Design of Pipe Rack involves considerable planning and cor-ordination with other engineering groups. Rack Design involves following activities.
- Pipe rack width calculation.
- Pipe tiers elevation calculation.
- Pipe rack piping layout
- Accessways on pipe rack
- Cable tray arrangement
- Future Space
- Spacing between rack beams
- Pipe rack loading to civil
- Flow meters in pipe rack
- Pipe supports on pipe rack.
- Utility stations from pipe rack
- Expansion loops on pipe racks
Racks shall be designed to give the piping shortest possible run and to provide clear head rooms over main walkways, secondary walkways and platforms. Predominantly process lines are to be kept at lower tier. Utility & hot process lines on upper tier.
Generally the top tier is to be kept for Electrical cable trays (if not provided in underground trench) and Instrument cable ducts/trays. Cable tray laying to take care of necessary clearances for the fire proofing of structure.
Hot lines and cold lines shall be kept apart in different groups on a tier. Bigger size lines shall be kept nearer to the rack column.
Spacing Between Pipes
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 25 mm clearance, even if flange is not appearing. Actual line spacing, especially at ‘L’ bend and loop locations, shall take care of thermal expansion / thermal contraction / non-expansion of adjacent line. Nonexpansion / thermal contraction may stop the free expansion of the adjacent line at ‘L’ bend location.
Anchors on the racks are to be provided on the anchor bay if the concept of anchor bay is adopted. Otherwise anchors shall be distributed over two to three consecutive bays. Anchors shall be provided within unit on all hot lines leaving the unit.
Battery Limit Valves
Process lines crossing units (within units or from unit to main pipeway) are normally provided with a block valve, spectacle blind and drain valve. Block valves are to be grouped and locations of block valves in 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.
Branches in Headers
Stubs in sea water shall be from top of main header.
Supporting of piping on Pipe Racks and Pipe Ways involves providing rest, guides, limit stops and anchors in a piping header.
Guides on a pipe header shall be arranged as shown in following diagram.
|NPS||2″ to 3″||4″ to 6″||8″ to 14″||>= 16″|
|L : Meters||3 to 4||6 to 8||12 to 16||12 to 16|
|L1 : Meters||6||8||14||As per Stress Anslysis|
- “L1” is the minimum length necessary to permit the piping maximum thermal expansion of 120 mm.
- “B” is the dimension of loop as per stress analysis.
- Pipe racks and pipe ways usually have spans of 6 to 8 Meters. So for pipes 6″ and smaller, guides are required on each main beam. For pipes more than 6″ NPS, guides on alternate beams can be provided.
Maximum distance between supports shall not be higher than allowed particular material of construction at particular operating conditions. Refer company standards for the same.
- Maximum allowable spans for metallic piping.
- Maximum allowable spans for Cupro-Nickel Piping
- Maximum allowable spans for Plastic Piping.
For all pipes which require a slope towards a definite direction, the max allowable span should be such that max deflection of pipe is less than the level difference between two adjacent support points, in order to avoid pockets.
Supporting Small lines from Big Lines
For small bore piping, intermediate supports from adjacent large bore piping may be required as an allowable span for small bore piping can be is less than the distance between two consecutive supports.
The adjacent pipe chose as a supporting pipe must have diamter at least three sizes greater than the supported pipe, and never less than 4″.
- Supporting Pipe : 4″ / 6″ / 8″
- Corresponding Supported Pipe : 1.5″ / 2″ / 3″