Thermal Valves & Actuators

Elastomers struggle with fluid compatibility concerns and will degrade in extreme temperatures. In a direct performance comparison with an incumbent elastomeric solution on a large commercial aircraft engine, a metal bellows solution increased the life by more than 10x, surpassing 1 million cycles without failure and with no performance drift. Additionally, metal bellows do not create particulate, reducing strain on system filters.

When it comes to the stroke of the bellows, the type and volume of thermal media are the most impactful design considerations to arrive at the desired performance. Material selection for the housing and bellows, wall thickness, and diaphragm design are all considerations made when tailoring response curves as they impact how quickly the thermal media is exposed to the system temperature. When it comes to the flow and pressure drop experienced across a thermal valve, the fluid flow path geometry is also a primary consideration, requiring computational fluid dynamics (CFD) to properly analyze.

Senior Metal bellows developed our first thermal valve in the 1990s and has since gained experience producing thermally actuated spool valves and poppet cartridge valves, beyond just actuators, for numerous aerospace applications. Senior can also design pressure relief capabilities into our thermal valves.

These devices have high repeatability with negligible stiction. Long-term set-point drift is minimized through all-metal construction, preventing thermal media contamination and leakage. There is a consistent hysteresis loop resulting from the spring force of the bellows requiring slightly more heat energy to lift-off than to return to its cool position. This delay in reaction makes the valve behave in a steady and reliable manner, as minor fluctuations in system temperature do not lead to chattering and seal wear, and limit rapid responses.

The temperature/environmental-limited component of any edge-welded metal bellows thermal valve/actuator is the thermal expansion media itself, which is selected based on the needs of the application. Successful valve/actuator testing has been performed with media at more than 800°F (430°C).

All system-wetted materials of the thermal valve/actuator are metal, so they are compatible with coolants, hydraulics, propellants, and other specialty fluids. Inconel is available where stainless steel compatibility is a concern.

The thermal media is contained within a hermetically sealed edge-welded metal bellows, so it will not leak throughout the life of the valve/actuator. Leakage of system fluid through the valve seal will depend on the type of valve and surrounding fluid parameters but is consistent with other valve technology. Senior will work with our customers to minimize the leakage through the valve seal, which can be confirmed through testing. Note, this leakage will remain consistent through the life of the part as there is no wear.

Senior metal bellows can readily design for desired customer interfaces with ports/fittings. Because they are maintenance-free and have a self-contained passive thermal motor, they are designed for “set-and-forget” placement in restricted areas during assembly, simplifying the overall system layout.

Typical qualification testing involves pressure testing, operational verification at temperature, and endurance testing. Depending on the life requirements of the hardware, accelerated testing procedures may be proposed. Additional tests such as impulse and vibration testing are also available and Senior Metal Bellows will review any desired qualification testing proposed by a customer.