Maintenance-Free Accumulators & Reservoirs

The separator between the precharge gas is a hermetically welded bellows assembly, which eliminates precharge migration, elastomer wear, and the need for recharging—sustaining system pressure and volume over the full life of the system.

Yes, edge-welded bellows are autogenously welded metal diaphragms, so if the base material is compatible with the fluid, then so are the bellows. Properly specified bellows handle hypergolic and cryogenic propellants, glycol/water coolants, synthetic hydraulic fluids (i.e phosphate esters), and inert gases. Senior Metal Bellows engineering teams review materials for compatibility with both the system fluid and environment to ensure longevity.

Helium offers favorable compressibility and easy detection; nitrogen is selected when program constraints favor it. Both are supported, but Senior Metal bellows will propose the most appropriate precharge gas during a review of the application based on real gas analysis.

The size of an accumulator/reservoir is based on the required operating parameters, which in turn impact the selection of the source of energy (i.e. gas precharge, spring, or active pressure supply). Based on this information, an engineer can:

• Define operating envelope: dependent on the minimum and maximum system pressures, temperature range, fluid properties, duty cycle, environment (vacuum, radiation, vibration), and required volume displacement. When a gas precharge is required, Senior Metal Bellows sizes the accumulator to account for the real gas properties and performs FEA to optimize the pressure vessel geometry.
• Determine materials & mass: Choose alloys for corrosion resistance and strength while considering weight impacts where applicable.
• Incorporate customer desired system interfaces: Port sizes, fittings, mounting methods.

Both accumulators and reservoirs are used to store fluid, but they have distinct applications:

• Accumulator: Accumulators provide on-demand fluid pressure to a fluid system (like when an actuator needs to be extended), accommodating volume fluctuations and smoothing pressure pulsations to make up for the non-instantaneous speed of pumps as the fluid needs to move. They do this by having energy stored in the form of compressed gas, where the metal bellows is used to separate precharge gas from system fluid, with no gas permeation as the volume/pressure changes.

• Reservoir: Reservoirs store fluid to accommodate continuous system volume fluctuations, but bellows reservoirs are unique from conventional reservoirs in that they also always supply energy to the system. Edge-welded metal bellows have inherent spring force, so a reservoir using this technology will constantly have active pressure on the system without necessarily needing an additional spring. When located on the low-pressure side of a fluid system, bellows reservoirs are often used to prevent cavitation of the system pump by ensuring a consistent supply of fluid and supplying a minimum input pressure. When the fluid system has a fixed volume, they may also be used as volume compensators to manage volume changes due to thermal expansion and system fluid loss, preventing over-pressurization and maintaining system pressures.

Because of their (relatively) low pressure requirements, reservoirs have reduced qualification requirements per the FAA/EASA, so a clear distinction between reservoirs and accumulators should be maintained when specifying hardware for such applications.

While there may be existing hardware designs that are appropriate for one’s application, custom ports, mounting fittings, and brackets/ bracket interfaces can be tailored to the requirements of the system.

Deliverables are dependent on the program needs shared by the customer. Senior Metal Bellows conforms to AS9100 and provides traceability information with shipments as a default, but may also supply acceptance testing results as required, including helium leak, proof pressure, and weight results, supporting regulated or mission-critical programs. If new hardware is developed for a customer, design reviews and qualification testing is also documented and shared as appropriate. Senior Metals bellows is happy to discuss the documentation needs of a program and provide the best options to accommodate.