Patent Application
20080141775
The invention relates to a diagnostic method for a final control element driven by auxiliary power according to the features of the preamble of claim 1.
Control valves, as the type of final control elements in question, are generally known in automation and process engineering as extremely important elements in the control and regulation of processes. Their reliability is a crucial factor in the quality of the overall control process. Faults occurring during operation can result in failure of the entire system, with high maintenance costs as a consequence. Hence early diagnosis and thereby detection of faults in the valve can prevent such failures, and consequently also reduce the costs that arise from replacing, as a precaution, valves that are still working perfectly.
In particular, leaks from valves in the closed state are of significant diagnostic interest. The sealing action of the valve seating is reduced by ageing processes or dirt accumulation, and the process medium continues to flow through the valve despite a closed valve being signaled to the outer world.
Such leaks can be detected, for example, by a flow-rate sensor connected downstream that is installed additionally in the process line. Such a sensor is very expensive, however, and there is a high cost involved in fitting the sensor. In addition, the power consumption of the flow-rate sensor is normally so high that it cannot share the supply for the valve controller, but requires an additional supply line. Thus such a sensor is normally only installed when it is already required for the process control system. In addition, it is known from the dissertation of Sebastian Maria Mundry “Zustandsüberwachung an Prozessventilen mit intelligenten Stellungsreglern” [“Monitoring the status of process valves using intelligent positioners”], Shaker Verlag, Aachen, 2002, that flow-rate sensors for measuring the maximum flow rate are not suitable for reliable detection of the low flow rates from leaks.
It is also known from the same publication that the flow of a fluid under pressure through a narrow aperture produces an acoustic signal as a result of various physical effects. For instance, the high flow rates that arise cause severe turbulence after the aperture, and the pressure drop in the flow results in cavitation. The turbulence and the collapse of the cavitation bubbles produce an acoustic signal that is directly dependent on the flow rate and the fluid properties. At low rates, the signal is composed of individual acoustic pulses generated by the collapse of the individual cavitation bubbles, and develops into white noise at high rates. This acoustic signal is overlaid by the general process sounds in the plant, which are produced by pumps, general flow noises, chemical processes etc.
As these process sounds propagate in the pipeline system of the plant, the sounds are attenuated by different amounts depending on their frequency. The high frequencies, in particular, are strongly attenuated, so that generally process noises are only detectable at the valve as low-frequency acoustic signals (in the kHz range). Thus the sounds produced by the leak can be discriminated from the general process sounds by measuring in higher frequency ranges. It is known from Leak Detection Service, Maintaining a Successful Valve and Trap Leak Detection Program using the Valve-Analyser System, The 10th Annual Predictive Maintenance Technology National Conference, Nov. 9-12, 1998, that valve service companies currently use ultrasound sensors in order to measure the acoustic signals directly at the valve, i.e. close to the noise source. In addition, these signals are also compared with the signals from ultrasound sensors installed further upstream and downstream in the pipeline system. A leak can then be detected from these signals, and, with suitable calibration, it is even possible to determine the size of the leak for the valve from the signal level.
Furthermore, EP 1216375 B1 and WO 00/73688 A1 disclose detecting the structure-borne noise on the valve casing or on parts directly connected to this, and supplying this information to the positioner, in which it is evaluated and processed. The valve is continuously monitored, with the electronics and position signal already available in the positioner being shared for the diagnosis. The publications also disclose that high-frequency signals (>50 kHz) are analyzed, and that the ultrasound spectrum in the closed state is compared with a signal in the slightly open state. The latter methods can be applied equally well to reducing the ambient noise without comparative measurements needing to be made at different points in the direction of flow and against the direction of flow. Although sharing the use of the position-sensor electronics and the position signal does reduce the installation costs for this diagnostic system, the ultrasound sensor head itself must still be fitted on the valve as an additional external unit.
Hence the object of the invention is to record significant status signals of the final control element to be monitored, at minimum possible expense and under continued use of the existing means necessary for the conventional use of the final control element.
This object is achieved according to the invention by the means of claim 1. Advantageous embodiments of the invention are given in the dependent claims.
The invention is based on a final control element driven by auxiliary power, in which the auxiliary power is supplied by means of an actuator for converting an electrical manipulated variable into a physical variable of the auxiliary power, where the elements of the actuator form a control loop.
According to the invention, the actuator, in detecting status signals from the final control element, is operated as a sensor in a direction of action of the signal flow that is opposite to its conventional direction. Here, the perturbing effect of the final control element on an element in the control loop of the actuator is detected, selected and evaluated as a disturbance variable. In detail, a mixed signal composed of the actual signal from the actuator and status signals from the final control element is picked up at the signal-receiving control-loop element.
Advantageously, by using the actuator, which is present for conventional purposes, as a sensor for the perturbing effect of the final control element, it is possible to dispense with additional control elements and components for signal detection.
According to another feature of the invention, the sensor signal is derived from the control deviation of the actuator. In this case, the mixed signal that can be picked up at the signal-receiving control-loop element is advantageously separated from the stationary component of the actual signal from the actuator, except for the control deviation.
According to another feature of the invention, the sensor signal is derived from the control variable of the actuator. In this case, the sensor signal is separated from the stationary component of the actual signal from the actuator by filtering higher frequency signal components.
The invention is described in more detail below with reference to an exemplary embodiment and the requisite drawings, in which
In a first embodiment of the invention, the positioner 9 is designed on the basis of the jet/baffle-plate principle known per se. As shown in
During conventional use, vibrations are excited in the process valve 2 as a function of its operating status. The excitations can have various causes as mentioned in the introduction, and result in acoustic signals appearing in different frequency ranges. For instance, acoustic signals in the region of several kilohertz indicate a leak, whereas low-frequency acoustic signals point to vibrations of the process valve 2.
These acoustic signals propagate in the process valve 2 and are fed back into the pneumatic system of the actuating mechanism 6 via the elements directly connected to the process valve 2. In this case, the acoustic signals are mainly transferred via the valve rod 7 onto the membrane in the actuating mechanism 6 and into the housing of the actuating mechanism 6, which amplify these signals like a large loudspeaker membrane and transfer them to the pneumatic fluid. Inside the actuating mechanism 6, in particular, strong amplification of the acoustic signal takes place in the pneumatic fluid of the drive chamber.
The acoustic signals also propagate via the pneumatic amplifier 20 into the pneumatic-fluid supply line 19 and the jet 16. The fluctuations in pressure in the pneumatic system caused by these signals produce a mechanical vibration of the balance beam 15 via the jet/baffle-plate system, which propagates into the plunge coil 14, which enters and re-emerges from the induction cups 13 tracking the vibration. An alternating magnetic field is generated in the electromagnet in the process, which induces an alternating voltage in the plunge coil 14, which, superimposed on the current equivalent to the setpoint value, can be picked up at the terminals 12 and can be provided for analytical processing. This means that additional sensors can be dispensed with. Thus leak detection can be implemented as a pure software solution in the positioner 9.
In addition to leak detection, it is also possible to use the procedure described above to evaluate and analyze sounds other than the flow sounds described above. These include particularly, but not exclusively, vibrations of the process valve 2, leaks in the drive system in the actuating mechanism 6 or in its supply lines, which, similarly to leaks in the process valve 2, can become perceptible as sounds in the pneumatic fluid, or other fault sources, which a technician would currently identify on-site by listening. It can be provided in this case to process these additional sounds by suitable acoustic analysis in the device or by transferring the sounds to a central device where they can be analyzed by a technician, without the technician needing to go to the site of the process valve 2. Whenever a strong, unusual sound arises, for example at the process valve 2, this can be transferred to the central device in the form of an acoustic file for diagnosis. Both manual and automated analysis of the received acoustic file can be provided in the central device.
According to a further feature of the invention, it is provided that the status data of the final control element is derived from the amplitude spectrum of the fed back acoustic signals. In this case, the occurrence of characteristic spectral images is used to infer associated status conditions of the final control element.
According to an alternative feature of the invention, it is provided that the status data of the final control element is derived from the levels of the fed back acoustic signals. This feature is based on the knowledge that the status of the final control element can already be inferred just from the intensity of the fed back acoustic signal.
According to another alternative feature of the invention, it is provided that the status data of the final control element is derived from characteristic patterns of the fed back acoustic signals. This is based on the knowledge that certain status conditions of the final control element can be assigned a respective characteristic acoustic pattern, which when identified in the fed back acoustic signal indicates the respective status.
In addition, in order to specify the diagnosis more precisely, the current status of the final control element and/or actuating mechanism can be used, for instance whether the process valve 2 is in the open or closed position, auxiliary power present/not present. This current status can be derived, for example, from the setpoint/actual signals or from general information about the system.
In a second embodiment of the invention, as shown in
During conventional use, an electrical signal is provided by the control amplifier 23 so that the control pressure on the pneumatic side of the I/P converter 24 equals the defined setpoint pressure. The pressure-regulated I/P converter 24 is thereby an actuator, which is a control-loop element in the control loop for positioning the actuating mechanism 6 for the process valve 2.
The acoustic signals emanating from the process valve 2 are fed back into the pneumatic system in amplified form as already described above, and propagate in the pneumatic system. In the process, pressure fluctuations at the pressure sensor 25 of the pressure-regulated I/P converter 24 are converted into an appropriate electrical alternating quantity.
In one embodiment of the invention, the alternating quantity is derived at the point labeled with reference number 31 in the control circuit from the control variable of the pressure-regulated I/P converter 24. In this case, the sensor signal is separated from the stationary component of the actual signal from the pressure-regulated I/P converter 24 by filtering higher frequency signal components.
In an alternative embodiment of the invention, the alternating quantity is derived at the point labeled with reference number 32 in the control circuit from the control deviation of the pressure-regulated I/P converter 24. In this case, the mixed signal that can be picked up at the signal-receiving control-loop element is advantageously separated from the stationary component of the actual signal from the pressure-regulated I/P converter 24, except for the control deviation.
Both embodiments share the feature that an existing control-loop element is used for another purpose. The pressure sensor 25 is already a component of the pressure-regulated I/P converter 24 already known, and is used to regulate the control pressure for the actuating mechanism 6.
1 pipeline
2 process valve
3 valve seating
4 closing body
5 process medium
6 actuating mechanism
7 valve rod
8 yoke
9 positioner
10 position sensor
11 communications interface
12 terminal
13 induction cup
14 plunge coil
15 balance beam
16 jet
17 storage device
18 control unit
19, 21 pneumatic-fluid supply line
20 pneumatic amplifier
22 summation
23 control amplifier
24 I/P converter
25 pressure sensor
31, 32 signal pick-up
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2006 059 938.1 | Dec 2006 | DE | national |
