DAP's multitasking RTOS allows in-place valve diagnoses

Despite the advent of direct control via plant-wide PC-based networks, critical process flows are still controlled by valves little changed from 50 or even 75 years ago. Traditional valves persist because they're simple, inexpensive and efficient, but they also retain an equally historical weakness—small changes in friction, caused by galling or uneven wear, can generate resistive f...

02/01/2002


Despite the advent of direct control via plant-wide PC-based networks, critical process flows are still controlled by valves little changed from 50 or even 75 years ago. Traditional valves persist because they're simple, inexpensive and efficient, but they also retain an equally historical weakness—small changes in friction, caused by galling or uneven wear, can generate resistive forces that eventually prevent opening or closing, which may compromise quality or safety. Consequently, maintaining control valves continues to be a crucial job in process applications.

The challenge in maintaining control valves, however, is that they tend to be in plants' dirtiest, wettest, most inaccessible, and most potentially hazardous locations, where traditional diagnostic equipment often can't survive or even fit. As a result, traditional valve maintenance requires regularly shutting down plant sections, removing and testing valves, and then reinstalling them, which can take hours or days, and still risk producing unrepresentative diagnoses.

To help users save time and labor with increased in-place diagnoses and maintenance, Black Diamond Engineering (Sandy, Ut.) recently developed a PC-based tool that evaluates control valves without taking them offline. To protect this in-place diagnostic system, Black Diamond started with an industrially hardened PC with an Intel Pentium III processor running Microsoft Windows in a waterproof, dust-proof, magnesium alloy, NEMA 4-rated enclosure. Its disk drive doors and other openings are covered, and it can withstand a one-meter drop onto concrete without losing data.

Reaching real-time

However, though its PC-based solution was affordable, Black Diamond still had to find a way to make its relatively slower, cycle-gobbling Windows functions available once each millisecond. This is the basic speed needed to measure and control real-time processes, and essentially simulate the control loops that operate valves and collect real-world performance data.

Black Diamond's engineers eventually opted for data acquisition processor (DAP) boards for their in-place valve evaluation system from Microstar Laboratories (Bellevue, Wa.). These DAP boards offer an onboard microprocessor that runs a multitasking, real-time operating system optimized for real-time data acquisition and control. DAPL, the DAPs' onboard operating system, simplifies communications by providing more than 100 commands optimized for data acquisition and machine control. This DAP-based intelligence extends the Windows user interface's power by executing all processor-intensive routines in real-time, and performing data reduction, so the PC's software can handle unusually demanding applications.

Securing, saving data

Meanwhile, Microstar's DAPs allowed Black Diamond's engineers to develop its front-end and data analysis tool—Benchmark Control Valve Diagnostics—without worrying about Windows' ability to service real-time applications.

With nearly 100 test systems units installed worldwide, Black Diamond's compact test system also provides valve record management, valve setup/calibration, pass/fail standards, report generation, database access and a series of air-operated control valve diagnostic tests.

Black Diamond adds there's no danger of losing data, no matter how many computer cycles are dedicated to foreground application, or even if Windows crashes. The data acquisition processor continues to run the special routine that sends control signals to the valve, and then measures its response independent of the central processor.

To examine a valve, Benchmark takes control of it using an analog signal—a digital signal method is now in development—and moves the valve through a series of prescribed motions. The instrument measures diaphragm pressure and displacement to determine diaphragm positioning accuracy and seal integrity. Even when maintenance is required, there's usually no need to remove the valve from the line.

For more information, visit www.mstarlabs.com or visit www.blkdmdeng.com , or visit www.controleng.com/freeinfo .


Author Information

Jim Montague, news editor jmontague@cahners.com




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