In focus…Cryogenic valves in aerospace applications
When it comes to the use of valves in aerospace applications, you probably think of diminutive sizes used in flying equipment such as R&D launchers, satellites or rovers.
These types of components usually require extensive development work and small quantities in production for prototypes or testing purposes, perhaps resulting in just one or a handful of units for the operation itself. Only satellite launchers might require the higher quantities that are of interest to an OEM of serial valves.
Yet demand is far higher when it comes to ground operations. Cryogenic valves are mainly used for launcher filling or rocket engine test benches. Typical media are oxygen and hydrogen at cryogenic or ambient temperatures, depending on the position along their supply line. Helium may also be used for purging, and nitrogen for additional fire protection, but both in comparably smaller quantities than the combustibles.
Distributed storage of huge quantities of gases is advised to lower the risk of explosion. This necessitates a large-scale piping network to transport the media to the place of use – mostly at cryogenic temperatures and in vacuum-jacketed lines. As rapid delivery is essential, valves with large nominal diameters (up to DN 300), high operating pressures (up to 50 bar) and optimised flow coefficient, respectively, high KVS value (up to 2,300qm/h) are essential. Over time, STÖHR for example has developed valves in XXL sizes for bulk gas service and special industrial applications to provide the high volume supply they require.
Bellow sealing provides the highest possible tightness to the environment and is therefore the only recommended form of sealing when there’s a risk of inflammability. Special body material guarantees the requirements of hydrogen and cryogenic temperatures down to 20 Kelvin. In order to avoid potential cold leaks during operation – which can occur at these temperatures, creating an uncontrolled release of gas into the vacuum – forged stainless-steel blocks or vacuum melted alloys are preferable.
Forged qualities allow comparably high operating pressures at these large diameters, as well as reducing wall thickness in the case of smaller body dimensions. Many valve parts besides the body require special material qualities, such as seamless tubes or special alloys.
Resistance to challenging environments
With many aerospace launch or test sites being located in coastal areas, customers frequently specify seawater-resistant designs. Resistance is mainly required for pneumatic actuators, which are made of stainless steel with mounting parts that are adequately covered against corrosion.
Other sites may call for different environmental preparations, such as specialised electric/electronic equipment for installation at low environmental temperatures (down to -50°C) or a specially designed exterior coating to protect the valve from desert conditions such as sand blast and high temperatures.
With short timeframes for closing or opening operations becoming a standard feature, large valve sizes also require fast shutter speeds. This development reflects the fact that large masses of parts need to be moved quickly, and the valve cone may need to travel long distances of up to 150mm. Flow-efficient valves require cones to have large, in-built hubs to enable the throughput of sufficient gas.
Combining high operating pressure with large nominal diameters requires a high torque – up to 1,500Nm – to press the valve cone tight at seat against the pressure of the medium. Pneumatic actuators, which are standard in this field, are required in twin or even triple execution to generate such forces. And plant engineers face an additional challenge – supplying sufficient operating air for their control, as opening and closing speeds are more often determined by the operating air supply than by the valve itself.
Ensuring safety at all times
As both the launch and the test site are usually evacuated and operated remotely for safety reasons, manual interference during operation is excluded. Manual override facilities at the actuators are rarely required.
Conversely, the ability to lock the valves during the service period between two operations is often required to prevent unforeseen activity during the maintenance operation and, of course, to protect both service staff and equipment. This facility can be provided either by a mechanical blockade of the valve spindle, or by interrupting the air supply to the actuator. In operations where manual valves are required and a standard handwheel proves too large for the installation space, an optional gearbox may be a reasonable alternative.
Standard cryogenic valves must be installed in an upright position with the actuator directed to the top, and a maximum angle of 30º inclined to the side. Exceed this and the liquid level could rise inside the stem, causing freezing of the actuator and operational failure. When it comes to the retrofit or extension of existing gas plants, the valve may require a free installation position with the stem installed horizontally or even upside-down to properly fit into the existing plant.
Source: STOHR
Very large valve sizes come with additional challenges, such as Category IV certification in accordance with DGRL 2014/68/EU (PED) by a notified body. Large actuators that weigh up to one metric ton require special knowledge and tools when it comes to the delicate final step in the assembly process. Transporting this heavy load into a pool of liquid nitrogen for the cold test is the final challenge generated by the production line of special valves.
When it comes to customer-driven final acceptance testing, these types of valves are often subject to various additional controls beyond the OEM’s standard quality plan, all supervised by a third-party inspector. Assessments may include X-ray testing of all weldings at pressure-bearing parts, bubble testing – which involves dipping the part into a water basin – or control of tightness at seat at cryogenic temperatures, which is tested by dipping the part into liquid nitrogen. Some customers also require response-time tests under various operating conditions, or endurance tests for parts ordered for the first time.
To STÖHR, nothing seems impossible. We continue to push our product development to new levels, which represent exciting opportunities for a range of applications – and potential solutions for fresh challenges within the gas industries. As well as in the aerospace sector, we see further opportunities for our XXL valves in newly developing hydrogen infrastructure projects: upstream, midstream and downstream.
