc.

An introduction to cryogenic valves

Cryogenic valves safely transport and assist in containing cryogenic liquids and gases in applications that operate at extremely low temperatures. Cryogenic valves must be designed to precise tolerances and be of an extremely high quality in order to safely and reliably contain these cryogenic liquids and gases.
 
^ Cryogenic valves are used in air separation plants to produce gases such as nitrogen, oxygen, argon, etc.

Article by Arie Bregman, DFT Valves
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Applications across many industries rely on valves specifically designed for cryogenic applications to maintain the safe handling of extremely cold liquids and gases. Equipment in refrigeration, air separation, chemical processing, medical and food processing fields all use cryogenic valves for purposes central to these industries’ functions.

In this article, we explore some of the key points of specifying cryogenic valves, as well as discuss some of their common applications in industry.

Where do cryogenic temperatures begin?

Air separation and gas industry standards generally specify that cryogenic temperatures to be at or below –150°C (–238°F). Temperatures above this range up to 3°C (37°F) are referred to as “refrigeration” temperatures. However, sometimes a more rigid approach to cryogenic applications is followed, and continuous temperatures below –73°C (–100°F) are seen as cryogenic operating conditions. No matter these specifications, all cryogenic applications require specific valve design, testing, and preparation.

Gases liquefy at different temperatures. For example, nitrogen, an inert gas, liquefies at a temperature of –196°C (–320°F). In contrast, hydrogen, a flammable gas, liquefies below –253°C (–423°F), making it useful for different applications. All gases have their own, unique liquid transition temperatures.

Cryogenic temperatures are usually expressed in terms of Kelvin units. 0 Kelvin is equal to –273.15°C (–459.67°F), and is referred to as absolute zero (the temperature at which all molecular activity stops). The Kelvin scale correlates exactly with the Celsius scale by just a difference of 273.15; therefore, 100 K is equal to –173.15°C.

Materials of construction

In-valve operating temperatures below –40°F require special design considerations, such as the selection of appropriate materials for valve construction. Most carbon steels, such as the casting grades WCB, WCC, and WC5, are too brittle for use in continuous temperatures below –20°F. Low-carbon steels like LCB and LCC are only effective down to –40°F.

For temperatures lower than –40°F, many engineers make use of austenitic stainless steels, which contain high levels of chromium and nickel and low levels of carbon. Austen-itic steels are more ductile, easily weldable and suitable for many cryogenic temperature applications.

Polymers, rubber compounds, and other soft materials that are typically used for seats or seals can become excessively hard in cryogenic conditions, causing them to function more like metals than elastomers or polymers. Valve designs that include external stems (and hence stem seals) such as ball, butterfly, gate or globe valves must also incorporate an extension to the bonnet to move the stem seals to a warmer area. Doing so will remove the seals from direct cryogenic liquid contact and work to preserve their elasticity and pliability.

Stem extensions are required for both manual and automated shutoff valves. The valve operator will also have seals or include greases that will be negatively impacted by the cryogenic temperatures and therefore must be above the pipeline insulation or out-side of the cold box enclosure to ensure continued usability.

Piping systems require a lot of welding. Typically low-carbon grade metals such as 316L (extra-low carbon) stainless steel are specified for the components that will welded. This low carbon specification will include piping flanges as well as the body materials for the valves to be used.

Special cleaning/concerns

Special cleaning is required to clean cryogenic valves and piping. This cleaning process is designed to remove oils, greases, and fibers, which may interfere with your system’s functionality or presenta flammability hazard depending on the service.

After washing the valve’s component parts in a solvent cleaner, workers must inspect them for any residues the cleaning procedures may have missed. In most cases, the valves and pipes should not contain any assembly greases or oils, although there are some approved lubricants for oxygen service components that will not harm the system with their presence.

Some customer specs may not allow for the presence of any lint or fibers within the valve. Even if some fiber is allowed, no sys-tem should contain fibers greater than 1/8” (3 mm) in length. In oxygen systems, the presence of lint can be quite problematic due to the risk of flammability. It is critical to thoroughly inspect all the components that go into the system to make sure they are free of any and all contaminants. Black light testing allows inspectors to easily identify lint, fibers, or other residue that needs to be removed.

Leakage

Another concern related to cryogenic valves is leakage, both internal and external.

Internal leakage is a function of the valve’s design, and the seating components of the valve need to be designed to seal properly at extremely cold conditions. Creating an effective, durable seal under cryogenic operating conditions is the main function of the valve and is required to maintain your system’s safety and operating efficiency. The seals should be constructed to provide a durable and reliable closure over the course of their expected service lifetime.

Most external leakage will come from the valve stem. However, the presence of flanged connections can increase the opportunities for external leakage to occur. To minimize this, opportunities for external leakage, many companies specify weld end connections for their valves. These welded valves will include butt welding ends or socket welding ends on the valves, depending on the size and pressure class of the valves.

Cryogenic Applications

Valves built specifically for cryogenic environments can be used in a wide range of applications. These include: air separation (nitrogen, oxygen, hydrogen, helium, argon, and carbon dioxide gases); aerospace applications; medical research; super conductivity research and applications.

About the Author

Arie Bregman is Vice President and General Manager for DFT Inc. located in Exton, Pennsylvania, U.S.A. Arie has spent over 35 years in the valve industry, with companies such as Nibco, Jamesbury and Neles. Arie holds a Master of Science degree in Mechanical Engineering from Worcester Polytechnic Institute. He is active in the Valve Manufacturers Association (VMA) in the U.S., serving on the Education and Training and the Technical Committee and is also the Chairman of the VMA Board of Directors.

Arie Bregman
Arie Bregman

 

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