Data Acquisition Processor Helps Provide In-Place Diagnosis of Control Valves

A data acquisition processor is one of the keys to the ability of a new instrument to provide in-place diagnosis of control valves. In-place diagnosis is a major breakthrough because removing a control valve in a process plant for testing in a repair shop is expensive: it usually requires shutting down process equipment for hours or even days. The new instrument makes this unnecessary by providing a hardware platform that can operate right next to the valve, withstanding rugged plant conditions and performing diagnostic tests in a matter of minutes. A key to the performance of the new instrument is the use of a data acquisition processor that runs mission-critical real-time operations independent from the non-real-time graphical user interface. This approach makes it possible to deliver the performance and reliability of high-end dedicated hardware while using relatively inexpensive industry-standard personal computer hardware and software.

Benchmark Control Valve Diagnostics

In-place diagnosis is a major breakthrough because removing a control valve in a process plant for testing in a repair shop is expensive: it usually requires shutting down process equipment for hours or even days. The new instrument makes this unnecessary by providing a hardware platform that can operate right next to the valve, withstanding rugged plant conditions and performing diagnostic tests in a matter of minutes... [while it] provides the economy and ease of use of industry-standard hardware and software along with true real-time data acquisition and control. It takes control of the valve... and moves it through a series of prescribed motions. It measures diaphragm pressure and displacement in order to determine diaphragm positioning accuracy and seal integrity. ...The result is a dramatic reduction in downtime and maintenance expenses.

[Benchmark Control Valve Diagnostics, developed and sold by Black Diamond Engineering, Inc., includes a powerful user interface under Windows that never compromises the time-critical parts of the application.]

Process plants have seen a dramatic shift in their control architecture, with advanced control strategies being implemented through plant-wide networks of computers operating as direct control systems. But, where the rubber hits the road, critical process flows are still controlled by valves that differ little in function or appearance from those in plants that operated 50-75 years ago. The same diaphragm, with a spring applying constant pressure in one direction and pneumatic pressure being applied in the other to open the valve, is still used to control the flow. This basic design has persisted because it's simple, inexpensive and efficient. Yet, control valves have a basic weakness. Small changes in friction, caused by galling or an uneven wear pattern, can generate resistive forces of a magnitude that prevent the valve from opening or closing. The results, depending on the flow that is being interfered with, can range from a small reduction in quality to a serious safety hazard. As a result, maintaining control valves is one of the most important jobs in a modern process plant.

Challenges in diagnosing control valves

The challenge is that control valves tend to be in the dirtiest, wettest, hardest to reach and often most potentially hazardous locations in the plant. Traditional diagnostic equipment won't survive. In fact, in many cases it won't even fit in these locations. For that reason, the traditional approach to valve maintenance is to shut a section of the plant down on a regular basis, remove valves and take them to the repair shop for evaluation, make repairs if necessary, return them to their original location and reinstall them. This process typically takes hours and can even take days in the case of valves that are particularly difficult to install or diagnose. While shutoffs exist to cut off flow while the valve is off line, the danger of an environmental leakage is always present when a line is broken. Finally, there is the basic problem that exists in attempting to diagnose anything in an unrepresentative setting. When you remove the valve and put it in a different setting, you can never be totally certain that it is operating exactly as if it were in the field. For example, there may be a problem with a control loop that would cause the valve to fail in operation but not prevent it from working perfectly in the repair shop.

In an effort to solve this type of problem, Black Diamond Engineering, Sandy, Utah, has developed a control valve diagnostic system capable of evaluating the performance of control valves without taking them off line. This saves the plant downtime and the time of skilled maintenance personnel required to remove them and take them to the repair shop. Just as important, the ability to diagnose control valves on line provides a far more certain analysis because it avoids a possible misdiagnosis that can result from removing the valve from its working environment. Finally, eliminating the need to remove the control valve except for cases where it truly does require repair can greatly reduce the possibility of environmental leakage by maintaining the integrity of the process line.

Overcoming major design obstacles

Black Diamond had to overcome several major design obstacles in developing their diagnostic instrument. The first was developing an instrument with the ability to survive in difficult conditions where control valves often are found: confined, wet, hot and exposed to corrosive materials. The developers of the device overcame this obstacle by making use of a hardened computer in a waterproof, dustproof, magnesium-alloy NEMA 4 enclosure that covers disk drive doors and all other openings. This computer is rated for the harshest industrial environments and will withstand a one-meter drop onto concrete without loss of data. In order to maintain a reasonable price, Black Diamond Engineering selected a personal computer that uses an industry-standard architecture, including an Intel Pentium III processor, and runs the Microsoft Windows operating system.

But the use of an industry standard solution raised the issue of how to deliver the required real-time control and data acquisition capabilities. Essentially, the instrument simulates the control loop that normally operates the valve while collecting data that evaluates its performance under real world conditions. In measuring or controlling a real-time process, computing power has to be available when it is needed, every millisecond or so. The problem is that Windows is not a real-time operating system. Operating systems like Windows use up a lot of cycles on the PC platform, and when they take control of the CPU they hold onto it for a long time in machine control terms. This means that if data acquisition and control were to run under Windows, the unit would be vulnerable to a situation where Windows was occupied with other tasks which would make it impossible to provide reliable test results.

Keeping the instrument affordable

Black Diamond engineers searched for an alternative that would help to keep the cost of the machine affordable. Then they discovered data acquisition processor (DAP) boards for the PC platform from Microstar Laboratories, Bellevue, Washington. The unique capability offered by DAPs is an onboard microprocessor that runs a multitasking, real-time operating system optimized for high-performance real-time data acquisition and control applications. DAPL, the onboard operating system used in DAPs, simplifies communications by providing over 100 easy to use commands optimized for data acquisition and machine control. This intelligence on the DAP board extends the power of the Windows user interface by executing all processor-intensive routines in real time and performing data reduction so that the software on the PC can handle more demanding applications than usual. Onboard intelligence led the engineers to choose a DAP for their instrument.

The key advantage for Black Diamond engineers in developing their instrument is that they are able to develop a powerful front end and data analysis tools without worrying about the ability of Windows to service the real-time application. The user can even run other programs such as a spreadsheet to analyze data or a word processor to prepare a report while the tests are underway. There's never any danger of losing data regardless of how many computer cycles are dedicated to the 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 measures its response totally independent of the central processor. Microstar engineers worked with Black Diamond to customize their board to meet special packaging constraints. They modified the layout so that the board could be shortened to fit special hardware.

Real-time acquisition and control on the plant floor

The resulting instrument, called Benchmark Control Valve Diagnostics, provides the economy and ease of use of industry-standard hardware and software along with true real-time data acquisition and control. It takes control of the valve, using an analog signal (a digital signal is in development), and moves it through a series of prescribed motions. It measures diaphragm pressure and displacement in order to determine diaphragm positioning accuracy and seal integrity. When the valve is performing within specifications, as in the vast majority of cases, there is no need to remove it from the line. Even in situations where maintenance is required, it can usually be accomplished without removing the valve. Only where serious problems exist does the valve need to be removed for repair. The result is a dramatic reduction in downtime and maintenance expenses.

Black Diamond Engineering has proven that first-class field and shop valve testing doesn't have to be difficult and time consuming. The company has nearly 100 units installed around the world. The compact test system provides comprehensive valve record management, simplified valve setup/calibration, pass/fail standards, automatic report generation, database access and a complete series of air-operated control valve diagnostic tests. A complete line of powerful supporting software products is also available. This application shows the power of a new generation of processor-independent data acquisition and control devices for the PC platform that brings real-time functionality at a far lower price point than was ever possible in the past.