The Smart Evolution
Smart is much more than just two-way communication and remote calibration
By Nanda Kumar
In the book Animal Farm, author George Orwell wrote: “All animals are equal, but some are more equal.” When it comes to smart technology in process control, the word smart has been abused more than used in its true connotation.
Smart devices came into commercial use in the late 1980s and early 1990s. The market enthusiastically welcomed the new concept because it offered users features not previously available. Consumers were willing to pay a few extra dollars for these added advantages.
Various industries used smart to differentiate the embedded intelligence available in the new technology from traditional solutions. When applied to computers, smart stands for self-monitoring analysis and reporting technology, which enables the PC to predict the future failure of hard disk drives.
Smart homes offer telephone control, integrated security, safety systems, and automated/independent living through networked devices connected to a central intelligence.
In process control technology, HART protocol became the catalyst and de-facto carrier of intelligent commands and feedback.
In the case of process instrumentation, when smart instruments hit the market for the first time, users were attracted by the power of HART or similar protocols that provided a way for two-way communication and remote calibration, which reduced the time efforts and expenses associated with start-up and maintenance.
Doing away with the walkie-talkie was a paradigm shift for the field technician, even if you ignore all the other benefits of the new technology.
However, as users embraced smart technology for process control, electronics, and IT technology developed on the fast track, user demands grew, pressing vendors to offer more value for the same price.
Thus arrived the second generation of smart devices. These offer much more than just two-way communication and remote calibration. Diagnostic became the key word to differentiate the leaders in the race from the rest.
Three devices contribute to more than 60% of a typical process plant’s total maintenance expense: transmitters, valve positioners, and flow devices.
Most of the smart transmitters available in the market offer the following benefits:
Automatic compensation for ambient temperature and line static pressure changes ensures accuracy of the final output signal.
Characterization of each transmitter by its own individual algorithm developed in simulations of most likely used environments.
No bench checks; pre-configuration to specifications substantially reduces loop verification times.
Compensation for the effects of temperature and pressure in the final output signal all but eliminates errors associated with conventional transmitters.
Constant self-testing significantly improves process reliability and performance.
Minimizing or eliminating drift ensures product quality standards are met.
Local configuration, using the smart communicator, enables on-the-spot re-ranging, calibration, and troubleshooting.
Lower transmitter-database management costs, efficient maintenance, extensive diagnostics, and reduced inventory requirements and start-up costs enable product payback in a matter of days.
A Foundation fieldbus (smart) pressure transmitter can detect plugged impulse lines and is a great feature for the end user, while making root cause analysis of a loop failure. When connected to asset management software, the transmitter generates an alarm to the operator pinpointing the reason for losing the transmitter, thus saving time and effort in determining cause of failure, reducing down time and start-up time.
Benefits of HART in Smart Systems
HART process alerts for pressure and/or temperature reduce trouble-shooting time and unnecessary trips to the field, especially when combined with AMS. In addition, scaled process variables make reading the output more intuitive with the process.
Valves dictating flow
Valve positioners are an area whereby just writing smart, end users get a wide variety of devices. Many of the cost-effective models available in the market provide only auto calibration or just a few more features.
The auto calibration feature is indeed a great benefit. Yet when plant management wants to reduce process variability, reduce down time, increase plant availability and efficiency, and profitability, control valves are the primary target, and advanced smart features help achieve these objectives at a reasonable cost.
The following features of a true smart positioner are available on the market now, and help achieve management objectives in the plant:
Diagnostic tests include the Dynamic Error Band Test. It plots analog I/P signal vs. valve travel from -5 to 105% travel; 8,000 data points resolution; analyzed data available with minimum/average/maximum dynamic error; dynamic linearity; and zero and full range travel.
The Step Response Test plots valve travel vs. time; high resolution of 8,000 data points; tests possible with maximum 30 steps; and analyzed data available for dead band, dead time, and percent overshoot. Overlaying with previous graphs to compare performance is possible.
The Performance Step Test is a preprogrammed 29-point performance step-response test. It provides the user a standardized test to evaluate their valve performance
The Valve Signature Test plots -5 to 105% of valve travel vs. actuator pressure; high resolution of 8,000 data points; and provides analyzed data for friction, bench set, spring range, and seat load.
Online, in-service diagnostics show problems, possible causes, and recommended actions:
Valve friction measurement and trending.
Valve dead band test.
I/P and relay integrity tests can detect problems such as plugging in the I/P nozzle due to dirty air, I/P calibration shifts, and I/P or relay soft parts failures.
Air mass flow tests can detect actuator or pneumatic tubing leaks.
Supply pressure test can detect insufficient air supply or drop in air supply when large travel steps occur.
Travel deviation test can identify valve travel calibration shifts or stuck control valve.
Scheduler feature can automate online, in-service tests.
Functional features for smart flow valving include:
Custom characterization using 21 points with flexibility of varying the X-axis (set point) and Y-axis (travel target) parameters, which provides a highly accurate characteristic curve.
Travel histogram, or bar chart display, determines where the valve sits at most of the process time to identify whether the valve is correctly sized for the application.
Multiple preset tuning sets plus user adjusted and expert tuning.
Performance tuner feature to determine the optimum tuning parameters for the total valve, actuator, positioner assembly.
Batch runner automates multiple tasks like configuration, calibration, and diagnostics, performed on multiple valves with no user intervention.
Event messenger feature allows e-mail notification of alert or alarms to a remote PC or pager.
Data synchronization aligns two databases by LAN or cable.
Audit log holds complete record of all activities by user, tag, event, date, and time. It also sorts and filters data for ISO documentation requirements.
Constructional features of the newer smarter positioners feature three pressure sensors, beam balanced relay, and a fully encapsulated and powerful microprocessor.
They provide a stable, highly reliable platform for the intelligence to deliver the desired results in harsh environment with IS or ex-proof classifications.
Beam balance type relays in smart positioners allow minor loop feedback (MLFB) into the microprocessor control algorithm by way of relay valve stem- position sensing with an additional Hall Effect sensor.
MLFB is the primary damping element, which allows higher forward path gains resulting in faster response and accurate performance.
Spool design relay, available in most low tier devices, has inherent sticking problems and higher dead band. It does not have the capability to dampen the response and could result in oscillations if gain increases to improve valve performance. These offer poor response on large size / high friction valves.
Smart flow devices
Today’s flow-measurement devices come loaded with features that make the best Swiss knife feel shy. They have multiple sensors and microprocessors that convert the sensed values into the most valuable process data available online at the click of a button.
Mechanical smartness still counts as a flowmeter with a v-shaped apparatus in the stream shows, so electronics are not the only way to improve performance in difficult situations.
The V-Cone flowmeter shapes the velocity profile mechanically to reflect a perfect well-developed flow prior to measurement. This precludes long downstream vortices and achieving nearly flat and stable digital signal output, which has low amplitude and high frequency.
The V-Cone (above) forms very short vortices as the flow passes the cone. These short vortices create a low amplitude, high frequency signal for excellent signal stability. The V-cone’s contour-shaped cone attracts the flow without impacting it against an abrupt surface and minimizes headloss. As a result, the beta edge of the cone is not subject to wear by dirty fluids. Because it remains unchanged, V-Cones rarely, if ever, require recalibration. Smart electronics and smart mechanics enable this flow measuring device to render accurate readings when other meter types couldn’t meet standards.
Indeed, it worked where the meter set had an inlet pipe with a 45 degree elbow, 12" pipe upstream, reduced to a 10" pipe, through a gate valve bolted directly to the meter.
A propeller meter had failed in the same situation. The move avoided expensive re-piping.
ABOUT THE AUTHOR
Nanda Kumar (firstname.lastname@example.org) lives and works in Dubai, United Arab Emirates, and has 20 years’ experience in sizing, selection, and marketing flow control and flow measurement devices, especially for the oil and gas industry in the Middle East.
Scintillation technology bests NIST: Optical technology has sensor external to the process and behind windows. http://www.isa.org/intech/050501
Inferred mass flow finally gets respect: Coriolis is best of course, but enhanced old-school technology works. http://www.isa.org/intech/040301
No fire in the hole: Desirable alternative to classic electrical servo control loop for all clean gas and liquid delivery applications. http://www.omac.org/intech/030301
The two phases of Coriolis flow: Exploring problems and solutions in flowtube design. http://www.isa.org/intech/050101
HART Communication Foundation http://www.hartcomm.org