Authored by:
Paul Hazlitt
Director of Engineering
Servometer
Cedar Grove, N.J.
Edited by Kenneth J. Korane
ken.korane@penton.com
Key points:

• Changes in temperature, pressure, or operating conditions can cause contained fluid to expand.
• Expanding fluid can lead to leaks and damage to sensitive equipment.
• Edge-welded metal bellows compensate for volumetric changes and handle temperature extremes and rough operating environments.
Resources:

Servometer

In many fluid-power and fluid-handling applications, the gas or liquid inside equipment and lines can sometimes expand beyond normal design limits. This can be due to machine operations or caused by the surrounding environment, such as changes in temperature or pressure.

Often, expanding fluid leads to trouble. It typically increases internal pressure which can result in blown seals, leaks, broken sensors and instrumentation, and premature wear. The cost of downtime, repairs, and environmental cleanup often exceeds that of the replacement parts themselves.

To compensate for fluid expansion, accumulators with rubber bladders are one option. But they’re often not suitable due to space or weight constraints, temperature extremes, or corrosive operating environments. Engineers should consider edge-welded bellows assemblies as a viable alternative. They maintain proper flow during normal operation and expand and contract to compensate for volumetric changes of the liquid or gas.

Bellows basics
An edge-welded bellows is, in effect, a precision bubble. It consists of a series of profiled metal diaphragms that completely collapse under external pressure or internal vacuum, maximizing the movement of the assembly. Thus, it can compensate for significant volume changes in a relatively small space. Other types of metal bellows have shorter strokes for a given size, thus offering less volume compensation. For instance, hydraulically formed (hydroformed) bellows typically travel about 20% of their free length, compared to 90% for an edge-welded metal bellows. Thus, edge-welded units can provide the same volume compensation with smaller and lighter assemblies.

Edge-welded bellows are manufactured by welding stamped, metal diaphragms into a long, flexible assembly. Strips of sheet metal are stamped into the shape of the diaphragm (typically round, oval, or rectangular) with the pressure, stroke length, spring rate, and temperature helping determine the thickness and material required to meet application demands.

The shape of the inside and outside edges and ripples in the diaphragm are crucial to performance. The ripples must be consistent to ensure accurate spring rates and long cycle life. For nested ripple-edge welded bellows (a version commonly used for volume compensation), diaphragms are positioned back-to-back (male to female) to pair the inside-diameter holes. Once in contact, they are welded together. Deep weld penetration is key to a longlasting, leaktight joint. Depending on the manufacturer and material, bellows can be welded using plasma, laser, arc, or electron-beam welding processes. Machine-vision systems are increasingly used to improve weld accuracy and consistency.

The bellows sections are stacked together until the assembly reaches the necessary length. Then, the ODs of the convolutions are welded. Copper rings can be inserted between the convolutions to ensure that heat from the welds does not distort or change material properties in the adjacent metal. And the bellows can be heat treated to strengthen the material.

End plates or flanges can be added to the ends of the bellows assembly. Once completed, edgewelded bellows are helium leak tested to ensure the assembly completely seals.

Sizing
To size edge-welded bellows for applications, first define the operating conditions, including operating pressures, temperatures, and maximum volume the bellows must compensate for. By knowing the amount of volume to be compensated for and understanding the size envelope the bellows compensator needs to fit into, one can then specify the bellows size. Subsequently, the mean effective area for the bellows can be analyzed to determine the height, diameter, stroke length, and length requirements. Engineers at reputable metal-bellows manufacturers can assist in this process.

Because edge-welded bellows respond to internal and external pressure, the specific application should determine if the bellows will extend or retract. For example, a high-pressure application would lend itself to a design with external pressure on bellows inside a thick metal housing. This lets the housing contain the pressure while the bellows compress and nest. In lower-pressure applications, the bellows could see internal pressure and expand as needed.

Pressure can be a limiting factor, although a two-ply bellows (two diaphragms stacked and welded together), or liquid or gas-filled assemblies are possible solutions. For example, if the outside of the bellows is exposed to hydraulic fluid at 5,000 psi (350 bar), the inside of the bellows can be pressurized with nitrogen to reduce the differential pressure and stress on the bellows.

The rate of expansion can also be important. Violent pressure spikes or extremely fast volume changes can alter design requirements, compared to applications with slow cycles and a volume that changes with ambient temperature or altitude.

Hardware considerations
The bellows and housings can be constructed of the same material if they will be exposed to the same media. While drawn housings are the most cost effective to manufacture, they are limited in size and material. Typically, machined housings offer the most design flexibility and ensure the proper wall thickness to handle pressure spikes and extremes. Guides can be built into the ends of the bellows to prevent side-to-side movement inside the housing. This increases cycle life. Guide materials should be compatible with the media or specially coated.

Custom fittings can also be integrated into the assembly. Threaded ports can be added to one or both sides of the bellows. The application, along with industry norms, determines the thread type. Standard NPT (National Pipe Thread) designs have tapered threads commonly used for low to moderate pressures in commercial applications in North America. The threads are typically taped and seal by deformation of the thread onto the pipe.

SAE and UNF O-ring and coned metal-to-metal seals are commonly used in automotive and aerospace applications. Here, engineers should research O-ring material compatibility and temperature capabilities. Metal-to-metal seals are typically one-time use, but sealing cones in various materials might also be an option.

For ultrahigh-vacuum applications, widely used VCR fittings create a metal-to-metal seal that prevents leaks. International threads are also becoming more common. BSP (British Standard Pipe) and metric threads are used in more designs as the global industrial market expands.

If the bellows must compensate for barometric pressure, they can be manufactured with a vent hole. They can also be filled with liquid for cooling or, as mentioned previously, to alter the differential pressure and increase pressure capabilities. While additional ports on each side are an option, a central tube can be as effective, lower costs, and increase reliability by reducing welding and potential leak paths.

To monitor conditions, sensors can be included in the design. Position sensors such as potentiometers and LVDTs mounted to the bellows can measure movements and report data to a PLC or controller. Pressure sensors, pressure switches, and flow controllers can report conditions at locations before and after the volume compensator. The data can be used to adjust valves or other devices further downstream. Bellows can also act as a sensing element, where a volumetric change alters the length or displaces a liquid inside the bellows.

Materials
Due to the bellows all-metal construction, engineers can select materials that ensure media compatibility without temperature limitations. Here are some common materials.

Stainless steel generally offers the best price-to-performance ratio of any bellows material. It provides ample strength and comes in standard and custom designs in a range of thicknesses.

316L stainless steel is often the first choice. It is readily available, offers good chemical compatibility with most liquids and gases, and is relatively inexpensive. The material is recommended for cryogenic and nonmagnetic applications, and for corrosion resistance in acidic environments. Operating temperature range is from –420 to 800°F.

AM350 stainless is a high-strength alloy with an operating range from –100 to 800°F. The metal is slightly magnetic and suited for nonacidic environments. It has an excellent fatigue life, with bellows typically lasting at least 100,000 cycles. AM 350 is also the preferred material for ultrahigh-vacuum (UHV) environments. The industry standard leak rate of 1 × 10-9 cc/sec (1 cc of volume in 30žyears) is commonly specified, but lower leak rates are possible for certain applications.

Titanium bellows work well in applications requiring high strength, lightweight construction, and media compatibility. Titanium is often used in the aerospace industry where every ounce is critical to the overall design. While low-end temperature ranges are limited, titanium bellows can be used inside aircraft that have temperature controlled areas. Other typical uses include invasive medical devices and test equipment.

Inconel is a nickel-based alloy with better corrosion resistance and temperature capabilities, compared to stainless steel. It can withstand temperatures exceeding 1,000°F, with Alloy 625 offering the highest standard ratings. Inconel bellows are generally designed for applications that require excellent corrosion resistance, such as down-hole valves or tools exposed to hydrogen sulfide.

Hastelloy is an alloy with chemical compatibility superior to most materials. It’s called for in applications with highly specialized and corrosive media. Hastelloy is an option in pharmaceutical, process, and chemical equipment requiring high vacuum or pressure. Temperature rating is –420 to 1,000°F.

Applications
Edge-welded bellows have been used for volume compensation in settings ranging from subsea depths to outer space. For example, they help handle pressure changes as research vessels descend to the ocean floor. And the bellows, combined with sensors, compensate for the pressure differential between seawater and hydraulic fluid. Bellows are also used to compensate for fluid-pressure changes in subsea transformers.

Hydraulic systems for critical aircraft flight controls use bellows to compensate for volumetric changes. As temperature and pressure change with altitude, the bellows stabilize hydraulic pressure and prevent premature wear and failure.

Industrial applications also take advantage of such devices. Refrigeration systems create a temperature change in the cooling fluid, such as ammonia. To compensate for thermal spikes, the bellows add surface area and expand to cool the fluid.

And in many pipelines and fluid circuits, when a valve quickly opens or closes, a pressure pulse as high as 10 times the normal operating pressure travels down the line. Often called a water-hammer arrestor, an edge-welded bellows can be integrated into a drawn housing and mounted in the fluid line. It expands during a pressure spike to absorb the shock wave, providing a cost-effective way to protect commercial and residential fluid systems.

© 2012 Penton Media, Inc.