Interested in linking to "HMIs evolve in process control"?
You may use the Headline, Deck, Byline and URL of this article on your Web site. To link to this article, select and copy the HTML code below and paste it on your own Web site.
Operating Display Problems
Graphical shortcomings have contributed to human errors and incidents, some with severe consequences. For example, regulatory inspectors identified several HMI-related issues as contributing to the July 1994 explosion and fire at the Texaco Pembroke refinery in Wales, U.K. Under the fluid catalytic cracking unit (FCCU) control system that existed on the day of that incident, any imbalance in liquid flow through the FCCU could lead to liquid accumulation. Therefore, it was important that any imbalance in liquid flow be detected, so the mass flow of the unit could be returned to a balanced position. The plant was well equipped with alarms, which showed where liquid was accumulating, but it was more difficult to assess the relative flows through the vessels and the overall mass balance of the unit. The process of fractional distillation requires that one raw material be divided into many fractions.
While it was easy to assess the unit feed rate, the various outputs of the process were spread over five product streams. This caused a practical problem because the accumulated outputs of the system may be spread across several different control display units, and the overall output of the unit wouldn’t be readily apparent unless the control system were configured to meet this need.
Unfortunately, this need reportedly wasn’t met at Texaco Pembroke. There were no displays providing an overview with an appropriate time scale on the FCCU. Therefore, it was difficult to obtain a complete picture of the whole or large sections of the process. In a mostly display-screen-based operating system, the provision of good overview displays is especially important because the operator doesn’t have a continuously available set of panel indicators.
During the incident, no one from the operations department had a complete picture of the FCCU. The actual FCCU graphics on the operating displays weren’t best designed or configured to help operators control the process. The operating graphics on the FCCU contained limited amounts of process data per graphic, and didn’t use color and intensity to highlight process data. Some graphics contained details of the internal structure of plant items. However, displaying the structure of plant items is only useful if measurements or derived information, such as pressure, temperature and flow, also are displayed to give the operator information relevant to plant status. At times, the text was unnecessary in the FCCU graphics. Text takes up large amounts of space on a graphic, and there were instances where the same information could have been better indicated by color change, according to U.K. Health and Safwety Executive (HSE) investigators, reporting on the disaster in "Investigation into the Explision and Fires on the Pembroke Cracking Co.'s Plant at the Texaco Refiner."
The HSE investigators' recommendations on human factors are of immediate interest in designing HMIs, especially the report's Recommendation #3: Display systems should be configured to provide an overview of the condition of the process including, where appropriate, mass and volumetric balance summaries.
Other studies into these types of human errors have identified that 30% of errors are attributed to the operator not becoming aware of an abnormal situation due to information overload, vague or misleading information, inappropriate levels of detail and operator vigilance decrement. Further research identified that an additional 20% of errors were attributed to the operator’s inability to identify the root causes in a timely manner due to insufficient knowledge, lack of operator experience, conflicting priorities, inaccurate labeling or information presentation, and excessive mental tasks. Many of these issues can be resolved by a well designed HMI.
Many of these traditional problems were initially introduced by poor guidelines from DCS manufacturers and even worse project implementation practices by vendors. Fortunately, vendors have continued to evolve the DCS, and now the graphics are high resolution, and are becoming web based. Unfortunately, the vendors again haven’t invested in human factor education (See Figure 4 below), and now promote 3-D objects that take over 60% of the screen. This means that 60% of the screen’s fixed data is unfortunately in the foreground, while real data useful for detecting and diagnosing abnormal situations is shoved to the remaining 40% of space in the graphic’s background.
FIGURE 4: HMI DESIGN MISTAKES REDUX
A lack of human factor education had led HMI makers to promote 3-D objects that fix more 60% of a screen’s data in the foreground, while real data useful for detecting and diagnosing abnormal situations shoved into the remaining 40% of the graphic’s background.
Ergonomics Can Resolve Operator Errors
In addition, the process industry hasn’t responded to the human error problem by following guidance that’s now available for alarm management. EEMUA, the organization that produced the well known "Alarm Management Guidelines," Publication 191, has also produced a similar guideline for graphics called "Process Plant Control Desks Utilizing Human-Computer Interfaces: a Guide to Design, Operational and Human Interface Issues," Publication 201. This document hasn’t had the same publicity or exposure, and needs further detail, but it’s a great start to improving the standards and current HMI or HCI practices. As knowledge and papers available on this subject are examined, a researcher will discover it’s not that information is lacking.
ControlGlobal.com is exclusively dedicated to the global process automation market. We report on developing industry trends, illustrate successful industry applications, and update the basic skills and knowledge base that provide the profession's foundation.