Senior Application Engineer Parker Hannifin Hydraulic Accumulator Div. Rockford, Ill.
Hydropneumatic accumulators are widely used in industrial and mobile hydraulics. They supplement pump flow, provide auxiliary power, suppress shocks and pulsations, and compensate for thermal expansion and internal leaks. And they are used in braking, ride control, steering, dead-engine pilot, and tensioning systems on off-road equipment.
Beyond these basic functions, engineers are finding new uses for accumulators because they save energy, reduce equipment costs, lower operating expenses, and prolong equipment life. And new designs now let accumulators tackle jobs previously deemed impractical. Here are a few examples of the new payoffs and benefits.
With today’s high fuel costs, accumulators are increasingly used as rechargeable hydraulic batteries to recover and store energy in mobile and stationary equipment. Excavators are a typical case in point.
An excavator’s massive lift arms generate a great deal of force when lowered. Conventional hydraulic systems control this motion by throttling fluid through valves and shunting it to the reservoir. But charging an accumulator with the high-pressure hydraulic fluid instead means previously wasted energy is stored and can subsequently be used to supplement pump flow to lift the excavator arms and load.
This energy recovery makes it possible to reduce pump size by 25%. In turn, the diesel engine driving the hydraulic system can be 25% smaller, and resulting fuel cost savings can be as high as 30 to 35%.
The same techniques are being used in oil fields where hydraulic power units drive single-acting cylinders that pump oil from wells. The pump supplies high-pressure fluid that retracts the cylinder rod, but the weight of the piston rod and attached stringer rods pressurizes fluid on the extend stroke, powering a hydraulic motor that drives a pump. Diverting this high-pressure fluid to charge an accumulator lets it supplement pump flow on the next cylinder cycle. So engineers can use a smaller pump, electric motor, and reservoir. Energy savings of 15 to 20% are not uncommon in these applications.
Accumulators are critical parts of new hydraulic-hybrid drives that offer significant fuel savings and higher operating efficiency in vehicles that make frequent stops and starts. For example, Parker’s RunWise drive for Class 8 vehicles replaces the standard transmission with a hydromechanical hybrid drive that recovers braking energy previously wasted as heat.
During braking, a hydraulic pump/motor pumps high-pressure fluid into an accumulator. The fluid compresses nitrogen gas in the accumulator to maintain high system pressure. When the brake pedal is released, the stored fluid in the accumulator is pumped back through a power-drive unit, which transmits torque to the driveshaft and propels the vehicle.
Thus, energy stored in the accumulator supplements the flow of hydraulic fluid from the dieseldriven pump to accelerate the vehicle. This has the potential to significantly reduce fuel consumption in public buses, delivery vans, and refuse trucks.
A key to making these systems economically viable is new accumulator shells made of carbon and glass-fiber composites. Composite accumulators weigh significantly less than their steel counterparts, making for lighter vehicles that either improve fuel economy or carry heavier loads. For instance, a RunWise 22-gallon composite accumulator rated for 5,500 psi weighs 140 lb. This compares favorably to a comparable steel accumulator weighing 900 lb. The composite accumulators also tend to be more compact and easier to install in vehicles.
When valves shift or center, it often sends hydraulic shocks through the lines. This causes unwanted noise, vibration and, possibly, hose and fitting failures. Another common source of undesirable noise is pressure pulsations from piston pumps. In both instances, adding a small accumulator near the source attenuates shock and noise.
For instance, Parker’s Pulse-Tone shock suppressor consists of: an inner radial chamber with a series of 0.5-in.-diameter holes; a compressed coil spring surrounding the inner chamber; an outer radial chamber dotted with 0.03-in.-diameter holes; and an elastomeric bladder around the outer chamber. A 0.25-in. gap separates the chambers.
In operation, oil flows through the inner radial chamber, spring, and outer chamber. The 0.03-in. holes maximize flow but keep the bladder from extruding through the outer chamber. Pulsations pass through the holes, and strike and deflect the nitrogen-charged bladder. It is this deflection of the bladder that reduces shock and noise.
Generally, the chamber surrounding the bladder is charged with nitrogen to 50 to 60% of the hydraulic operating pressure. The combination of a large bladder that can oscillate at high frequencies, and the short distance the pulsations travel once they enter the unit contribute to the unit’s effectiveness.
An advantage of the in-line Pulse-Tone is in systems where hydraulic pressure drops below precharge pressure, such as where the pump is unloaded at low pressure. Standard accumulators are not acceptable in these applications.
One place shock suppressors are used is on top-end yachts that often have large, central hydraulic systems with piston pumps generating 4,000 psi and flows to 100 gpm. They power stabilizers, bow/stern thrusters, and other hydraulic devices, and the yachting crowd will not tolerate fluid-borne noise rumbling about their multimillion- dollar vessels. Ship builders mount an in-line shock suppressor at the pump outlet to prevent pulsations and noise from traveling through the hydraulic lines. Typical noise reduction is more than 6 dB.
Adding accumulators to equipment can increase operating speed and productivity without a larger power unit. This is readily accomplished if the circuit has dwell time.
Two factors determine accumulator size when the goal is higher speed. One is maximum and minimum system pressures. The other is the volume of fluid the accumulator must deliver to supplement pump flow and reduce cycle rate. Pump size determines minimum dwell time. If, for instance, an accumulator delivers 308 cu in. of fluid during the machine work cycle, the pump must be large enough to refill the 308 cu in. during the dwell period.
Take, for example, a machine with a total cycle time of 16 sec — 8 sec to cycle a 4-in. bore, 12.5-in. long cylinder with a 1-in. rod; and 8 sec of dwell time. The operator wants higher productivity, but the dwell segment cannot change because workers need time to load and unload the machine. However, adding an accumulator increases machine production from 3.75 to 5.0 ppm, a 33% increase. (See the accompanying sidebar, Sizing Accumulators, for more information on engineering calculations.)
Many t ypes of equipment have hydraulic systems that require high flow for short periods, followed by a few seconds of dwell time. Examples include die casting, injection molding, and rubbermolding machines. Without an accumulator, during the dwell period pump flow is diverted to the reservoir and does no work. In such cases, adding an accumulator harnesses this flow and can significantly reduce the size of the pump and electric motor.
Let’s look at an application that requires 2,000 psi and cycles a cylinder in 8 sec, followed by 8 sec of dwell time. In this instance, total flow from the pump is 1,000 cu in. or 32.45 gpm. A simple hydraulic system designed for this job would require a 41.7-hp motor, a motor starter, approximately a 100-gallon reservoir, as well as valves and filters.
Adding a 15-gallon accumulator charged to 3,000 psi to the system would not change machine performance. However, design requirements would decrease to a 9.3-gpm pump, 30-gallon reservoir, and 17.9-hp motor. Not only would this revised design reduce power-unit component costs by up to 60%, energy consumption and operating costs would also be significantly less. The installed and operating cost savings with and without an accumulator are shown in the Accumulator Value Comparison table.
Thus, incorporating accumulators not only makes for betterengineered systems, they improve efficiency and deliver big payoff benefits in terms of energy savings, reduced initial and operating costs, higher payloads, and longer equipment life.
Hydropneumatic accumulators use compressed gas to apply force to hydraulic fluid. Identical in their operating principal, piston, bladder, and diaphragm accumulators use different mechanisms to separate the gas from the fluid. Here’s an overview of the different designs and performance characteristics.
Piston accumulators have a cylindrical body sealed by a gas cap with a charging valve at the gas end, and a hydraulic cap at the fluid end. A lightweight piston separates the gas and hydraulic sides. This design offers high efficiency and units come in a wide range of sizes. Advantages include extremely high flow rates, a wide temperature range, high compression ratios, the ability to withstand external forces, and the units work well with gas bottles.
Bladder accumulators feature a nonpl e ated, flexible rubber bladder housed within a steel shell. One end has a valve stem molded into the bladder and fitted with a gas valve. This part of the bladder passes through the shell and is held in place by a jam nut. A poppet valve, normally held open by a spring, mounts in the other end of the steel shell. The poppet asssembly prevents the bladder from extruding out the hydraulic port during shutdown. The units are suitable for most applications, are dirt tolerant, respond quickly, and work well with water and low-lubricity fluids.
Diaphragm accumulators use a one-piece molded diaphragm mechanically sealed to a high-strength metal shell. The flexible diaphragm separates gas and fluid, and a button molded to the bottom of the diaphragm prevents it from extruding out the hydraulic port. These units are compact and lightweight, simple, inexpensive, dirt tolerant, and respond quickly. Note that bladder and diaphragm accumulators are generally preferred in applications with rapid cycling or contaminated fluid.
Engineers typically specify accumulators based on fluid pressure and the volume of fluid they must deliver to a system. They are generally rated by pressure capacity and the gas volume when all fluid has been expelled. Accumulator sizing calculations depend on the application. For instance, a typical equation for supplementing pump flow, auxiliary power, energy storage, leakage compensation, and holding applications is:
where V1 = accumulator size, gallons; Vw = liquid volume discharged from the accumulator, gallons; P3 = maximum system pressure, psi; P1 = precharge pressure, psi; P2 = minimum system pressure, psi; f = charge coefficient; n = discharge coefficient; and 0.95 is the efficiency factor. Note that polytropic exponents f and n vary with the gas, pressure, compression and expansion times, and gas temperatures.
However, real-world factors affect results. For instance, hydraulic fluid entering an accumulator compresses and heats the gas. Unless the gas cools to ambient temperature, the fluid volume entering the unit may be less than calculated, so the amount discharged may be less as well. System efficiency may be a guess, too. At best, zeroing in on the best unit is an iterative process, and calculations can be tedious.
Experts now recommend software- based calculators available from major manufacturers. For instance, Parker’s inPHorm sizing software (available at parker.com/accumulators) streamlines the process of specifying hydraulic accumulators.
Users input application requirements, and the program guides them through a step-by-step selection process. The software, for instance, includes a precharge tool that calculates the effects of temperature on pressure. Another tool calculates the capacity of an existing accumulator based on operating conditions. And inPHorm sizes accumulators for various applications including supplementing pump flow, auxiliary power source, holding/ leakage compensation, shock suppression, piston-pump pulsation dampening, and attenuating shock. It generates model numbers of all accumulators meeting the circuit parameters, offers on-line references for more information on products, as well as CAD files in various formats.
Gas bottles reduce accumulator size and cost
Gas bottles are opening new applications for accumulators, particularly where large capacity is required. They work by increasing the gas capacity of a small accumulator. For instance, assume an application requires a 45-gallon accumulator. And that maximum and minimum system pressures are relatively close (much like in a die-casting machine) and the accumulator delivers only 4.4 gallons of fluid. If working pressures range between 2,000-psi maximum and 1,700-psi minimum, a 44.48-gallon accumulator would be required. The list price of a 45-gallon accumulator is approximately $14,000. Instead, using a 15-gallon accumulator and two 15-gallon gas bottles reduces the accumulator cost by $5,770. The added advantage is that gas bottles seldom require maintenance.
In addition to saving money with smaller accumulators, gas bottles save space. They can mount remotely and in any orientation. And instead of handling a large accumulator that weighs up to 2,100 lb, the smaller unit weighs less than 700 lb. Finally, the accumulator seal kit costs less. Seals for a large 50-gallon accumulator can cost $1,000, depending on the compound, whereas those for a 15-gallon accumulator cost less than half that amount.
Accumulator shells made of carbon and glass-fiber composites are a critical part of new hydraulic-hybrid drives. They weigh significantly less than steel accumulators, making for lighter vehicles that improve fuel economy.
Pulsations pass through the inner and outer chambers of the Pulse-Tone shock suppressor and strike and deflect the nitrogen-charged bladder. This deflection reduces shock and noise.
Hydraulic accumulators are increasingly used to save energy and reduce capitalequipment and operating costs.
Results shown here are based on 50-ton load, 8 10-in. cylinder, 4-sec extension and return, and 20 sec of dwell. With an accumulator, there is a 6-hp efficiency gain from the pump. Based on a 2,000-hr work year, a system without an accumulator wastes 8,948 kW-hr compensating for the 6-hp loss. Thus, adding an accumulator cuts annual operating costs by $832.00, in addition to the $6,525 upfront equipment savings.