Regulations and standards for hydraulic accumulators

Accumulators are pressure vessels that contain gases and liquids at high pressures — often at several thousand psi — which demands caution when they are made, handled, and used. To ensure the safety of operators and equipment, various regulations govern their design, manufacture, and installation.

Unfortunately, there is no recognized global certification standard that applies universally, regardless of where and how an accumulator is used. Rather, accumulators and gas bottles are subject to safety laws, regulations, and ordinances valid in the state or country where they are installed. Further complicating matters, other regulations apply to specific industries such as mining, shipbuilding, and aerospace.

Thus, certifications for accumulators vary — often quite significantly — depending on the application and location. Sorting through the requirements that may be encountered is a cumbersome task. So here’s a brief look at the two fundamental design codes and several common certifications engineers are likely to encounter.

Base design codes
Although many countries have their own regulations and quality standards for hydraulic accumulators, most refer to one of two base design codes. The oldest and most referenced design code for pressure vessels is from the American Society of Mechanical Engineers. Originally a standard for manufacturing boilers for steam locomotives, the ASME Boiler and Pressure Vessel Code Section VIII, Division 1 has evolved into requirements for pressure vessels and accumulators in the U. S. This section requires:
• Certification on vessels with internal diameters of 6 in. or greater.
• Certified vessels carry the “U” symbol on them as evidence they were designed and manufactured according to the Code. The “U” symbol is an international designation of design and manufacturing quality.
• Accumulators must be made from materials that meet ASME specifications for traceability.
• A 4:1 ratio of burst pressure to rated pressure. This design factor is with respect to the minimum tensile strength of the material.
• Each vessel must be marked with its design pressure at the Minimum Design Metal Temperature (MDMT) for that vessel.
• Vessels are manufactured under an approved quality system, like ISO 9001.
• An approved third party observes all hydrostatic testing.

The 4:1 design factor is mandatory for all accumulators with ASME Certification, except those that comply with a specific rule within the Code called Appendix 22. Appendix 22 permits accumulators manufactured with forged shells and openings of a specified maximum size to be certified with a 3:1 ratio of burst to rated pressure.

The second base design code is the Pressure Equipment Directive (PED), in force since May 2002 in the European Union. The Directive (designated 97/23/EC) applies to the design, manufacture, testing, and conformity assessment of pressure equipment and related assemblies that operate above 0.5 bar. The directive requires:
• Operating fluids must be in Group 2 (nonhazardous).
• Certified vessels must receive a CE mark if: volume is greater than 1 liter and pressure capacity (the product of service pressure and volume, PS × V) exceeds 50 bar-liter; or service pressure PS exceeds 1,000 bar. In general, all accumulators larger than 1 liter must be CE marked.
• Certified vessels must be made from materials that meet PED specifications for traceability.
• A 2.8:1 ratio of burst to rated pressure. This design factor is with respect to the minimum yield strength of the material.
• Certified products must pass a low-temperature Charpy test (temperature to be determined by application or customer). In Canada, per local inspection authorities, mining applications are specified to have an MDMT of –40°F. Other applications will be determined by the lowest temperature the accumulators will see.
• Vessels are manufactured under an approved quality system, like ISO 9001.
• All hydrostatic testing to be witnessed by an approved supervisory body. Manufacturers can become self‑certified.
• Once installed, national laws govern equipment and accumulator inspection as well as operational safety.