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Detecting Loop Electrical Integrity Issues

by | Mar 8, 2012 | Measurement Instrumentation, Pressure, Technologies

Jim Cahill

Jim Cahill

Chief Blogger, Social Marketing Leader

Advanced diagnostics are improving our lives in many ways from healthcare to transportation. And in our world of process automation, they are fundamentally changing the way we operate and manage our plants. In an earlier post, A High-Speed, Statistical View for Better Decision Making, I shared how statistical process monitoring (SPM) techniques can help spot plugged impulse lines, furnace flame instability, DP level agitation loss, pump / valve cavitation, turbine blade wear, pressure transient detection, and distillation column flooding. These conditions can lead to unsafe situations, unplanned downtime, and equipment failure if not averted in time.

Emerson’s Abhishek Prakash pointed me to an article I had not yet seen, Advanced diagnostics embedded in a pressure transmitter can detect integrity issues. It highlights an electrical Loop Integrity Diagnostic in Rosemount 3051S Series HART transmitters.

Rosemount 3051S Series Loop Integrity DiagnosticAbhishek noted that the Loop Integrity Diagnostics monitor the voltage between the terminals of the transmitter and sends notifications of unwanted changes on the electrical loop. Any changes in the impedance in the electrical loop caused by degradation of wiring, corrosion of terminals, water in terminal housing or an unstable power supply will cause the current-voltage relationship of a 4-20mA loop to deviate from its normal levels. If this deviates beyond a pre-set limit, the transmitter sends out a HART alert, which can be captured by the AMS Device Manager software or DeltaV system.

The article’s authors, Emerson’s Erik Mathiason, Jessica Lo, and Natalie Strehlke, describe how the diagnostic works:

Power advisory diagnostics monitor transmitter terminal voltage and sends notifications of unwanted changes on the electrical loop. Characterisation of the electrical loop is completed when the diagnostic is enabled.

The transmitter then automatically drives the terminal voltage to its low (20 mA) voltage and high (4 mA) voltage outputs. The characterisation constructs a linear line between the two points… and sets high and low voltage deviation limits that are parallel to that line.

In this blog’s safety category, are numerous posts about safety coverage, probability of failure on demand (PFD), and Failure Modes, Effects and Diagnostics Analysis (FMEDA). For the 3051S Series of transmitters, the authors highlight the:

…industry’s highest diagnostic coverage with a Safe Failure Fraction of 96.7 percent and the lowest dangerous undetected failures, resulting in the lowest PFD transmitter on the market. To achieve this rating, a number of internal transmitter focused diagnostics were added, such as power consumption and mA output. This transmitter can detect external failures that occur in the electrical loop and process that may otherwise go undetected. Electrical loop diagnostics is where the transmitter learns the voltage characteristics of the loop and alerts the operator if something changes.

To see the diagnostics in action, visit the RosemountMeasurement YouTube channel and watch the video, Ensuring Electrical Loop Integrity with Rosemount 3051S Pressure Transmitter.

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The opinions expressed here are the personal opinions of the authors. Content published here is not read or approved by Emerson before it is posted and does not necessarily represent the views and opinions of Emerson.