01 November 2002
The human touch
Analytical process predicts, prevents operator errors.
By Ellen Fussell
When it comes to preventing hazardous events in process industries, analyzing and pinpointing control points are crucial to avoid catastrophe. One obvious crucial control point is the operator.
Human intervention can be a savior in the control process or wreak havoc-if an operator is tired, for example. In fact, human error was the most common cause of accidents in transportation and chemical industries, as ility Engineering in Tampere, Finland, revealed in its February 2002 annual report on accidents in chemical and process industries. (See October InTech, page 132, for details on the study.)
Analyzing how to prevent hazards relies to some extent on predicting human error and developing processes to prevent it. The operator's ability to keep the process safe relies on training, process feedback, and his general condition-morale, workload, and fatigue-said Edward Marszal, principal engineer at Exida in Sellersville, Pa.
In a generic operator response situation, an operator would first read an alarm message, diagnose the root cause of the alarm, and then perform the response action. It's possible, Marszal said, that in the very first task, reading the alarm, the operator could accidentally read the wrong tag number or description from the alarm display. If this happens, the sequence that follows will be wrong; the operator will diagnose the wrong problem and will not arrive at the correct solution.
HUMAN RELIABILITY ANALYSIS
Marszal is a proponent of one hazard analysis technique called human reliability analysis (HRA) to pinpoint the root cause of failures and prevent them. HRA begins with analyzing tasks such as who's responsible for performing and approving tasks, describing the task they're performing, and the available time required to perform the task.
The HRA is a process of calculating the probability that the operator will not perform a task properly. The result of the HRA is the human error probability (HEP), or the probability (say, 1 in 10 or 1 in 10,000) that the operator will fail to perform the task correctly. "A full HRA is a long and involved process to calculate the HEP that requires you to look at what you're asking the operator to do and break it down into a long list of steps," Marszal said. One method of breaking down this sequence of events that can result in human failures is called an event tree (see related story, page 38).
The operator needs to look at an alarm, press a button, and decide which switch to turn. "With each one of these events you'll be able to get the probability from a database that the operator will fail to do it properly," Marszal said. "Then that long string of probabilities is compiled into one number based on how they're logically related. That number is the HEP for that task."
One example of such a calculation might occur in a refinery, where pumps feed the surge drums-a holding tank that holds material to smooth out the flows. If the operator lets the tank run empty, this creates a dangerous condition. When the level gets too low, an alarm goes off.
So acknowledging the fact that there's an alarm is the first step, then reading the alarm properly and getting the right tag number is the second, and making the decision of the root cause of that alarm is the third step, Marszal said.
HAZARDS IN FOOD PROCESSING
"A big part of ensuring food safety is the observation of the operator and the people on the plant floor," said Paul Moylan, process solutions marketing manager at Rockwell Automation in Cary, N.C. "They're the ones responsible for noticing conditions that might threaten food safety and taking corrective actions," he said.
Rockwell helps minimize human error by using consultants to analyze control points. The Food and Drug Administration and United States Department of Agriculture are enforcing a hazard management technique for the process control industries called hazard analysis for critical control points (HACCP). They have identified segments in food processing-seafood, meat, poultry, and juice-that point to the most critical control points.
All food processing will eventually have to meet HACCP, Moylan said. "It says typically you would get a consultant to evaluate your food production process and ensure all the good manufacturers' sanitary practices are being met-regulations and guidelines on cleaning and manufacturing," he said.
Once the HAACP team is established, companies such as Rockwell evaluate where there's a threat to food safety in the production process, such as microbiological contamination. One threat could be the temperature of a storage location. It could be a potential for metal contamination being introduced into a cookie.
In assessing a company's hazardous areas, a consultant would go through and create a HACCP team to evaluate the critical control points and put together a plan, say, in food storage temperature. "Once you identify the critical control points and the threats to food safety, there's an ongoing partnership that will ensure you're following procedures," Moylan said.
Another way Rockwell complies with HACCP regulations is by electronically managing manual work instruction. Tools such as e-procedure allow the electronic creation of standard operating procedures and work instructions to automate sequencing, Moylan said.
"If an operator observes a fan over an open mixer that's dropping metal shavings into the mixer," Moylan said, "he enters that information into a computer terminal that sequences through a series of instructions to be carried out to fix the problem. He should notify maintenance to fix the fan and recommend enclosing the mixer. That observation is stored electronically and stores the instructions they followed to resolve the problem."
The electronic signature shows who responded to the observation, more efficiently meeting requirements of the company's critical control points and who's authorized to carry out corrective actions.
"Any time you try to put new policies and procedures in place, you measure its success by its rate of adoption," Moylan said. "The faster you adapt and adopt the more successful you are."
To minimize human failures and adapt to new procedures such as electronic recording, Moylan said, it's crucial to make the transition easy, with as little change as possible to the existing interface. "If today I push a button to start a machine, tomorrow I don't want to have to navigate through five HMI screens and look for the green button," he said.
"It's OK to give me an HMI screen when I see the same picture, and the button looks like it did before. Yesterday I had a manual button, and today I have an electronic button, but it still looks the same," he said.
The key is to minimize the impact of the daily routine and functions that people go through. It's impossible to make change without impacting that, he said. "Try to minimize how much you change, but you have to change, so you'll still need to take training into account." IT
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