Chapter 6
Workplace Design
6.2.5.2 Distances between Adjacent Pieces of Equipment
The distance between adjacent pieces of equipment again depends on how they are used. If the separation is merely to permit a person to pass safely between equipment, the minimum distance is determined by the width (or depth) of the largest user. In most cases, the minimum distance is specified to be 60 cm (24 in.). Clearly, if the equipment is hot or sharp or if footing is uncertain, a safety factor must be added. Normally, an additional 8-13 cm (3-5 in.) is sufficient.
If, however, the distance between equipment is determined by the task that the operator must perform, it depends on the job performed. Figure 6-7, for example, illustrates a technician squeezing between two pieces of equipment to access an instrument. In this case, merely fitting between the equipment is insufficient.
Figure 6-8 illustrates the minimum dimensions that are necessary to permit a maintenance technician to work on a pump in a squatting position. The minimum clearance is again determined by the largest technician (95th percentile) in a squatting position to perform the job.
6 2.5.3 Ladders, Stairs, Ramps, Walkways, and Platforms
Ladders, stairs, ramps, walkways, and platforms are fundamentally important in the design of process plants. Process plants require workers to operate and maintain equipment above grade. Above-grade access can be accomplished in many ways, but in most plants, operators climb to the equipment using ramps, stairs, and ladders. And, once at elevation, workers access equipment using walkways and platforms.
The proper design of stairs, ramps, ladders, walkways, and platforms can affect the safety and efficiency o fwork above grade. Most government agencies responsible for the safety of process plants have enacted legislation (United States: OSHA, 1997; Norway: NPD, 1977; Great Britain: HSE, 2001) that lays out the minimum requirements for the design of ladders, stairways, ramps, walkways, and platforms. Many standards organizations have also addressed the issue (ABS, 1998) and several textbooks have reported design specifications for this equipment (Eastman Kodak, 1983; Grandjean, 1988). The purpose of this section is to advise the reader what design issues are important and guide the user to appropriate standards.
Stairs, Ladders, and Ramps
It is generally agreed that the design and use of a ramp, stairway, or a ladder depends on two major issues: the angle of ascent/descent and the nature of the task. Most research suggests that ramps should be used when the angle of ascent/descent is between O° and 20°. Angles above 15° are difficult to negotiate if material is being manually pushed or pulled. So, power equipment is required. Stairs are installed when the angle of in ascent/descent is between 30° and 50° to the horizontal (OSHA, 1997), and ladders are used from 50° to 90°.
Variations from traditional designs occur often in process plants. Eastman Kodak (1983), for example, describes a stair ladder, with a slope of between 50° and 75° (Figure 6-9), which is used for operators to move "between several levels on an occasional basis." In addition, ladders for floating tanks can assume angles between O° and 90° (Figure 6-10). So, they can be considered ladders or stairways and designed accordingly.
When facilities permit a wide range of options, the optimum slope of a stairway is determined by the energy required to use it. Grandjean (1988) cites research that found that the least energy consumed when climbing stairs was with a gradient of 25-30° and tread height (h) and depth (d) that met the formula
2h + d = 630 mm
The design characteristics of a stairway are shown in Figure 6-11. Data from Eastman Kodak (1983) suggests that, as the slope of the stairway increases, the riser height should increase and the tread depth should decrease. Table 6-1 provides data from three sources that illustrate the wide range of recommendations on step height, tread depth, and tread overlap for the North American or European user populations. For stairways installed for use by other populations, the dimensions in Table 6-1 would vary. Designers should consult the regulations in each jurisdiction to ensure that stairways meet the requirements of the user population.
Quite apart from the angle of ascent and descent, how a person ascends or descends determines, to a large extent, what equipment is used. Ladders are cheaper to install than stairs. So, in most plants, ladders, not stairs, are the default method of getting from one level to another.
Where
A = Step Height
B = Riser Height
C = Step Depth
D = Tread Depth (C + E)
E = Overlap
TABLE 6-1
Recommended Stairway Dimensions from Three Sources
| Reference | Step Height | Tread Depth | Overlap |
| mm (in.) | mm (in.) | mm (in.) | |
| Eastman Kodak (1986) | 160-190 (6.5-7.5) | 280-300 (11-12) | 25-40 (1-1.5) |
| Grandjean (1988) | 170 (6.7) | 290 (11.4) | |
| ABS (1998) | 203*-229 (8-9) | 229-276* (9-10.9) | 22 (0.9) |
| * = recommended |
But, ladders are generally harder to climb than stairs. So stairs, not ladders, should be installed, if
- Personnel are required to carry large tools or pieces of equipment up or down thestructure.
- Equipment must be accessed or personnel evacuated during emergencies (e.g.,battery limit stations).
- Hazardous material must be carried manually between levels.
- Equipment is frequently accessed (at least once per shift on the average).
While climbing stairs is considered a good form of exercise, stair climbing can also promote wear and tear to the body if stairs are used excessively or not designed properly. Field data suggest, for example, that workers complain more about leg and knee discomfort when stairs have steeper angles and many back and forth transitions. Depending on the way plant areas are accessed on a day-to-day basis (e.g., surveillance rounds, preventive maintenance), a decision to provide elevators or power lifts should be made. Based on the authors' experience, the facility is a candidate for a powered lift of some type if all of the following conditions apply.
- The height of the climb is more than 10m (32.5 ft).
- The climb is performed by the same person more than four times per shift (on average).
- Equipment and tools are often carried up and down.
Other combination of climbing heights and conditions would have to be analyzed to determine whether elevators are indicated using a combination of energy expenditure and musculoskeletal analyses.
Another way to reduce the degree of discomfort from using stairs is to find an alternate way to move tools and equipment up and down the structure. When available, cranes can be used to lift and lower tools, materials, and equipment. Small elevators, sometimes referred to as dumbwaiters, can be installed for routine use. Finally, one of the best ways to reduce the wear and tear from ladders and stairs on workers is to reduce their use. Often, moving equipment to grade (e.g., sampling stations or field panels) or changing the work processes (e.g., rounds protocols) can reduce worker exposure to stairs and ladders.
As a final note, stairways and ladders, if not properly designed, can contribute to slips and falls. It is important that the stairway and ladder covering is designed to maximize friction in all types of weather and for the uses intended