Field of the Invention
[0001] This invention relates to gas precharged accumulators for use in hydraulic pressure
fluid systems. Such hydraulic fluid systems may be used to power various given devices.
For example, high pressure hydraulic fluid systems may be used to retract aircraft
landing gear or to start auxiliary power units in aircraft.
Statement of the Prior Art
[0002] In the prior art there are typically three types of accumulators. One type uses a
piston and cylinder design, another type uses a metal bellows design and the other
type uses a bladder or flexible diaphragm. The piston design tends to be generally
more lignt weight and has a relatively simple design. In contrast, althougn the metal
bellows design is more complex, it has tile advantage of being able to seal the gas
within the chamber much better than the piston design. Although the diapnragm or bladder
design provides an excellent seal, it does not have sufficient strength to accomodate
the same high differential pressures as the other designs. Earlier models of 2841D
[0003] these designs typically used air as the gas medium under pressure. The air preferably
had water and corrosives removed in order to improve performance and increase useful
life. In high pressure applications (i.e., approximately 3,000 psi or greater) both
designs typically use pressurized nitrogen gas. Nitrogen has the advantage over air
in that nitrogen does not tend to corrode the material forming the chamber of the
accumulator and being inert does not react with the accumulator material. It is for
this reason that nitrogen has found wide acceptance in high pressure accumulator applications.
[0004] Nitrogen and ordinary air used as a precharge gas ordinarily give satisfactory performance
under moderate pressure and moderate temperature applications; however, under high
pressure and very low temperature applications, both nitrogen and air lose a significant
amount of energy. This reduces the gas volume and pressure available to exert force.
These reductions necessarily reduce the power that can be applied to the hydraulic
fluid to actuate the device. Consequently, at low temperature and high pressure applications,
the accumulator size must be increased to contain more gas in order to provide the
required energy to the system. However, even at at a higher temperature and lower
pressure than these, nitrogen and air still lose a significant amount of the energy
available to power a device.
[0005] Consequently, the accumulators must be rather large in order to operate at such low
temperatures and high pressures thus increasing their weight considerably. The weight
and size of such accumulators clearly present significant disadvantages in aircraft
applications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006]
Figure 1 shows a typical metal bellows accumulator charged with helium gas according
to the disclosure of the present invention.
Figure 2 illustrates the accumulator states for various minimum and maximum operating
temperatures as used in the accumulator volume equation.
Figure 3 is a comparison of accumulator volume for nitrogen as compared to helium
at specified minimum and maximum operating temperatures for specified minimum operating
pressure requirements.
SUMMARY OF THE INVENTION
[0007] It is a principal object of the present invention to provide a precharged gas accumulator
having a minimum volume of gas therein for given temperature and pressure ranges.
[0008] It is another object of the present invention to provide a method for determining
the accumulator gas volume required for operation of the accumulator within a given
temperature range and given pressure range.
[0009] It is yet another object of the invention to provide a gas accumulator of minimal
size and weight for accumulator pressure applications in excess of 3,000 pounds per
square inch.
[0010] Briefly, in accordance with the invention, there is provided an accumulator using
helium as the precharging gas. Helium has not heretofore been used because it is very
difficult to seal (due to its small molecular size); this is especially true with
piston and diaphragm type accumulators. However, it has now been determined that metal
bellows accumulators are able to seal the helium gas therein for a satisfactory period
and number of. operating cycles. It has further been determined in accord with the
invention that helium behaves more like an ideal gas than the other gases used in
prior art accumulators. Thus, at relatively low temperatures and high pressures, helium
gas has more energy available to power the system. Although helium has a decided advantage
over other gases at such low temperatures and high pressures at which the other gases
may liquefy, it has also been shown that even at higher temperatures and lower pressures
than these the other gases exhibit some liquid-like characteristics (although they
have not turned into liquids); but helium does not have any such liquid-like characteristics
at these temperatures and pressures and thus has more energy available to power the
hydraulic fliud system. This translates into a lower required volume of gas in the
hydraulic fluid system, and therefore a smaller accumulator may be used. Consequently,
this reduces the size and weight of the accumulator and makes the entire hydraulic
fluid system more practical and advantageous in many aircraft applications for which
it would not otherwise ordinarily be feasible. For example, a helium accumulator system
may be substituted for gas cartridge systems currently used to eject weapons from
aircraft.
[0011] In line with the present invention, an accumulator with minimum volume (and resulting
weight and size advantages) for a particular application can be used. This is in accord
with a devised relationship between certain parameters. These include displacement
of hydraulic fluid needed to actuate a given device, the compressibility factor of
the accumulator gas used, the minimum and maximum operating temperatures of the accumulator
gas and the minimum and maximum operating pressures of the accumulator gas. Using
this relationship, the accumulator size and weight can be optimized for a given application
resulting in significant size and weight savings for the accumulator.
[0012] Other objects and advantages of the present invention will be apparent upon reading
the following detailed description and upon reference to the drawings.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0013] Referring now to Figure 1, there is shown a metal bellows accumulator generally designated
by the numeral 10. The accumulator 10 is adapted for use in hydraulic fluid systems
used to power a given device. For example, in aircraft applications, such accumulators
10 may be used in hydraulic systems to retract landing gear or eject missiles and
other weapons. The accumulator 10 has a housing 12. The housing 12 contains a chamber
14 which, in turn, contains precharged helium gas 16 under pressure. The helium gas
16 may be compressed to pressures in excess of 3,000 psi. The housing 12 also contains
an inlet tube 18 to precharge the chamber 14 with the helium gas 16. The tube 18 is
sealed after precharging the chamber 14. An inlet valve (not shown) may alternatively
be provided in the inlet tube lb. The Inlet valve closes off the chamber after a desired
quantity of helium gas 16 has been admitted into the chamber 14.
[0014] A metal bellows 2U is preferably positioned at one end of the chamber 14. The metal
bellows 20 allows expansion of the helium gas 16 in the chamber 14 and also transmits
the force exerted by the gas pressure to the hydraulic fluid 22. A guide 24 prevents
cocking of bellows 20. The housing 12 is preferably provided with a hydraulic fluid
port 26 which allows hydraulic fluid to be admitted into the housing and allows the
fluid to make contact with the metal bellows 20. The port 26 is also preferably in
one end of the metal bellows 20 as shown in Figure 1. The metal bellows 20 can thereby
displace a desired volume of hydraulic fluid out of the housing 12 and into the hydraulic
fluid lines (not shown) of the system in order to activate a device. A pressure gauge
28 may also be provided.
[0015] Figure 2 shows that accumulator 10 has different performance characteristics under
different conditions of temperature. Specifically, at the lowest operating temperature
shown, the accumulator 10 has the least amount of power available to displace the
required quantity of hydraulic fluid. Thus, it is imperative that at the lowest operating
temperature shown, the accumulator 10 has sufficient volume of gas under pressure
tnerein to enable it to displace the required hydraulic fluid volume with sufficient
force to power a given device. Figure 2 also shows that because p
7 is greater than p
3 (i.e. the final pressure at the highest temperature is greater than the final pressure
at the lowest temperature), at the highest temperatures shown the accumulator 10 has
the greatest amount of energy availaole to power a given device. In order to minimize
the size of the accumulator 10, the volume of the chamber must be optimized to enable
a desired volume of fluid of a required energy content to be displaced at both minimum
and maximum temperatures of operation.
[0016] Since there is a considerable difference in the accumulator sizes required for an
isentropic versus an isothermal expansion process, it was critical to establish the
actual nature of the gas expansion process. Referring again to Figure 2, empirical
data has shown that the gas expansion is approximately an isentropic process for rapid
discharges of hydraulic fluid and that P
2 = P
4 = P
6. As a result of this research, a formula was derived which can yield the minimum
accumulator size required for a given application. The formula is:
where Va = accumulator volume = oil (hydraulic fluid) volume plus gas volume in the accumulator,
Vo = hydraulic oil (fluid) required to power a given device,
T2 = lowest accumulator gas temperature before oil discharge,
P2 = accumulator gas pressure at T2,
Z2 = Compressibility factor, a function of T2 and P2,
T3 = accumulator gas temperature after oil discharge,
P3 = accumulator gas pressure at T3,
Z3 = compressibility factor, a function of T3 and P3,
T6 = nighest accumulator gas temperature before oil discharge,
P6 = accumulator gas pressure at T6,
Z6 = compressibility factor, a function of T6 and P6.
[0017] Comparison of the accumulator size required when using nitrogen gas vs. helium gas
as the charge medium shows a substantially smaller accumulator size when using helium
gas therein. The savings in size when using helium gas are approximately 35 to 55
percent when the maximum operating pressure is 8000 psi and when the minimum operating
pressure is approximately 4.00U to 5,000 psi. Figure 3 compares the accumulator volume
for given ranges of minimum operating pressures and minimum and maximum operating
temperatures. It is apparent that substantial savings in both size and weight can
result from the use of helium accumulators in high pressure and low temperature ranges
as are common in modern high performance aircraft applications. It must be noted,
however, that at system pressures less than 3,000 psi, there is not a substantial
advantage in using helium in place of nitrogen for the purpose of reducing the accumulator
size and weight.
[0018] Although helium has been used in detecting leaks in certain high pressure systems,
it has not previously been used as the operating gas in such systems due to its very
small molecular size and resulting tendency to leak past any conventional type of
seal (and so its use as a leak detector). Indeed, the high energy capabilities and
advantages of helium in high pressure and wide (and low) temperature range applications
was not addressed by the prior art and therefore prior art systems did not investigate
production of a system capable of using helium. Thus, the present invention extends
the applicability of relatively small and lightweight gas precharged accumulators
into high pressure systems having system pressures in excess of 3,000 psi.
[0019] Thus, it is apparent that there has been provided, in accordance with the invention,
a helium charged accumulator system that fully satisfies the objectives, aims, and
advantages set forth above. While the invention has been described in conjunction
with specific embodiments set forth above, it is evident that many alternatives, modifications,
and variations will be apparent in light of the foregoing description. Accordingly,
it is intended to embrace all such alternatives, modifications, and variations that
fall within the spirit and scope of the appended claims.
[0020] The invention may be summarized as follows:
1. An accumulator for use in hydraulic systems, comprising:
a housing;
a chamber mounted within said housing;
helium gas sealingly contained under pressure in said chamber;
means for transmitting the force of said helium under pressure to hydraulic fluid
in order to displace a desired volume of the hydraulic fluid in the system.
2. The accumulator of 1 wherein said means for transmitting comprises a metal bellows.
3. The accumulator of 1 wherein said housing has a hydraulic pressure port for allowing
the hydraulic fluid ingress to and egress from said housing so that a desired volume
of the hydraulic fluid may be displaced within the system from said housing by said
means for transmitting the force of said helium gas.
4. An accumulator for use in high pressure hydraulic systems, comprising:
a housing;
a chamber mounted within said housing, said chamber being expandable in at least one
dimension;
helium gas sealingly contained under pressure in said chamber
means for transmitting the force of the helium under pressure to hydraulic fluid in
order to displace a desired volume of the hydraulic fluid in the system;
means for allowing precharging of said chamber with said helium.
5. The accumlator of 4 wherein said housing includes a hydraulic fluid pressure port
to allow the fluid access to said means for transmitting so that said means for transmitting
can displace a desired volume of the hydraulic fluid in the system.
6. The accumulator of 4 wherein said means for transmitting the force of said helium
gas comprises a metal bellows mounted in said chamber.
7. An accumulator for use in high pressure hydraulic systems, comprising:
a housing;
a chamber mounted within said housing, said chamber sealingly containing helium gas
under pressure;
an inlet tube connected to said chamber for precharging said chamber with said helium
gas;
a metal bellows mounted within said housing, said bellows allowing expansion of said
helium gas in said chamber for transmitting the force of said helium under pressure
to the hydraulic fluid in order to displace a desired volume of hydraulic fluid in
the system;
d. The accumulator of 7 wherein said bellows has a hydraulic fluid port for allowing
the hydraulic fluid to contact said metal bellows in order to allow said metal bellows
to displace a desired volume of the hydraulic fluid in the system;
9. An accumulator for use in high pressure hydraulic fluid systems, comprising:
a housing;
a chamber within said housing, said chamber sealingly containing a gas under pressure,
the size of said chamber being substantially in accord with the equation:
where Va = accumulator volume = oil (hydraulic fluid) volume plus gas volume in the accumulator,
V0 = hydraulic oil (fluid) required to power a given device,
T2 = lowest accumulator gas temperature before oil discharge,
P2 = accumulator gas pressure at T2,
Z2 = Compressibility factor, a function of T2 and P2,
T3 = accumulator gas temperature after oil discharge,
P3 = accumulator gas pressure at T3,
Z3 = compressidility factor, a function of T3 and P3,
T6 = highest accumulator gas temperature before oil discharge,
P6 = accumulator gas pressure at T6,
Z6 = compressibility factor, a function of T6 and P6..
means for allowing expansion of said gas in said chamber in order to transmit the
force of said gas under pressure to the hydraulic fluid to displace a desired volume
of hydraulic fluid in the system from said housing.
10. The accumulator of 9 wherein said gas comprises helium.
11. The accumulator of 9 wherein said means for allowing expansion of said gas to
transmit the force of said gas comprises a metal bellows mounted in said chamber.
12. The accumulator of 9 wherein said housing has a hydraulic pressure port for allowing
the hydraulic fluid to contact said means for transmitting the force so that a desired
volume of hydraulic fluid may be displaced in the system.
13. The accumulator of 9 further including an inlet valve connected to said chamber
for precharging said chamber with said gas under pressure.
1. An accumulator for use in hydraulic systems, comprising:
a housing;
a chamber mounted within said housing;
helium gas sealingly contained under pressure in said chamber, said helium gas being
at a pressure in excess of approximately 3,000 psi throughout a range of operating
temperatures and pressures;
means for transmitting the force of said helium under pressure to hydraulic fluid
in order to displace a desired volume of the hydraulic fluid in the system throughout
the range of temperatures and pressures.
2. The accumulator of Claim 1 wherein said means for transmitting comprises a metal
bellows.
3. The accumulator of Claim 1 wherein said housing has a hydraulic pressure port for
allowing the hydraulic fluid ingress to and egress from said housing so that a desired
volume of the hydraulic fluid may be displaced within the system from said housing
by said means for transmitting the force of said helium gas.
4. An accumulator for use in high pressure hydraulic systems, comprising:
a housing;
a chamber mounted within said housing, said chamber being expandaDle in at least one
dimension;
helium gas sealingly contained under pressure in said chamber said chamber containing
only helium gas, said helium gas under pressure being at a pressure in excess of approximately
3000 psi throughout an entire operative range of operating temperatures and pressures;
means for transmitting the force of the helium under pressure to hydraulic fluid in
order to displace a desired volume of the hydraulic fluid in the system throughout
the range of temperatures and pressures;
means for allowing precharging of said chamber with said helium.
5. The accumlator of Claim 4 wherein said housing includes a hydraulic fluid pressure
port to allow the fluid access to said means for transmitting so that said means for
transmitting can displace a desired volume of the hydraulic fluid in the system.
6. The accumulator of Claim 4 wherein said means for transmitting the force of said
helium gas comprises a metal bellows mounted in said chamber.
7. An accumulator for use in high pressure hydraulic systems, comprising:
a housing;
a chamber mounted within said housing, said chamber sealingly containing substantially
only helium gas under pressure, said helium gas under pressure being at a pressure
in excess of approximately 3,000 psi throughout an entire operative range of operating
pressures and temperatures;
an inlet tube connected to said chamber for precharging said chamber with said helium
gas;
a metal bellows mounted within said housing, said bellows allowing expansion of said
helium gas in said chamber for transmitting the force of said helium under pressure
to the hydraulic fluid in order to displace a desired volume of hydraulic fluid in
the system[;j throughout the range of operating pressures and temperatures including
minimum and maximum temperatures of approximately 80o F, and 1500/V.
d. The accumulator of claim 7 wherein said bellows has a hydraulic fluid port for allowing
the hydraulic fluid to contact said metal bellows in order to allow said metal bellows
to displace a desired volume of the nydraulic fluid in the system.
9. An accumulator for use in high pressure hydraulic fluid systems, comprising:
a housing;
a chamber within said housing, said chamber sealingly containing a gas under pressure,
said gas under pressure in excess of approximately 3000 psi throughout an entire operative
range of operating temperatures and pressures the size of said chamber being substantially
in accord with the equation:
where Va = accumulator volume = oil (hydraulic fluid) volume plus gas volume in the accumulator,
Vo = hydraulic oil (fluid) required to power a given device,
T2 = lowest accumulator gas temperature before oil discharge, .
P2 = accumulator gas pressure at T2,
Z2 = Compressibility factor, a function of T2 and P2,
T3 = accumulator gas temperature after oil discharge,
Pi = accumulator gas pressure at T3,
Z3 = compressibility factor, a function of T3 and P3,
T6 = highest accumulator gas temperature before oil discharge,
Pb = accumulator gas pressure at T6,
Z6 = compressibility factor, a function of T6 and P6.
means for allowing expansion of said gas in said chamber in order to transmit the
force of said gas under pressure to the hydraulic fluid to displace a desired volume
of hydraulic fluid in the system from said housing, throughout the range of operating
temperatures and pressures.
10. The accumulator of claim 9 wherein said gas comprises helium.
11. The accumulator of claim 9 wherein said means for allowing expansion of said gas
to transmit the force of said gas comprises a metal bellows mounted in said chamber.
12. The accumulator of claim 9 wherein said housing has a hydraulic pressure port
for allowing the hydraulic fluid to contact said means for transmitting the force
so that a desired volume of hydraulic fluid may be displaced in the system.
13. The accumulator of claim 9 further including an inlet valve connected to said
chamber for precharging said chamber with said gas under pressure.
14. The accumulator of Claim 9 wherein the range of operating temperatures and pressures
include a minimum temperature of approximately 80°C, and a maximum temperature of
approximately 150°F.