J. ABID and A. SARWAT, Antonoil Service, Basra, Iraq
This article provides deep insight to piping layout engineers, presenting a holistic/systematic approach for pipe spacing based on different pipe supports—e.g., rest, guide, shoe and clamps (rather than assuming spacing based on 50-mm or 75-mm clearance between pipes). Equations have been developed to calculate the principal pipe spacing. Through a simple example, pipe spacing is calculated and shown in graphical form for quick comparison. Finally, consolidated tables are presented for (simple) upstream projects and for (complex) downstream projects, with sizes ranging from 2 in.–24 in. The effect of insulation thickness is also considered. This article does not cover pipe spacing containing flanges.
A pipe rack is the main artery of a process unit. It connects all equipment with lines that cannot run through adjacent areas. Pipe spacing in a pipe rack depends on pipe outside diameter, insulation (if present) and thermal expansion. However, pipe spacing should also consider different pipe supports to avoid interference with adjacent pipes and/or minimize changes in pipe rack width [previously calculated during the front-end engineering design (FEED) stage].
Pipe spacing has a direct effect on the width of the pipe rack. If the piping designer underestimates the pipe spacing (pipe rack width) during the FEED or proposal stages, it might result in the shifting/relocation of adjacent above-ground equipment and/or below-ground pipe or cables during the detailed engineering phase. In brownfield/revamp jobs, this relocation of adjacent equipment/pipes is practically impossible and can require cantilever extensions or additional tiers in an already congested pipe rack. Conversely, if designers grossly overestimate the pipe rack width by considering too much contingency/future requirements (without any valid reason or justification), it can result in wastage of premium space in the pipe rack.
Symbols and abbreviations are shown in FIG. 1.
Methodology. The overall process can be broken down into four steps. As shown in FIG. 2, the first step is to identify possible pipe supports used in the oil and gas industry. Then, define the evaluation criteria/equation to calculate the principal spacing. Finally, prepare a matrix table for each case and consolidate the information according to the agreed criteria.
Identification of significant pipe supports. For layout purposes, the following cases/credible scenarios are considered for this article:
Note: Pipe supports without significant lateral/radial dimensions (U-bolt, line stop, trunnion support, etc.) are not considered here.
P = R1 + 75 mm + R2 (1)
This is the base case/minimum requirement and must be satisfied.
PG = R1 + 3 mm + G1 + 25 + R2 (2)
Adjacent pipe guides should be staggered to minimize pipe space requirements.
WS = S1 + 3 mm + G2 + 25 + R2 (3)
CS = max [(S1 + 3 mm + G2 + 25 mm + R2), (C1 + 25 mm + R2)] (4)
FC = C2 + 25 mm + R2 (5)
Rest, guide and welded shoes are widely used in both upstream (U/S) and downstream (D/S) projects, while clamped shoes and flat clamps are mainly used in D/S projects.
Evaluation criteria and consolidated tables. This step is the most important and challenging as it requires compromise between minimum requirements (an average of Case 1–Case 5) vs. the conservative/worst-case design (a maximum of Case 1–Case 5). Based on experience and research, general trends for different pipe supports arrangement and spacing requirements are listed below:
So, the principal spacings of pipes are determined in Eqs. 6 and 7:
Pipe spacing-upstream, A-U/S = max (P, PG, WS) (6)
Pipe spacing-downstream, A-D/S = [max (P, PG, WS, CS) + FC] / 2 (7)
Tables 1A and 1B in FIG. 3 are based on Eqs. 6 and 7.
Effect of insulation thickness. If either or both pipes are insulated, the insulation thickness should be added to the center-to-center distance, P (not to A-U/S or A-D/S). In this article, the total insulation thickness of 25 mm (i.e., a 12.5-mm insulation thickness for each pipe) is considered for U/S projects (Eq. 8):
Insulated pipe spacing-upstream, A-U/SINS = max (P + 25 mm, A-U/S) (8)
Similarly, for D/S projects, the total insulation thickness of 50 mm is considered (Eq. 9):
Insulated pipe spacing-downstream, A-D/SINS = max (P + 50 mm, A-D/S) (9)
Tables 1B and 2B in FIG. 3 are based on Eqs. 8 and 9. Similarly, tables can be developed for different insulation thickness (in steps of 25 mm), if required.
Why this concept is important for downstream projects. Although this concept of optimized spacing is valid for both upstream and downstream projects, it is of paramount significance for downstream projects due to the longer length of pipe racks. In a typical refinery or petrochemical plant, the length of the main pipe rack can be ~1 km or longer, while pipe racks in an upstream project (early production facility or degassing station) are usually only a few hundred meters long. The reduced/optimized width of each pipe bent in a pipe rack would result in significant material (tons of steel) savings, especially in downstream projects.
Example. This example calculates the pipe spacing between NPS 14-in. and NPS 8-in. bare pipes based on Eqs. 1–7 and shown in FIG. 4 for visual representation. The horizontal axis shows the centerline distance between pipes (in mm), while the vertical axis show various pipe supports configurations as well as suggested pipe spacing for U/S and D/S projects in the oil and gas industry. As shown, the pipe spacing is 396 mm for upstream projects and 405 mm for downstream projects.
Consider the same NPS 14-in. pipe has 25-mm thick insulation in an upstream facility. So, spacing P now becomes 363 + 25 = 388 mm, which is less than A-U/S (396 mm). As per Eq. 9, the consolidated pipe spacing between the 14-in. insulated pipe and the 8-in. bare pipe remains 396 mm—this is shown in Table 1B in FIG. 3. In fact, A-U/S can accommodate a total insulation thickness up to 33 mm (396 mm – 363 mm).
Takeaway. The consolidated tables would decrease contingency/future space requirements during FEED, as principal spacing already includes possible support types and insulation thicknesses of 25 mm and 50 mm (for U/S projects and D/S project, respectively). This would also minimize pipe rack changes during the engineering, procurement and construction (EPC) stage.
This concept is based on limited available data. It is expected that a leading consortium like Process Industry Practices (PIP) will further develop and benchmark data for the benefit of knowledge sharing in the oil and gas industry. HP
JAMSHAID ABID is a Piping Lead at Antonoil Service, Majnoon Oilfield, Iraq. He holds a BS degree in mechanical engineering from the University of Engineering & Technology Taxila, Pakistan. Abid has written two articles for Hydrocarbon Processing magazine and articles at LinkedIn related to piping design and layout. In his free time, he enjoys yoga and meditation.
https://www.hydrocarbonprocessing.com/magazine/2020/september-2020/columns/supply-chain-how-to-reduce-piping-inventory/
https://www.hydrocarbonprocessing.com/magazine/2019/march-2019/columns/project-management-estimate-skid-and-package-dimensions/
ADNAN SARWAT has more than 25 yr of project and maintenance/turnaround experience in both the downstream and upstream sectors while mainly working in the Middle East with international oil companies. As a mechanical engineer, he has a passion for using his practical experience to train and mentor fellow and young engineers.