Increase human reliability
Human factors considerations are relevant to all aspects of the design and operation of process plants. The topic is, however, a vast one and the account given here is necessarily limited. The approach taken is to consider in particular the process operator and then to touch on certain other aspects such as communications, maintenance and construction.
- Human Factors in Process Control
Although modern control systems achieve a high degree of automation, the process operator still has the overall immediate responsibility for safe and economic operation
of the process. There are different philosophies on the extent to which the function of safety shut-down should be removed from the operator and assigned to automatic trip system. In general, the greater the hazards, the stronger the argument for protective instrumentation. This question is considered in more detail later. But, whatever approach is adopted, the operator still has the vital function of running the plant so that shut-down conditions are avoided.
The job of the process operator is therefore a crucial one, but it is also rather elusive in that it presents the engineer with a type of problem with which he is not normally required to deal. The study of industrial jobs and work situations is the province of ergonomics or, its American near-equivalent, human factors. It is appropriate, therefore, to consider the contribution which this discipline can make to the problems associated with the work of the process operator.
It should be said, however, that the chemical industry in general appears to make little use of human factors in this area. There is considerable willingness to do so amongengineers, but perhaps also some lack of appreciation of the scope of human factors as a discipline.
- Human Factors in System Design
The pace of technological change and the scale of systems have now become so great that it is often not possible to rely on evolutionary trial and error to achieve the proper adaptation of human tasks. Instead, it is necessary to try to foresee the problems and to design to overcome them. The discipline that is concerned with this on the human side of systems is human factors.
The development of human factors has been strongly influenced by the problems of large, complex man-machine systems that occur in the fields of defense, aerospace andcomputers. Much of the fundamental research and design experience are in these areas.An important area of study in the early work on human factors was the compatibility of man and machine, with its emphasis on ‘knobs and dials’. More recent work has laid a greater stress on system design. In consequence, the ergonomist has concerned himself increasingly with all stages of the design process, particularly the early stages where the crucial decisions are made.
Figure 1. Human factors activities in system design (Lees, 1974c) (Courtesy of Taylor & Francis Ltd)
Human factors is now established, therefore, as an aspect of systems engineering. An outline of the human factors activities in system design is shown in Figure 1. Two main points may be noted: (1) human factors play a role at all stages of the design; and (2) the decisions taken early on, such as those on allocation of function, are especially important and the design process is a highly interactive and iterative one. Only one iteration loop is shown, but in fact iteration occurs at all stages of the design process.Important emphases in human factors, therefore, are the system criteria and the system design process.
The engineer unfortunately frequently misunderstands human factors. The view of the subject as being concerned with knobs and dials is entrenched. The application of human factors is too often ineffective because it is called upon too late in the day to fulfill a rescue or cosmetic function. Its greatest contribution should in fact come earlier, particularly at the allocation of function stage.
- Themes in Human Factors
Much of the early work on human factors was concerned with physical tasks, but in more recent years, the emphasis has been increasingly on mental tasks. This is certainly more relevant as far as the process operator is concerned, since his job is essentially decision-making. Table 1 shows themes get involved in human factors.
Table 1. Selected topics in human factors
Information sampling
The question of the sampling and processing of information by the human operator is of great importance. Work in this area has emphasized: the ability of man to accept information coming through many sensory channels and coded in many different ways, and his ability to compensate for errors in the information; the differences in the amounts of information which can be handled by the various channels; the sampling of information and the updating of his mental model of the environment; the effect of information overload, resulting in selective omission of parts of the task; and the characteristics of memory, particularly short-term memory such as is exercised in remembering a telephone number to make a phone call.
Skill
The ergonomist’s approach to a particular job tends to be to enquire into the skill involved, including the nature of the skill, its acquisition through the learning process and its disintegration under stress.
Stress, fatigue
The effects on skill of various forms of stress such as fatigue, workload and anxiety, have been investigated both on account of the importance of these effects in themselves and of the light which they throw on the nature of skill as it degrades under stress. An important finding is that skilled performance tends to improve with moderate stress, but that beyond a certain threshold, which varies greatly with the individual, it deteriorates rapidly.
Decision-making
The characteristics of human as opposed to mathematically optimal decision-making have been studied. Interesting results are man’s tendency to make decisions that are based on rather small samples, that is to jump to conclusions, and to make decisions that are biased towards optimism, that is to gamble on beating the odds.
Display
Much work has been done on displays, in terms both of detailed design of dials, etc., and of the display layout, and it is perhaps this aspect which the engineer most readily identifies as human factors.The classification of the uses of displays is listed as follows:
(1) Indicating.That is the operator perceives one of two binary states.
(2) Quantitative reading,that is the operator requires a precise numerical value.
(3) Check reading, that is the operator requires confirmation that value lies within an acceptable range.
(4) Setting, that is the operator manipulates his machine controls in order to achieve a predetermined display state.
(5) Tracking, that is the operator carries out an on-going control task in order to achieve certain display conditions which may vary as a function of time.
These are very different uses and a display which is optimal for one is not necessarily so for another. There is some work which suggests that some 75% of industrial applications is accounted for either by check reading alone or by check reading and setting combined.
Control-display relations
Control-display relations can be important. In a given culture, there tend to be expectations of particular relations between control movements and display readings. Atypical stereotype of a controldisplay relation is shown in Figure 2. Although an operator can be trained to use equipment that embodies faulty controldisplay relations
in its design, he may tend under stress to revert to the expected relation. Violation of the stereotype can result in severe penalties.
Figure 2. Expected relationships between control and display movements (E. Edwards and Lees, 1973) (Courtesy of the Institution of Chemical Engineers)
The problems of man-computer systems have been studied quite extensively. These include allocation of function between man and machine, man-computer interaction and
man computer problem-solving. For many functions the emphasis has moved away from early attempts at complete automation towards computer-assisted operator decision-making.
Man-computer systems
A great deal of progress has been made toward improving and evaluating the reliability of hardware systems; however, the place where systems most frequently fail is in the interface of humans with the system. Human reliability is generally much lower and more difficult to control than hardware reliability.
- Approach to human error
The traditional approach to human error has been in terms of human behavior, and its modification by means such as exhortation or discipline. This approach is now being superseded by one based on the concept of the work situation. This work situation contains error-likely situations. The probability of an error occurring is a function of various kinds of influencing factors, or performance shaping factors.
The approach to the work situation has itself undergone development. Three phases may be distinguished. In the first phase, the concern was with error-likely situationsand performance shaping factors in general and on the application of ergonomic and human factors principles. The second phase saw greater emphasis on cognitive anddecision-making aspects of the task. The third phase seeks the root causes in the organizational and, more generally, socio-technical background. The approaches taken to human error may therefore be summarized as:
(1) behavioral approach;
(2) work situation approach:
a. general work situation,
b. cognitive features,
c. organizational features.
Any approach that takes as its starting point the work situation, but especially that which emphasizes organizational factors, necessarily treats management as part of the
problem as well as of the solution.
- Increase human reliability
Measures for the prevention of human error, and an overall strategy for this, are considered under the headings of the organization, the job and personal factors. In this strategy, the organization is considered in terms of the safety climate, standard setting, monitoring, supervision and incident reports; the job is considered in terms of task analysis, decision-making, man-machine interfaces, procedures and operating instructions, the working environment, tools and equipment, work patterns and communication; and personal factors are considered in terms of personnel selection,
training, health assessment and monitoring.
A five-step strategy is described for modification of human behavior in relation to accidents and hazards. These steps are: (1) identification and analysis of accidents and hazards; (2) revision of safety rules concerning working behavior; (3) development of a plan of measures; (4) implementation of the plan and (5) follow-up−assessing the effectiveness of the measures taken.
Here, several measures to increase human reliability are introduced in detail.
Psychological stress greatly affects human perform. The effective way to deal with this problem is monitoring. Coupled with programs to monitor the level of stress should be employee assistance programs to help people cope with and reduce the stress related to personal problems. An expanded effort ensuring that individual physical and mental capabilities, education and training, and motivation are properly matched with job and task requirements can aid in improving the overall reliability of most systems.
The frequency of human error can also be reduced by better design of input and output systems. The design engineer with a knowledge and understanding of both the physical and mental limitations of system users can reduce operator errors by applying sound ergonomic principles to the design of machine controls, switches, keyboards, and other input devices. Similarly, instruments, gauges, and various other types of readouts out system outputs that carefully consider the capabilities and limitations of the operators and output monitors can enhance the communication between the human and mechanical components of the systems.
People perform better if parameters such as temperature, humidity, light levels, noise levels, and other environmental factors are maintained within comfort limits. Air conditioning, good illumination, soundproofing, and other creature comfort measures tend to reduce both psychological and physiological stresses and thus allow humans to work at higher levels of performance for longer periods. These measures are probably more important, albeit more difficult to implement, in combat vehicles and construction equipment than they are in administrative settings.
Because the ability to change basic human characteristics is rather limited, the most significant improvements in overall system reliability may be made by reducing the role of the human, especially in those situations where human performance is most error prone. The development of artificial intelligence and sophisticated, computerized control systems offer tremendous potential for controlling human errors by the skillful integration of man and machine, in situations and environments where human error rates may not be acceptable, the primary role of humans may well be to provide system oversight and monitoring of key automated functions and work in parallel with automated control systems. Whereas basic human capabilities tend to remain relatively fixed, machine capabilities continue to expand at very rapid rates.
Quiz:
- Which opinion on human factors is right? A
- human factors plays an important role at all stages of the design
- human factors is no longer an important issue in process control because of automation
- human factors is not influenced by the complexity of the system
- It is unnecessary to try to foresee human factors and to design to overcome them in system design process.
- Human reliability can be increased in the approach of_D___.
- Organization
- Job
- personal factors
- all of the above
- Which one is the effective to deal with psychological stress to reduce human error in industry? B
- Change working schedule
- Monitoring
- Communication
- Training
Reference:
Stephans, R. A.; Stephenson, J., System safety for the 21st century. Wiley-Interscience Hoboken, NJ: 2004.
Mannan, Sam Lees' Loss Prevention in the Process Industries, Volumes 1-3 (3rd Edition).. Elsevier.