EP2672126B1

EP2672126B1 - Method for determining the presence of undissolved gas in a hydraulic system

Info

Publication number: EP2672126B1
Application number: EP13170090.8A
Authority: EP
Inventors: Klaus Leonard Witt
Original assignee: Sun Test Systems BV
Current assignee: Sun Test Systems BV
Priority date: 2012-06-04
Filing date: 2013-05-31
Publication date: 2015-07-08
Anticipated expiration: 2033-05-31
Also published as: NL2008927C2; EP2672126A1

Description

The invention relates to a method for determining the presence of undissolved gas in a hydraulic system comprising a main circuit with a primary reservoir containing a medium, and a by pass circuit connected to the main circuit and having a threshold pressure valve which opens at a threshold pressure.
The hydraulic system will in general be part of an apparatus, e.g. an airplane. If there is undissolved gas in the medium of the hydraulic system 9, the working of the hydraulic system 9 can be deteriorated so that the functioning of the apparatus 11 can be hindered. It will be clear that in certain applications of the hydraulic system 9, e.g. an airplane, this can lead to very serious and even life threatening situations.
A conventional method for determining the presence of undissolved gas in a hydraulic system is known from EP 2 302 270 A1 .
It is an object of the invention to provide an improved or at least alternative method for determining the presence of undissolved gas in a hydraulic system comprising a main circuit with a primary reservoir containing a medium, and a by pass circuit connected to the main circuit and having a threshold pressure valve which opens at a threshold pressure. Said method comprises the steps of:

A. connecting a testing apparatus comprising a secondary reservoir of the medium and a pump to the main circuit,
B. completely fillng the primary reservoir,
C. after the primary reservoir is completely filled, pumping the medium from the secondary reservoir into the main circuit to increase the pressure of the medium until a pressure equal to or larger than the threshold pressure is reached in the by pass circuit so that the threshold pressure valve opens,
D. continuing the pumping of the medium such that the threshold pressure valve remains open and continuing said pumping until a substantially constant flow rate of the medium pumped from the secondary reservoir into the main circuit is reached,
E. over time measuring the flow rate of the medium pumped from the secondary reservoir into the main circuit, and
F. determining the presence of undissolved gas on basis of the measured flow rate.

The method allows to determine the presence of undissolved gas in the by pass circuit. The presence of undissolved gas can be determined on basis of the measured flow rate during the period from the opening of the threshold pressure valve until the moment that a substantially constant flow rate is reached. These measurements over time of the flow rate provide an indication of the presence of the undissolved gas in the by pass circuit. The measured flow rate can be compensated for the compressibility of the hydraulic system being free from undissolved gas. The measured flow rate can be compared with a reference flow rate. The reference flow rate can be obtained by subjecting a reference hydraulic system being free from undissolved gas to the steps A-E. The reference hydraulic system can be the same hydraulic system which is subjected to the method.
In an embodiment of the method, step E comprises over time measuring the flow rate of the medium pumped from the secondary reservoir into the main circuit during the steps C and D.
In an embodiment of the method, the pressure of the medium in the hydraulic system is released just before step C is started.
In an embodiment of the method, the medium in step D is pumped at a constant pressure equal to or larger than the threshold pressure into the main circuit.
In an embodiment of the method, the hydraulic system during normal operation comprises a low pressure part and a high pressure part, the by pass circuit is located in the high pressure part and the method comprises pumping the medium of the secondary reservoir into the high pressure part upstream of the by pass circuit.
In an embodiment of the method, the testing apparatus is connected to the main circuit via a first fluid connection located upstream of the by pass circuit and a second fluid connection located downstream of the by pass circuit.
In an embodiment of the method, the steps C and D comprise pumping the medium into the main circuit only via the first fluid connection.
In an embodiment of the method, during the steps C and D the second fluid connection is used as a return flow of medium to the secondary reservoir.
In an embodiment of the method, a further threshold pressure valve is provided in the return flow of medium to the secondary reservoir. The further threshold pressure valve may be an adjustable threshold pressure valve.
In an embodiment of the method, the testing apparatus comprises a flow rate measuring system for measuring the flow rate of the medium pumped from the secondary reservoir into the main circuit.
In an embodiment of the method, the testing apparatus comprises a pressure measuring system for measuring the pressure of the medium pumped from the secondary reservoir into the main circuit.
In an embodiment of the method, the primary reservoir is a piston type reservoir and step B comprises filling the piston type reservoir until the piston reaches its end stop.
In an embodiment of the method, step B comprises the sub steps of;

B1. pumping the medium from the secondary reservoir into the main circuit until the primary reservoir is completely filled,
B2. releasing the pressure on the primary reservoir,
B3. pumping the medium from the secondary reservoir into the main circuit while over time measuring the volume and the pressure of the medium pumped into the main circuit, and
B4. determining the presence of undissolved gas in the main circuit by comparing the measured volume with the measured pressure.

In an embodiment of the method, the sub steps B1 and B3 comprise pumping the medium into the main circuit via the first fluid connection and/or the second fluid connection.
Embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings in which corresponding reference symbols indicate corresponding parts, and in which:

Figure 1 schematically shows an embodiment of a testing apparatus connected to a hydraulic system and determining the presence of undissolved gas in the hydraulic system;
Figure 2 schematically shows a graph of the measured flow rate of medium pumped from the testing apparatus into the hydraulic system in the theoretical situation that the by pass circuit would be endless stiff and would be absolutely free from undissolved gas,
Figure 3 schematically shows a graph of the measured flow rate of medium pumped from the testing apparatus into the hydraulic system in the situation that the by pass circuit is free from undissolved gas.
Figure 4 schematically shows a graph of the measured flow rate of medium pumped from the testing apparatus into the hydraulic system in the situation that the by pass circuit contains undissolved gas, and
Figure 5 schematically shows a graph with a linear relation between the volume and pressure.

Figure 1 shows a testing apparatus 1 provided with a secondary reservoir 3 containing a medium and a pump 7 connected to a hydraulic system 9 of an apparatus 11 to be tested e.g. an airplane.
The hydraulic system 9 comprises a main circuit 50 provided with a primary reservoir 13 containing the medium and a by pass circuit 60 having a threshold pressure valve 62 which opens at a threshold pressure.
A first system 51, e.g. a flight control system of an airplane, is provided in the main circuit 50. A second system 61, e.g. a utility circuit (such as a landing system), is provided in the by pass circuit 50.
The primary reservoir 13 comprises a gas bleed valve 15 which is used to evacuate undissolved gas from the medium, e.g. oil, in the primary reservoir 13.
If there is undissolved gas in the medium of the hydraulic system 9, the working of the hydraulic system 9 can be deteriorated so that the functioning of the apparatus 11 can be hindered. For example, a second pump 17 which during normal operation of the apparatus 11 is used to bring the medium in the high pressure part 72 of the hydraulic system 9 at the required high pressure e.g. 100-350 Bar cannot work if gas is sucked into the second pump 17.
If there is gas in the first system 51 or the second system 61, there is a high risk that said system will not function correctly. It will be clear that in certain applications of the hydraulic system 9, e.g. an airplane, this can lead to very serious and even life threatening situations.
The high pressure part 72 of the hydraulic system 9 is used via a first piston 19 to bring the medium at the primary reservoir 13 at a lower return pressure with a second piston 20 during normal operation of the system.
The hydraulic system 9 is connected via a first fluid connection 23 and a second fluid connection 21 to the testing apparatus 1 during testing. The testing apparatus 1 is provided with connectors 25, 27 to connect to the apparatus 11.
Furthermore, the testing apparatus 1 is provided with a pressure measuring system 5 to over time measure the pressure in the medium and a flow rate measurement system 31 to over time measure a flow rate of medium flowing from the secondary reservoir 3 into the hydraulic system 9. Also a filter 29 to filter the medium is provided.
The first valve 33, second valve 35 and third valve 37 may be used to control the working of the testing apparatus 1. Each of said valves 33, 35, 37 can be opened and closed independently from each other. A further threshold pressure valve 80 is provided in a fluid connection bypassing the third valve 37. The further threshold pressure valve 80 is an adjustable pressure valve.
For determining the presence of undissolved gas in the hydraulic system 9 the following steps are performed.
In step A, the testing apparatus 1 is connected to the main circuit 50 of hydraulic system 9.
In step B, the primary reservoir 13 is completely filled.
In step C, after the primary reservoir 13 is completely filled, medium is pumped from the secondary reservoir 3 into the main circuit 50 to increase the pressure of the medium until a pressure equal to or larger than the threshold pressure is reached in the by pass circuit 60 so that the threshold pressure valve 62 opens.
In step D, the pumping of the medium is continued such that the threshold valve 62 remains open and said pumping is continued until a substantially constant flow rate of the medium pumped from the secondary reservoir 3 into the main circuit 50 is reached.
In step E, the flow rate of the medium pumped from the secondary reservoir 3 into the main circuit 50 is measured over time.
In step F the presence of undissolved gas is determined on basis of the measured flow rate.
The presence of undissolved gas is determined on basis of the measured flow rate during the period from the opening of the threshold pressure valve 62 until the moment that a constant flow rate is reached. These measurements over time of the flow rate provide an indication of the presence of the undissolved gas in the by pass circuit 60. The measured flow rate can be compensated for the compressibility of the hydraulic system being free from undissolved gas. The measured flow rate can be compared with a reference flow rate. The reference flow rate can be obtained by subjecting a reference hydraulic system being free from undissolved gas to the steps A-E. The reference hydraulic system can be the hydraulic system 9 which is subjected to the method.
The figures 2-4 show different situations wherein the hydraulic system of fig. 1 is subjected to the method according the invention. In the graphs shown in the fig. 2-4, the horizontal axis indicates the time (t) and the vertical axis the measured flow rate (f) of the medium pumped from the secondary reservoir 3 into the main circuit 50. In the graphs of fig. 2-4, the threshold pressure valve 62 opens at X. In the graphs of the fig. 3 and 4, a substantially constant flow rate is reached at Y.
Figure 2 shows a graph of the measured flow rate of medium pumped from the testing apparatus 1 into the hydraulic system 9 in the theoretical situation that the by pass circuit 60 would be endless stiff and would be absolutely free from undissolved gas. In practise, hydraulic systems always have a certain degree of compressibility. Furthermore, it is in practise not possible to get a hydraulic system absolutely free (100% free) from undissolved gas. Therefore, when reference is made to a hydraulic system (or a part thereof, such as the by pass circuit) being free from undissolved gas, a hydraulic system (or a part thereof, such as the by pass circuit) as in practise would be considered to be free from undissolved gas is meant. This is sometimes also referred to as being substantially free from undissolved gas.
Figure 3 shows a graph of the measured flow rate of medium pumped from the testing apparatus into the hydraulic system in the situation that the by pass circuit is free from undissolved gas. When compared with fig. 2, the peak in fig. 3 is caused by the compressibility of the by pass circuit. It is noted that a small part of the peak may be caused by the fact that the by pass circuit will not be absolutely free (100% free) from undissolved gas. As mentioned before, this if further neglected because the aim of the method is to determine whether the by pass circuit in practise would be considered as being free from undissolved gas.
The area A1 indicates the difference between the area under the graph during the period between X and Y when compared with the graph of fig 2. The area A1 therefore is considered to correspond with the compressibility of the by pass circuit 60.
Figure 4 shows a graph of the measured flow rate of medium pumped from the testing apparatus into the hydraulic system in the situation that the by pass circuit contains undissolved gas.
The area A2 indicates the difference between the area under the graph during the period between X and Y when compared with the graph of fig 2. The area A2 therefore corresponds with the compressibility of the by pass circuit 60 and the undissolved gas present in the by pass circuit 60. When the graph of fig. 4 is compensated for the compressibility of the by pass circuit 60 (for example by deducting A1 from A2), an indication of the amount of undissolved gas in the by pass circuit 60 is provided.
In practise, one can provide a reference flow rate by subjecting the hydraulic system 9 being free from undissolved gas to the steps A-E. The data of the over time measured flow rate forms the reference flow rate. Once the hydraulic system 9 is at a later stage subjected to the step A-F, the over time measured flow rate can be compared with said reference flow rate. If the over time measured flow rate and the reference flow rate are the same, there is no undissolved gas present in the by pass circuit 60 of the hydraulic system 9. If there are differences between the over time measured flow rate and the reference flow rate, there is undissolved gas present in the by pass circuit 60 of the hydraulic system 9. The degree of difference between the measured flow rate and reference flow rate provides an indication of the amount of undissolved gas in the by pass circuit 60.
For example, the graphs of fig. 3 and 4 can be compared to determine whether there is undissolved gas present in the by pass circuit 60. A difference between said graphs provide an indication that there is undissolved gas in the by pass circuit 60. More specifically, the period from the moment that the threshold pressure valve 62 opens (at X) until a constant flow rate is reached (at Y) of both graphs are compared. The difference in the area under the graphs during said period provides an indication of the amount of undissolved gas in the by pass circuit 60. The larger said area of fig. 4 is when compared with said area of fig. 3, the more undissolved gas is present in the by pass circuit 60. As indicated before, it is also possible to compare specific parts said areas with each other.
In the embodiments shown, the medium is in step D pumped at a constant pressure equal to or larger than the threshold pressure into the main circuit 50, more specifically the medium is pumped at a predetermined constant pressure equal to or larger than the threshold pressure.
The hydraulic system 9 during normal operation comprises a low pressure part 71 and a high pressure part 72, the by pass circuit 60 is located in the high pressure part 72 and the method comprises pumping the medium of the secondary reservoir 3 into the high pressure part 72 upstream of the by pass circuit 60.
The testing apparatus 1 is connected to the main circuit 50 via a first fluid connection 23 located upstream of the by pass circuit 60 and a second fluid connection 21 located downstream of the by pass circuit 60.
The steps C and D comprise pumping the medium into the main circuit 50 only via the first fluid connection 23. During the steps C and D the second fluid connection 21 is used as a return flow of medium to the secondary reservoir 3. In said situation the valves 33 is open and the valves 35 and 37 are closed. The return flow passes the valve 37 via the further threshold pressure valve 80.
The primary reservoir 13 is a piston type reservoir and step B comprises filling the piston type reservoir until the piston reaches its end stop.
Step B comprises the sub steps of;

B1. pumping the medium from the secondary reservoir 3 into the main circuit 50 until the primary reservoir 13 is completely filled,
B2. releasing the pressure on the primary reservoir 13,
B3. pumping the medium from the secondary reservoir 3 into the main circuit 50 while over time measuring the volume and the pressure of the medium pumped into the main circuit 50, and
B4. determining the presence of undissolved gas in the main circuit 50 by comparing the measured volume with the measured pressure.

If there is a linear relation between the volume and pressure, there is no undissolved gas in the main circuit 50. This is shown in fig. 5. If there is no linear relation between the volume and pressure, there is undissolved gas in the main circuit 50. In figure 5, the horizontal axis indicates the measured pressure (p) and the vertical axis the measured volume (v).
The sub steps B1, and B3 comprise pumping the medium into the main circuit via the first fluid connection 23 and the second fluid connection 21. The medium may be pumped into the main circuit 50 mainly via the second fluid connection 21.
While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practised otherwise than as described. The descriptions are to be intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

Method for determining the presence of undissolved gas in a hydraulic system
comprising a main circuit (50) with a primary reservoir (13) containing a medium, and a by pass circuit (60) connected to the main circuit and having a threshold pressure valve (62) which opens at a threshold pressure, wherein the method comprises the steps of:
A. connecting a testing apparatus (1) comprising a secondary reservoir (3) of the medium and a pump (7) to the main circuit,

B. completely filling the primary reservoir (13),

C. after the primary reservoir (13) has completely been filled, pumping the medium from the secondary reservoir (3) into the main circuit (50) to increase the pressure of the medium until a pressure equal to or larger than the threshold pressure is reached in the by pass circuit so that the threshold pressure valve (62) opens,

D. continuing the pumping of the medium such that the threshold pressure valve (62) remains open and continuing said pumping until a substantially constant flow rate of the medium pumped from the secondary reservoir (3) into the main circuit (50) is reached,

E. over time measuring the flow rate of the medium pumped from the secondary reservoir (3) into the main circuit (50), and

F. determining the presence of undissolved gas on basis of the measured flow rate.
Method according to claim 1, wherein step E comprises over time measuring the flow rate of the medium pumped from the secondary reservoir (3) into the main circuit (50) during the steps C and D.
Method according to claim 1 or 2, wherein just before step C is started, the pressure on the medium in the hydraulic system is released.
Method according to any of the preceding claims, wherein step F comprises compensating the measured flow rate for the compressibility of the hydraulic system being free from undissolved gas.
Method according to any of the preceding claims, wherein step F comprises comparing the measured flow rate with a reference flow rate.
Method according to claim 5, wherein step F comprises comparing the measured flow rate with the reference flow rate of a reference hydraulic system being free from undissolved gas and subjected to the steps A-E.
Method according to any of the preceding claims, wherein in step D the medium is pumped at a constant pressure equal to or larger than the threshold pressure into the main circuit (50) .
Method according to any of the preceding claims, wherein the hydraulic system during normal operation comprises a low pressure part (71) and a high pressure part (72), the by pass circuit (60) is located in the high pressure part and the method comprises pumping the medium of the secondary reservoir (3) into the high pressure part upstream of the by pass circuit.
Method according to any of the preceding claims, wherein the testing apparatus (1) is connected to the main circuit (50) via a first fluid connection (23) located upstream of the by pass circuit (60) and a second fluid connection (21) located downstream of the by pass circuit.
Method according to any of the preceding claims, wherein the steps C and D comprise pumping the medium into the main circuit only via the first fluid connection (23).
Method according to any of the preceding claims, wherein during the steps C and D the second fluid connection (21) is used as a return flow of medium to the secondary reservoir (3).
Method according to claim 11, wherein a further threshold pressure valve (80) is provided in the return flow of medium to the secondary reservoir (3).
Method according to any of the preceding claims, wherein the testing apparatus comprises a flow rate measuring system (31) for measuring the flow rate of the medium pumped from the secondary reservoir (3) into the main circuit (50).
Method according to any of the preceding claims, wherein the testing apparatus comprises a pressure measuring system (5) for measuring the pressure of the medium pumped from the secondary reservoir (3) into the main circuit (50).
Method according to any of the preceding claims, wherein the primary reservoir (13) is a piston type reservoir and step B comprises filling the piston type reservoir until the piston reaches its end stop.
Method according to any of the preceding claims, wherein step B comprises the sub steps of;
B1. pumping the medium from the secondary reservoir (3) into the main circuit (50) until the primary reservoir (13) is completely filled,

B2. releasing the pressure on the primary reservoir,

B3. pumping the medium from the secondary reservoir into the main circuit while over time measuring the volume and the pressure of the medium pumped into the main circuit, and

B4. determining the presence of undissolved gas in the main circuit by comparing the measured volume with the measured pressure.
Method according to claim 16 and in combination with claim 9, wherein the sub steps B1 and B3 comprise pumping the medium into the main circuit (50) via the second fluid connection (21).
Method according to claim 16 or 17 and in combination with claim 9, wherein the sub steps B1 and B3 comprise pumping the medium into the main circuit (50) via the first fluid connection (23).