Table 2 Seven variations of intersection and union

[A]

V - A - O

Only visible, but not approachable and operable. This happens when obstacles and other pipes prevent engineers from accessing equipment and pipelines.

[B]

A - V - O

Only approachable, but not visible and operable. This happens when obstacles and other pipes block displays and controls.

[C]

O - V - A

Only operable, but not visible and approachable. Although remote control is possible, we did not consider this variation.

[D]

V ∩ A − O

Visible and approachable, but not operable. This happens when controls or valves are mounted too high, too low, or too far away to reach and operate.

[E]

V ∩ O − A

Operable and visible, but not approachable. The same as [C]. We did not consider this variation.

[F]

V ∩ O − V

Approachable and operable, but not visible. This happens when controls and valves are mounted behind the display, and engineers have to bend their arms to operate them. However, any blindness operation is not allowed in our assumption.

[G]

V ∩ A ∩ O

Visible, approachable, and operable—the ideal situation.