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01 January 2004

Advance from reactive and preventive to predictive

Connect devices from multiple suppliers to a single asset management system.

By Mark Menezes

When combined with open digital protocols and asset management software, advanced transmitter diagnostics allow users to not only identify device component failures, but also to diagnose the performance of the complete measurement system. These performance diagnostics allow users to evolve from reactive and preventive to predictive maintenance, minimizing maintenance effort and maximizing uptime.

After diagnostics have identified and isolated a problem, new best practices can minimize future occurrences.

Open and asset facilitate

Diagnostics in field devices are of limited utility unless the user can economically and conveniently access diagnostics alerts and information. For example, how useful is a diagnostic that detects plugged impulse lines if the diagnostic only annunciates at the transmitter faceplate?

How much more useful is that diagnostic information if it registers at the maintenance shop PC along with the online user manuals and context-sensitive remedial actions?

Digital protocols allow a field device to transmit additional information-including diagnostics-on the same pair of wires that carry the process variable. This minimizes incremental cost. By using only open protocols, such as HART and Foundation fieldbus, the user can connect devices from multiple suppliers to a single asset management system (AMS), consolidating all diagnostic information. This not only minimizes

capital, installation, and training costs, but also simplifies maintenance and streamlines regulatory compliance. HART field devices can connected in parallel to both a legacy distributed control system (DCS) and the AMS. HART is a hybrid protocol-the process variable is the 4-20 mA signal, while additional information such as diagnostics are digitally superimposed at a high frequency. Although the AMS is capable of stripping out and interpreting this digital information, the legacy DCS is unaffected by the digital signal, treating it as high-frequency noise.

Meter faults, noise, failures

Magnetic flowmeters-"magmeters"-are widely used in the food & beverage and other process industries to measure flows of water and water solutions. They offer significant advantages when compared with other flow technologies-cost effective, obstruction-less, widely applicable, reliable, accurate. Most liquid applications also fall within magmeter limitations-low-to-medium pressures and temperatures, conductive liquids, and slurries.

In any magmeter application, the objective is to maximize the signal, while minimizing extraneous noise. The most common sources of noise in magmeter applications are faulty grounding, high process noise, and intermittent electrode failure. Unfortunately, from the flow signal alone it is impossible for a user to distinguish between noise from these three causes. More importantly, it is also impossible to distinguish between a noisy flow signal and a truly noisy flow rate. So, extraneous noise can mask a genuine increase or decrease in flow variability. Severe noise in control applications can lead to unnecessary valve travel and wear, so the user may be tempted to apply excessive damping, further masking true process variability and reducing control effectiveness.

What causes faulty grounding? Magnetic flowmeters utilize Faraday's Law, measuring the electric field generated by a conductive fluid moving in a fixed magnetic field. To ensure that any electric potential is due solely to the Faraday effect, the fluid must somehow ground to ensure that it has zero electric potential upon entering the flowmeter. A grounding strap, a grounding ring, or a dedicated grounding electrode can accomplish this. Unfortunately, it is common for the integrity of a ground to lapse over time-a wire can break, a connection can coat over or corrode. When this happens, power supply hum can enter the system, resulting in a noisy flow signal.

Process noise occurs when:

• the fluid contains high solids or entrained bubbles, common with pulp stock or mining slurry flows
• chemical reactions that generate electric potentials occur in the process fluid, common with metal slurries or chemical additives
• the fluid contains large particles or rocks that physically contact or rub the electrode, again common with pulp stock or mining slurries

Although the user cannot distinguish a noisy flow signal from a truly noisy flow rate, the microprocessor in the smart magmeter transmitter can by analyzing the frequency of the noise. Direct-current magmeters drive their coils at a fixed frequency-for example, 6 hertz. The microprocessor will primarily scan this drive frequency, but it can also periodically check for noise at other frequencies. For example, signals at lower frequencies are typically the result of process noise, while a faulty ground will cause a signal at a frequency of 50-60 hertz.

While the remedial action for a faulty ground is fairly straightforward-fix the ground-it is not so obvious for high process noise. In some applications, the user can try to operate at a higher drive frequency, for example 30 hertz, to avoid the noise. However, if noise remains high, the best solution to achieving an acceptable signal-to-noise ratio is to increase the signal itself by replacing the existing magmeter with a high signal magmeter. These devices incorporate much heavier windings in the flow tube, and as a result can handle a much higher coil current from the transmitter-as much as ten times higher. High signal mags provide a very strong signal, requiring minimal damping, even in applications with very high noise. In one case at Falconbridge Mine in Ontario, a high signal magmeter worked successfully in an underground reticulation system operating with an 82% to 84% solids paste.

In any magmeter, the two electrodes used in the flow tube should provide a signal with equal amplitude, yet opposite polarity, because they are located directly across from each other. Unfortunately, leakage of process fluid or moisture into an electrode terminal compartment will cause the electrode to short intermittently, causing a noisy signal. Eventually, the electrode will fail entirely, causing a consistently low reading. Similarly, some process conditions can cause intermittent or permanent coating of the electrodes. Again, without a diagnostic or some independent reference, the user may not be aware of this problem-especially serious in safety, quality, or environmental applications.

Detect plugged impulse line

A pressure transmitter connects to the process by sensing-impulse-lines and a valve manifold. If the transmitter fails, the user can simply isolate the transmitter using the valves in the manifold, and replace it. However, modern microprocessor-based pressure transmitters are extremely reliable, and transmitter failures are increasingly rare. Not so rare are failures in the impulse lines themselves, particularly plugging and freezing. A plugged or frozen impulse line causes slow process response-a rapid increase in pressure may take minutes to register at the transmitter, preventing the control system from responding to process variability.

To detect plugged or frozen impulse lines, the smart differential pressure (DP) transmitter must continuously monitor the very rapid fluctuations in pressure caused by fluid turbulence. Although these variations are too small and fast to register at the DCS, the sensor readily detects them at the process. Because the diagnostic takes place in the transmitter itself, the update rate to the DCS is not important, but a fast sensor-100 milliseconds or less-is vital. Many pressure transmitters are too slow by a factor of two to five.

To perform this diagnostic, the transmitter must learn the process and characterize an OK condition over the operating range. Once the process is learned and the diagnostic is online, a change in short-term variability will alarm at the AMS screen.

If the impulse lines frequently plug or freeze in a given application, the user should try to eliminate or minimize these lines. Historically, users mounted their transmitters at grade for easy maintenance access, typically necessitating long impulse lines. The newest smart transmitters, however, have been specifically designed to facilitate direct mount in that they have extremely long stability (more than ten years before requiring recalibration); extremely high robustness and reliability; immunity to zero shift, and hence there is no need to rezero; and optional separation of sensor and liquid crystal display/terminations, allowing the user to mount the sensor directly at the process and the operator interface and field terminals at grade.

Vortex favored over some

In applications where reliability and safety are key, some users prefer vortex flowmeters to DP flowmeters. Vortex flowmeters use a bluff body-called a "shedder bar"-to generate vortices (the von Karman effect), and measure the frequency of vortex formation. Vortex meters do not require periodic maintenance, are easy to install correctly, and offer good accuracy over very wide flow ranges. The latest generation vortex flowmeters have no ports, crevices, or gaskets. As a result, they are much more reliable than older designs, especially in dirty or corrosive fluids, or in applications with wide swings in process temperature. They are also mass balanced, with dynamic signal processing, for improved resistance to vibration and pulsation.

Because new design vortex flowmeters rarely suffer from plugging or vibration, the most common remaining problem is misapplication. If a vortex meter sizes and configures for a specific set of flow conditions-flow rate, viscosity, density-but actual conditions turn out to be very different, the meter may not provide a reliable flow signal. Performance diagnostics can isolate application problems and help the user reconfigure the meter for the new application conditions.

There are of course applications where vortex technology will not work at all-for example, slurries and high viscosity fluids-but most problems resolve by either changing the meter configuration and filter settings or by changing the meter size. A smaller meter will read lower flows, while a larger meter will read higher flows. Also, for a given flow, a smaller meter will produce a stronger signal, while a larger meter will minimize pressure loss.

Changing the size of an installed meter is unfortunately very expensive. Not only must the meter itself swap out, so must the required straight up- and downstream pipe. Because this straight pipe must be of the same diameter as the meter, replacing a meter with one of a different size requires the user to remove existing insulation, cut the pipe and weld in a reducer and expander, X-ray the welds, and reapply the insulation.

This will not only be expensive, but will require a minimum twenty-four-hour shutdown. Instead, users should take advantage of newer reducer technology. A reducer vortex meter includes built-in pipe reduction and expansion, and the supplier has flow calibrated it for no sacrifice in accuracy. As a result, the user can install either a 6-inch or a 4-inch vortex meter in a 6-inch line, with no changes to piping, and can easily switch between them if the optimum flow range changes.

Temperature drift alert

In critical temperature applications, the user can install a redundant sensor (RTD). The transmitter will compare the two RTDs, and alert the user if they drift apart. If one RTD fails entirely, the transmitter can switch its output to the other transmitter.

Although users have done this for years with two traditional transmitters, the uncertainties in the system-different sensors, transmitters, DCS inputs-can be ±2°C or worse at commissioning. Of course, the user cannot detect a drift smaller than this uncertainty, limiting confidence in the measurement. Today, a single smart transmitter can connect to a dual-element RTD, eliminating most of these sources of uncertainty. If the system matches the sensors to the transmitters using Callendar van Dusen constants, the sensitivity of the drift detection can improve to ±0.5°C.

We are talking here only about diagnostics currently available in transmitters. Others exist and more are planned for other field devices and are also available through the AMS, including DP level for the detection of damaged or leaking remote seals, pH for the detection and prediction of faulty or drifted electrodes, Coriolis flow for the detection of slug flow or tube coating, and valves that can detect stiction and wear that may have progressed to the requiring rebuild stage.

Diagnostic capabilities can also be located in the asset management software, which can combine alerts from multiple field devices to provide new, loop-based diagnostics.

All these performance diagnostics and best practices can provide significant improvements in process availability. To ensure that they receive maximum benefit from advanced diagnostic capabilities in their field devices, users need to use open protocols such as HART or Foundation fieldbus.

Also, those wishing to leverage this technology should gain experience with asset management software, make sure that any control system they are considering can fully utilize advanced device diagnostics, and communicate with their suppliers.

What diagnostics are available now and what is on tap for the future? MP

Behind the byline

Mark Menezes has degrees in chemical engineering and business. He has fourteen years of experience in process automation and is a past vice president of the Toronto ISA Section. He works at Emerson Process Management as the measurement business manager for Canada. Write him at mark.menezes@emersonprocess.com. This article is from a larger piece that he wrote on diagnostics. Read the entire piece including a brief case study at www.isa.org/intech/modernprocess/diagnostics.