Accumulators are pressure vessels that store hydraulic energy and deliver that energy back to the system on demand. This is analogous to the way a car battery stores energy. In hydro-pneumatic accumulators, compressible gas (nitrogen) is used to keep stored fluid pressurized. Hydro-pneumatic accumulators are used extensively in the global fluid power industry. In process plant operations, accumulators have multiple uses: Pulsation dampeners, emergency power source, thermal expansion, transfer barrier for fluid separation, as prefill for large volume press applications, noise attenuation, leakage compensators, dispensers for pressurized lubricants, auxiliary power, and others.
There are different designs to separate gas from fluid. Bladder accumulators use a flexible closed bladder. Diaphragm accumulators use a flexible open diaphragm (membrane). Piston accumulators use a moveable piston with a system of seals. Float accumulators allow a buoyant valve to open and close the accumulator when necessary.
For seamless high pressure bladder accumulators, chrome-moly steel has been used extensively for more than 40 years. This material has a high tensile strength, equivalent to AISI 4130. It lends itself well to forming and machining and has a burst pressure more than 350% of design, that being 3.5:1 safety factor conforming to the ASME Code, Section VIII, Div. I. for unfired pressure vessels.
Various applied coatings can enhance resistance to rust, corrosion, and abrasion. Most notable is the internal coating of phenolic resins. This material is an excellent choice to resist many common process fluids and has shown excellent chemical resistance. It is easily applied, and for bladder accumulators, aids in applications where lubrication from the working fluid is poor. Because of the smooth surface, it helps prevent the bladder from adhering to the internal wall surface during operation.
Electro-less nickel plating is also very popular. This coating is applied approximately 2-mm thick internally and externally. Electro-less nickel has proven to be a very durable material, cost effective, and has been shown to be a viable alternative in many applications to the significantly more costly stainless steels, while maintaining the strength integrity of the carbon steel. This variation, as with the phenolic coated variation, can be combined with 316 stainless steel connections. These connections include the fluid and gas ends of the accumulator, along with the all of the internal component parts. An added feature of nickel plating also protects the accumulator on the outside, for those applications where water wash down or corrosive vapors may come into play. Electrolytic action between the stainless steel and electro-less nickel is seldom seen.
There are several accumulator manufacturers that will produce accumulator housings using 316 stainless steel. However, because 316 stainless steel does not have the tensile strength of high carbon steels, the wall thickness must be adjusted to a thicker dimension to equal the strength of chrome-moly steel, especially for accumulator designs over 500 psi maximum allowable working pressure. Stainless steel housing hydraulic accumulators are usually special order, both in the piston and bladder configurations and therefore may have extended delivery times.
The most common and most widely used of all hydraulic accumulators are for the fluid power market. These accumulators are typically designed to operate up to 6000 psi. Both the piston and bladder manufacturers provide the fluid power industry with variations from 1.5 in.3 displacement up to 120-gal capacity. These are the most common accumulators selected by OEM manufacturers for injection molding machines, rubber presses, die casting machines, machine tools, presses, and a variety of automated machines and process lines with extensive use in the land based and offshore oil industry for safety valves and production applications.
The next classification of hydraulic accumulators are those of the 500-psi design, bladder type, large capacity up to 140-gal and larger. These bladder accumulators are most commonly found in process areas where large piping networks are distributed through one or several buildings. These networks carry a variety of fluids, such as water, fuel oil, lubricants, and chemicals. These accumulators are sized and placed in specific locations to primarily eliminate shock. Secondary applications of these large capacity accumulators are for thermal expansion/contraction, pump start up and shut down, and for fire protection.
Smaller accumulators known as the diaphragm type are used for mostly pulsation dampening where flow rates are relatively low, on line sizes under 1-in. diameter. These accumulators are usually non repairable and must be disposed of when failure occurs. Repairable diaphragm accumulators are now common with some manufacturers. The diaphragm accumulator is a relatively low cost device, rated up to 3000 psi, but can be used on low pressure applications as well. They are very reliable products, and with proper pre-charge maintenance, will maintain their reliability. These accumulators may be phenolic coated internally or coated internally and externally with electro-less nickel plating. They also have optional elastomers for specific fluid compatibility. Some manufacturers offer these in 316 stainless steel housings at reduced pressures, usually 500 – 1000 psi operating pressures. The design is simple—they have the anti-extrusion plug built into the bladder and with the repairable design, bladders can be replaced in minutes.
Properly sizing the accumulator is the first element of reliability. One must size the accumulator according to the application, taking into consideration flow rate, maximum operating temperature, maximum and minimum pressures, and of course fluid compatibility. Boyle’s Law of Gases is the primary formula for sizing. Specific formulas, depending on the application, expand from the primary formula shown below. Compression of gas, temperature and cycle times are factored in the expanded formulas.
P1 = Initial Pressure
V1 = Initial Volume
P2 = Final Pressure
V2 = Final Volume
After correctly sizing the accumulator for its particular application, materials must be properly chosen for compatibility. Proper elastomers are chosen depending on the fluid and the operating temperature. From there, connection sizes can be chosen with a host of options: For the 500 psi design bladder accumulators, normally the standard ANSI four bolt flanges are the standard, based on commercial piping connections, from 150# to 600#. For high pressure accumulators, over 500 psi designs, piston or bladder, the choices are female NPT, SAE, BSP or SAE split flange connections.
P1 V1 = P2 V2