US3856048A

US3856048A - Hydropneumatic accumulator

Info

Publication number: US3856048A
Application number: US00370808A
Authority: US
Inventors: J Gratzmuller
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-01-29
Filing date: 1973-06-18
Publication date: 1974-12-24
Anticipated expiration: 1991-12-24
Also published as: CH533769A; CA942162A; BE762192A; JPS5288708U; FR2120383A6; FR2121340B2; NL7101249A; AT307843B; NL152637B; FR2121340A2; SE376277B; GB1321586A; JPS5842641Y2; FR2076812A5; DE2103552B2; ES387667A1; DE2103552A1

Abstract

This invention relates to a hydropneumatic accumulator comprising a piston and cylinder arrangement defining in the cylinder a first compartment in which is provided a gas which is under pressure and which has a density less than that of nitrogen and a second compartment in which liquid is provided. Helium is the gas preferably used, such that, on the one hand, a much larger volume of the liquid can be stored in the accumulator at a predetermined pressure than is possible using nitrogen, and, on the other hand, the variations in pressure due to variations in temperature is much less than with nitrogen. The invention is particularly applicable to hydraulic controls for electric circuit breakers.

Description

United States Patent [191 Gratzmuller Dec. 24, 1974 HYDROPNEUMATIC ACCUMULATOR [76] Inventor: Jean Louis Gratzmuller, 66

Boulevard Maurice Barres, Hauts-de-Seine, France Related US. Application Data [63] Continuation of Ser. No. 110,951, Jan. 29, 1971,

abandoned.

[30] Foreign Application Priority Data [58] FieldofSearch ..138/30,31;267/113,118, 267/122, 124

[56] References Cited UNITED STATES PATENTS 2,170,890 8/1939 Allen 138/31 1 IITIIIIIHHIII 2,747,370 5/1956 Traut 138/31 2,829,672 4/1958 Bleasdale... 138/31 2,999,680 9/1961 Eiseman, Jr... 267/64 R 3,064,686 11/1962 Gratzmuller 138/31 3,326,241 6/1967 Mercier 138/30 Primary Examiner-Charles A. Ruehl Attorney, Agent, or Firm-Lilling & Siege] [57] ABSTRACT This invention relates to a hydropneumatic accumulator comprising a piston and cylinder arrangement defining in the cylinder a first compartment in which is provided a gas which is under pressure and which has a density less than that of nitrogen and a second compartment in which liquid is provided. Helium is the gas preferably used, such that, on the one hand, a much larger volumeof the liquid can be stored in the accumulator at a predetermined pressure than is possible using nitrogen, and, on the other hand, the variations in pressure due to variations in temperature is much less than with nitrogen. The invention is particularly applicable to hydraulic controls for electric circuit breakers.

9 Claims, 6 Drawing Figures FATENTED DEC 24 1974 saw 3 m 5 AmEOV ZO UEDJO wmnwwwmm PATENTED UEC24 I974 sum u (5 g PRESSURE DECREASE FIG. 4

HYDROPNEUMATIC ACCUMULATOR This is a continuation of application Ser. No. 110,951, filed Jan. 29, 1971, now abandoned.

BACKGROUND OF THE INVENTION It is known that hydropneumatic piston accumulators are substantially made up of a sealed cylinder divided by a piston into two compartments of volumes which are inversely variable, one compartment enclosing a cushion of gas under pressure, forming an elastic buffer, the other compartment containing a liquid, generally oil, which is thus stored and always available at the pressure established by the cushion of gas.

Such accumulators are widely used in hydraulic control installations (e.g., hydraulic circuit breaker controls). where they ensure that there is always available a predetermined minimum volume of oil at a predetermined minimum pressure. In these installations, the accumulators are generally periodically recharged or reinflated with oil by a pump.

The gas used as an elastic cushion in conventional accumulators was originally air, but the oxidizing effects of the oxygen content of the air were sometimes damaging under high pressure, so that, for a number of years, nitrogen has been almost universally used in preference to air, as it is inert and very cheap. In practice, the large proportion of nitrogen in the air allows the characteristics of air-inflated accumulators to be compared to those of nitrogen-inflated accumulators, so that the following remarks apply equally to both gases. 1

Until the present day, the pressures frequently used in hydraulic control plant comprising oil-pneumatic accumulators were of the order of about 100 to 300 kg/cm and accumulators using nitrogen were satisfactory.

However, certain applications now require working pressures above 300 kg/cm and, for example, of the order of 100 to 700 kg/cm and as much as 1,000 kg/cm It will be these pressures, above 300 kg/cm which will be designated high pressures" in the followmg.

It was proved that, for these high pressures, and even starting from 300 kg/cm conventional nitrogen accumulators had certain drawbacks, and it was necessary to increase their dimensions, and consequently their price, to a substantial degree if it was desired to have available a volume of oil under pressure which was substantially unchanged in relation to medium-pressure accumulators. Again, high temperatures (e.g. of 50C, which may be encountered in outdoor installations exposed to the sun) proved to be very disadvantageous in high-pressure nitrogen accumulators, causing a loss in available energy which is proportionally much greater than in the case of medium pressures.

For the sake of simplicity it may be stated that the difficulties arising at high pressures with conventional nitrogen accumulators are due to the loss of compressibility of the nitrogen in proportion as the pressures increase. Consequently, the nitrogen fulfils its function of an elastic cushion less and less efficiently and tends to behave progressively as a liquid as the pressures rise. At pressures between 200 and 300 kg/cm the loss of compressibility of the nitrogen is already of the order of percent.

In fact Mariottes (Boyle s) law PV-C" is only a limit law, applicable to the perfect gaseous state, a state more closely approached by known gases in relation as they move away from their critical point. In the case of high-pressure accumulators, used at ambient temperatures between 40 and 9509C, the physical properties of nitrogen move considerably away from those of a perfect gas, resulting in a serious deterioration in the elastic qualities of the gas.

It is an object of the present invention to obviate or mitigate drawbacks such as outlined above and allow hydropneumatic high-pressure accumulators to be produced which are capable of storing and replacing more energy (e.g. 50 percent more energy) than a conventional nitrogen accumulator of the same dimensions and functioning at the same pressure.

The present invention is a hydropneumatic accumulator comprising a sealed cylinder having a piston slidably located thereon and defining with the cylinder a first compartment enclosing a gas under pressure and a second compartment enclosing a liquid, the improvement that the gas enclosed in said first compartment is a gas of a density less than that of nitrogen.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic view of an accumulator according to the invention in an hydraulic control installation;

FIG. 2 is a graph showing the volumes of oil available as a function of the pressures in an accumulator according to the invention, and in a conventional nitrogen accumulator at a temperature of 20C.

FIG. 3 is an analogous graph, but drawn for temperatures of +50 and 35C;

F IG. 4 is a diagrammatic view in section of an electrical circuit breaker fitted with a static hydropneumatic accumulator according to the invention;

FIG. 5 is a diagram showing the advantages of accumulators according to the invention for static applications; and

FIG. 6 shows an embodiment of a hydropneumatic accumulator according to the invention and, diagrammatically, the hydraulic installation on which this accumulator is mounted.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 illustrates a hydropneumatic accumulator which comprises a sealed container formed by a cylinder 3 and two end parts 5 and 7. The cylinder 3 has a piston 13 slidably located therein, the piston 13 defining with the cylinder 3, two compartments 9 and 11. Compartment 9 encloses a gas under pressure i.e., according to the invention a gas of a density less than that of nitrogen, such as helium, hydrogen or neon, while compartment 11 contains oil.

It has proved that such gases reveal a loss of compressibility at the high pressures envisaged which is much lower than that encountered with nitrogen in conventional accumulators, such gases being moresimilar to perfect gases than nitrogen in the conditions of operation envisaged.

Again, helium is preferable chosen, as it is not flammable, is chemically neutral and is easily obtained. It will be shown later that, in spite of the high cost of helium compared to nitrogen (about 15 times dearer), an

accumulator inflated with helium is less costly than one inflated with nitrogen, for identical performances.

The construction of such an accumulator is conventional, and needs no further detailed description.

FIG. 1 also shows the essential elements of a conventional working circuit of such an accumulator. The oil compartment 11 is connected by a pipe 15 to a motor apparatus such as a jack 17, a rod 19 of which can activate any controllable device e.g. the mobile contact of an electrical circuit-breaker, if the installation is used as an hydraulic circuit-breaker control. A valve 21 interposed in the'pipe 15 allows the jack 17 to be selectively linked to the high pressure of the accumulator or to a low-pressure tank or container 23. A pump 25 draws oil from tank 23 and recharges compartment 11 of the accumulator with oil under pressure. It may thus be said that in this application, the accumulator operates as an active appliance, receiving or restoring energy.

A pipe 27, equipped with a stop-valve 29 allows compartment 9 of the accumulator to be supplied with gas at the initial pre-inflation pressure, and also allows the accumulator to be recharged if there is a gas leak.

There will now be a brief survey of the method of operation of hydropneumatic accumulators in order to bring out better the advantages attained by the invention. If for example it is desired to provide an accumulator capable of delivering oil at a pressure between 400 and 600 kg/cm the compartment 11 is empty or almost empty of oil, i.e., the piston 13 is at that moment against or in the vicinity of the lower end part 7 of cylinder 3, and oil is introduced into compartment 11 by means of pump 25, this oil pushing back piston 13 and compressing the gaseous cushion 9.

After introduction of a determined volume V of oil, the pressure reaches the maximum pressure selected, e.g., 600 kg/cm and the pump stops. Thus from this point there is in the accumulator a reserve of energy, readily available, corresponding to the outlet of a volume V of oil at a pressure between 600 and 400 kg/cm In practice, of course, variation in ambient temperature must be taken into account, as this will cause the pressures to vary, and the above indications have been given for reasons of simplification, it being assumed that the operating temperature is constant.

FIG. 2 shows, in the form of a graph, the functioning of an accumulator as described above. The volumes of oil in cm introduced into the accumulator, or capable of being restored by it, are entered as abscissae, and the corresponding pressures in kg/cm as ordinates.

Curve A relates to a conventional accumulator charged with nitrogen, curve B to an accumulator according to the invention charged with helium, and curve C is a theoretical curve representing the functioning of an accumulator charged with a perfect gas which obeys exactly, for the high pressure considered, Mariottes (Boyles) law PVC". These three curves are drawn for an identical temperature of +20C and for an identical accumulator of a total internal capacity of 1,000 cm, capable of delivering oil at a pressure between about 400 and 600 kg/cm Considering curve A first, the abscissa shows that the accumulator was pre-inflated with nitrogen at a pressure of about 400 kg/cm Oil is then introduced under pressure into the oil compartment. Point 33 on the curve shows that, after introduction of 123.5cm of oil the pressure of the accumulator is raised to about 500 kg/cm and, after introduction of 206cm of oil (point 35 of the curve) the pressure reaches about 600 kg/cm In short, with such a conventional nitrogen accumulator, there is provided a reserve of energy constituted by the output of 206cm of oil at a pressure between about 600 and 400 kg/cm (at a temperature of 20C). It should be noted here that the volumes indicated on the curves are those actually occupied by the oil at the pressure under consideration; in fact, at these high pressures, the oil is relatively compressible (about 3/100 for about ,600 kg/cm Curve B will now be examined relative to an accumulator of identical construction and capacity, but charged according to the invention with a gas less dense than nitrogen, helium in the example chosen (density of helium relative to air; 0.137; density of nitrogen relative to air; 0.97).

The origin of curve B is identical to that of curve A, both accumulators being pre-inflated to the same initial pressure of about 400 kg/cm It is seen at point 37 on curve B that 180.5cm (instead of l23.5) of oil introduced reaches a pressure of about 500 kg/cm The final pressure of about 600 kg/cm (point 38) is reached after introduction of 303 cm of oil. The result is that with such a helium accumulator a reserve of energy is available in the form of an outlet of oil of 303 cm at a pressure between about 600 and 400 kg/cm instead of only 206 cm under the same conditions with a conventional nitrogen accumulator. The energy gain is thus in the region of 50 percent, with the better compressibility of helium compared to nitrogen, at the high pressures under consideration.

The theoretical curve C, representing the phenomenon if the accumulator could be charged with a perfect gas (PV-C) has above all the object of showing the loss of compressibility which arises with nitrogen, this loss being much less with helium (or with another less dense gas such as hydrogen). The point 39 on curve C corresponds to a pressure of about 600 kg/cm and to a volume of oil of 333 cm It may be deduced from this that the nitrogen accumulator allows only 206/333 62 percent of the maximum theoretically storable energy to be stored, while the helium accumulator allows 303/333 91 percent of this maximum theoretical energy.

A gas less dense than nitrogen, which is likewise suitable for an accumulator according to the invention is hydrogen (density relative to air; 0.069), whose curve (not shown) would be located substantially between those for nitrogen and helium. Point 40 appearing in Flg. 2, for a pressure of about 600 kg/cm corresponds to a volume of available oil of about 255 cm, in an accumulator inflated with hydrogen, i.e. an increase in available energy of 24 percent relative to a conventional nitrogen accumulator.

Finally, neon (density relative to air; 0.674) could also be used advantageously in an accumulator according to the invention.

As a real gas becomes more similar to a perfect gas in proportion as it moves away from conditions corresponding to its critical point, it is of interest to examine the critical constants of the different gases considered above. These constants are indicated in the following table to which has been added the density of the gas relative to air at about normal conditions.

critical critical density Temp. C pressure kg/cm helium -268 about 2.25 0.137 hydrogen 240 about 12.8 0.069 neon 205 about 29.0 0.674 nitrogen l47.l about 33.5 0.967

This clearly shows that the gases selected, particularly helium, will be much further removed from their critical point at the high pressures envisaged and for the current ambient temperatures (40C to 50C) hence the gain in compressibility.

It may also be noted that helium, despite its cost, which is about times higher than that of nitrogen, allows hydropneumatic accumulators to be made more economically than those filled with nitrogen. In fact, if the same available reserve of energy were required, the total internal capacity (and thus the dimensions) of a conventional nitrogen accumulator would have to be about 50 percent greater than those of a helium accumulator according to the invention, which would increase the cost price by about 50 percent, while the increase in cost price due to the use of helium in the place of nitrogen is less than 7 percent.

These conclusions are valid for the isothermal functioning of the accumulator, at a constant ambient temperature but, as has been indicated before, the performances of hydropneumatic accumulators are modified as a function of ambient temperatures.

In FIG. 3, curves D and E are the function curves respectively at 35C and +50C of a helium accumulator according to the invention, having served to trace the curve B in FIG. 2. Curves F and G are the curves, respectively at 35C and +50C of a conventional nitrogen accumulator of the same capacity, having served to trace the curve A in FIG. 2.

In both cases, the accumulator was initially preinflated to a pressure of 400 kg/cm at a temperature of C, as in the case of FIG. 2, and the total interior capacity of the accumulator is 1,000 cm, as indicated before.

Naturally, variations in temperature cause the preinflation pressures to vary (pressures initially established, e.g. at 20cm when the accumulator is empty of oil), as well as the quantities of oil available. These variations, visible on the curves in FIGS. 2 and 3, are reproduced in the following table:

variation in pre-inflation in kg/cm as a function Quantity of oil in cm Volume of oil output Helium Nitrogen from 600 kg/cm accumulator accumulator 135 cm 482 kg/cm 408 kg/cm 230 cm 422 kg/cm It is seen, at this temperature of 35C that not only is the advantage of 70 percent in output of available oil 'which existed at +C retained, but that the pressures are between 600 and 408 kg/cm for Nitrogen, whereas they are between 600 and 422 kg/cm (still with the greater output) for helium. The gain in available energy is thus again improved.

The result of the above is that, with an accumulator according to the invention, inflated with helium, the following advantages are attained compared to a conventional accumulator inflated with nitrogen.

1. a gain in quality, stemming from the reduced pressure drops;

2. a gain in available energy of at least 50 percent when functioning at a constant ambient temperature;

3. a gain in available energy of at least 70 percent when operating at very variable ambient temperatures, and for example, in a range between +50C and 35C;

4. A cost increase which does not exceed about 7%,

for a minimum energy gain of 50 percent.

Finally, it may be said that, even when operating at a constant ambient temperature, the expansion is never absolutely isothermal, so that the increase in energy produced is even greater than that indicated above, for the drops in pressure are less in reality.

With reference to FIGS. 4 and 5, there will now be described the application of a helium accumulator acof temperature contained in the accumulator at 600 kg/cm 35C +20".C +50C 35C +20C +50C nitrogen 278 400 471 358 206 135 helium 330 400 449 445 Thus it is seen that an accumulator of a total internal capacity of 1,000cm, pre-inflated to 400 kg/cm at 20C, and whichwould be used in a temperature range cording to the invention to static work consisting in keeping a fluid at a predetermined pressure and particularly, in keeping in the liquid state a liquefiable dielectric gas serving to insulate a circuit-breaker.

The circuit-breaker shown in FIG. 4 has already been described and illustrated in French Patent 1,430,333. It is enough to recall that a cut-off chamber 52, in which a fixed contact 62 is mounted, and in which a mobile contact 56, actuated by an hydraulic jack 66, is filled with a liqueflable dielectric gas, e.g., SF6 or Freon. This dielectric gas is kept permanently in the liquid state by the pressure exerted by a piston accumulator 78.

The accumulator 78 comprises a cylinder 94 in which is slidably located a piston 80 defining with the cylinder 94 a gas compartment 82, filled with a gas less dense than nitrogen, particularly helium, under pressure, and a liquid compartment 76 filled with the above mentioned dielectric gas in the liquid state. The liquid compartment 76 communicates with the internal volume 69 of the cut-off chamber 52 by a narrow-bore tube 74.

A valve 84 allows the accumulator to be reinflated and, by means of a monometer 86, the maintenance of the pressure of the dielectric fluid in the circuit-breaker can be checked. In the above mentioned Patent, it was indicated that nitrogen was chosen to fill the gas compartment 82, i.e. to constitute the cushion of elastic gas of the accumulator. It was also indicated that the manometer 86 could be used to automatically control operations for re-establishing the pressure.

According to the present invention, nitrogen is not used to form the gaseous elastic cushion, but a gas less dense than nitrogen, and in particular, helium. It will be shown in the following that, due to the helium, the pressure limits of the dielectric fluid necessary for correct functioning of the circuit-breaker can be naturally respected without having recourse to an automatic system for controlling and regulating the pressure, as might be necessary with an accumulator inflated with nitrogen.

As indicated above, the variations in temperature accepted in France for circuit-breakers are from --35C to 50C.

Moreover, in the case of a liquefied dielectric gas (SP6) circuit-breaker, in the closed condition of the circuit-breaker, the passage of the current may raise the temperature of the SP6 to 30C.

Consequently, the extreme temperatures for the liquid may vary from 35C (cold, open) to +80C (hot, closed) i.e. a temperature differential of the liquid of 115C. Of course, this differential of 115C will only affect the liquid contained in the cut-off chamber, whereas the accumulator itself will only be subjected to variations in external temperature i.e. from 35C to +50C.

The variations in pre-inflation pressures have been indicated above as a function of temperature, in the case of a nitrogen accumulator, and in that of a helium accumulator, both pre-inflated to 400 kg/cm at a temperature of 20C.

It is sufficient to recall that the said variations are from 278 to 431 kg/cm for nitrogen, and from 330 to 449 kg/cm for helium, between -35C and +50C.

It is very evident that when a circuit-breaker of the type shown in FIG. 4 is set up, methods must be provided to keep the pressure in the dielectric liquid between predetermined limits, starting from a mean pressure, despite the variations in temperature.

The variations are mainly due to the expansion of the gas and also to the expansion of the liquid in the cut-off chamber.

One of the known methods consists in using an accumulator of very large volume i.e. with a large-volume gas compartment. Thus, if it is assumed that the variations in volume of the liquid due to temperature fluctu ations cause a volume variation of in the gas compartment, the corresponding pressure variations would be of the order of 10%. But such a method would lead to the use of accumulators of very large dimensions, which would be unnecessarily expensive, and would only compensate for the expansion of the liquid, to the exclusion of that of the gas.

It is seen from now on, however, that the use of a helium accumulator according to the invention will reliably allow limitation of the pressure variation due to gas expansion to 449 330 l 19 kg/cm whereas, with a conventional, nitrogen accumulator, they would be 471 278 193 kg/Cm Considering the case of a circuit-breaker in which the dielectric is liquid SP6, the liquid has a coefficient of expansion is 4 10 per degree C, which, for a temperature differential of l 15C, results in a volume variation of the liquid of 4.5 percent. The compressibility of liquid SP6 is only 10 per kglcm and is thus relatively insignificant. It will not be taken into account in the following.

It will be assumed that an accumulator of a volume of 1,00Ocm is selected, as in the example quoted above.

It will also be assumed that, for a determined installation, a pressure of 600 kg/cm is fixed as a maximum acceptable hot pressure, both with a nitrogen and with a helium accumulator.

Under these conditions, at the minimum temperature, and with constant volume, the pressure will drop to 430 kg/cm in the case of helium (i.e. the decrease in pressure would be 28 percent), while it will drop to 339 kg/cm in the case of nitrogen pressure decrease 43 percent.

These results are illustrated in FIG. 5, where the percentages of pressure decrease are shown horizontally, based on the maximum acceptable pressure when the temperature decreases.

According to this PIG., may be made:

A. It will be impossible with any accumulator of the volume selected to respect a pressure decrease below 28 percent;

B. If the acceptable pressure decrease is between 28 and 43 percent, a helium accumulator of a sufficiently high volume according to the invention will fulfil the requirements set, and will thus ensure, in a static manner and without any regulation apparatus, that the pressure is kept between the selected limits.

On the contrary, no conventional nitrogen accumulator, whatever its volume, will be able to satisfy such Conditions.

C. The nitrogen will only be able to maintain the pressure for decreases above 43 percent.

To summarise, the nitrogen accumulator will only be suitable in zone C of FIG. 5. The helium accumulator will be suitable in zones B and C. No accumulator will satisfy the requirements of zone A.

By means of an experimental example, the advantage of the accumulator according to the invention will be illustrated, in comparison with conventional accumulators. A circuit-breaker will be postulated, in which the volume of dielectric liquid (SP6) is such that the volume variations (hot and cold) of the liquid are 1,000 cm, i.e. the accumulator must be able to absorb or restore 1,000 cm of liquid.

the following observation A pressure-decrease latitude ratio of 50 percent is chosen (e.g. pressure between 600 and 300 kg/cm so that this pressure can be attained by both types of accumulator (Zone C, FIG.

The experiments results show that a helium accumulator of a total capacity of 3.5 litres is sufficient, whereas the conventional nitrogen accumulator, giving the same latitude of performance, should be of l 1.2 litres, or more than 3 times greater.

With reference to FIG. 6, there will now be described a hydro-pneumatic accumulator according to the invention, provided with reference means indicating the position of the piston in the cylinder of the accumulator, these means enabling control of the filling of the accumulator with oil as a function of the volume of oil in the latter, and not as a function of pressure.

A cylinder 103 of an accumulator 101 has a piston 113 slidably located therein, the piston 113 defining with the cylinder 103 a gas compartment 109 filled under pressure with a gas less dense than nitrogen, this gas preferably being helium, and a liquid compartment 111 filed with oil which supplies a working circuit 117.

The helium accumulator is provided with an extension rod 127 which is integral with piston 113, and which passes through a base 105 of cylinder 103, through a sealed joint 129.

The outer end of the rod 127 preferably having a widened part 131 forming a cam, can actuate control keys of one or more electrical contacts 133-433 incorporated in the supply circuit of an electric motor 135 driving an oil filling pump 125.

An accumulator, with a rod which controls electrical contacts, has already been described in French Patent 1,181,955.

In the embodiment shown in FIG. 6, contact 133 is a start switch for motor 135, while contact 133 is a stop switch for the same motor. Due to this arrangement, it is possible to keep permanently in the accumulator a reserve of oil under pressure whose volume is between two predetermined limits, which are fixed by the position of cam 131 of the emerging rod relative to the keys of the switches 133-133. The position of cam 131 is preferably adjustable on rod 127, in order to regulate the tripping positions of switches 133-133 There will be indicated by way of example in the following the features of a hydropneumatic accumulator capable of producing under all circumstances a volume of 1,000 cm of oil, when the temperatures vary between -35C and +50C.

A. When the oil recharging is controlled as a function of the position of the piston in the accumulator, e.g. by means of the emerging-rod system described above, the total capacity of the accumulator should be 3,500 cm in the case of an accumulator filled with helium, and 11,200 cm in the case of a nitrogen accumulator, for a pressure decrease of 50 percent. The variations in pressure, for the temperature variations indicated above, would then be between about 600 and 300 kg/cm, i.e. the maximum decrease in pressure would be 50 percent.

It was seen in the above, with regard to the static" application of the accumulators, that pressure decreases of less than 28 percent could not be obtained under the conditions indicated, with any accumulator, whether helium or nitrogen. It was also seen that a nitrogen accumulator was unable to ensure pressure decreases of less than 43 percent, whereas the helium accumulator could itself ensure a pressure regulation between 28 and 43 percent, and, of course, beyond. This is why there was chosen in the above example a pressure decrease of 50 percent which is acceptable to both types of accumulator.

From the above example it can be seen that the capacity of the nitrogen accumulator should be more than three times greater than that of the helium accumulator, for identical performance. Naturally, with equal capacity, the helium accumulator would give pressure decreases well below those of the nitrogen accumulator.

B. In the case when the oil recharge is controlled as a function of the pressure in the accumulator, for example by means of manostatic control of the filling pump, as described in the first application, the pressure decrease may be reduced much more.

Thus for pressures between about 500 and 600 kg/cm the total capacity of the accumulator should be 8.186 cm with a helium accumulator and 15.610 cm with a conventional nitrogen accumulator (the preinfiation pressures being respectively about 464 kg/cm and 458 kg/cm at 20C). Here again it is seen that a helium accumulator can have a volume half that of the conventional nitrogen accumulator.

Naturally, the invention is in no way limited to the examples described; it is capable of numerous variations accessible to the specialist, depending on the applications envisaged, and without going beyond the scope of the invention.

Having thus set forth the nature of the invention what I claim herein is:

l. A hydropneumatic accumulator comprising a sealed cylinder having a piston slidably located therein and defining with the cylinder a first compartment and a second compartment, the second compartment enclosing a liquid and the first compartment enclosing a body of gas compressed to a maximum operating pressure of at least 200 kglcm said gas having a density less than that of nitrogen.

2. An accumulator according to claim 1, in which the gas enclosed in the first compartment is helium.

3. An accumulator according to claim 1, in which the gas enclosed in the first compartment is hydrogen.

4. An accumulator according to claim 1, in which the gas enclosed in the first compartment is neon.

5. An accumulator according to claim 1 in which the gas is compressed to a maximum operating pressure of between 300 and 1,000 kg/cm 6. In an hydropneumatic accumulator comprising a sealed cylinder having a piston slidably located therein and defining with the cylinder a first compartment enclosing a gas under pressure and a second compartment enclosing a liquid, the improvement that the gas enclosed in the first compartment is a gas of a density less than that of nitrogen, and the liquid enclosed in the second compartment is a liquefiable gas kept in the liquid state by the pressure exerted by the gas contained in the first compartment.

7. An accumulator according to claim 6,'in which the gas enclosed in the first compartment is helium.

8. An accumulator according to claim 6, in which the liquid enclosed in the second compartment is a liquefiable dielectric gas kept in the liquid state.

9. An accumulator according to claim 6, in which the liquid enclosed in the second compartment is liquefied ENTTED STATES PATENT OEETEE QERHHQATE 0F CQRREQTIUN PATENT NO. 3,856,048

D E I December 24, 1974 !NVENTOR(S) 1 Jean Louis Gratzmuller it is certified that error appears in the ab0veidentified patent and that said Letters Patent are hereby corrected as shown below:

In 7 Foreign Application Priority Data", change the French application number "70.03103" to 70,03l05 fiigned and geaied this f f Day 0? Auust1975 [SEAL] G Azresr:

RUTH C. MASON C. MARSHALL DANN Atmsling Officer ('ummissr'mu'r nfl'alcmx and Trademarks

Claims

1. A hydropneumatic accumulator comprising a sealed cylinder having a piston slidably located therein and defining with the cylinder a first compartment and a second compartment, the second compartment enclosing a liquid and the first compartment enclosing a body of gas compressed to a maximum operating pressure of at least 200 kg/cm2, said gas having a density less than that of nitrogen.

5. An accumulator according to claim 1 in which the gas is compressed to a maximum operating pressure of between 300 and 1, 000 kg/cm2.

6. In an hydropneumatic accumulator comprising a sealed cylinder having a piston slidably located therein and defining with the cylinder a first compartment enclosing a gas under pressure and a second compartment enclosing a liquid, the improvement that the gas enclosed in the first compartment is a gas of a density less than that of nitrogen, and the liquid enclosed in the second compartment is a liquefiable gas kept in the liquid state by the pressure exerted by the gas contained in the first compartment.

7. An accumulator according to claim 6, in which the gas enclosed in the first compartment is helium.

9. An accumulator according to claim 6, in which the liquid enclosed in the second compartment is liquefied hexafluoride of sulphur.