Hydraulic energy storage
By Chris Grosenick
right) Accumulators provide backup power
for brakes, landing gear, emergency applications,
and APU starting. The average pneumatic charge
in an accumulator is 1,000- to 2,000-psi pressure.
While accumulators are closely identified with hydraulic systems, they find applications in other aircraft systems too. They come in a number of capacity sizes, and depending on the system application, are either charged with a gas or use mechanical force to store energy in the form of pressurized fluid. Pneumatically pressurized accumulators are used primarily in hydraulic systems, and mechanical accumulators are used in various applications like fuel for APU starting and grease for stab trim lubrication systems. Accumulator construction varies and has evolved over the years, with the cylindrical shape being the most predominant.
Principles of operation
Accumulators are simple devices that are constructed of a piston, a cylindrical sleeve, and two end caps. The piston is free to move through the entire length of the cylinder sleeve, similar to a rod-less piston in a hydraulic actuator. Pressure from the aircraft hydraulic system enters the fluid side and forces the piston toward the pneumatic end of the cylinder. As the piston is forced away from the fluid end, it compresses the trapped gas on the pneumatic side. When the pressures equalize, the piston stops moving and the accumulator is now storing a predetermined amount of pressurized fluid. A check valve from the pressure supply, and selector/shut-off valves keep the pressurized fluid trapped until it is needed to perform work. The main physical principles at work here are the theoretical incompressibility of one fluid (hydraulic oil) and the highly compressible nature of another fluid (nitrogen or air).
Most hydraulic accumulators are cylindrical with a pneumatic and fluid side separated by an internal free-floating piston. Depending on the available real estate inside an aircraft, the pneumatic side may use a tubing run to locate the pressure gauge and servicing valve. Older accumulator styles were spherical with bladder type diaphragms to separate the pneumatics from the hydraulics. Some accumulators are constructed using a spring instead of pneumatic pressure to provide the force to move the piston. This type is used mainly in low pressure applications such as APU fuel systems, grease dispensing mechanisms, and pump suction-side hydraulics.
Another type of accumulator is the self-displacing variety. This accumulator has three chambers with two piston heads attached together by a common rod. This type of accumulator is used in hydraulic systems where reservoir volume is small or speed of operation is important. Fighter planes and helicopters have this configuration due to tight spaces and small hydraulic system volumes. The latest technology in accumulators is the helium charged bellows type, which is being used in advanced generation fighters. This accumulator is maintenance-free, and requires replacement if the pneumatic charge leaks out, or becomes otherwise unserviceable. Accumulator capacities range from 500 cubic inches (C-5, self-displacing) to 50 cubic inches (many aircraft applications), and hydraulic system design determines what capacity is required.
Accumulators provide backup power for brakes, landing gear, emergency applications, and APU starting. They are also used as system dampers, absorbing pressure spikes in hydraulic systems with large volumetric output piston pumps. In the dampening role, the accumulator is plumbed into the pressure tubing downstream from the pump(s), and the capacity for this function is usually 100 cubic inches. Accumulators in the 10- to 25-cubic-inch capacity range are used as local dampers, and depending on system requirements, subsystems like flight controls or landing gear may need protection from pressure spikes or plumbing design-induced flow characteristics. Large aircraft have at least one brake accumulator, and some have up to four.
Brake accumulators are used for ground towing operations, and as such, they get a fair amount of use. With an inflight hydraulic system loss, the brake accumulator can become the difference between staying on the pavement and becoming a 100-ton mud buggy. Another component of many brake systems is a return system compensator. This device is a spring-operated accumulator that compensates for volume changes in the brake return system when the parking brake is set, or when system design requires expansion room in the return tubing. Temperature changes in this trapped fluid situation can cause elevated pressures (heat) or loss of brakes (cold). Most commercial aircraft use electrical power for APU starting, but the military uses hydraulic accumulator starting on many airframes. With hydraulic starting, the design philosophy is such that when the troops are in the middle of nowhere, they can hand-pump an accumulator and get out of town without having to deal with potentially dead batteries.
Accumulator servicing and troubleshooting
There are two general servicing rules to keep in mind. First, an accumulator must be purged of fluid charge before servicing. Second, follow the servicing instructions provided by the aircraft manufacturer to make sure the accumulator will have the correct capacity or dampening characteristics.
Servicing charts are usually stuck to the aircraft in the vicinity of the servicing valve, and the gas of choice is nitrogen. Accumulators develop pneumatic leaks in the tubing, gauges, and servicing valve. When this happens, the piston moves toward the pneumatic port until there is no room on the pneumatic side for the correct charge. This problem manifests itself when constant servicing is required or the number of component cycles is less than expected. Another symptom of leakage is rapid pneumatic pressure drop-off after servicing. One way to troubleshoot this problem is to attach the pneumatic pressure source to the servicing valve and apply 2,000- to 3,000-psi pressure. Use leak detection fluid (soapy water) to find the pneumatic leak; many times the servicing valve is leaking because the swivel nut has been overtorqued past the 50- to 70-inch-pound limit and the valve seat is damaged. Depressurize and repair the leak, and reapply 2,000-to 3,000-psi pneumatic pressure. Cycle the components until they no longer operate and then service the pneumatic side to the correct pressure. This process forces the excess fluid out of the accumulator so there is enough volume to get the correct pneumatic charge.
Another problem with accumulators is internal leakage. This is harder to spot in aircraft with vented or air pressurized reservoirs, but it is quite evident in aircraft equipped with piston type reservoirs. When the system is pressurized, fluid leaks past the piston seals and fills the pneumatic cavity, and when the system is depressurized, gas leaks the other way into the hydraulic system. If this type of leak is suspected, crack open the servicing valve and if a large amount of fluid foams out, the piston seals need replacement. It is acceptable to have a small amount of fluid escape when the servicing valve is opened, because pistons are designed with grooves and holes to hold a small amount of hydraulic fluid for seal and cylinder wall lubrication.
Sometimes a piston gets stuck against the cylinder wall, and no amount of pressure will break it free. This is usually easy to spot because the pneumatic pressure will not change, and components will not operate without system pressure applied. Check mounting clamps too, they usually have to be torqued to specific values, and if they are too tight it could deform the cylinder enough to bind the piston. Accumulators used in the dampening mode are harder to troubleshoot for a stuck piston because there is no obvious problem. Depending on the aircraft, removal and bench check may be the only way to find a stuck piston in this type of application. Accumulators have to be bled when changed, and this is especially true for the self-displacing kind. Because of the return chamber and added fluid volume, any air trapped in the pressure or return chamber affects the pneumatic charge by allowing more pneumatic volume than is required. Trapped air in the fluid spaces compresses more than fluid and will force the piston toward the fluid end. Aircraft using this type of accumulator have unique hydraulic bleeding procedures to eliminate this problem.
On some aircraft, correct reservoir servicing is dependent on accumulator fluid charge. Check the reservoir servicing instructions to see if the accumulator has to be dumped before fluid is added, or an overboard dump of fluid could occur. Always consult the aircraft maintenance manual before doing any of these procedures to make sure they can be accomplished safely. Many maintenance manuals do not contain detailed accumulator troubleshooting procedures, so system and component knowledge is very useful in these situations.
Accumulators store large amounts of pressurized gas and fluid and are hazardous to the untrained and uninformed. The average pneumatic charge in an accumulator is 1,000- to 2,000-psi pressure. This is more than enough pressure to puncture the skin and cause severe health problems - even death. Another hazard is the fully charged accumulator. With 3,000 psi or more fluid pressure stored, a small leak can cut through clothing and skin like a razor. Never use your bare hand to check for any hydraulic or pneumatic leaks, rely on sight and hearing to locate leaks. Generally, large pneumatic leaks are immediately evident using soapy water, and hydraulic leaks under pressure will create a mist that looks like smoke. Always wear eye protection when servicing any pneumatic component. Sometimes gauges, plumbing, and components give way when being re-pressurized, and an unexpected air release from the servicing equipment could damage your eyes.
Respect the energy stored in these devices and the hazards they impose, and always consult the maintenance manual for aircraft specific practices and cautions when working on pneumatic and hydraulic systems.
Army TM 55-1520-240-23-2, CH-47D Maintenance Manual
Boeing 757 Maintenance Manual
F-15 Hydraulic Systems, The Boeing Company and Chris Grosenick