Flow measurement diagnostics

BACKGROUND OF THE INVENTION

The present invention relates to fluid process control and monitoring systems. In particular, the present invention relates to diagnostics for such systems.

Fluid flow meters are used in industrial process control and monitoring environments to measure fluid flow and provide flow signals for flow indicators and controllers. Inferential flow meters measure fluid flow in a pipe by measuring a pressure drop near a discontinuity within the pipe. The discontinuity (primary element) can be an orifice, a nozzle, a venturi, a pitot tube, a vortex shedding bar, a target or even a simple bend in the pipe. Flow around the discontinuity causes both a pressure drop and increased turbulence. The pressure drop is sensed by a pressure transmitter (secondary element) placed outside the pipe and connected by impulse lines or impulse passageways to the fluid in the pipe. Reliability depends on maintaining a correct calibration. Erosion or buildup of solids on the primary element can change the calibration. Impulse lines can become plugged over time which isolates the pressure transmitter from the process such that the transmitter is no longer able to track the pressure and adversely affects the operation of the transmitter.

Disassembly and inspection of the impulse lines is one method used to detect and correct plugging of lines. Another known method for detecting plugging is to periodically add a "check pulse" to the measurement signal from a pressure transmitter. This check pulse causes a control system connected to the transmitter to disturb the flow. If the pressure transmitter fails to accurately sense the flow disturbance, an alarm signal is generated indicating line plugging. Another known method for detecting plugging is sensing of both static and differential pressures. If there is inadequate correlation between oscillations in the static and differential pressures, then an alarm signal is generated indicating line plugging. Still another known method for detecting line plugging is to sense static pressures and pass them through high pass and low pass filters. Noise signals obtained from the filters are compared to a threshold, and if variance in the noise is less than the threshold, then an alarm signal indicates that the line is blocked.

<patcit id="pcit0001" dnum="WO0050851A"><text>WO/0050851</text></patcit> is considered the closest prior art and this describes a fluid flow meter which diagnoses the condition of its primary element or impulse lines connecting to a differential pressure sensor. A difference circuit coupled to the differential pressure sensor has a difference output representing the sensed differential pressure minus a moving average. A calculate circuit receives the difference output and calculates a trained output of historical data obtained during an initial training time. The calculate circuit also calculates a monitor output of current data obtained during monitoring or normal operation of the fluid flow meter. A diagnostic circuit receives the trained output and the monitor output and generates a diagnostic output indicating a current condition of the primary element and impulse lines.

SUMMARY

A flow meter for measuring flow of a process fluid includes a sensor configured to provide a sensor output signal related to flow of the process fluid. Circuitry is configured to determine a statistical parameter related to sensor output signal. A diagnostic output is provided based upon the determined statistical parameter and the sensor output signal. Such a flow meter of the invention is described in independent claim 1. A method of diagnosing operation of a flow meter is described in independent claim 12. Preferred embodiments of the invention are given in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

<ul id="ul0001" list-style="none"><li><figref idref="f0001">FIG. 1</figref> is a simplified diagram of a process control or monitoring system.</li><li><figref idref="f0002">FIG. 2</figref> is a simplified block diagram showing a cross-sectional view of the transmitter shown in <figref idref="f0001">FIG. 1</figref>.</li><li><figref idref="f0003">FIG. 3</figref> is a graph of standard deviation of a differential pressure signal versus a differential pressure signal.</li><li><figref idref="f0004">FIG. 4</figref> is a graph of standard deviation of a differential pressure signal versus a differential pressure signal.</li></ul>

DETAILED DESCRIPTION

<figref idref="f0001">FIG. 1</figref> is a simplified diagram of a process control or monitoring system 10 in which process piping 12 carries a flow 14 of process fluid. A flow transmitter 16 is configured to sense the flow 14 and provide an output related to the flow 14. In the example shown in <figref idref="f0001">FIG. 1</figref>, the output is provided to control room 18 over a two wire process control loop 20. Loop 20 can operate in accordance with any protocol. Example standard protocols includes 4-20 mA signals, the HART(R) communication protocol or Fieldbus protocols. However, the present invention is not limited to such communication techniques, including wireless techniques, and can even operate in a stand alone device.

Process variable transmitter 16 senses flow using any appropriate technique. Example techniques include measuring a differential pressure across a restriction in the pipe 12, magnetic based technologies, pitot tubes, vibrating sensors, etc.

In accordance with the invention, transmitter 16 provides a diagnostic output based upon the flow 14 and noise in a sensor output signal. In one specific configuration, a correlation between standard deviation of the sensor signal and the sensor signal is used for diagnostics. The diagnostic output can be used internally to transmitter 16, or can be provided externally, for example by transmission over loop 20.

<figref idref="f0002">Figure 2</figref> is a simplified block diagram of transmitter 16 shown in <figref idref="f0001">Figure 1</figref>. In the diagram of <figref idref="f0002">Figure 2</figref>, transmitter 16 is illustrated as including a differential pressure sensor. In such a configuration, a difference in pressures P1 and P2 is applied to the transmitter 16 and can be used to infer the rate of flow 14 through process piping 12 shown in <figref idref="f0001">Figure 1</figref>. An orifice plate 47 is placed in the process piping and includes an orifice 49 formed therein. The orifice plate 47 creates a differential pressure which is the difference between pressures P1 and P2 as the flow 14 is forced to flow through the restricted orifice 49. The invention is not limited to such a flow measurement technique.

Pressures P1 and P2 are applied to isolation diaphragms 52 and 54 through process coupling 53 and 55, respectively. The isolation diaphragms 52 and 54 form respective cavities 56 and 58 in the transmitter 16. The cavities 56 and 58 are isolated from the process fluid and contain an isolation fluid. Small capillary tubes 60 and 62 lead from respective cavities 56 and 58 to differential pressure sensor 50. The capillary tubes 60 and 62 transfer the pressure from pressure applied by P1 and P2 to the differential pressure sensor 50 while isolating the pressure sensor 50 from the process fluid.

Differential pressure sensor 50 can operate in accordance with any technology. One example technology uses a deflectable diaphragm which forms a variable capacitance. The pressure sensor 50 couples to transmitter circuit 70 and provides a sensor output to transmitter circuit 70. Transmitter circuit 70 can comprise any type of circuit configuration. Typically, a transmitter circuit 70 includes a microprocessor or other digital controller along with a memory 72. The memory 72 stores program instructions, configuration information, temporary variables and the like. Transmitter circuit 70 includes measurement circuit 74 which provides an output related to flow of the process fluid. The flow can be determined using any appropriate technique. Diagnostic circuit 76 is also implemented in transmitter circuit 70. The diagnostic circuit 76 includes circuitry 77 to determine a statistical parameter related to the sensor output signal 78. The statistical parameter can be indicative of noise in the sensor output signal 78 and, in one specific embodiment, the statistical parameter comprises standard deviation. Output circuitry 86 receives the flow signal and provides an output on two wire process control loop 20 in accordance with a desired signal and protocol.

In the configuration shown in <figref idref="f0002">Figure 2</figref>, the diagnostic circuit 76 receives the flow signal from measurement circuit 74 along with the sensor output 78 from sensor 50. As used herein, the flow signal and sensor signal can be processed prior to receipt by diagnostic circuit 76. Based upon the flow signal 80 and the sensor output 78, diagnostic circuit 76 provides a diagnostic output 82. The diagnostic output 82 can be used locally by transmitter 16 and can be provided externally, for example by transmission over two wire process control loop 20. The diagnostic output is indicative of a condition of one or more components of transmitter 16, or other components coupled to the industrial process 10 shown in <figref idref="f0001">Figure 1</figref>. For example, the diagnostic output 82 can provide an indication that one of the process couplings 53, 55 is becoming plugged. This information can be used alone, or in combination with other line plugging diagnostics to identify the plugging of process couplings 53, 55. Another example diagnostic is indicative of wear in the restrictive orifice plate used to create the differential pressure. This wear can cause edge corrosion in the plate resulting in inaccurate flow measurements. Similarly, loss of the isolation fill fluid carried in capillaries 60, 62 and cavities 56, 58 can be detected. The diagnostic output 82 can also be indicative of the composition of the process fluid flowing through pipe 12 shown in <figref idref="f0001">Figure 1</figref>. Change can be due to different fluids, changes in the components, such as contaminants in the fluid such as due to aeration or the addition of solids, etc.

The particular algorithm used to correlate the sensor signal and the flow signal with the diagnostic output can be chosen as appropriate. In one example embodiment, the diagnostic circuitry 76 identifies noise in the sensor output signal and provides the diagnostic output based upon this noise and the measured flow rate. In particular, a relationship between the noise level in the sensor signal and the flow rate during steady state flow conditions is used to provide the diagnostic output 82. The standard deviation is indicative of the noise in the sensor output signal. <figref idref="f0003">Figure 3</figref> is a graph of the standard deviation of a differential pressure sensor output versus flow. In this example, the process fluid comprises water at approximately room temperature.

<figref idref="f0004">Figure 4</figref> is a similar graph using a different size orifice plate to create the differential pressure. In the case of both <figref idref="f0003">Figure 3</figref> and <figref idref="f0004">Figure 4</figref>, there is an approximately linear relationship between the standard deviation of the differential pressure signal and the differential pressure signal itself. For example, as the differential pressure increases by 20% due to a change in flow rate, the standard deviation also increases by approximately 20%. Note that there is some variation in the relationship between standard deviation and differential pressure over the range of flow rates, particularly at low flow rates as the baseline noise becomes a more significant part of the total noise. However, typically the increase or decrease of this standard deviation is less than the increase or decrease in the mean differential pressure value. In some applications, the relationship between standard deviation and the sensor signal can be characterized by a more complex relationship, such as a polynomial with additional terms.

The particular relationship between standard deviation of the sensor signal and the sensor signal itself can be determined for the particular flow measurement technology. Once the relationship is determined for nominal operating conditions, the relationship can be monitored to identify a particular diagnostic condition.

In one particular embodiment, the signal 80 provided to the diagnostic circuitry 76 is the sensor output signal 78, or is directly related to the sensor output signal 78. The noise determination can be made using any technique and is not limited to the standard deviation discussed above. However, in one specific embodiment, the noise is determined based upon the standard deviation. In another example specific embodiment, the noise signal is compared to the mean of the process variable. Such a configuration is advantageous because the calculations are relatively simple. Typically, the noise of interest is in the range of from approximately 1 Hz to approximately 30 Hz. Noise signals lower than this can be difficult to identify due to slow changes in the process. In a further example technique, the diagnostic circuitry 76 includes a high pass filter 79 to filter the process variable prior to calculation of the standard deviation. Any appropriate technique can be used to determine noise in the sensor signal. Examples include a digital band pass filter that provides an RMS value of the noise that can then be related to the mean sensor signal. Another example technique to determine noise includes using a Fast Fourier Transform (FFT), wavelets, etc.

During operation, a nominal relationship between the noise signal and the sensor output signal can be stored in memory. This relationship can be determined empirically by observing operation of the transmitter 16 during normal conditions, or can be determined during manufacture and stored in memory 72. Once the transmitter 16 is placed into normal operation, the diagnostic circuitry 76 calculates the noise in the sensor signal and uses this information along with the sensor output signal and performs a comparison with the relationship stored in memory 72. The relationship stored in memory 72 can be, for example, a polynomial equation, a table of data points, etc. This information can also be updated over process control loop 20 or through other programming techniques.

Based upon the diagnostic output 82, the diagnostic information can be transmitted over process control loop 20. In another example, the diagnostic output Is used to provide a local alarm, correct measurements in the process variable output from transmitter 16, used to schedule maintenance, etc. The diagnostic output can also be used in conjunction with other diagnostic techniques to provide more accurate diagnostic information.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the scope of the invention as defined in the attached claims. Any type of flow measurement technology can be used in conjunction with transmitter 16 including magnetic flow meters, vortex or corioles based meters, etc. For example, sensor 50 can comprise a magnetic sensor for a magnetic flow meter, a vibration based sensor, etc.