Metal-bellows accumulator handles the extremes

Edge-welded metal bellows accumulators are at the cutting edge of hydro-pneumatic accumulator technology, according to Peter Kloft and Daniel Rau, research and development engineers with HYDAC Technology in Sulzbach/Saar, Germany.

Compared with other designs such as diaphragm, bladder and piston accumulators, the gas chamber in metal-bellows units is usually hermetically sealed and the metallic barrier between gas and fluid sides permits no permeation. This results in a design that has extraordinary media compatibility, an extremely wide operational temperature range, is maintenance free and maintains full performance for its entire service life.

In a presentation at this year’s IFK Fluid Power Conference in Aachen, Germany, the researchers discussed one of HYDAC’s newest versions, which mounts on a jet engine. It withstands extreme vibrations, shock loads and heat, and features an indicator that lets technicians easily check the integrity of the metal bellows.

In general, HYDAC distinguishes between two different basic designs:

Corrugated metal bellows have only a limited compression ratio and, therefore, need a large installation space in relation to their displacement volume. But due to their lower costs compared with edge-welded metal bellows, they often serve as dampers.

Edge-welded metal bellows are widely used as compensators in vacuum applications such as in semiconductor production equipment. They are manufactured by welding single round blanks punched out of a coil at the inner and outer circumference.

Edge-welded metal bellows have a significantly higher compression ratio than do corrugated versions and are thus suited for energy storage as well as every other accumulator application. The metal blanks are formed to reduce stresses and deformations in the areas at the welding seams. Along with the welding process and testing, these are the most important design and production considerations.

The metal bellows act like a spring, so engineers must account for this spring force. Force versus compression in a typical metal bellows is mainly linear until it reaches a block position. When handling low precharge pressures, as required for a suction line accumulator, this force has to be considered during the design phase and added to the precharge pressure.

New design

HYDAC was awarded a contract to design and produce an edge-welded metal bellows accumulator for the suction line in an aircraft hydraulic system. The accumulators must maintain pressure inside the suction line above a certain limit, allowing the pumps to speed up without cavitation; and during shut-down of the pumps avoid high-pressure peaks generated by decelerating fluid flow. The accumulators are installed at the jet engine, close to the inlet port of the pumps.

Aircraft applications are mainly characterized by extreme temperatures, for example from -75° to 160° C, and challenging shock and vibration profiles. The ability to withstand severe vibration in case of a fan blade loss, high media and temperature ranges, large displacement volumes combined with high flow rates, and the option to have a maintenance free accumulator lead to edge-welded metal bellows as the preferred design.

Defect indicator

Using metal bellows as a barrier between gas and fluid sides, the gas chamber should be hermetically sealed. Therefore it is difficult to check the integrity of the accumulator with a gauge or similar device. Every seal will add some leakage to the gas side which, as a consequence, will reduce service life or require maintenance. Any kind of pressure measurement will also require measuring the gas temperature, especially in applications with wide temperature swings—such as in aircraft. For instance, if charging the accumulator at 20° C with the nominal precharge, the precharge pressure between the lowest and highest temperatures will vary more than 350%.

Evaluating all these effects, HYDAC decided to build in a different type of indicator which lets technicians determine the integrity of the metal bellows without any seals, and without pressure or temperature measurement. If the metal bellows breaks it will expel gas and in permit some ingress of fluid into the gas chamber. HYDAC incorporated a pressure resistant, gas-tight glass into the upper part of the accumulator. Such glass windows are widely used in the process engineering and chemical industries. These glass windows are helium leak tested with a leakage rate lower than 1 × 10-8 mbar-l/sec. Inside, a white fabric treated with a special chemical substance will change to a highly visible red color if contaminated with hydraulic fluid. The indicator is held a small housing about 10 mm in diameter, which is made by additive manufacturing from stainless steel.

Jet-engine requirements

To qualify a design for the aircraft application, the accumulator was subjected to rigorous testing. To verify compliance with vibration requirements, the accumulator was tested in all three axis directions on one of the most powerful vibration tables in Germany.

A unique requirement for an accumulator installed on a jet engine is the fire test. The test was carried out according to a customized generic fire test specification standard which is based on ISO 2685. During this test the specimen has to withstand a flame temperature of 1050° C for 5 and 15 minutes—while it vibrates along its weakest direction at a 33 Hz frequency and with a minimum acceleration of 8 g. The unit successfully passed the tests, showing no external leakage of the accumulator or the glass interface. The glass showed some surface cracks but didn’t disintegrate or loosen.

The suction line accumulator was also endurance tested between 0 and 15 bar (absolute) for more than 1 million cycles, over the complete temperature range. Overall results showed the edge-welded metal bellows accumulator could fulfill its job even in harsh environments. Due to the high temperature and media resistance, as well as being maintenance free, this class of accumulators can be installed in applications well beyond the capabilities of accumulators using elastomeric or plastic barrier materials. Derived from aircraft applications, these accumulators are being used more and more in process engineering and chemical industry applications, in braking circuits, and as dampers in hydraulic circuits.

Compared with diaphragm, bladder and piston accumulators, edge-welded metal bellows accumulators are more expensive up front, but a lower service-life cost make up for the higher acquisition price. Higher-volume production and automated punching and welding will lower the costs of metal bellows in the future. With reduced costs, metal bellows accumulators will gain more and more market share in all kind of mobile and industrial applications.