By Gilbert Welsford Jr. – Founder, ValveMan.com. Posted by Sarah Bradley.
Overview
Cryogenic valves are common in pipe-lines handling Liquefied Natural Gas (LNG), Compressed Natural Gas (CNG), liquid nitrogen, helium, oxygen, or nitrogen. By compressing the service media, the pressure of the piping systems builds up, with some processes reaching 750 psi.
Handling cryogens requires high safety standards. The design and manufacture of cryogenic valves adheres to API 598 standards to prevent leakages around the valve seals, gaskets, and the stem seat. The different components of the valve contract or expand at different rates when in contact with the cryogen, as they are made from diverse materials. While handling cryogens, there are notable pressure variations in the piping system, causing heat build-up around the valve. Cryogens are unstable and can instantaneously convert into explosive gases under atmospheric conditions. Valves, pipes, and other cryogen processing equipment require insulation to prevent the rapid transformation of service media into gases.
Ball Valves for Cryogenic Applications
Ball valves provide quick and tight shut-off capabilities, preventing leakages and fugitive emissions when handling cryogenic media. There are two types of ball valves for cryogenic fluid service based on seat construction. The two ball valve types are:
Floating Ball Design: Suitable for use with small pipe diameters (up to 8 inches bore diameter) and low-pressure applications (Class 300 and below). These ball valves have vent holes on the upstream side which relieve the excess pressure that builds up in the cavity.
Trunnion-Mounted Ball Design: Valves in this category have complex structures and powerful sealing parts to control cryogenic fluid service in larger pipelines operating at higher pressure (above Class 300).
Ball Valves for Cryogenic Applications
Ball valves provide quick and tight shut-off capabilities, preventing leakages and fugitive emissions when handling cryogenic media. There are two types of ball valves for cryogenic fluid service based on seat construction. The two ball valve types are:
Floating Ball Design: Suitable for use with small pipe diameters (up to 8 inches bore diameter) and low-pressure applications (Class 300 and below). These ball valves have vent holes on the upstream side which relieve the excess pressure that builds up in the cavity.
Trunnion-Mounted Ball Design: Valves in this category have complex structures and powerful sealing parts to control cryogenic fluid service in larger pipelines operating at higher pressure (above Class 300).
Stem Designs
Ball valves for cryogenic fluid service have extended stems and bonnets. With this design, the stem packing is above the cryogenic fluid path. The elongated stem acts as an insulating column. By pressurizing the column, the cold liquid changes into a warm vapor. The vapor then creates a gas gap below the seals, protecting the valve from damage due to localized freezing. The insulation column protects the service fluid from the influx of atmospheric heat that destabilizes the physical properties of the cryogen.
Materials for Cryogenic Ball Valves
Cryogenic temperature ranges vary from one application to the other. When selecting a ball valve, evaluate the impact of temperature deviation on the reliability and deformation of valve com-ponents. The popular materials for cryogenic service valves are stainless steel, aluminum alloy, brass, bronze, Monel, copper alloy, and zirconium. Ball valves used in applications with temperatures above -148ºF (-100 ºC) are manufactured from ferritic stainless steel. Austenitic stainless steel ball valves are suitable for applications operating at -148ºF (-100 ºC) and below. Copper and aluminum alloys are preferable for low-pressure, small diameter cryogenic ball valves.
Cryogens have multiple physical and chemical characteristics which affect the performance of valves. As such, ball seats should be chemically inert to minimize corrosion, provide adequate sealing, and possess low expansion coefficients. Some cryogenic applications will use metal-to-metal seats, while most applications utilize Polytetrafluoroethylene (PTFE). The only disadvantage with PTFE seating is that they shrink under low temperatures, holding the ball tightly. This increases frictional resistance between the ball and the valve seat, raising the operating torque. Such ball valves are fitted with spring mechanisms that compensate for the shrinkage of PTFE seats.
Trunnion-mounted ball valves have complex sealing mechanisms. These valves have seals between the body and the valve seat retainer. At ambient temperatures, O-rings provide adequate sealing. However, as the temperatures drop, the rubber O-rings become ineffective. Under such conditions, the cryogenic ball valves use expandable PTFE U-rings or graphite packings. Graphite packings, like metal seats, are fitted with springs to minimize the effects of cold shrinkage.
Cryogenic Ball Valve Actuation
Designing the actuation of cryogenic ball valves is as essential as choosing sealing components. Like standard valves, cryogenic ball valves are manually or power-actuated. The operator needs to deliver sufficient torque to open or close the valve without causing mechanical damage. Choosing the appropriate actuation method depends on the desired speed of operation, the orientation of the piping system (enough actuation space), frequency of operation, and duty cycles. By comparison, actuated ball valves operate faster – supplying uniform actuating forces throughout their service lives – than their manually-actuated counterparts.
Testing Cryogenic Valves
Cryogenic ball valves operate under harsh conditions and are prone to failures that institute leakages and explosions. Cryogenic ball valves undergo rigorous tests, vital for establishing their structural strength and leak tightness. Subjecting cryogenic valves to ambient temperature when testing or assembling reduces their sealing performance. Manufacturers assemble and test valve parts in special environments where cool temperatures are maintained. These tests include:
- Seat Leak Tests: This is a pneumatic pressure test. The valve is subjected to 1.1 times its allowable maximum pressure. The seat should not indicate any signs of leakages at this pressure.
- Shell Strength Tests: The shell (body) of the valve is subjected to a hydro-static or pneumatic pressure whose magnitude is 1.5 times the maximum operating pressure. The body of a sound ball valve should not rupture under this test condition.
- Shell Leak Tests: A pneumatic test with the test pressure that is 1.1 times the maximum system operating pressure.
- Cryogenic Prototype and Production Tests: These tests are conducted as per the BS 6364 standards.
Guidelines for Selecting Cryogenic Service Ball Valves
Selecting ball valves for cryogenic fluid service goes beyond flow rates and actuation speeds. Any ball valve destined for cryogenic application must overcome all the hurdles caused by the low temperature of the service fluid. Aside from that, it has to be reliable, durable, safe, and easy to maintain. These guidelines will help technicians to make better decisions when designing or sizing valves for cryogenic applications.
Understanding the properties of the service medium, the cryogenic temperature range, and the desired level of control are vital factors that will dictate the type of valve manufacturing materials and the nature of sealing material to use. The ball valve must provide superb sealing for the entire cryogenic temperature range of the application. Seals should have excellent corrosion resistance.
Consider the method of connection to existing pipelines. Generally, cryogenic valves use welded and flanged joints. Technicians should therefore check if the relief vents on unidirectional ball valves are in good working conditions. To do this, one should verify if the selected valve conforms to industrial standards (pressure and leakage classes). Finally, a technician should also confirm if the actuation system satisfies the operating requirements of the pipeline.
Conclusion
Cryogenic service ball valves are preferred for their quick action, longevity, and excellent sealing characteristics. They are available in sizes standard for application in different cryogenic industrial processes.
Fugitive emissions and leakages are undesirable in cryogenic applications. Therefore, materials for manufacturing ball valves must be sturdy, durable, and lightweight. The elongated valve stems are designed to be blow-out-proof. By optimizing the thermal efficiency of the valves, the cryogens are prevented from changing their physical phases.
About the Author
Gilbert Welsford Jr. is the founder of ValveMan.com and a third-generation valve entrepreneur. He has learned valves since a young age and has brought his entrepreneurial ingenuity to the family business in 2011 by creating the online valve store, ValveMan.com. Gilbert’s focus is building on the legacy his grandfather started, his father grew, and he has amplified.