When a hydraulic circuit needs an accumulator, how do you choose which style? Accumulators are used in a wide variety of applications and for various specific reasons such as: supplemental flow which assists the pump, pulsation dampening, shock absorption, emergency flow, makeup oil to account for losses in a hydraulic system, and many others. There are two major groups, or construction types, of accumulators: mechanical and hydropneumatic. Mechanical accumulators are loaded with either a spring or a mass. Hydropneumatic accumulators contain both a dry side (compressed nitrogen) and a wet side (hydraulic fluid). Hydropneumatic accumulators are the most common and therefore are the focus of this article.
There are three hydropneumatic accumulator types: bladder, diaphragm or piston. When selecting an accumulator for the application, many system and performance requirements should be considered. Accumulators are difficult to apply because nearly all the following factors should be considered in each application: failure mode, total output volume, flow rate, fluid type, response time, shock suppression, high frequency cycling, mounting position, external forces, sizing information, certifications or conformance to standards, safety, and temperature effects.
One extremely important question in the application of accumulators is "What happens if the accumulator fails?" The failure mode is radically different for piston accumulators as compared to the bladder or diaphragm accumulator style. Some applications absolutely cannot accept the complete failure of the accumulator, such as happens with the bladder or diaphragm design. Other applications prefer the bladder or diaphragm’s immediate method of failure. Piston accumulators fail slowly over a long period of time which makes their failure mode more appropriate in certain cases. Piston accumulators also have an inherently higher total output volume relative to their overall size, which makes them more useful in circuits where space is limited.
For a given system pressure, the flow rates for piston accumulators generally exceed those for bladder or diaphragm designs. But flow is limited by a maximum piston velocity, which should not exceed 10 ft/sec to avoid seal damage. In high-speed applications, a bladder or diaphragm accumulator is the better choice.
In some applications such as servo systems, the response time of the accumulator is critical. When the required response time is less than 25 msec, a bladder or diaphragm accumulator should be used.
Fluid type is also a concern when selecting the accumulator style. Since bladder and diaphragm accumulators are more resistant to contamination, they are more suited for water applications as water systems tend to carry more contaminants. Piston accumulators are preferred for exotic fluids or where extreme temperatures are experienced, as they have fewer compatibility issues without a bladder.
There are many things to consider when selecting the most appropriate type of accumulator. Don’t risk making the wrong decision; give Quality Hydraulics & Pneumatics, Inc. a call and one of our Certified Fluid Power Specialists will help guide you in making the best decision for any application.