Process Noise, Standard Deviation & Mean

Using instruments with Advanced Diagnostics is one of the three main strategies I commonly present to achieve process control and instrumentation reliability.  There is a lot of diagnostic information available about your process that is not currently being captured.  Accessing it allows you to fully develop a Predictive Maintenance (PdM) program for your process.  Basic diagnostics are helpful, but advanced diagnostics should be considered.

When the HART protocol was initially developed is was predominately used to calibrate and configure field instruments.  As it evolved basic diagnostic information was added.  However, basic diagnostics would only report issues with instrument performance, like electronics temperature.   Advanced diagnostics not only monitors the instrument, it monitors the process.

Advanced Diagnostics are realized in pressure transmitters through Statistical Process Monitoring (SPM).  SPM reports the mean and standard deviation of the pressure at the sensor (not just the pressure reported to the control system).  I often use the analogy of cruise control on a car.  SPM monitors what is happening at the tires, not just the speed.  Advanced diagnostics can be used to detect plugged impulse lines, pump cavitation and loss of impellers on a mixer.

Temperature transmitters also provide benefits with advanced diagnostics.  Hot Backup and Sensor Drift Alert dramatically improve process reliability.  Hot Backup will automatically switch from one sensor to another, if an error is detected.  This can be applied with one dual-sensor or two single sensor RTDs.  Because temperature sensors will drift over time, monitoring this drift and providing an alert allows you to keep you process online.

Process level measurement tends to be the most difficult to make reliably.  Guided wave radar level transmitters with advanced diagnostics monitor the ratio of the signal gain compared to the noise gain.  This is known as Signal Quality Metrics.  As the waveguide becomes fouled, the ratio will decrease, as more energy is being absorbed by the contamination.  Once it drops below a certain threshold, it will provide an alert, letting you know the waveguide needs to be cleaned.

Advanced diagnostics are communicated via the HART variables (or Fieldbus registers) and can be used on any host system with digital I/O cards.  If your current host does not include any kind of asset management software, this can be added as a stand alone system.  Naturally, the most critical measurements should be added first, with additional devices added over time.

Your Reliability Resolution for 2012 should include your process control and instrumentation.  Instruments with advanced diagnostics is one strategy to consider.  Engineering Best Practices and New Technology, two other strategies I recommend, will be presented in future posts.  Stay tuned!

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4 Responses to Reliability with Advanced Diagnostics

  1. Bert Mulder says:

    Would it be possible to monitor the membrane of a pressure repeater?
    We are using a 3051S with a pressure repeater consisting of an 99.9% pure solid silver membrane. Due to the properties of the medium, this membrane lasts 8 to 16 months. It would be of great help to be able to monitor the status and predict when to replace the instrument.

  2. Bert: All processes have a noise or variation signature and the Statistical Process Monitoring algorithm detects changes in the signature. I gather from your comment the medium in your process eventually penetrates the membrane of the repeater. It is likely this event can be detected by the SPM algorithm. Since you are currently using a 3051S, I would replace it with a 3051S with Advanced Diagnostics to test it. I hope this helps!

  3. Bert Mulder says:

    Thank you for your quick response.
    I will take it up with our maintenance manager to have a test-unit. If this works, I would like to apply it to several transmitters.
    We now check the membrane three or four times a year, with every time risking to damage the transmitter or coming into contact with the medium. We of-coarse take all necessary precautions, but there is always a small risk.
    Would you know a situation where there is any experience with this kind of application of SPM.

  4. Bert: I am not aware of anyone using SPM for this application. If this is successful, it is worth sharing in an application success story! Please let me know if you need any help with local Emerson support. Thanks!

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