Human Factor in Plant Safety

8/12/2019 Human Factor in Plant Safety

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HUMAN FACTORS IN PROCESS PLANT SAFETY

David A. StrobharSenior Engineer

Beville Engineering, Inc.

280 Regency Ridge Drive, Suite 2000Dayton, Ohio 45459(937) 434-1093

Abstract

Human factors engineering is the branch of science dedicated to the analysis and designof man-machine systems. Human factors principles are discussed relative to the

enhancement of personnel safety at oil and chemical processing facilities. Application ofdata on human characteristics to both physical (e.g., lifting, equipment access) and

psychological (e.g., decision making, risk perception/avoidance) concerns is shown.Implications for the design of warning systems and emergency procedures are

highlighted.

INTRODUCTION- WHAT IS HUMAN FACTORS

Its only human. How often do we hear that phrase, either when talking about ourselvesor other people? And yet, what does it mean? What does being human entail? Are there

certain attributes or characteristics that all people possess? The phrase would indicate thatthere are things about us humans that, if not universal, are at least generalizable.

The U.S. Air Force found in World War II that the sophisticated machines of war were

being limited by the people who operated them. Aircraft could not be consistently flownat design specifications because of operator error. In fact, more pilots died in WWII

during training than in combat, often due to poor selection or system design. In mostcases, the design of the man-machine system interface (e.g., instrumentation) did not

account for human characteristics and limitations.

Due to the operator-limited nature of their systems, the Air Force began to incorporatecharacteristics of the human operator into the design of the system. They established what

became known as the sacred six, those six instruments that form the basic informationfor flying an aircraft. The realized that the space requirements of the cockpit could vary

up to one-half a foot if the design was made to accommodate the 95 instead of just the 90percentile male adult. The application of human factors technology has allowed the

aerospace industry to progress from the Wright B Flyer to the F-14 Tomcat withoutincreasing the number of operators (i.e., pilots) required. The definition and integration of

human characteristics and limitations into the design of systems became the branch ofscience known as human factors engineering.


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The Science of Human Factors Engineering

A cockpit is not a chemical plant, so how does human factors apply to process safety?While there are obvious differences between a chemical plant and an airplane, there are

similarities in humans. The model in Figure 1 represents key man-system interactions. In

the model, the person assimilates information in order to make a control action. Theentire activity occurs in some environment. Two distinct aspects of the model haveimplication for chemical process safety, the physical or environmental interactions and

the psychological characteristics of the individual.

Figure 1. Man-system interactions.


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The physical characteristics of humans have been defined over the years in primarilyempirical ways. Large numbers of individuals have been measured and tested, with the

results statistically compiled and grouped together. Human size variations, differences instrength limits, responses to heat and cold have all been analyzed and compiled. The

concern for the physical aspect of human characteristics is generally referred to as

ergonomics or anthropometrics.

Considerable data exists on anthropometric (human-measurement) considerations. Figure

2 shows some of the data available on the variances within individuals. In the figure, thestandard forward reach for the 5

ththrough 95

thpercentile adult male is provided, along

with the standard deviation, required aperture for the reach, and required height. Usingthis and similar types of data, access requirements for maintenance can be specified,

producing a design that minimizes openings and yet enhances maintainability.


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Beyond human physical considerations is the psychological element; that is, what isgoing on inside the persons head. Many accidents in process plants are attributed to

operator error, and most of that to poor judgment or operator decisions. The ability toeffectively make the right decision is influenced by a number of variables.

In order for any information to be processed, it must first be received. The individualmust attend to some aspect of their environment and examine the information. In aprocessing unit, this often means looking at a display. The ability to detect a signal in a

noisy environment has been studied extensively, with the probability of detecting thesignal decreasing with increasing noise. In general it falls to warning systems to direct the

individuals attention to a problem or some aspect of the environment. However, theprobability of detecting an alarm is a function of the number of alarms. The more alarms,

the less the probability of detecting the actuation of any one alarm.

Once the information has been received, it must be processed in the persons conscious orshort-term memory. First, everyones short-term memory is a capacity-limited system,

only able to hold about seven chunks of information at a time. Second, the individualmust have available mental energy or workload capacity in order to manipulate the

information. A certain amount of mental capacity is given over to stress management,leaving the individual less available mental workload reserve during periods of high

stress. Third, information must often be exchanged with long-term memory. However,the ease with which a person can retrieve information is a function of how it is stored, the

role of training. People in general are much better at making relative versus absolutediscriminations/judgments. Relative judgments place less of a demand on long-term

memory and mental workload requirements. Complex systems then should account forhuman short-term memory limitations, mental workload requirements, how knowledge is

stored, and how people make decisions.

Human factors engineering then is concerned with the various aspects of a personinteracting with a complex system. The physical area of human factors deals with access

to equipment, lifting requirements, and generally the necessary sizing of the processhardware. The psychological or cognitive area of human factors is concerned with how

the individual assimilates information (training, displays), how the individual makesdecisions (procedures, training, displays), and how the individual takes action (controls).

Example of human factors engineering: analysis of emergency procedures

The creation of good emergency procedures is an area that requires analysis of both the

psychological and physical interactions between men and machines. Beville Engineeringwas requested to look at the emergency procedures for a set of units in an oil refinery

with the goal to make them more usable. The overall objective was to protect thepersonnel on the unit and to minimize the impact of process disturbances. Four general

criterion were developed for the emergency procedures analysis:


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Validity- The quality of the information contained in a procedure is a measure ofits validity. Are the technical requirements for controlling the situation contained

in the procedure? Does the procedure describe actions that a person can physicallyaccomplish in the period of time required? Do the procedures have any adverse

side effects, causing other problems in correcting the primary one?

Applicability- The degree to which existing procedures cover or account foranticipated emergencies is a measure of their applicability. Are all anticipated

events covered? Are logical combinations of events covered? Upon whatassumptions were the procedures based and are the assumptions reasonable, at all

times?

Usability- The ease with which the correct procedure can be selected and appliedis the measure of its usability. It does no good to have a technically perfect

procedure that no one knows when to use or is not readily available for the personto use. Do the operators have the know when as well as the know how?

Understandability- The degree to which the intended user of the procedure

comprehends the contents is the measure of its understandability. Are theprocedures written in clear, concise language appropriate for the user? Are

conditional events formatted in such a way that the user understands theconsequences of not meeting the conditions?

Existing emergency procedures were walked-through with the operators to determine

if, or to what degree, the procedures met the four criteria. Operator psychological andphysical requirements were examined as part of the process, with some interesting

results. The original procedures were similar in style to that in Figure 3. Four majorproblems were noted in the procedures.

First, the procedures require that the user know what is wrong in order to select

the correct procedure. The operator cannot simply begin to take action, as theprocedure is tied into plant events rather than plant symptoms. The procedure is

not tied into a warning system, nor does it define how to confirm that theprocedure is the correct one chosen.

Second, the procedure is written in a prose format, which makes conditional

statements (if/then) long and wordy. What if the conditional is not met, should theoperator simply continue or is there a different action. A large amount of

information is required to be processed with the prose style.

Third, there are a large number of ambiguities in the procedure. One part instructsthe person to retain enough products in the system. How much is enough? Is the

definition of enough the same for the person who wrote the procedure and theperson using the procedure? If a difference in interpretation exists and the unit is

damaged, who is at fault? The procedures are forcing absolute discriminations tobe made and not relative discriminations.


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Fourth, certain steps have multiple actions. Remember that short-term memory is

capacity-limited. It has been found that the probability of omitting an actionincreases exponentially with the number of actions per step.

A revised version of the same procedure is presented in Figure 4. The procedure is inflowchart format, allowing conditionals to be explicitly presented and skipped if thecondition is not met. Shape coding has been used to identify the different requirements of

the user.


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Reformatting of the procedure addressed the psychological interaction of the user withthe system. However, the criteria of validity require that the procedure actually be able to

be performed or carried out. In at least one instance, the physical design of the hardwaremade it questionable as to whether the operator could perform the task.

At one point, the operators indicated a need to close a block valve manually (Figure 5a).Closing the valve was hampered by the fact that the operators first had to stand on thecheck valve to reach the valve wheel and that the valve wheel was poorly positioned. The

poor footing, valve wheel position, and the large size of the valve resulted in up to 45minutes for the operator to close the block valve. Using ergonomic principles, an

enhancement was proposed, with a platform constructed for better footing and the valvewheel positioned for maximum strength (Figure 5b).

Summary- Human Factors Engineering as a Science for Enhanced Process Safety

Human characteristics can be defined and accounted for the design of systems.Appropriate consideration for both the physical and psychological similarities that we all

posses enhance the usability of modern systems. Human factors can be utilized toimprove aspects of training, displays, controls, and plant hardware. The ability to decide

what to do and the ease with which the action can be carried out are all the domain of thehuman factors engineer. At the disposal of the human factors engineer is a database on

human physical and psychological characteristics.

Application of human factors principles to the creation of good emergency procedureshas the potential for significant system safety improvement. Well human engineered

emergency procedures (1) improve the operators ability to correctly identify what needsto be done in an emergency and (2) ensure that the operator can physically carry out the

required actions.


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Human Factor in Plant Safety