Accumulators have proven to be valuable in many fluid power applications when properly selected and installed. The selection process, while fairly straight forward, does involve several important factors. Often, system planners specify familiar components used in previous designs without considering unfamiliar characteristics of new systems.
System requirements
To select the correct accumulator for an application, you must have the following information:
Flow rate and total fluid volume required — This requires pump, piping, cylinder, and other component specifications.
Maximum system working pressure — Calculated peak demand, as well as intermittent and momentary spikes, should be noted. All components that affect pressure, such as pumps, valves, cylinders, and the often-ignored piping, must be taken into account.
Minimum system working pressure — To prevent damage in bladder accumulators, the minimum pressure should be at least 1/4 that of the maximum pressure.
Ambient, minimum, and maximum fluid temperatures — Actual operating temperatures within the accumulator often go well beyond those calculated. If this occurs, the accumulator can act as a heat sink.
Fluid specifications — These can be obtained from suppliers and often are available with the material safety data sheets.
Cycle timing — Whether hours or milliseconds, work and recovery times are important.
With this information, you can move to the next step and determine:
Pressure — The accumulator's maximum allowable working pressure should meet or exceed the system's maximum pressure.
System spikes must be identified and accounted for. In general, accumulators are available in 150 psi to 10,000-psi models, with 3000 psi and 6000 psi among the most popular. Never use the accumulator's proof pressure or design burst pressure in your assessment. Conversely, installing an accumulator with a maximum pressure rating far exceeding the system's maximum pressure may add unnecessary cost.
Design code — Some applications require the accumulator to meet ASME or other design codes. The location or type of system often may require a specific inspection agency or quality standard. Code requirements should be determined prior to specification. Only some accumulator manufacturers can meet most design codes or have most agency approvals.
Sizing — The selection of the proper size accumulator is important for efficient operation. If too small, there may be insufficient capacity to do the job. Sizing depends on the type of application, and calculations are based on variations of Boyle's law. Sizing formulas are available on most accumulator manufacturers' websites, in catalogs, and many sizing programs are also available.
Fluid compatibility — As with all hydraulic seals, it is important that the system fluid be compatible with the accumulator's elastomer compound. Several compounds are available. Compatibility charts are found on accumulator and rubber manufacturers' websites, catalogs, and in a number of industry wide publications.
Temperature — Each type of elastomer compound has associated minimum and maximum temperatures. Ensure that the system does not exceed this range.
Precharge — The proper nitrogen gas precharge is critical to the operation of any accumulator. Generally, precharge is a percentage of the minimum or maximum working pressure of the system and determined by the type of application. The calculation is part of all sizing formulas.
Placement — The connection, placement, and orientation of the accumulator can affect system efficiency. Although suppliers can provide recommendations, it is the designer's responsibility to decide on the best arrangement.
This discussion was provided by Jeffry Schneider, president, Accumulators Inc., Houston. For more information, visit www.accumulators.com.
