GB2148397A

GB2148397A - Double enclosure pressure vessel

Info

Publication number: GB2148397A
Application number: GB08421824A
Authority: GB
Inventors: George Orloff; Ian Hale Lewin
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-09-19
Filing date: 1984-08-29
Publication date: 1985-05-30
Also published as: GB8421824D0; GB8325037D0; GB2148397B

Abstract

The pressure vessel comprises a fluid-containing inner enclosure (1) having an inlet (8b) and an outlet (8a) for the fluid. For example, the pressure vessel may be a hydropneumatic accumulator in which case the inner enclosure is divided by a movable partition (2) into two portions (14, 15) containing respectively a hydraulic liquid and a gas. For enhanced safety in the event of structural failure the inner enclosure is surrounded by an outer fluid- containing enclosure (6). Fluid can enter the outer enclosure through a passage presenting an impedance to flow, such as a restrictor orifice (7). In this way a predetermined pressure can be maintained in the outer enclosure under normal operation thereby reducing the stress in the inner enclosure. The fluid-which may be liquid or gas-for the outer enclosure may be obtained from either portion of the inner enclosure or from a source external to the pressure vessel. <IMAGE>

Description

SPECIFICATION Pressure vessel This invention relates to a pressure vessel comprising a fluid-containing enclosure having means enabling the ingress and egress of fluid from outside the vessel.

Pressure vessels are used generally in hydraulic systems for storing fluid under pressure.

An example of a pressure vessel is a hydropneumatic accumulator in which the enclosure is divided by a movable partition into two mutually sealed portions containing respectively a hydraulic liquid and a gas. Hydropneumatic accumulators are commonly used for storing energy and, in particular, to cater for fluctuations in demand for the hydraulic liquid.

Unfortunately, however, conventional accumulators are potentially very dangerous because a structural failure will result in the sudden release of the stored energy. If the stored energy in the vessel is high - and this is not unusual - the consequences of such a structural failure could be catastrophic. Usually the major risk will be associated with the explosion of an enclosure containing gas when fragmentation could directly injure people in the vicinity or else cause structural damage to another pressurised enclosure nearby, for example, the pressurised cabin of a commercial aircraft flying at high altitude.

However, where high temperatures occur the sudden loss of gas may be more acceptable than the outburst of hydraulic liquid but there is still the risk of fragmentation.

Known methods of reducing the risk of structural failure involve reducing the working stresses, determined by calculation, by increasing the thickness of the enclosure walls, and also by performing periodically non destructive tests on the pressure vessel.

However, the real value of the stress cannot be determined accurately because stress concentrations tend to occur due to minute imperfections on the inner surfaces of the enclosure. These imperfections are, by their very nature, difficult to detect and can induce a sudden failure after a number of normal pressure cycles. In addition there are other aspects of a metallurgical nature, such as hydrogen embrittlement, which serve only to worsen the situation.

According to the present invention there is provided a pressure vessel comprising a first fluid-containing enclosure having means enabling the ingress and egress of fluid from outside the vessel, a second fluid-containing enclosure surrounding said first enclosure, and a passage communicating with the second enclosure whereby fluid can be conveyed between the outside and the inside of said second enclosure to maintain under normal operation a predetermined fluid pressure within the second enclosure, said passage comprising means adapted to impede the flow of fluid therethrough.

A pressure vessel in accordance with the invention has a second (outer) fluid-containing enclosure surrounding a first (inner) fluid-containing enclosure. Fluid is able to enter and leave the outer enclosure through the passage communicating therewith, but the flow of fluid is restricted by impedance means.

With this arrangement a predetermined fluid pressure can be maintained in the outer enclosure. This is advantageous because the pressure difference between the outer and inner enclosure can thus be reduced thereby minimizing stress and increasing the life of the inner enclosure.

However, in the event of structural failure of either the inner or the outer enclosures, the double enclosure configuration has the important advantage of enhanced safety because it significantly reduces the risk of fragmentation under a variety of failure modes. At the very least the outer enclosure can be regarded as a containment guard in the event of the inner enclosure failing.

However, because of the reduced stress in the inner enclosure, in many cases the outer enclosure is more likely to fail first, but in this situation the impedance means is effective to restrict the outburst of fluid into the environment and therefore an accident which might otherwise have had catastrophic consequences, although still undesirable, can occur far more safely without fragmentation of the pressure vessel.

The fluid - and indeed the fluid pressure in the outer enclosure may be the same or different to that in the inner enclosure and may be obtained from a source external to the pressure vessel. Alternatively, when the fluid in the outer enclosure is the same as that in the inner enclosure it may be obtained directly from the inner enclosure. In this case the passage extends between the first and second enclosures whereby in response to a fluid pressure change in the first enclosure fluid is conveyed between the two enclosures until the fluid pressure in both enclosures is substantially equalized.

The pressure vessel may comprise a movable partition in the first (inner) enclosure dividing said first enclosure into two mutually sealed portions of variable volume. In the case of a hydropneumatic accumulator, for example, the fluid in one portion of the inner enclosure is a gas and the fluid in the other portion is a hydraulic liquid.

In one example of a hydropneumatic accumulator in accordance with the invention the passage for conveying the fluid extends between the outer enclosure and the portion of the inner enclosure containing liquid so that the outer enclosure also contains liquid. As mentioned previously, the impedance means acts to restrict the outburst of liquid in the event of a structural failure of the outer enclosure. Therefore the consequence would be no worse than a pipe failure under fatigue conditions and accordingly would occur far more safely than in known svstems. Moreover the inner enclosure has a long life since only minor dynamic pressure differences will occur between the inner and outer enclosures, the more major stress occuring at the lower inflation pressure of the gas which happens when the hydraulic system is de-energised.

Alternatively in a hydropneumatic accumulator in accordance with the invention the passage for conveying the fluid may extend between the outer enclosure and the portion of the inner enclosure containing gas so that the outer enclosure also contains gas. In this case in particular it is preferable for the volume of the outer enclosure to be small compared with the volume of the inner enclosure.

Under normal operation the pressure in both enclosures rarely falls below the initial inflation value, which typically may be one half of the maximum working pressure of the hydraulic liquid and, in consequence, the fatigue cycle pressure amplitude is reduced.

In both examples of the hydropneumatic accumulator discussed above the partition may be in the form of a piston with a peripheral sealing ring. An advantage of a double enclosure configuration in accordance with the invention is that the life of the sealing ring is increased because the substantially equalized pressure between the inner and outer enclosures means that the clearance between the piston and the wall of the inner enclosure is substantially constant, whereas in the prior art fluctuations in pressure cause the pressure vessel to 'breathe' and premature failure of the sealing ring can then occur because of the consequential variation in clearance between the piston and the internal wall of the pressure vessel.

It is emphaized that the outer enclosure of a pressure vessel in accordance with the present invention may contain whichever fluid and pressure are most suitable for the circumstances. Thus the outer enclosure may be provided with an independent passive pressure system, provided with instrumentation detecting initial leakage of the inner enclosure in which the fluid is at elevated or ambient pressure. Alternatively the outer enclosure may be provided with an independent active pressure-regulating system which sounds a warning or causes shut down of the whole system when the required pressure conditions can no longer be maintained. If cooling is desired, a more straightforward pressure system can be used, as discussed in more detail below.

It is noted here that in all the examples discussed herein the impedance means may comprise, for example, one or more restrictor orifices, a cascade of capilliary tubes, or a plug of porous material.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic sectional view of a hydropneumatic accumulator in accordance with the invention, Figure 2 is a schematic sectional view of an different form of hydropneumatic accumulator in accordance with the invention Figure 3 is a schematic sectional view of a modified form of the hydropneumatic accumulator shown in Fig. 1, Figure 4 is a schematic sectional view of a modified form of the hydropneumatic accumulator shown in Fig. 2, Figure 5 is a diagrammatic sectional view of a pressure system comprising a pressure vessel in accordance with the invention Figure 6 is a diagrammatic sectional view of a different pressure system comprising a pressure vessel in accordance with the invention, and Figure 7 is a diagrammatic sectional view of a further pressure system comprising a pressure vessel in accordance with the invention.

The hydropneumatic accumulator shown in Fig. 1 may be used in aircraft or industrial hydraulic systems to save weight and cost of pumping equipment in supplying bursts of energy. The accumulator has an inner enclosure comprising a generally cylindrical shell 1 which should, in general, be capable of withstanding three times the inflation pressure in the vessel. In this particular embodiment the inner shell 1 is made, for example, of 3mm thick chromium vanadium alloy steel. The inner enclosure is surrounded by an outer enclosure comprising a similar, but larger, generally cylindrical shell 6 which may be made of 6mm thick chomium vanadium alloy steel. Because the outer shell 6 is thicker than the inner shell 1 it is capable of withstanding a greater pressure, say three times the maximum working value. The internal diameter of the inner enclosure may be between 50mm and 300mm, for example 75mm, and in this case the internal diameter of the outer enclosure may be, for example 83mm. The overall length of the accumulator may be approximately 560mm. The inner enclosure is divided into two portions 14, 1 5 of variable volume by a floating separator piston 2 surrounded by a sealing ring 16. The lower portion 14 contains the working hydraulic liquid, for example a mineral hydraulic fluid under pressure supplied from an external pump (not shown) via inlet 8b. The lower portion 14 of the inner enclosure also has an outlet 8a which communicates with an external system (not shown) to which hydraulic liquid under pressure is to be supplied.

A restrictor orifice 7, which may be for example 0.5mm in diameter and 2mm long, extends through the inner shell 1 between the lower portion 14 of the inner enclosure and the outer enclosure whereby the hydraulic liquid in portion 14 can flow, but only in a restricted manner, into the outer enclosure until the'liquid pressure in both enclosures is substantially equalized.

To facilitate initial filling of the outer enclosure the outer shell 6 is provided with a bleeder screw 4 at its upper end allowing air to escape during the filling operation. After initial filling the bleeder screw 4 is closed.

The upper portion 1 5 of the inner enclosure contains a gas, for example, nitrogen under pressure. The gas is introduced through an inlet 3 in the top of the accumulator.

The same inlet 3 is used to introduce a lubricating liquid 1 2 into the inner enclosure for lubricating the sealing ring 1 6 of the separator piston 2. A short tube 11 extending downwardly from the top of the inner enclosure serves to establish the level of the lubricating liquid when it is introduced with the separator piston 2 in its uppermost position.

The top end of the inner shell 1 is fastened to the outer shell 6 by a retaining nut 5 and sealed by an O-ring 1 9a. At its lower end the inner shell 1 is sealed by an O-ring 1 9b and retained by a circular circlip 1 3. An extended portion of the inner shell constituting an end fitting protrudes through the lower portion of the outer shell and is sealed by an O-ring 1 7.

A lower axial retaining ring 10 is screwed onto a shoulder machined on the end fitting of the inner shell 1, but a clearance d is left with the outer shell 6 to allow for relative axial movement between the shells due to differential expansion.

Under steady state conditions the pressure of the gas in the upper portion 1 5 of the inner enclosure and the pressure of the liquid in the lower portion 14 are equal, which means that the hoop stress in the inner shell 1 is zero when the separator piston 2 is away from the bottom abutment. Axial stresses are also reduced in proportion to the end thrust areas of the end fittings exposed to the pressure in the outer enclosure.

Under dynamic conditions there exists a small differential pressure across the inner shell 1 which is caused by the hydraulic resistance-capacitance effect in its coupling to the outer enclosure. This is believed to be of negligible effect because of the stiffness of the liquid and the low resistance offered by the restrictor orifice 7 to the small cross flow occuring under normal conditions. In the event of the outer shell 6 fracturing, the resistance to flow will, however, be sufficient to remove the incident from the list of castastrophic failures.

It is noted here that in a modified form of the accumulator shown in Fig. 1 the impedance to flow between the inner and outer enclosures may be provided, for example, by a cascade of restrictor orifices, by capillary channels or tubes, or by a porous medium.

The hydropneumatic accumulator shown in Fig. 2 is different in appearance to that described above with reference to Fig. 1, but is similar conceptually in that the outer enclosure contains the working hydraulic fluid. This accumulator is a so-called bladder accumulator and comprises an inner enclosure comprising two generally hemispherical inner shell portions 21 a, 21 b and an outer enclosure comprising similar, but larger, generally hemispherical outer shell portion 26a, 26b. The inner enclosure is divided into two portions 34, 35 of variable volume by an elastic membrane or bladder 22 made for example of rubberized canvas. The hemispheres are pulled together into close abutting relationship by a screwed ring 25. The hemispheres 21a, 21b of the inner enclosure are sealed together by abutting the edge of the bladder 22.The hemispheres 26a, 26b of the outer enclosure are sealed together by an O-ring 202. Additionaly oan O-ring 201 between the upper hemispheres 21a and 26a and an O-ring 203 between the lower hemispheres 21 b, 26b seals the outer enclosure from the atmosphere.

The lower portion 34 of the inner enclosure contains the working hydraulic liquid supplied from an external pump (not shown) via inlet 28. In this example the inlet 28 also serves as an outlet in that it also communicates with an external system (not shown) to which hydraulic liquid under pressure is to be supplied. A perforated diaphragm 20 serves as abutment to the inflated bladder when the hydraulic pressure is atmospheric, but does not materially restrict cross flow of fluid to and from portion 34.

A restrictor orifices 27a which may, for example be 0.5mm in diameter, extends through the lower hemisphere 21 b between the lower portion 34 of the inner enclosure and the outer enclosure. Also an orifice 27b in the lower hemisphere 21 b and an orifice 27c in the upper hemisphere 21a communicate with a common annular channel 29 extending around the full circumference of the upper hemisphere 21a thereby interconnecting the upper and lower portions of the outer enclosure. Thus the hydraulic liquid in portion 34 can flow, but only in a restricted manner, into the outer enclosure until the pressure in both enclosures is substantially equalized.

To facilitate initial filling of the outer enclosure the upper hemisphere 26a is provided with a bleeder screw 24 allowing air to escape during the filling operation. After initial filling the bleeder screw 24 is closed for normal operation.

The upper portion 35 of the inner enclosure contains a gas, for example, nitrogen under pressure. The gas is introduced through an inlet 23 at the top of the upper hemisphere 26a.

Although different in construction, this bladder accumulator with hydraulic liquid in the outer enclosure has the same advantages as the piston accumulator described above with reference to Fig. 1.

The hydropneumatic accumulator shown in Fig. 3 is a modification of the piston accumulator shown in Fig. 1 and hence the same features are identified by the same reference numerals. In the present example, however, the outer enclosure contains gas rather than hydraulic liquid and to this end the restrictor orifice 7 now extends between the upper portion 1 5 of the inner enclosure and the outer enclosure. In this case the gas in portion 1 5 can flow, but in a restricted manner, into the outer enclosure until the gas pressure in both enclosures is substantially equalized.In other respects this piston accumulator is similar in construction to that described above with reference to Fig. 1 and similar advantages apply in this case except that since the outer shell is also inflated, for example, to half the maximum working pressure of the hydraulic liquid, its fatigue cycle amplitude is diminished in proportion. Also, whereas the gas in the outer enclosure is more compliant than liquid so that the volume of the outer enclosure should be kept to a minimum, there is nevertheless a significant benefit in the event of the outer shell 6 failing because the rate of gas discharge will be limited by the critical pressure ratio established across the restrictor orifice 7.

The hydropneumatic accumulator shown in Fig. 4 is a modification of the bladder accumulator shown in Fig. 2 and hence the same reference numerals are again used to identify the same features. In this example, however, the outer enclosure contains gas rather than hydraulic liquid and in this sense is analogous to the example described above with reference to Fig. 3. In this case the restrictor orifice 27a extends through the upper hemisphere 21a between the upper portion 35 of the inner enclosure and the outer enclosure and consequently gas from portion 35 can flow, in a restricted manner, into the outer enclosure until the gas pressure in both enclosures is substantially equalized. In other respects this bladder accumulator is similar in construction to that described above with reference to Fig.

2 and its advantages are equivalent to those of the piston accumulator with gas in the outer enclosure described above with reference to Fig. 3.

Whereas it will be evident from the description so far that a pressure vesel having a double enclosure in accordance with the present invention may take various forms, it is noted here the means for obtaining the pressure may also vary according to circumstances.

In the previous embodiments the fluid in the outer enclosure was obtained directly from the inner enclosure. There will now be described with reference to Figs. 5 to 7 several different arrangements comprising a double enclosure pressure vessel wherein the fluid in the outer enclosure is obtained from a source external to the pressure vessel.

The arrangement shown in Fig. 5 is a double enclosure pressure vessel with a passive system for maintaining a predetermined pressure in the outer enclosure. Again the pressure vessel comprises an inner enclosure 731 surrounded by an outer enclosure 732.

The inner enclosure 731 stores the working hydraulic fluid at a pressure P1 and is connected to the main hydraulic system (not shown) by an inlet/outlet 700. The outer enclosure is part of an independent pressure system and additionally the pressure therein can be monitored. The outer enclosure 732 is supplied with a hydraulic liquid - which may be different from that in the inner enclosure 731 - from an external source S such as a miniature pump providing a restricted supply to the outer enclosure 732 through inlet 738.

Additionally or alternatively a restrictor orifice R, or, for example a capillary tube, may be provided between the source and the inlet 738. The outer enclosure 732 can thus be pressurized to a value P2.

The system may also include, for example, a rapid response relief-valve 735 connected to a reservoir 736, and instruments 733, 734 to monitor the pressure P2 and to initiate an alarm and or shut down routine in case of emergency.

The preference for the failure mode can depend on the choice of values for P1 and P2.

a) P1 P2 b) P1-- P2 c) P, P2 In case a) the outer enclosure can be regarded merely as a containment guard in the event of an inner enclosure failure, with a preferred evacuation mode through relief valve 735.

Case b) can be regarded as equivalent to those of the double enclosure accumulators discussed previously.

In case c) the failure mode probable is that of implosion of the inner enclosure. The relief valve 735 deals with thermal expansion of the fluid in the outer enclosure and the instrumentation and alarm systems will indicate/operate on reduction of pressure from P2.

The modified arrangement shown in Fig. 6 is a double enclosure pressure vessel with an active pressure comparing system.

In this embodiment the control and monitoring system for the pressure P2 in the outer enclosure 832 is hydromechanically activiated by an electrically driven pressure compensated pump 844 supplied from reservoir 836 and delivering fluid to the outer enclosure 832. The pressure P1 in the inner enclosure 831 is used as a datum, measured. by instrumentation 837 and changed into an electric signal by pressure transducer 838, then amplified by amplifier 839 to produce a command signal at the differential network 841.

There is also a feed-back path which senses the pressure to the outer enclosure system, changes it to an electrical signal at the pressure transducer 834 which is modified at amplifier 840, and is compared with the command signal given by amplifier 839 at the differential network 841. Read-out instruments 833 are provided for setting the system.

The difference between the signals, or control error is amplified by the closed loop feedforward path containing an operational amplifier 842 and a pressure control electrohydraulic servo-valve 843, the output of which is fluid pressure, fluid being supplied to the outer enclosure 832 through inlet 838. The servo-valve 843 also serves to impede the flow of fluid into the outer enclosure. However, to further impede the fluid supply flow a restrictor orifice R or a capillary tube may be included between the servo-valve 843 and the inlet 838 which also introduces a lag in the achievement of pressure P2 The magnitude of error at the fee-forward path, say upstream of the operational amplifier 842 can be used for setting-off the alarm and initiating the shut-down routine.

The sensitivity of the circuits determines the tolerance level of the system. However, integrating components can be introduced, for example, to produce a tolerance on duration of error. Additionally, sensitivity can also be varied by the output of the pressure compensated pump 844. These are measures which are well within the scope of a skilled person in the art and so no further details will be given here.

Finally the arrangement shown in Fig. 7 is a double enclosure pressure vessel employing an active flow monitoring and cooling system.

In this embodiment the system comprises a constant delivery pump 945 driven by a synchronous electric motor. The impedance to flow in this case is obtained by using as the pump 945 a constant flow source with high output impedance. Fluid is obtained from reservoir 936 and delivered to the outer enclosure circuit, downstream of which is a metering orifice 946 setting up a pressure P2 sensed by instrumentation 933 and pressure transducer 934 used for the alarm/shut-down routine. Alternatively, the instrumentation 949 etc., may comprise an electrohydraulic flowmeter 947,948 or a contamination counter 950. Signals will be dependent on changes of measured P2, flow, pulse counts, bought about by leakages into or out of the monitoring circuit.

Claims

1. A pressure vessel comprising a first fluid-containing enclosure having means enabling the ingress and egress of fluid from outside the vessel, a second fluid-containing enclosure surrounding said first enclosure, and a passage communicating with the second enclosure whereby fluid can be conveyed between the outside and the inside of said second enclosure to maintain under normal operation a predetermined fluid pressure within the second enclosure, said passage comprising means adapted to impede the flow of fluid therethrough.

2. A pressure vessel as claimed in Claim 1, wherein the passage extends between said first and second enclosures whereby in response to a fluid pressure change in said first enclosure fluid is conveyed between the two enclosures through said passage until the fluid pressure in both enclosures is substantially equalized.

3. A pressure vessel as claimed in any of the preceding claims, wherein the impedance means comprises a restrictor orifice.

4. A pressure vessel as claimed in any of the preceding claims, wherein the volume of the second enclosure is small compared with the volume of the first enclosure.

5. A pressure vessel as claimed in any of the preceding claims, wherein a movable partition is present in the first enclosure dividing said first enclosure into two mutually sealed portions of variable volume.

6. A pressure vessel as claimed in Claim 5, wherein the partition comprises a rigid piston.

7. A pressure vessel as claimed in Claim 5, wherein the partition comprises a deformable membrane.

8. A pressure vessel as claimed in any of Claims 5 to 7, wherein each of the two portions of the first enclosure contains a different fluid.

9. A pressure vessel as claimed in Claim 8, wherein the fluid in one portion of the first enclosure is a gas and the fluid in the other portion is a hydraulic liquid.

10. A pressure vessel as claimed in any of Claims 5 to 9, wherein the passage for conveying the fluid extends between the one portion of the first enclosure and the second enclosure, the second enclosure containing gas.

11. A pressure vessel as claimed in any of Claims 5 to 9, wherein the passage for conveying fluid extend between said other portion of the first enclosure and the second enclosure, the second enclosure containing hydraulic liquid.

12. A pressure vessel as claimed in any of the preceding claims, wherein the second en closure comprises two portions joined in sealed relationship.

1 3. A pressure vessel as claimed in any of the preceding claims wherein the first enclosure comprises two portions joined in sealed relationship.

14. A pressure vessel as claimed in any of the preceding claims, wherein the second enclosure is adapted to withstand a greater pressure than the first enclosure.

1 5. A pressure vessel substantially as herein described with reference to any of Figs.

1 to 7 of the accompanying drawings.