Satish Lele
lelepiping@gmail.com

Pipe Rack Sizing and Loading Data
  • Configuration: This is the selection of "fit-for-purpose." Each main run, minor run and branch run must be looked at to determine its configuration. Will it be an overhead rack or a sleeper way? Will each be single deck (layer) or multiple deck? Will the support be a single column ("T") support or multi-column support? How many columns? A second part of the configuration issue effects pipe racks in the process units themselves. This is the question of whether or not the pipe rack will support equipment such as Air Coolers (Fin Fans). Another part of configuration is the issue of intersections. Poor planning on this issue can cause problems later with the piping.
  • Height: How high should each run of rack be? Should they be elevated or low sleepers. The sleepers are concrete with an imbedded steel plate on the top. For sleepers, they need to be off the ground to allow for maintenance and drainage also to prevent corrosion. For elevated multi-level racks what should the separation be? For elevated racks you must plan the height and the separation of the whole system together. A key element in the determination of separation is the line sizes to be carried on the racks.
  • Width: This requires a detailed study of the total piping systems for the whole plant based on pipe rack routing. In the past, a study (called a "Transposition") was done to, as best you could, account for each line on each pipe rack. From this study, a berth sequence was established and the line spacing set. A percentage was added as an error factor and then the clients "future" reserve was added. This then constituted the minimum rack width. The final width would be set after all racks were "sized" and then some might be rounded up in width for consistency, based on the materials of construction/fabrication method.
  • Spacing: This issue can be addressed after the transposition has been completed. The transposition identifies all the rack piping from the largest to the smallest From this the average line size for each leg of the rack system can be established. With the pipe size information (largest, smallest and average pipe size) the number and spacing of the pipe support bents can be set. A cost tradeoff is evaluated and made between more pipe supports spaced closer together or fewer pipe supports and some sort of intermediate support system.

Pipe Rack and Structural form basic framework to hold equipments and Piping.

    Pipe Rack work is done in 3 ways.
  • Concrete : In this case reinforced cement concrete structure is erected. The reinforcement is provided by tensile strength of steel bars and compression strength is provided by Cement. The columns and beams are provided with insert plates, to weld steel members to it. It is time consuming as all work has to be done on site and the concrete takes long time to cure.
  • Steel : This is based on steel structural members like Beams, Channels and Angles. Most of the fitting and aligning work is done in shop and shipped to site. It is then bolted on site. Design and Erection work is very fast. However, periodic maintenance and painting is required, if environment is corrosive.
  • Steel covered by Concrete : The steel structure bears all the load while cement cover protects it from corrosion. The steel structure does not melt in case of fire.
Most of the structure is made of vertical columns and horizontal beams, connected to columns. The equipment and piping rests on beams. Sufficient gap is provided between beams to lift the equipments and for passage of pipes. Columns are generally 5 meters away and height of beams is 4 meters above the floor level. Cross bracings are provided to have stability to the structure. This is where anchor points are located, especially in pipe racks. Most of the pipe supports are created from steel structural members.
Modular Pipe rack: Sometimes clients look for modular pipe rack / modular structure for their plants. Module is a series of standard units that function together. Structural Frames completely fitted with pipes, Cable trays and miscellaneous equipment. If the project site is at remote location, then it will be very difficult to get good local contractor. Parallel construction activities are possible (foundation and module fabrication) and gain in time schedule Controlled construction environment is possible Controlled quality controlled.
In some countries, labor cost at site is more than labor cost at fabrication / module assembly shop. So, if you fabricate and fit all the pipes, cable trays etc. at shop, then you can save the project cost.
Data collection for pipe rack design: Pipe rack is the main artery of any plant. This carries the pipes and cable trays (raceways) from one equipment to another equipment within a process unit (called ISBL pipe rack) or carries the pipe and cable trays from one unit to another unit (called OSBL pipe rack).
    Data Collection for different types of pipe rack:
  1. Conventional / Continuous Pipe rack: Continuous Pipe racks (conventional pipe rack) system: This is essentially a system where multiple 2-dimensional (2D) frame assemblies (commonly called bents), comprised of two or more columns with transverse beams, are tied together in the longitudinal direction utilizing beam struts (for support of transverse pipe and raceway elements and for longitudinal stability of the system) and vertical bracing to form a 3D space frame arrangement. Pipe racks supporting equipment such as air-cooled heat exchangers must utilize the continuous system approach.
    Data collection for pipe rack design: Due to the “fast track” nature associated with most of the projects, often the final piping, raceway, and equipment information is not available at initiation of the pipe rack design. Therefore, as a Civil/Structural Engineer, you should coordinate with the Piping group, Electrical, Control Systems, and Mechanical groups to obtain as much preliminary information as possible. When received, all design information should be documented for future reference and verification. In the initial design, the Engineer should use judgment when applying or allowing for loads that are not known, justifying them in the design basis
    • Plot plans and equipment location plans.
    • 3D model showing piping layout, cable tray layout, Pipe rack bent spacing and elevation of support levels in the transverse direction , Elevation of longitudinal beam struts and locations of vertical bracing. and location of pipe bridge, if any.
    • Piping orthographic drawings.
    • Vendor prints of equipment located on the rack, e.g., air coolers and exchangers. The vendor prints should include the equipment layout, mounting locations and details, access and maintenance requirements, and the magnitude and direction of loads being transmitted to the pipe rack.
    • Electrical and control systems drawings showing the routing and location of electrical and instrumentation raceways and/or supports.
    • Underground drawings that show the locations of buried pipes, concrete structures and foundations, duct banks, etc. in the area of the pipe rack.
    • Pipe rack construction material (Steel, Cast-in-situ concrete, Pre-cast concrete) shall be as per project design criteria.
    Design loads consideration:
    • Piping Gravity load (D): In the absence of defined piping loads and locations, an assumed minimum uniform pipe load of 2.0 kPa should be used for preliminary design of pipe racks. This corresponds to an equivalent load of 6 in (150 mm) lines full of water covered with 2 in (50 mm) thick insulation, and spaced on 12 in (300 mm) centers. This assumption should be verified based on coordination with the Piping Group, and concentrated loads should also be applied for any anticipated large pipes. When the actual loads and locations become known, as the project develops, the structural design should be checked against these assumed initial load parameters and revised as required. A concentrated load should then be added for pipes that are 12 in (300 mm) and larger in diameter. The concentrated load P should be:
      P =(W - s x p x d), s = Spacing of pipe rack bent, p = pipe weight considered (kPa), d = pipe diameter W = pipe concentrated load.
      Where consideration of uplift or system stability due to wind or seismic occurrences is required, use 60% of the design gravity loads as an "all pipes empty" load condition.
      Loading due to hydrostatic testing of lines should be considered in the design if applicable. Coordinate the testing plan(s) with Construction, Startup, and/or the Piping Group as necessary, in order to fully understand how such loads will be applied to the pipe rack structure. Under most normal conditions, multiple lines will not be simultaneously tested. The hydro-test loads do not normally need to be considered concurrently with the other non-permanent loads, such as live load, wind, earthquake, and thermal. Typical practice is to permit an overstress of 15% for the hydro-test condition. Because of these considerations, the hydro-test condition will not normally govern except for very large diameter pipes.
    • Electrical Tray and Conduits (D): Electrical and control systems drawings and/or the project 3D model should be reviewed to determine the approximate weight and location of electrical trays, conduits, and instrumentation commodities. Unless the weight of the loaded raceways can be defined, an assumed minimum uniform load of 1.0 kPa should be used for single tier raceways.
    • Self weight of Pipe rack (D): The weight of all structural members, including fireproofing, should be considered in the design of the pipe rack.
    • Weight of Equipment on pipe rack (D): Equipment weights, including erection, empty, operating, and test (if the equipment is to be hydro-tested on the pipe rack), should be obtained from the vendor drawings. The equipment weight should include the dead weight of all associated platforms, ladders, and walkways, as applicable. Special Loads: Special consideration should be given to unusual loads, such as large valves, expansion loops, and unusual piping or electrical configurations.
    • Live Load (L): Live load (L) on access platforms and walkways and on equipment platforms should be considered, as applicable.
    • Snow Load (S): Snow load to be considered on cable tray and on large dia pipes. This load shall be calculated per project approved design code and project design criteria. Generally, you need to consider 100% snow load on top tier and 50% on other tier of pipe racks.
    • Wind Load (W): Transverse wind load on structural members, piping, electrical trays, equipment, platforms, and ladders should be determined in accordance with project approved design code. Longitudinal wind should typically be applied to structural framing, cable tray vertical drop (if any), large dia pipes vertical drop (if any) and equipment only. The effects of longitudinal wind on piping and trays running parallel to the wind direction should be neglected.
    • Earthquake Loads (E): Earthquake loads in the vertical, transverse, and longitudinal directions should be determined in accordance with the project design criteria. Vertical, transverse, and longitudinal seismic forces generated by the pipes, raceways, supported equipment, and the pipe rack structure should be considered and should be based on their operating weights. Pipes must be evaluated for seismic loads under both full and empty conditions and then combined with the corresponding gravity loads.
    • Friction Loading (Tf): Friction forces caused by hot lines sliding across the pipe support during startup and shutdown are assumed to be partially resisted through friction by nearby cold lines. Therefore, in order to provide for a nominal unbalance of friction forces acting on a pipe support, a resultant longitudinal friction force equal to 7.5% of the total pipe weight or 30% of any one or more lines known to act simultaneously in the same direction, whichever is larger, is assumed for pipe rack design. Friction between piping and supporting steel should not be relied upon to resist wind or seismic loads.
    • Anchor and Guide Loads (Ta): Pipe racks should be checked for anchor and guide loads as determined by the Pipe Stress Group. It may be necessary to use horizontal bracing if large anchor forces are encountered. For conventional pipe rack systems, it is normally preferred to either have the anchors staggered along the pipe rack so that each support has only one or two anchors, or to anchor most pipes on one braced support. For initial design, when anchor and guide loads are not known, use a longitudinal anchor force of 5.0 kN acting at mid span of each bent transverse beam (refer project design criteria). Guide loads are usually small and may be ignored until they are defined by the Pipe Stress Engineer. For non-continuous pipe rack systems, piping may be transversely guided or anchored at both cantilever frames and anchor bays. Longitudinal anchors may be located only at anchor bays.
    Framing of Continuous/Conventional Pipe rack:
    • Frames: Main pipe racks are usually designed as moment-resisting frames in the transverse direction. In the longitudinal direction, there should be at least one continuous level of beam struts on each side. For pipe racks with more than one tier, the beam struts should be located at a level that is usually equal to one-half tier spacing above or below the bottom tier. Vertical bracing in the longitudinal direction should be provided to carry the longitudinal forces, transmitted through the beam struts, to the base plate / foundation level.
    • Transverse Beam: Transverse beams must be capable of resisting all forces, moments, and shears produced by the load combinations. Transverse beams are generally a moment-resisting frame, modeled and analyzed as part of the frame system. The analysis model must reflect the appropriate beam end conditions. In the design of beams, consideration should be given to
      • Large pipes that are to be hydro-tested.
      • Anchor and friction load with large magnitude
    • Central Spine: For steel pipe racks with spans of more than 6 m, a center spine consisting of a system of horizontal braces and struts located at mid span of each level of piping should be considered . This additional light horizontal framing greatly increases the capacity of the transverse pipe support beams to resist friction and anchor forces, and also serves to reduce the unbraced length of the beam compression flange in flexure and to reduce the unbraced length of the beam about the weak-axis in axial compression. This concept reduces the required beam sizes and provides a mechanism for eliminating or minimizing design, fabrication, or field modifications that could otherwise be required due to late receipt of unanticipated large pipe anchor forces.
    • Longitudinal Beam Strut: For typical continuous pipe rack systems, the longitudinal beam struts should be designed as axially loaded members that are provided for longitudinal loads and stability. Additionally, the longitudinal beam struts that support piping or raceway should be designed for 50% of the gravity loading assumed for the transverse pipe or raceway support beams, unless unusual loading is encountered. This 50% gravity loading will account for the usual piping and raceway take-offs. Normally, the gravity loading carried by the beam struts should not be added to the design loads for the columns or footings since pipes or raceway contributing to the load on the beam struts would be relieving an equivalent load on the transverse beams.
      For any continuous pipe rack system where the anticipated piping and raceway take-offs are minimal or none, the 50% loading criteria does not apply. In such cases, the beam struts should be designed primarily as axially loaded members. Do not provide beam struts if they are not needed for piping or raceway support, or for system stability. Conversely, the 3D model should be checked to verify that beam struts subjected to unusually large loads (such as at expansion loops) have been given special consideration. All longitudinal beam struts, including connections, should be designed to resist the axial loads produced by the longitudinal forces.
      When designing the longitudinal beam struts for flexural loads, the full length of the beam should be considered as the unbraced length for the compression flange.
    • Vertical Bracing: When moment-resisting frame design is not used in the longitudinal direction, vertical bracing should be used to transmit the longitudinal forces from the beam struts to the foundations. Knee-bracing or K-bracing is most often used for this purpose. Unless precluded by equipment arrangement or interferences, bracing should be placed equidistant between two expansion joints. Design calculations and drawings must reflect a break in the beam strut continuity between adjacent braced sections through the use of slotted connections or by eliminating the beam struts in the bays designated as free bays. The maximum length of a braced section should be limited to 48m to 50m. If the braced bay is not located equidistant from the free bays, the maximum distance from the braced bay to a free bay should be limited such that the maximum total longitudinal growth or shrinkage of the unrestrained segment does not exceed 40 mm.
    • Column: The columns must be capable of resisting all loads, moments, and shears produced by the load combinations. A moment-resisting frame analysis should normally be used to determine the axial load, moment, and shear at points along the columns. The frame analysis model should be based on the following:
      • Consider column base as hinge.
      • Use 4 bolt connections for safety purpose.
      For design of steel columns subjected to flexural loads, the distance between the base and the first transverse beam or the knee brace intersection should be considered as the compression flange unbraced length.

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