With the routing diagram, a dimensioned cross section is developed at the bent that will carry the most piping, which is bent No 12 in the example shown in following drawing.
When all the distances have been established between all lines on each level, including allowances for future growth and conduit, the only remaining dimension to be set is the distance from the first line in the rack to the vertical column centerline.
Column sizes are furnished by the civil/structural engineers.
The last step is to add up all the dimensions and round off to the next whole number—for example, 20 ft (6,100 mm) rather than 19 ft 3 in (5,850 mm).
To illustrate, if the pipe requires a much greater area in the rack, the designer would work with the structural engineers to determine whether the pipe rack should be two 30-ft (9,150-mm) wide levels or three 20-ft (6,100-mm) wide levels.
This decision affects the cost of the structure and pipe and must be made carefully. The width of the rack shall be 6M, 8M or 10M for single bay and 12M, 16M or 20M for double bay having 4 tiers maximum.
Another Method : First estimate number of lines as described. Add up the number of lines up to 18” diameter in the most dense section of the piperack. The total width in meters (W) will be : W = ( f x N x S ) + A meters Where f, safety factor = 1.5, if the lines have been laid out as described in initial evaluation.
Where f, safety factor = 1.2, if the lines have been laid out as described under development. N = number of lines below 18” diameter S = average estimated spacing between lines in millimeters. Usually – S = 300 mm (230 mm if lines in piperack are smaller than 10” ) A = additional width required meters for [Lines
larger than 18” + Future lines + Instrument and electrical cable trays + Any slot for pump discharge lines 500 mm – 1 meter] . The total width is thus obtained. If W is bigger than 9M usually two piperack levels will be required. Note : At the beginning of a job, `W` should usually include 30 – 40% of clear space for future lines. The width of the piperack may be increased or determined by the space requirement, and/or access to equipment arranged under the pipe rack.