Pipe rack can be considered as the main artery of a plant piping system. It offer a means to group and route plant piping in an organized manner. Pipe rack design is one of the significant aspects of plant design and a good design contrbutes to integrity and reliability of the plant and the associated structures. Piping inside the process plants are preferably routed on pipe racks and piping in the off-site areas are preferably routed on pipe sleepers. 

Pipe rack design development

Every plant will have its own unique pipe rack layout and design to comply with the specific plant requirements. The configuration and size of piperack is established based on the overall plot plan arrangement, process requirement and plant economy. Early in plant layout development, during the conceptual stage, the main piperacks must be sized to establish their overall foot print and elevations. Height and number of pipe rack levels should be established, along with the location and approximate size of instrument and electrical trays. As piping design goes through the detailed development process and more information becomes available, detailed rack layouts should be finalized. These will include finalizing the pipe rack dimensions, spacing of pipes on the rack, rack column spacing, expansion loop requirements and location of anchor bays. Pipe rack design requires considerable planning and coordination with all disciplines to ensure that all the relevant requirements are taken into consideration including the constructability aspects and any impact on the adjacent process units. 

pipe rack section
PIPE RACK TYPICAL SECTION

In the initial stages of plot plan development, a piping study is required to be carried out to identify the various lines that will be routed on the pipe rack. This study is required to estimate the pipe rack width and elevations and pipe rack runs required in various areas of the plant. For the piping study to be intitiated the following inputs are required:

  • Process and Instrumentation Diagrams - P&IDs (In the initial stages a preliminary P&ID will suffice).
  • Line list indicating the line sizes, operating and design conditions, insulation requirements and any other specific process requirements such as lines with no pockets, lines requiring slope, orifice meter runs etc.
  • Based on the line list, stress critical lines should be identified and a preliminary stress analysis must be carried out to identify requirement of expansion loops and anchor bays.
  • Pipe racks carrying hydrocarbons may require fire-proofing of structural members to ensure that critical structural elements are not at risk.

Pipe rack design should consider sufficient space for future expansion as defined in the piping design basis. The space for future expansion may vary between 15-30% depending on the owners requirements. The space for future expansion may govern the overall pipe rack width and may neccessitate provision of another tier on the pipe rack if the future space requirements cannot be accomodated within the pipe rack width.

Because the pipe rack is located in between the process units, it is required to be erected before the erection of the adjacent process units. Consequently, the dimensional and load inputs of rack design to structural department becomes one of the early requirements during the detailed design phase. 

Pipe Rack Configurations

The pipe rack can be visualised as splitting the plant units into several areas. The pipe rack may take various shapes such as ‘straight’, ‘L’, ‘T’, and ‘C’ or ‘U’ depending on the plot site conditions, equipment arrangement and the process requirements. Overall pipe rack configuration will be based on the location of incoming and outgoing lines (battery limits) and routing of lines between the process units. While designing pipe rack runs in the plot, the designer may consider applying one of the following commonly used configurations. In many cases the overall layout could be a combination of these basic configurations.

Single Pipe Rack Layout

straight pipe rack configuration

Comb Type Pipe Rack Layout

comb pipe rack configuration

Double Comb Type Pipe Rack Layout

double comb pipe rack configuration

U Type Pipe Rack Layout

u type pipe rack configuration

Pipe Rack Design Considerations

Minimum Pipe Rack Elevation and Area under the pipe rack

The elevation of bottom most tier of pipe rack is determined taking into consideration the following:

  • Headroom required over the main road.
  • Headroom required for access to equipment under the pipe rack.
  • Headroom under lines connecting the pipe rack and equipment located outside.
  • The size of the steel or concrete beam supporting overhead piping must be taken into consideration.

The area under the pipe rack may be used as a maintenance aisle and used for movement of fork lifts and mobile handling equipments. The minimum recommended maintenance aisle width is 4m over the entire stretch of the pipe rack.

Pumps handling toxic, flammable or combustible products pose a significant fire hazard and should not be located under the pipe rack structure. The pump end should be located outside the edges of the piperack. A minimum gap of 3m between pumps in flammable service and pipe rack structure is recommended. Pumps handling non-flammable products may be located closer or partially below the pipe rack.

Mutliple Level or Tier Racks

If there is a single level of pipe rack, it is recommended to group the utility lines in the centre of rack. Where piping has to be routed in multiple levels (tiers) racks, lines carrying hazardous and toxic service shall be placed on the lower tiers. Utility piping should be routed on upper tiers. Large bore piping should be preferably located on the lower tiers and in the proximity of the pipe rack columns (along the pipe rack edges) to reduce the bending moments on the pipe rack beams and optimize the pipe rack structural design. It is also recommended not to place any pipes over the main columns as it will prohibit addition of further levels on the rack in the future if required.

Electrical and Instrument cable trays must be located on the topmost tier to have a clear separation between piping and cables. Further, there should be separation between the electrical cables and instrument cables. Instrument and electrical trays may be placed on the utility level if space permits and acceptable as per the piping design basis.

The gap between the tiers shall ensure that the branch requiring the largest vertical dimension can be routed through the gap between the two tiers. While finalising the gap between the two tiers, due consideration shall be given to any additional dimensions required for insulation, pipe shoes, height of structural member etc. All branch lines from the main lines on pipe rack should be routed at the same top of steel (TOS) elevation for good aesthetics.

Pipe spacing and location of pipes on rack

The minimum separation between two adjacent pipes shall be based on a minimum gap of 25mm between the flange of larger diameter pipe and outside of the smaller diameter pipe. It is recommended to maintain this separation even if there are no flanged connections on the pipe rack. If flanged connections are required to be provided they should be staggered. The separation between two adjacent lines may have to be increased to take care of large thermal movements based on the recommendations of stress analysis. If the pipes are insulated, the thickness of insulation shall be taken into consideration for spacing between the pipes.

Another factor that should be taken into consideration for placement of pipes on the rack is the size of expansion loop required based on results of stress analysis calculations. Lines requiring largest legs to absorb expansion should be placed along the outermost edges to accommodate larger dimensions of expansion loops. This approach permits the expansion loops to be housed within the pipe rack framework or with minimum projection (or cantilever) outside the pipe rack structure. Three dimensional loops are preferred over two dimensional loops as 3D configuration provides the flexibility in nesting the loops.

It is recommended to place lines connecting to equipments on the side of the rack which is closer to the equipment. Lines with orifice runs having orifice flanges installed should be preferably placed close to the edge of the rack and near a pipe rack column to allow convenient access by ladder.

Relief header should be provided with slope towards the KO drum. From KO drum to the flare stack, the flare header should be self-draining to the KO drum. To elminate pockets, the relief header and flare header may be placed at a higher elevation on the main piperack on an extended column with T-support or a cantilever arrangement on the extended column.

Lines in gas service should have their branch connections connected to the top of headers. Liquid lines may have their branch connections from the top or bottom.

Pipe rack width and column/support spacing

It is common practice in most engineering specification to limit the pipe rack width to 6m. If the piping does not fit this rack width, it is recommended to design the pipe rack with multiple levels or tiers. The spacing between pipe rack columns is normally maintained at 6m. Though these width and span limits are commonly used, it is not uncommon to design pipe racks with much larger widths and spans. If small bore lines are routed on the pipe racks, intermediate beams should be provided to facilitate supporting of small bore lines which have shorter support spans. Many company standards recommend a minimum line size of 2 inches NPS on the pipe rack. This is because it is in many instances more economical to increase the pipe size rather than providing additional structural steel to support small bore piping.

It is recommended not to support small bore lines from large bore lines. Most company specifications restrict this practice. In some situations if it becomes necessary to support small bore pipes from large bore piping, the following configuration may be used, subject to owners approval. In this configuration the horizontal steel member is connected by U-bolts to two large diameter pipes and acts as a cradle to support the small bore pipes. This arrangement is also referred to as pick-up pipe support.

pick up support

Pipe support anchors bays should be optimized by grouping the anchors. Loading at anchor bays of the pipe rack must be provided to structural group for designing the pipe rack to withstand the anticipated loads.

Changes in pipe rack elevation for changes in direction

Where practically feasible, pipes running along the north-south and east-west directions should be run at different elevations. In other words, when there is a change in pipe rack direction or where another pipe rack branches of the main pipe rack, it is recommended to change the elevation of piping in two perpendicular directions. The difference in elevation may vary between 750mm to 1000mm but should take into consideration the minimum fitting to fitting dimension of the largest diameter pipe routed on the rack. If one of the pipe running on the rack is significantly larger than the other pipes, the change in elevation for the larger pipe can be negotiated using a 45 degree and 90 degree elbow instead of two 90 degree elbows to optimise the elevation difference between the two perpendicular racks.

pipe rack direction change

The elevation of piping run on the pipe racks is generally represented as Bottom of Pipe (BOP) and will be the same elevation as the Top of Steel (TOS) unless a pipe shoe is provided below the pipe, in which case the BOP of pipe gets elevated by the shoe height. Changes to pipe size on the rack are accomplished using flat bottom eccentric reducers such that the bottom of pipe (BOP) elevation remains constant.  

Where a single-tier pipe rack turns 90° and all lines can be kept in the same sequence in both directions, no elevation difference may be necessary. When the line sequence changes, introducing an elevation change at the turn makes it possible to change the sequence of pipe runs.

Equipments on top of pipe rack

Large air coolers should be preferably located on top of the main pipe rack to minimise space requirements. The other benefits that accrue by adopting the approach are as follows:

  • As air coolers are relatively not very heavy equipments, their weight may be more economically supported by utilising the already existing pipe rack structure.
  • As air coolers have a large footprint, supporting them from the rack can result in significant saving in plot size and make this space available for other units in the plant.
  • Locating air coolers above the pipe rack can result in shorter pipe runs and reduced structural steel as the support members can be taken from the existing pipe rack steelwork.
  • The increased elevation of the coolers ensures good air flow and reduces the possibiity of hot air recirculation.

Large air cooler banks above pipe racks require access for maintenance of fans, motors and other ancillaries associated with the cooler. Vendors recommendation shall be followed.

In situations where air coolers have to be located on the top of pipe racks, it is preferable to match the width of the pipe rack columns to the air cooler supports.