How do you size a pipe rack? What are the steps?

The height of a pipe rack is based on the minimum overhead clearance required under the lowest level of piping passing transverse under the rack (normally 12'-0" +/- or 4 meters). Then the vertical spacing of the North-South (N-S) and East-West (E-W) levels is determined by the largest average line size in the rack.  If your largest average line size is 12" NPS (a 12" long radius Ell is 18") then, two Ells welded together results in 3'-0' center to center of pipe (also 3'-0" bottom of pipe to bottom of pipe).  This means that the vertical spacing between the N-S and the E-W rack should be 3'-0". If you only require one level then you will have the original grade (ground) to lowest crossing level which may be 12'-0" plus 3'-0" to the first rack level for a total of 15'-0" For every rack level you add you must double the 3'-0" to allow for another crossing level between the normal rack levels.  The final overall height of the rack is determined by the number of levels required and any other special conditions or requirements. Special conditions include mounting Air Cooled Exchangers (Fin Fans) on top of the pipe rack or the requirement to have the flare line slope to free drain to the Flare Knock-out Drum.

On the other hand, the width of any given pipe rack is best determined early in the project during the Plot Plan development activity by doing a “Piping Study”. You must have well developed P&ID's and an understanding from the structural group on the type of supports construction (i.e.: bare steel, fire proofed steel, pre-fab concrete, or poured in place concrete). You then take the preliminary equipment arrangement (does not need to be to scale) and allow a clear space where you know the pipe rack will be located.  Then take every line from the P&ID that you know will need to run on the pipe rack and "sketch" it onto the pipe way area of the preliminary plot plan. As you go make sure you identify every line with the line number, line classification, line size and insulation if required. Do not make it detailed; keep it a very simple "stick" drawing. Do this with all lines. This drawing is also called a "Transposition" and the activity is called doing a "transposition." Once you have completed the transposition and all the lines shown then you can find the most congested point of the pipe rack. Study the most congested point to determine the most appropriate line sequence. 

Then you need to calculate the spacing between each two lines individually.  To do this you take one half the diameter of the flange of the largest of each two lines + one half the outside diameter of the other pipe + the thickness of any insulation on the smaller line if any + 1" spacing. Do this for all pipes in the congested part of the transposition. Now add the line spacing for a raw total. To the raw total you should add 30% for lines that may be added during Design Development. You should also ask the client how much "Future" rack space is wanted (normally 10%) for the finished plant. The raw total plus the 30% plus the 10% may give you a number that is acceptable for a one level rack.  However you may find it is more likely that you will need two or more main levels.  Work with the numbers, considering the overall space available, the span of the proposed rack and the loading to be placed on the span. 

Line spacing is (a) one half the diameter of the flange of the largest of each two lines +(b) one half the outside diameter of the other or smaller pipe + (c) the thickness of any insulation on the smaller line if any + (d)1" spacing.