Piston Accumulators

Definition and Functionality
The hydropneumatic piston accumulator is a device used to exchange energy using the hydraulic system to which it is connected.
At given moments, it lets energy escaping, the it accumulates it as pres- sure gas energy and, finally, it readily and integrally replenishes the sy- stem on demand, returning to the conditions of receiving again.
The piston accumulator consists particularly of two chambers, one of which is filled with gas under opportune pressure and the second one is connected to the hydraulic circuit.
The gas pressure must be chosen in relation to the conditions of the ac- cumulator work and represents the pre-charge pressure.
Constructional features
The piston accumulator consists of a steel cylinder, closed at both ends, in which slides an airtight aluminium piston.
This divides the internal of the cylinder in two chambers, one filled with pre-charge gas and the other with oil or, generally speaking, with fluid from the system (Fig 1).
• The piston is made of aluminium in order to have rapid response time and not to generate pressure peaks during rapid cycles. For lighting pur- poses, it’s also provided with cavity, visible in Fig 1. facing the gas cham- ber in order to increase the accumulation volume. Even the surface in contact with the oil has a concave cavity. The purpose of this cavity is so that the oil pressure acts on almost the entire surface of the piston and not only on one spot when the piston is against the bottom end cover in the oil chamber.
• Seal between piston and cylinder is guaranteed by a special multi- ring seals, which constitute the key characteristic elements for the effi- ciency of the accumulator. This type of seal has allowed the piston accumulator to have essential characteristics regarding air lightness, component longevity and stroking. In fact, the differential pressure ne- cessary to move the piston, which relates directly on the response speed of the accumulator, is contained in moderate values, contrary as occurs in most seals for standard pistons.
The maximum operating temperature with polyurethane seals is 80oC. It is possible to operate at temperatures up to 150oC, using Viton® seals and reduced piston, as the expansion factors of aluminium and steel are different; it is therefore necessary to compensate the thermal effect. It is also possible to use Teflon® gaskets for low temperatures (lower than – 60°C) or for low friction applications.
In piston accumulators, the duration and number of operations carried out without evidence of changes in pressure exceeding 5% of the pre- charge value overcome, without penetration, certain quantities of oil in the gas chamber.

It is usually preferable to assume the change of pre-charge as a para- meter for evaluating the longevity of the accumulator as long as this check is carried out rapidly and simply.
Through practical results, obtained by application experience, as well as laboratory tests, it was proved that 1,000,000 operations can be achieved without maintenance or recharge intervention.

• The cylinder body of the accumulator is made of low carbon steel, according to the mechanical characteristics of 97/23/EC.
The internal surface of the cylinder is honed to 0.2 micron of roughness. For special reasons, the cylinder and end covers can either be superfi- cially treated or made of stainless steel.

• The gas side end cover is screwed to the cylinder body; the seal is guaranteed by a toroidal gasket, complete with anti-extrusion ring.
In the standard version, this end cap has a threaded seat in which the pre-charged valve is placed.

• The oil side end cover is also screwed to the cylinder body and is com- plete with relative seal.
This end cap has a coupling to connect it to the system, either threaded or flanged, in accordance to the customer requirements. Fig. 1

• All the accumulators manufactured by EPE are tested according to PED standards.
The accumulators are tested at PT pressure which is equal to the maxi- mum working pressure PS, multiplied by 1.43, which allows to verify the absence of defects, which could cause flaws and deformities in the cy- linder and in the piston or gas or oil leak from the seals, threaded sections or valve.

The relieve pressure exceeds 1150 bar for model types, designed to work at a maximum pressure of 375 bar.