METHOD AND DEVICES FOR DETECTING OUTPUT LEAKS IN A DOUBLE-CONE AIR POWER AMPLIFIER

Description

Field of invention

[0001] The invention relates to a method for detecting outlet leaks in a double-cone air booster, as well as corresponding systems and / or devices. The aforementioned air boosters are widely used for the controlled compressed air supply of pneumatic actuators for process control valves, or other compressed air consumers.

[0002] Increasingly stringent requirements are being placed on such control valves regarding their ability to detect changes and, in particular, deteriorations in their operating conditions. This includes, for example, indications of leaks in the pneumatic system. In the long run, these leaks can cause significant costs and even compromise safety, as certain valve positions may no longer be accessible. They can also lead to increased noise levels. State of the art

[0003] An electropneumatic control system that includes a secondary pneumatic power stage is shown, for example, in EP 1 769 159 B1. This system uses, for example, a double-cone air power amplifier. Diagnostic capabilities relating to the compressed air system, and in particular to leaks, are not provided.

[0004] A pneumatic amplifier designed as a double-cone air power amplifier is described, for example, in publication DE 37 41 364 A1. No diagnostic options are described there either.

[0005] The publication WO 2018 / 035181 A1 discloses methods and devices for testing pneumatic signal amplifiers during operation. These methods allow, for example, conclusions to be drawn about the basic functionality of these devices. However, the methods and devices described therein cannot be used to determine whether there are leaks in the compressed air system.

[0006] A leak detection system is described in US 4,157,656 A. However, this requires the use of additional devices, which in turn require a pneumatic amplifier to amplify the output signal in such a way that it can be distinguished as clearly as possible from normal fluctuations.

[0007] Document US 2003 / 0208305 A1 describes a method for diagnosing a pneumatic control loop of a control valve. This includes detecting outlet leaks in an air booster located within the control loop. For this purpose, a control signal and the outlet pressure of the air booster are recorded. By comparing this with a calculated mass flow rate, a leak can be inferred.

[0008] Publication JPS55115603A describes a system that includes a fluid booster and deals with the compensation of pressure drops caused by a leak. Task

[0009] The object of the invention is to provide a method and system that is as uncomplicated as possible for detecting leaks in downstream consumers of a double cone air power amplifier. Solution

[0010] This problem is solved by the subject matter of the independent claim. Advantageous embodiments of the subject matter of the independent claim are identified in the dependent claims. The wording of all claims is hereby incorporated by reference into this description.

[0011] The use of the singular should not exclude the plural, and the same applies in reverse, unless otherwise stated.

[0012] The following section describes individual process steps in more detail. In a preferred embodiment of the invention, these steps are carried out in the specified order. However, the steps need not necessarily be performed in the specified order, and the process described may also include further, unmentioned steps.

[0013] To solve the problem, a method for detecting output leaks in a double-cone air booster is proposed. The method comprises the following steps: Initially, a reference curve is recorded that relates an input variable to an output variable. The input variable is a quantity that controls an output pressure of the double-cone air booster, e.g., the control pressure of the air booster, or a current if an I / P converter is connected upstream. The output variable is the output pressure of the double-cone air booster or a quantity controlled or derived from the output pressure, e.g., the position of the valve element if the air booster supplies compressed air to a pneumatic valve actuator.

[0014] During operation, at least one steady-state condition is determined. Only steady-state conditions preceded by a drop in the outlet pressure of the double-cone air booster are considered. The air booster and / or any upstream components are therefore still in exhaust mode. The input and output values ​​corresponding to the steady-state condition are determined and compared with the reference curve. If the outlet pressure corresponding to the determined output value is lower by a predefined tolerance than the outlet pressure expected according to the reference curve for the determined input value, an outlet leak is detected. This is equivalent to determining an input value corresponding to a determined output value that is higher by an equivalent predefined tolerance than would be expected according to the reference curve.

[0015] Outlet-side leaks are understood to mean leaks on the outlet side of the air power amplifier, i.e., leaks in downstream or downstream lines or compressed air consumers.

[0016] This method allows for the straightforward detection of outlet leaks in a double-cone air booster, particularly without additional components. This provides early indications of potentially necessary maintenance, thus saving costs associated with late-detected maintenance requirements.

[0017] Early detection of any leaks further reduces the costs of compressed air generation. It is also extremely important to identify whether certain valve positions can no longer be reached due to pressure loss caused by the leak. The consequences can be safety-relevant or affect the final product. This can be prevented or detected early using the described method.

[0018] A more meaningful result is achieved by determining a plurality of steady-state conditions. For each of these plurality of steady-state conditions, values ​​of the input and output variables corresponding to that steady state are determined. All these determined values ​​are then compared with the reference curve. This can prevent so-called "outliers" from causing erroneous leakage reports.

[0019] The reliability of leak detection is increased if an output leak is detected when, during a predefined measurement period, all output pressures corresponding to the measured output variables are lower by a predefined tolerance than the output pressures expected according to the reference curve for the measured input variables. This ensures that no leak is detected if at least one value measured during the predefined measurement period falls within the tolerance of the reference curve. False positives are thus significantly reduced.

[0020] The reliability of leak detection is further enhanced if an output leak is detected when, for a predetermined number of recently measured output variables, the output pressures corresponding to those variables are lower by a predefined tolerance than the output pressures expected according to the reference curve for the measured input variables. This also effectively suppresses false positives. This approach is preferable when suitable steady-state conditions are short-lived due to the operating environment. In such cases, it might not always be possible to guarantee a sufficient number of values ​​for accurate statistical analysis within a given measurement time. Therefore, it is more advantageous to specify the number of values ​​rather than the time required to measure them.

[0021] For further evaluation and documentation, it is advantageous if the procedure includes the further step of creating a data carrier (volatile or permanent) on which the determined values ​​and / or results and / or the reference curve are stored.

[0022] The problem is further solved by a device which is set up to carry out the procedure as already described.

[0023] The problem is further solved by a system for detecting outlet leaks in a double-cone air booster. This system comprises a double-cone air booster and a controller for controlling an input variable of the double-cone air booster, wherein the input variable is a quantity that controls an outlet pressure of the double-cone air booster. The system further comprises means for determining a steady state and means for acquiring the input variable and an output variable, wherein the output variable is the outlet pressure of the double-cone air booster or a quantity controlled or derived from the outlet pressure.The system also has means for recording a reference curve that relates the input variable to the output variable, means for comparing recorded values ​​with the reference curve, and means set up to execute the steps of a procedure as described above.

[0024] If the system includes an I / P converter, the input can be an electrical current, which is advantageous for controlling and / or regulating the dual-cone air power amplifier, as well as for data acquisition and evaluation. The I / P converter provides a control pressure for the dual-cone air power amplifier. The input is the current flowing through the I / P converter.

[0025] It is particularly advantageous if the I / P converter in the aforementioned system is based on the nozzle-impact plate principle. Such an I / P converter exhibits hysteresis behavior due to its electromagnetic components. This behavior is superimposed on the hysteresis-like behavior of the double-cone air power amplifier during the transition between supply and exhaust air operation, resulting in a more pronounced shape of the reference curve or characteristic curve. In particular, the supply and exhaust air curves differ more significantly.

[0026] The task is further solved by a positioner for a pneumatically driven control valve with a system as described above.

[0027] The task is also solved by a pneumatically driven control valve with a aforementioned positioner, and also by a process engineering plant with at least one aforementioned pneumatically driven control valve.

[0028] The task is also solved by a computer program, e.g. as part of a so-called firmware, comprising commands that cause the above-mentioned device or system or positioner to perform the above-mentioned process steps.

[0029] The task is ultimately solved by a computer-readable medium on which the aforementioned computer program is stored.

[0030] Further details and features will become apparent from the following description of a preferred embodiment in conjunction with the figures. The respective features can be implemented individually or in combination. The possibilities for solving the problem are not limited to this embodiment. For example, range specifications always include all intermediate values ​​(not explicitly stated) and all conceivable sub-intervals.

[0031] An exemplary embodiment is shown schematically in the figures. Identical reference numerals in the individual figures denote identical or functionally equivalent elements, or elements corresponding to each other with respect to their functions. Specifically, the figures show: Fig. 1 a schematic sectional view through a double-cone air booster with an additional spring in holding mode (state of the art); Fig. 2 a schematic sectional view through a double-cone air booster without an additional spring in supply air mode (state of the art); Fig. 3 a schematic sectional view through a double-cone air booster without an additional spring in exhaust air mode (state of the art); Fig. 4 a representation of the pressure profile and leakage flow during a test function at various leakage rates; Fig. 5 I-P characteristic curves of an air booster with an upstream I / P converter at various leakage rates; Fig. 6A a time course of the output pressure during a simulated operating situation; Fig. 6B the corresponding time course of the derivative of the output pressure; and Fig. 7 determined steady-state conditions in comparison to the IP characteristic curves from Fig. 5 .

[0032] First, the following will be used as a starting point Figuren 1 bis 3 The operating principle of a double-cone air power amplifier will be explained. This is state of the art, which is not claimed, but must be explained for better understanding.

[0033] Fig. 1 This shows the construction of a double-cone air power amplifier 100. This has a control input 105 for the control pressure. P st , an air inlet 110, which is supplied with compressed air at the pressure P z supplied with a vent outlet 115, which is open to the environment, and an outlet 120 for compressed air at the desired outlet pressure P a .

[0034] Between the control input 105 and the vent outlet 115 there is a first membrane 125 with the effective area A o , and between the vent outlet 115 and the outlet 120 there is a second membrane 130 with the effective area A u The effective area of ​​each membrane is approximately limited by the center of the membrane's ridges. These two membranes are connected by a control body 135, allowing them to move together. The control body 135 contains the exhaust air seat 140 in its center, which is closed or opened by the exhaust air cone 145.

[0035] Between the supply air inlet 110 and the outlet 120 is the supply air seat 150 with the cross-sectional area A z , which is closed or opened by the air inlet cone 155. The air inlet and exhaust cones 155 and 145 are rigidly connected so that they always move together, forming the double cone 160. This can be pre-tensioned by means of an additional spring 165, but this is not absolutely necessary.

[0036] Furthermore, an offset spring 170 is present, which controls the control body 135 and the two diaphragms 125, 130 in Fig. 1 The upward pressure ensures that the air booster only starts operating once a certain minimum control pressure is reached, which overcomes the force of the offset spring 170. The offset spring 170 also facilitates safe venting in case the pilot system, which supplies the control pressure, cannot vent completely (this is often the case, for example, with conventional nozzle-baffle plate IP converters). Furthermore, the lower effective surface 175 of the double cone is visible. Its significance is explained below.

[0037] In Fig. 1 The air booster is shown in hold mode. This means that the control pressure P st and output pressure P a are precisely sized so that the first and second diaphragms 125, 130, as well as the connecting control element 135, are in a central position in which both the supply air seat 150 and the exhaust air seat 140 are closed by the double cone 160. Therefore, there is no fluid connection between the outlet 120 and the supply air inlet 110 or the vent outlet 115, so that the outlet pressure P a is maintained. In practice, this state (holding mode) is rarely significant, since pneumatic consumers always exhibit a certain basic leakage, and a controlled air power booster must therefore switch to supply air mode to maintain a consumer state permanently.

[0038] In Fig. 2 A double-cone air power amplifier 200 is shown, which is derived from the Fig. 1 differs only in the absence of the additional spring 165. In Fig. 2 The air booster is in supply air mode. The control pressure P st is increased, the output pressure P a But not yet, which is why the two membranes 125, 130 and the control element 135 are pressed downwards. Therefore, the exhaust air seat 140 remains closed by the exhaust air cone 145, but the supply air cone 155 is pushed downwards out of the supply air seat 150, allowing compressed air to flow in from the supply air inlet 110 and increase the outlet pressure. P a Increased at exit 120.

[0039] Fig. 3 shows the same double-cone air power amplifier 200 as Fig. 2 , however, in exhaust air mode. The control pressure P st is lowered, so that the offset spring is 170 and the output pressure P a , The compressed air in the consumer is forced upwards by the two diaphragms 125 and 130 and the control element 135. Therefore, the supply air port 150 is closed by the supply air cone 155, while the exhaust air port 140 is opened by the exhaust air cone 145. This is because the exhaust air port 140 is also lifted, but the exhaust air cone 145, as part of the double cone 160, cannot follow, as it is connected to the supply air cone 155. Consequently, compressed air escapes from the consumer through the exhaust air outlet 115, resulting in a pressure at the outlet 120. P a is decreasing.

[0040] By varying the control pressure P st can the output pressure P a to any value between the ambient pressure and the supply air pressure P z be brought.

[0041] In the regulated state of the double-cone air booster, a force equilibrium exists for the control element 135. Without considering other forces, the following applies: P st * A o = P a * A u

[0042] For a more precise description, the force must also be considered. F z an additional spring (if present) and the effective force on the lower working surface of the double cone 160 A z The forces corresponding to the cross-section of the supply air seat 150 are taken into account, but only during supply air operation. During exhaust air operation, these forces are supported by the supply air seat 150 and absorbed by the housing. The force F The offset of the offset spring 170 must always be taken into account. Consequently, different force balances result for supply and exhaust air operation: Supply air operation: P a * A u + F offset + P z * A z + F z = P st * A o Exhaust air operation: P a * A u + F offset = P st * A o

[0043] The spring forces are involved F z and F The offset is assumed to be constant to a first approximation, since the double cone 160 and the control body 135 typically only move over short distances.

[0044] The following applies to the output pressure: in supply air mode: P a = P st ⋅ A o − P z ⋅ A z − F offset − F z A u in exhaust air mode: P a = P st ⋅ A o − F offset A u

[0045] At the same control pressure, the output pressure is therefore reduced in supply air mode compared to exhaust air mode. The reduction depends on the cross-section. A z of the air intake seat 150, the strength of the additional spring and the air intake pressure P z off. If one assumes the dependence of the output pressure P a from the control pressure P st A characteristic curve is thus formed, resulting in a hysteresis-like pattern.

[0046] To obtain such a characteristic curve, a test function with a curve such as the one in Fig. 4 The diagram shows how it can be used. The controller applies a slow upward ramp of the input value, and after a certain time, an equally slow downward ramp is applied. In the upper range of Fig. 4 The corresponding behavior of the outlet pressure can be seen. The thick line 410 shows the reference behavior without leakage, while the thin line 420 represents the behavior with a significant leakage (typical leakage flow rate several hundred nL / h - nL are standard liters). With a comparatively minor leakage of, for example, less than 50 nL / h, the resulting curve is indistinguishable from the reference behavior in this representation. It becomes apparent that the behavior differs primarily in the downward ramp. In the lower range of Fig. 4 are the associated leakage flows Q Leck applied for a large leakage (curve 430) as well as for a small leakage (curve 440).

[0047] Fig. 5 Figure 1 shows the characteristic curves of a double-cone air power amplifier, preferably with an upstream I / P converter based on the nozzle-baffle plate principle. This corresponds to the configuration frequently used in practice, for example, for the compressed air control of a pneumatically actuated control valve. The input variable is the current of the I / P converter, and the output variable is the output pressure of the air power amplifier. I In this representation, the initial value is plotted on the y-axis as a percentage. P a The x-axis is shown in bar. Both the double-cone air power amplifier and the I / P converter exhibit hysteresis-like behavior, which together results in the shape of the reference curve shown in bold. The upper (left) branch corresponds to supply air operation, while the lower (right) branch corresponds to exhaust air operation. As already explained, the output pressure in supply air operation is lower than in exhaust air operation at the same flow rate. The behavior with a small leakage differs only slightly from the reference curve. With a larger leakage, however, the entire branch for exhaust air operation shifts upwards (to the left), meaning that at a given flow rate, the pressure is lower compared to the reference curve. The present method utilizes these changes in behavior. Initially, such a reference curve is recorded and stored.

[0048] During operation, steady states are determined, i.e., states in which both the input and output variables change by a maximum of a specified tolerance within a given time period, e.g., by a maximum of 10 mbar within 5 seconds.

[0049] If such a steady state (also known as the operating point) is detected, a more detailed investigation is carried out. This involves checking whether the operating point was reached through a venting or purging process. As already explained, for the intended leak detection, only steady states achieved through purging are of interest, i.e., when the double-cone air booster is operating in exhaust mode. For this purpose, the pressure change rate, i.e., the first derivative of the output pressure, is considered and, for example, assigned a tolerance band. A tolerance band of, for instance, + / - 0.02 bar / s is suitable.

[0050] This is for a simulated operating situation in Figs. 6A und 6B shown: Fig. 6A shows the time course of the initial pressure, Fig. 6B the associated time course of the first time derivative of the initial pressure with the aforementioned tolerance band.

[0051] In Fig. 6A The detected stationary states are marked by diagonal crosses. If a stationary state occurs as a result of a penetration of the tolerance band in Fig. 6B If a steady state exists from above, it was achieved by venting and is therefore not considered further. However, if a steady state exists as a result of the tolerance band being penetrated from below (i.e., the derivative was negative immediately beforehand), it was achieved by venting.

[0052] These work points will be further considered and are included in Fig. 6A marked by circles. In Fig. 6B It can be seen that - at least in this simulated example - larger changes or jumps are often followed by a small correction, and that suitable working points are often only available for a short time directly after this correction.

[0053] In Fig. 7 It is possible to see how determined values ​​of the input and output variables, e.g., the current of the I / P converter and the output pressure of the double-cone air power amplifier, which belong to the steady-state conditions determined as described, are compared with the reference curve. A predefined tolerance limit is used for this purpose, which is specified in Fig. 7This tolerance limit is represented by a bold dashed line. It could, for example, lie halfway between the reference curve and an expected characteristic curve for a leakage that is considered just barely unacceptable. Measured values ​​to the left of this tolerance limit (or equivalently, above it) indicate significant leakage, as these operating points exhibit a lower output pressure at a given current than would be expected according to the reference curve. Conversely, no leakage can be detected at operating points whose corresponding measured values ​​lie below or to the right of this tolerance limit.

[0054] To compensate for statistical fluctuations and other disturbances, and to avoid frequent false alarms or erroneous diagnoses, it is advisable to consider a larger number of operating points and to choose a conservative condition for leak detection. In a typical example, the last 30 operating points can be considered. A leak would preferably only be detected if, without exception, all measured values ​​at these operating points are above or below the tolerance limit. glossary I / P converter

[0055] I / P converters are electro-pneumatic converters that generate an output air pressure depending on the current strength of an electrical input signal. I / P converter based on the nozzle-impact plate principle

[0056] I / P converters based on the nozzle-impact plate principle are described in detail, for example, in German patent application DE 198 18 336 C1. Such a converter comprises a coil, a magnetic yoke, and a rotatable armature designed as an impact plate. The impact plate can close and open an outlet nozzle, depending on the resulting torque due to the pneumatic force repelling the impact plate and the magnetic force attracting the armature. The system is also supplied with compressed air. By opening and closing the outlet nozzle in response to the current through the coil, which generates the magnetic force, an output air pressure is set that is proportional to the current. Standard liter

[0057] Standard volume (especially standard cubic meters, standard liters, etc.) is a unit of volume commonly used in pneumatics, process engineering, and gas technology. It is used to compare gas quantities at different pressures and temperatures (operating conditions, operating volumes). For this purpose, the gas quantities are converted to the same standard conditions, e.g., 0°C and 1 atm. (According to https: / / de.wikipedia.org / wiki / Normvolumen) Stationary state

[0058] A steady state, as used here, refers to a state of the double-cone air amplifier in which both the input and output variables change by a maximum of a specified tolerance within a given time interval. The input variable is a parameter that controls the output pressure of the double-cone air amplifier, e.g., the control pressure of the air amplifier, or a current if an I / P converter is connected upstream. The output variable is the output pressure of the double-cone air amplifier or a parameter controlled or derived from the output pressure, e.g., the position of the valve element if the air amplifier supplies compressed air to a pneumatic valve actuator. Reference sign

[0059] 100, 200, 500 Double cone air power amplifier 105 Control input 110 Supply air inlet 115 Venting outlet 120 Outlet 125 First diaphragm 130 Second diaphragm 135 Control body 140 Exhaust air seat 145 Exhaust air cone 150 Supply air seat 155 Supply air cone 160 Double cone 165 Auxiliary spring 170 Offset spring 175 Lower effective surface of the double cone 410 Reference output pressure curve during test function 420 Output pressure curve during test function with high leakage 430 Leakage flow curve during test function (high leakage) 440 Leakage flow curve during test function (low leakage) cited literature

[0060] Patent literature cited: DE 37 41 364 A1, DE 198 18 336 C1, EP 1 769 159 B1, US 4,157,656 A, US 2003 / 0208305 A1, WO 2018 / 035181 A1

Claims

1. Method for detecting outlet-side leaks in a double-cone air power amplifier (100; 200), comprising the following steps: 1.1 initially recording a reference curve relating an input variable to an output variable; 1.1.1 wherein the input variable is a variable that controls an outlet pressure of the double-cone air power amplifier (100; 200); 1.1.2 wherein the output variable is the outlet pressure of the double-cone air power amplifier (100; 200) or a variable controlled by or derived from the outlet pressure; 1.2 determining at least one stationary state during operation; 1.2.1 wherein only stationary states are considered which were preceded by a drop in the outlet pressure of the double-cone air power amplifier (100; 200); 1.3 determining values for the input and output variables, which values correspond to the stationary state; 1.4 comparing the determined values with the reference curve; 1.5 identifying an outlet-side leak if the outlet pressure corresponding to the determined output variable is lower by a specified tolerance than the outlet pressure expected according to the reference curve for the determined input variable.

2. Method according to the preceding claim, characterized in that 2.1 a plurality of stationary states are determined; 2.2 for each of these stationary states, values for the input and output variables are determined; and 2.3 these determined values are all compared with the reference curve.

3. Method according to the immediately preceding claim, characterized in that an outlet-side leak is identified if, during a specified measuring time, all output pressures corresponding to the determined output variables are lower by a specified tolerance than the output pressures expected according to the reference curve for the determined input variables.

4. Method according to claim 2, characterized in that an outlet-side leak is identified if, for a specified number of recently determined output variables, the output pressures corresponding to the determined output variables are lower by a specified tolerance than the output pressures expected according to the reference curve for the determined input variables.

5. Method according to any of the preceding claims, comprising the further step of creating a data carrier on which the determined values and / or results and / or the reference curve are stored.

6. Device configured to carry out the method according to any of the preceding claims.

7. System for detecting outlet-side leaks in a double-cone air power amplifier (100; 200), comprising: 7.1 a double-cone air power amplifier (100; 200); 7.2 a controller and / or regulating mechanism for controlling and / or regulating an input variable of the double-cone air power amplifier (100; 200); 7.2.1 wherein the input variable is a variable that controls an outlet pressure of the double-cone air power amplifier (100; 200); and comprising 7.3 means for determining a stationary state; 7.4 means for receiving the input variable and an output variable; 7.4.1 wherein the output variable is the outlet pressure of the double-cone air power amplifier (100; 200) or a variable controlled by or derived from the outlet pressure; 7.5 means for recording a reference curve which relates the input variable to the output variable; 7.6 means for comparing recorded values with the reference curve; and comprising 7.7 means configured to perform the steps of the method according to any of claims 1 to 5.

8. System according to the immediately preceding claim, characterized in that 8.1 the system comprises an I / P converter, 8.1.1 the I / P converter providing a control pressure for the double-cone air power amplifier (100; 200); and 8.1.2 the input variable being the current flowing through the I / P converter.

9. System according to the immediately preceding claim, characterized in that the I / P converter is constructed according to the nozzle-baffle principle.

10. Positioner for a pneumatically driven control valve, comprising a system according to any of claims 7 to 9.

11. Pneumatically driven control valve comprising a positioner according to the immediately preceding claim.

12. Processing plant comprising at least one pneumatically driven control valve according to the immediately preceding claim.

13. Computer program comprising instructions which cause the device of claim 6 or the system according to any of claims 7 to 9 or the positioner of claim 10 to perform the method steps according to any of claims 1 to 5.

14. Computer-readable medium on which the computer program according to the immediately preceding claim is stored.