Use Ergonomics To Improve Safety

During design always consider ease of construction and use.

By Dirk Willard, contributing editor

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One-hundred-and-fourteen people died when the skyways at the Hyatt Regency Hotel in Kansas City, Mo., collapsed on July 17, 1981. An investigation spearheaded by the Kansas City Star revealed that a simple design error for the support rods was to blame. The structural engineer's design was too complicated and too expensive to make, so the contractor took a shortcut. Workers complained that walking on the skyway was like being on the deck of a rolling ship. Nobody listened, though.

This accident exemplifies why you must design for people. That means you should consider ergonomics, which in simple terms is about reducing friction between people and machines. So, in this column, I'll offer a few pointers on plant construction.

Let's start with workspace design. By this I mean, for example, locating valve handles. "Human Engineering Guide for Equipment Designers," by Wesley Woodson and Donald Conover, University of California Press, Berkeley, Calif., 1970, states on pp. 2-112–124 the following limits for the average standing operator: 1) maximum arm extension, 32 in. horizontal; 2) maximum reach from extended elbow, 39 in. from the floor; 3) maximum reach above, 78 in. from the floor; 4) a sitting operator's maximum convenient arm reach, about 20 in., to the palm, horizontally. Locate a valve handle between shoulder height and the elbow; maximum force can be exerted from shoulder level for a standing operator and the elbow for a sitting operator. Orientation of a valve handle makes a difference, too. The best orientation for the valve stem is 90° from horizontal between 24 in. and 48 in.

Occupational Safety and Health Administration guidelines say to avoid work requiring stooping, squatting and lying down. When such a situation is unavoidable, the prone height minimums are 24 in. for people working on their backs and 18 in. for those on their abdomens. These values make no allowance for winter clothes or protective gear.

Now, let's consider consoles and controls. Here're some general concepts.

Glare from artificial or natural light can hamper reading an instrument or screen. Sometimes it's a combination of light. Hooding or orientation may help. For a typical distributed-control-system operator's sitting perspective, the screen angle from horizontal should be 60°. For a standing control panel, it's about 30° and a height of 36 in. is best.

Keep critical controls and field instruments within eyesight — this means no higher than 74 in. and no lower than 39 in. above the ground. Also consider orientation.

Contrast is important for alarms and indicators: black and white provides the strongest difference. The color and brightness of the background has more impact than its size.

Avoid color combinations susceptible to confusion because of the closeness of the hues — such as red and orange or purple and blue — or because of partial color blindness; everyone is partially color blind to one or more combinations of colors — mine is light blue on light green. Carefully choose colors. Combinations like green on red, orange on black or orange on white are worst. Use contrast to reduce reading errors. Cockpit designers have found that, regardless of lighting conditions, the difference in brightness between instrument controls and surrounds must be more than 7:1 and that pilots always should be able to see at least the pointer markers on controls.

Alarm overload is a continual problem. Reduce the number of alarms per page — less is more. (For more on dealing with alarm overloads, see: "Choose an Alarm Champion"; "Avoid the Domino Effect"; and "Adroitly Manage Alarms".)

Keep natural orientation in mind. Always place labels horizontally to make them easier to read; use common sense: up is up; down is down; right is right, etc.

Noise control is a common issue. Earplugs provide cheap protection but block out sounds that can give insights. Maintenance people need to hear equipment. Sounds are an important diagnostic tool — take it from someone who's suffered short-term tinnitus from tuning a burner. Because you can't prevent staff from listening, you must protect them by reducing the amplitude (decibels) of the noise. One old trick is using an acoustic blanket; something as simple as hanging baffle boards from the ceiling may suffice. In truth, noise often is a symptom of equipment failure; eliminating the noise altogether may be the best control.

If a detailed analysis, say, of a control room, is necessary, you must establish which operator is assigned to what equipment and how equipment and operators will communicate. This type of engineering is called equipment arrangement. I've covered this before (see: "An Alert Operator is a Safe Operator"). The best solution for most mundane tasks may be a simple hazard checklist including such questions as: Bumps? Burns? Pinching or mashing? Slipping? Noise? Fire or explosion? Electric shock? Atmosphere? Other dangers? Don't overlook issues such as spacing between pumps. Designing a control room requires a fine appreciation of the interaction between people and their machines. Creating an environment where harmony is achieved should be your goal.

Dirk Willard is a Chemical Processing contributing editor. You can e-mail him at dwillard@putman.net.

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