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.
Accumulators with a volume less than 1 liter, service pressure less than 1,000 bar, and pressure capacity less than 50 bar-liter fall within the guidelines of Sound Engineering Practice (SEP). Accumulators built to SEP must be manufactured to a known standard and have an authorized regulatory body approve the design and technical file. Because the accumulator is not held to the same standard as with PED, a CE stamp must not be applied to the vessel; however, the accumulator must be marked with SEP on the label.
Manufacturers may also offer dual-certified ASME/CE accumulators.
In addition to the basic design codes, here is a brief overview of the most-common country standards:
Australia: AS1210 is a standard based on ASME design requirements. It calls for additional hydrostatic testing. Accumulators must be tested at 1.5 times the design pressure for 30 min, plus an additional 1 min/mm of the shell/vessel thickness. All accumulators must be tested per AS1210 requirements and be marked with the certification number for use in Australia.
Brazil: Regulatory Rule NR-13 establishes minimum conditions for installation, operation, maintenance, and inspection of pressure vessels and boilers in Brazil. Both ASME and PED are acceptable codes, but ASME designs are more prevalent. When NR-13 is required, engineers can submit a technical file to Brazilian Registered Engineers (BRE) for approval. After approval, BRE will inspect the vessel at the installation. This could also include hydrostatic testing.
Canada: Canadian Registration Numbers (CRN) can be obtained by making an accumulator from ASME-certified material using ASME design standards, and then applying for the registration number. Each province has its own registration number, thus the final destination of the accumulator must be known. Some provinces accept alternative design codes like PED for specific markets such as farming and mining.
China: Accumulators shipped to China must be approved by the Special Equipment Licensing Office (SELO). SELO is based on ASME and PED standards. A complete audit is required and it starts with a proposal to receive Chinese certification. After that document is reviewed, test samples must be submitted. Once the test pieces are approved, inspectors conduct an audit at the accumulator manufacturing facility and review the documented quality-control manual. Further, they examine and review the heat codes and material traceability process. With Chinese approval, a manufacturing license is then issued. Additionally, paperwork before and after the purchase of the accumulator must be submitted to China for tracking purposes.
Russia: Accumulators shipped to Russia often require GOST approval. GOST currently accepts both PED and ASME standards. A technical file called a “passport” must be submitted with each accumulator shipment. Other countries that have adopted GOST standards, in addition to their own, are Armenia, Azerbaijan, Belarus, Georgia, Kazakhstan, Kyrgyzstan, Moldova, Tajikistan, Turkmenistan, Ukraine, and Uzbekistan.
American Bureau of Shipping (ABS): Accumulators on shipping vessels and oil rigs require this certification. Accumulators to be added to a Product Design Assessment Certificate must meet ASME design requirements, plus any additional ABS requirements. Manufacturers can obtain an ABS Certificate of Manufacturing Assessment and many accumulators are listed on the Bureau’s List of Type Approved Products. An ABS-approved inspector must witness testing at the manufacturer of all ABS-approved accumulators.
Det Norske Veritas (DNV): Off-shore Standard DNV-OS-E101 is often required for accumulators for oil and gas applications, particularly in the North Sea. Some major manufacturers stock accumulators with DNV Type Approval.
Because many other certifications exist, we recommend contacting engineers at a reputable accumulator manufacturer to ensure their products meet applicable standards.