A pipe rack is the main artery of a processing unit. It connects all equipment with lines that cannot run through adjacent areas. Because it is located in the middle of most plants, the pipe rack must be erected first, before it becomes obstructed by rows of equipment. Pipe racks carry process, utility piping, and also include instrument and electrical cable trays, as well as equipment mounted over all of these. Fig. 1, shows a typical pipe rack.
This is a small presentation on Pipe Rack and Rack Piping. It will be very helpful for beginners in the piping industry. This article will cover the following points in brief:
- Data required for Pipe Rack Development
- Pipe Rack design criteria
- Future Space
- Width of Pipe Rack
- Pipe Rack Loading
- Rack Piping
- Positions of Lines (Process & Utilities)
- Hot Lines & Cold Lines
- Bigger Size Lines
- Pipe Spacing
- Anchor Bay
- Unit Battery Limit
- Expansion Loops
- Pipe Route
Data Required for Pipe Rack Development
The primary data required for the detailed development of a pipe rack:-
- Plot Plan
- Client Specification
- Construction Materials
- Fireproofing requirements
- Statutory requirements
Pipe Rack Design Criteria
Shapes of Pipe Rack
There are various shapes of pipe racks like L/T/U/H/Z. These shapes shall be considered based on the area available.
Future Space Requirement in Pipe Rack
The total width of the pipe rack shall include 25% extra space for future expansion/modification in the unit for rack width up to 16 m and 10% for rack width above 16 m. The future space %age is normally based on client requirements.
Width of the Pipe rack
The width of the rack shall be 6 m, 8 m, or 10 m for single bay and 12 m, 16 m, or 20 m for double bay having 4 tiers maximum. The spacing between pipe rack portals shall be taken as 6m in general. However, it can be increased to 8m depending on the size of the pumps to be housed below the pipe rack.
Clearance criteria in Pipe rack
For units, clearance beneath pipe rack shall be 4 m minimum both in longitudinal and transverse directions.
For Offsite, clearance beneath pipe rack shall be 2.2 m minimum both in longitudinal and transverse directions.
Road clearance shall be 7 m for the main road and 5 m for the secondary road.
Rack Width Selection Criteria
Refer to Fig. 2 for details
Pipe Rack Loading
Pipe rack loads shall be given by stress group to Civil & structural discipline for pipe rack design.
- Sustain Load (Dead Load): Weight of piping, valve, and load insulation
- Thermal Load: Load by thermal expansion of piping & Reaction force by the internal pressure of expansion bellows
- Dynamic Load
- Occasional Loads by the vibration of piping & by wind and earthquake
- Sustained Load (Live Load): Liquid load for the hydrostatic pressure test
Guidelines for Rack Piping
Position of Lines in a Pipe Rack:
Predominantly process lines are to be kept at a lower tier and, utility & hot process lines on the upper tier.
Hot Lines & Cold Lines in a Pipe Rack:
Generally, hotlines & cold lines are to keep at different tiers or at different groups on a tier.
Pipe Spacing inside Pipe rack:
Minimum spacing between adjacent lines shall be decided based on O.D. of bigger size flange (minimum rating 300# to be considered), O.D. of the smaller pipe, individual insulation thickness, and additional 25mm clearance. Even if the flange is not appearing the min. spacing shall be based on the above basis only. Actual line spacing, especially at ‘L’ bend and loop locations, shall take care of thermal expansion/thermal contraction/non-expansion of adjacent lines. Non-expansion/thermal contraction may stop the free expansion of the adjacent line at the ‘L’ bend location.
Bigger Size Lines:
Large size lines (14” and larger) shall be arranged close to the column in order to decrease the bending moment of the beam. Water lines more than 30” shall not be routed over a pipe rack, these shall be routed underground.
Anchor Bay in a Pipe rack
Anchors on the racks are to be provided on the anchor bay if the concept of anchor bay is adopted. Otherwise, the anchor shall be distributed over two to three consecutive bays.
Anchors shall be provided within the unit on all hotlines leaving the unit.
Racks shall be designed to give the piping the shortest possible run and to provide clear head rooms over main walkways, secondary walkways, and platforms.
Generally, the top tier is to be kept for Electrical cable trays (if not provided in the underground trench) and Instrument cable ducts/trays. Cable tray laying to take care of necessary clearances for the fireproofing of the structure.
Battery Limit (ISBL):
Process lines crossing units (within units or from the unit to the main pipe way) are normally provided with a block valve, spectacle blind, and drain valve. Block valves are to be grouped and locations of block valves in the vertical run of pipe are preferred. If the block valves have to be located in an overhead pipe-way, staircase access to a platform above the lines shall be provided.
Expansion loops in a Pipe Rack
Refer to Fig 3.
The expansion loop is provided on the high-temperature lines. This information shall be given by the stress group. All the loops shall be located around one column only.
Make lines into a group and install a large size piping and high-temperature piping to the edge of the pipe rack.
When necessary to install an expansion loop on the condensate line, do it horizontally to prevent water hammering. But do as above if the horizontal loop is impossible.
To learn the piping stress concepts of pipe rack design visit the following article: Rack Piping for a Piping Stress Engineer
Online Course on PipeRack Piping Layout and Stress Analysis
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