The most common use for accumulators is to supplement pump flow. Some circuits require high-volume flow for a short time and then use little or no fluid for an extended period. Generally speaking, when half or more of the machine cycle is not using pump flow, the application is a likely candidate for an accumulator circuit.
The circuit shown uses several accumulators to supplement pump flow because the dwell time is 45 sec out of the 57.5-sec cycle time. This circuit’s 22-gpm fixed-volume pump operates on pressure during most of the cycle to fill the cylinder and the accumulators. Without the accumulators, this circuit would require a 100-gpm pump driven by a 125-hp motor. The first cost of the smaller pump and motor plus the accumulators is very close to that of the larger pump and motor. However, energy savings over the life of the machine make the pictured circuit much more economical.
This circuit shows a common application of an accumulator supplementing pump flow.
One drawback of using accumulators to supplement pump flow is that the circuit must operate at a pressure higher than needed to perform the work. In the circuit, a minimum of 2000 psi is necessary to perform the work. This means the accumulators must be filled to a higher pressure so they can supply extra fluid without dropping below the minimum pressure. This circuit uses 3000-psi maximum pressure to store enough fluid to cycle the cylinder in the allotted time and still have ample force to do the work. The flow control in the circuit is necessary to keep the cylinder from cycling too rapidly. An accumulator discharges fluid at any velocity the lines can handle while operating at whatever the pressure drop is when a flow path is opened.
The pictured circuit uses a fixed-volume pump and an accumulator unloading-and-dump valve. The valve forces pump flow to the accumulators when pressure drops approximately 15% below its maximum set pressure. At set pressure, the unloading valve opens and all pump flow bypasses to tank at 25- to 50-psi pressure drop. While the pump is bypassing, a check valve keeps the accumulators from unloading to tank. The dump valve (which is a high-ratio, pilot-to-close check valve) is held closed by pump idle pressure until the pump shuts down.
This information was excerpted from Chapter 16: Accumulators of our Fluid Power Ebook Edition 1. Read the entire chapter here.
The following is a brief round-up of some of the latest accumulator products available in the fluid power marketplace:
Miniature AC type hydro-pneumatic accumulators are made of steel and exceed 7000 psi. They are ideal for workholding and jigs. Available in two sizes with volumes of 13 cm3 (0.79 in.3) and 40 cm3 (2.44 in.3), they are easily integrated into the valve bank with a screw-in fitting and have an optional shut-off valve. They compensate for temperature-dependent volume variations, possible losses due to leakage, or to dampen oscillations in circuits with pressure-difference-controlled devices.
High-pressure bladder accumulator
Seamless bladder accumulator is rated for pressures to 10,000 psi. It features no moving seals to prevent contaminants from entering a system and is field-repairable. They are available in sizes from from 1 qt to 15 gal capacity. Standard materials include: standard, low-temperature (–40°F) and extreme low-temperature (–80°F) Buna-Nitrile; Butyl; Fluorocarbon; EPR; and Neoprene.
Accumulator isolator block
SBA Safety Blocks now feature a lockout mechanism for work environments that mandate a “lockout, tagout” procedure. They allow an accumulator to be isolated from a hydraulic system then discharged; the new lockout mechanism helps safeguard against accidental rejoining of the accumulator and system while maintenance is being performed. They work with bladder, piston and diaphragm accumulators. A full range of adaptors is available to suit all common port sizes and styles including SAE and BSPP straight port. Three different sizes – NG10, NG20 and NG32 – are available in a corrosion resistant finish.
Pleated bladder accumulators
Nacol Accumulators range in size from 1⁄5 pint to 40 gal, rated for 3000 and 5000 psi. They are available in carbon and stainless steel. “Top easy maintenance design” makes it possible to inspect the inside and replace bladders without removing them from pipings. Dynac gas valve acts as a safety device in case of fire or extreme heat. Safety vent releases remaining gas when accumulator is disassembled. Seamless, one-piece molded bladder resists fatigue and has a low degree of gas permeability.
Standard line of piston accumulators is available in pressure ratings of 3000 and 5000 psi in capacities from 1 pint to 30 gal. They come with bore sizes of 4.0, 6.0, 7.0, 9.0, and 12.0 in.; custom bores are available. Materials include carbon and stainless steel, aluminum, nickel and chrome-plated steel, and special paints and coatings. Standard porting includes SAE O–ring, NPT and NPTF, SAE 4–bolt flange, Code 61 and 62 and special ports.
CarbonWeight high-pressure bladder accumulators are 1⁄4 the weight of standard steel accumulators. They feature a spun carbon fiber-wrapped aluminum tank, a nitrogen gas compression bladder, and ports for fluid flow. They are available in 10, 15, 20, 22, 25, and 30 gal sizes for pressures to 6000 psi (414 bar). They are tested to 500,000 cycles and have been burst-tested to a safety factor greater than 3.4 times operating pressure.
Series GO piston-type accumulators provide auxiliary and emergency power and absorb or suppress hydraulic system shock pressures. They are available in sizes from 10 in.3 to 2.5 gal for pressures to 3000 psi. They offer rapid response rate using lightweight, low inertia aluminum piston. Recessed end cap receives piston for dashpot action when discharged. Safety gas vent prevents removal of end cap from charged accumulator. Standard charging valve is recessed and covered with cap for safety.
Hydraulic bladder accumulators are available in 1, 2.5, 5, 10, 11 and 15 gal sizes, for pressures from 3000 to 6000 psi with NPT, SAE and Code 61/62 porting options. For safety reasons, they cannot be disassembled under pressure and are interchangeable with other manufacturers. Bladder is available in standard rubber compounds, including Nitrile and cold-weather Nitrile. Standard porting is 11⁄4 and 2 in. NPT machined from alloy steel and coated for corrosion prevention.