Regulatory and Safety in Hydraulic Accumulators: A High-Pressure Responsibility

Hydraulic accumulators are vital parts of contemporary fluid power systems, serving as energy storage elements to stabilize pressure, dampen shocks, and provide a source of emergency power. However, their primary existence is as a pressure vessel, because accumulators store high-pressure hydraulic fluid and high-pressure compressed gas, both of which are hazardous.

To ensure appropriate and safe operation, it is essential to have a comprehensive understanding of the strict global regulations, as well as the safety protocols, operating procedures, and documents all required for operating these components. This is not just a means to avoid fines, it's to prevent catastrophic failure, injury, and downtime.

The Context of Regulation: Required Compliance Standards 

Accumulators are considered pressure equipment therefore the design, manufacture, and install earn compliance with international and regional directives. Compliance is a requirement for payoff purposes and to trade internationally.

1. International Design Codes (ASME)

In North America and many other regions in the world, meeting the ASME Boiler & Pressure Vessel Code (BPVC), Section VIII, Division 1, is the primary requirement.

ASME 'U' Stamp: Accumulators built to ASME code receive the 'U' symbol that signifies met global quality design and manufacturing compliance.

Design Factor: ASME requires a high margin of safety typically 4:1 design factor (the ratio between burst pressure and rated pressure) to allow a vessel to operate at a significantly lower pressure than it is rated.

Certification: To obtain certification, certain material traceability and a hydrostatic test witnessed by an approved third-party is needed.

2. European Compliance (PED)

The Pressure Equipment Directive (PED 2014/68/EU) applies to units sold or put into service in the European Economic Area (EEA).

CE Marking: Any accumulator that has a product of pressure (bar) times volume (liters) that exceeds a limit must have a CE Mark. This mark is the manufacturer’s declaration that specified equipment meets all applicable Essential Safety Requirements (ESR) of the directive.

Fluid and category: The PED has classified equipment depending on maximum pressure, volume, fluid type (gas/liquid), and hazard of fluid. This classification determines the appropriate conformity assessment procedure.

Sound Engineering Practice (SEP): Smaller accumulators that fall below the full scope of the directive shall still be manufactured in accordance with "Sound Engineering Practice" (SEP) and marked and have instructions as necessary without CE Marking.

Essential Operational Safety Guidelines

Adherence to the regulations set forth by the manufacturer is merely the first component of operational safety. The end-user or system operator is responsible for ensuring safety during the accumulator's operational life.

1. Pre-Charging: Nitrogen Only

The single most important safety rule regarding the maintenance of accumulators is the process of pre-charging.

Dry Nitrogen (N₂) ONLY: Accumulators must be pre-charged ONLY with dry nitrogen gas. Nitrogen is relatively inert.

NEVER Use Air or Oxygen: Under no circumstances should compressed air or oxygen be used. The chemical combination of high-pressure oxygen (or high-pressure air if present) with hydraulic oil (hydrocarbon) can create a violent fire or explosion.

Proper Pressure: The pre-charge pressure (P0) needs to be set correctly, generally with a specific charging rig based on the system minimum operating pressure. Incorrect pre-charge will lead to poor operating performance and reduced service life of component parts in the accumulator (bladders or seals).

Requirements for Inspection and Maintenance

Regular maintenance is necessary for safety and compliance with regulations, which may be required by local rules.

Periodic Inspection: Visual inspections for corrosion, leaks, or external damage should be conducted once a year, with special attention to mounting points and external fittings.

Hydrostatic Testing: The pressure vessel shell should be periodically taken out of service and hydrostatically (water) pressure tested for structural integrity. Such testing shall be performed, as required, every 5 - 10 years based on local rules and the size/classification of the accumulator.

Pre-charge Verification: The nitrogen pre-charge should be verified at prescribed periodic intervals (quarterly/annually, etc.) using a calibrated gauge, and always when the system is depressurized and provides accurate readings.

No Shell Repair: Never attempts to weld, repair or modify the accumulator shell. If the shell is damaged, it must be retired from service and replaced.

By complying with these regulatory standards and safety protocols, operators can take advantage of the tremendous capabilities of hydraulic accumulators while reducing the hazards associated with energy stored at high pressure.