Published on: March 10, 2020
If you have ever had an MRI, then you have benefited from the service of cryogenic solenoid valves. The superconducting MRI magnets that generate the field used to create your MRI images are cooled by liquid helium, or LHE. When liquified, helium’s working temperature is -452°F. Very few solenoid valves can operate successfully at such low temperatures. In fact, the temperature of LHE is within 5° of absolute zero (-459°F or 0° Kelvin)!
LHE is also commonly used to simulate the temperatures of deep space. Environmental chambers used to test components that are part of spacecraft are cooled by LHE. Superconducting computers use LHE to cool their microprocessors. More on solenoid valves for LHE service, later.
Several industrial atmospheric gases are available in liquid form including liquid argon (LAR), liquid nitrogen (LN2), liquid CO2 (LCO2), and liquid oxygen (LOX). Also, liquid propane (LP), is quite common and found on most barbecue grills. Liquified natural gas (LNG) is used to fuel some cars, trucks, and locomotives.
When it comes to purity levels, not all gases are created equal. Generally, industrial-grade gases are usually 99% pure. While this may seem like a high level of purity, it allows for 1% impurities, which can include moisture and particulate. The more ‘nines’ in the purity indication, the purer the gas. For example, a semiconductor-grade ultra-high purity gas is defined as 99.999% (or Five Nine) pure. The level of particulate and moisture content is exceptionally low, and the permissible particles extremely small. The greater the level of purity, the smaller the size and count of allowable particulates. On the other hand, merchant-grade gases are typically 95-99% pure. The impurities can include particulate, other gases, hydrocarbons, and moisture. When handling merchant-grade gases, it is always wise to include filtration and coalescing at the inlet of the system under design. Cryogenic liquids are similarly classified. With cryogenic systems, a filter at the system inlet is extremely important to protect all the components in the system from particulate, particularly the valves.
Controlling cryogenic fluids can be challenging. For the design engineer working on a system that must handle cryogenic fluids (liquified gases), choosing the right solenoid valve can be a daunting experience. General Purpose solenoid valves that work great at ambient temperatures may not last very long at cryogenic temperatures. Solenoid valves not specifically designed for cryogenic temperatures will have a very short service life, if they even work at all. For example, the pressure boundary seal must withstand cold temperatures while remaining resilient enough to ensure a leak-free seal, even if the valve itself is in a room temperature environment. Some elastomers, particularly urethane, will work at temperatures down to -65°F, but below this temperature, all elastomers will fail.
There are two methods to address the pressure boundary seal issue for solenoid valves operating at temperatures to -345° F (liquid nitrogen, or LN2). The more common seal is a spring-energized polytetrafluorethylene (PTFE) lip seal. The spring provides a constant loading force by pressing outward against the walls of the seal. The PTFE is resilient enough to form a leak-free seal against the body of the valve and the plunger tube. Another approach is to use graphite seals. While they are less expensive, and have a low working temperature, they are not dynamic like the lip seal. This makes them prone to leakage as the valve ages.
Valcor’s SV95-series valves utilize a proprietary seat and poppet design that provides exceptionally long service life. The unique tapered poppet ensures a leak-tight seal across the seat by slowly pushing into the body orifice over time, exposing new seal material to the seat as the valve endures tens of thousands of cycles of use. The poppet material is PTFE, which is the best choice since it is good to -345° F. To further enhance the service life of the SV95, Valcor employs rider rings on the plunger. These rider rings act like bearings to keep the plunger vertically oriented in the core tube, eliminating side to side contact during cycling, enabling exceptionally long operating life for the valve. This combination of design features continues to provide excellent performance in a wide range of applications, particularly in analytical instrumentation, cryogenic cooling systems, and environmental chambers.
Applications for oxygen and liquid oxygen (LOX) require a special construction and cleaning process. For best results, please contact Valcor Engineering for recommendations based on your process requirements.
Operating at temperatures below -345°F is when the going gets really tough. For these applications, no resilient poppet or body seal material will work. The solution for the pressure boundary seal is to weld the core tube to the body. This welded construction creates a hermetic seal and ensures that there is no leak path to the outside world. The floating seal valve, first patented by Valcor in 1956, incorporates the all-welded pressure vessel with a novel floating seal disc. The seal disc is typically made of polyimide and is suspended between two seats while loosely secured by the plunger. This is a direct-acting design, not relying on the fluid pressure to assist in opening or closing. The valve has a straight-through flow path, providing maximum possible flow capacity for the orifice of choice. Further, the seats incorporate a tapered flow exit path, creating a venturi that helps recover any flow lost to friction as it passes through the seats. Moreover, the leak-tight seal is enhanced as the valve cycles, by virtue of the seal disc lapping itself against the seat. The self-cleaning floating seal actually wipes across the seat, preventing any particulate from causing across-the-seat leakage. Additionally, the floating seal valve is bi-directional. Pressure can be applied to either port and the valve will function properly. The all-welded floating seal valve is capable of continuous duty service at temperatures to -457°F. No other valve on the market is capable of this degree of service.
Valcor Engineering utilizes the floating seal design in dozens of models designed for commercial, military, and space applications. These valves are compact and use relatively low power to actuate them. Many are capable of continuous duty service. Valcor’s SV91 is a perfect example of this design and are most commonly used on LN2 and LHE applications in the environmental test chamber, medical, and analytical instrumentation markets.
Are you working on a project that includes cryogenic systems, or looking for a better solenoid valve solution for existing designs? Valcor Engineering has the industry best-in-class cryogenic valves for your consideration.