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Previous Issue: New Next Planned Update: 1 September, 2007 Page 1 of 81 Primary contact: Abu-Adas, Hisham on phone 874-6908
2 References ........................................................................................................... 42.1 Saudi Aramco Standards....................................................................... 52.2 Industry Codes and Standards.............................................................. 5
3 General ................................................................................................................ 5
The purpose of this practice is to provide guidelines for steel piperack design foruse by engineers working on Saudi Aramco projects and Saudi Aramcoengineers.
1.2 Scope
This design guide defines the minimum requirements for the design of piperacksin process industry facilities at Saudi Aramco sites. It covers general designphilosophy and requirements to be used in the analysis and design of piperacks.Criteria presented herein pertain to loads, load combinations, allowable stresses,and superstructure and foundation design. Section 2 of this instruction includesreference codes, and Saudi Aramco Standards.
1.3 Disclaimer
The material in this Best Practices document provides the most correct andaccurate design guidelines available to Saudi Aramco which comply withinternational industry practices. This material is being provided for the generalguidance and benefit of the Designer. Use of the Best Practices in designingprojects for Saudi Aramco, however, does not relieve the Designer from hisresponsibility to verify the accuracy of any information presented or from hiscontractual liability to provide safe and sound designs that conform toMandatory Saudi Aramco Engineering Requirements. Use of the information ormaterial contained herein is no guarantee that the resulting product will satisfythe applicable requirements of any project. Saudi Aramco assumes noresponsibility or liability whatsoever for any reliance on the informationpresented herein or for designs prepared by Designers in accordance with theBest Practices. Use of the Best Practices by Designers is intended solely for,and shall be strictly limited to, Saudi Aramco projects. Saudi Aramco® is aregistered trademark of the Saudi Arabian Oil Company. Copyright, SaudiAramco, 2002.
1.4 Conflicts with Mandatory Standards
In the event of a conflict between this Best Practice and other Mandatory SaudiAramco Engineering Requirement, the Mandatory Saudi Aramco EngineeringRequirement shall govern.
This Best Practice is based on the latest edition of the references below, unlessotherwise noted. Short titles will be used herein when appropriate. Short titles will beused herein when appropriate.
2.1 Saudi Aramco Standards
Saudi Aramco Engineering Standards
SAES-A-112 Meteorological and Seismic Design Data
SAES-Q-005 Concrete Foundations
Saudi Aramco Best Practices
SABP-002 Spread Footings Design
SABP-006 Wind loads on Piperacks & Open FrameStructures
2.2 Industry Codes and Standards
American Concrete Institute (ACI)
ACI 318 Building Code Requirements for ReinforcedConcrete and Commentary
American Society of Civil Engineers (ASCE)
ASCE 7 Minimum Design Loads for Buildings and OtherStructures
Wind Load and Anchor Bolt Design for Petrochemical Facilities
Guidelines for Seismic Evaluation and Design of Petrochemical Facilities
American Institute of Steel Construction (AISC)
AISC Manual of Steel Construction – Allowable Stress Design (ASD)
American Society for Testing and Materials (ASTM)
ASTM A36 Specification for Carbon Structural Steel
ASTM A325 Specification for High strength Bolts forStructural Steel Joints, Including Suitable nutsand Plain Washers
ASTM A992/A992M Specification for Steel for Structural Shapes forUse in Building Framing
3.1 Piperacks are structures that support pipes and auxiliary equipment within andbetween process areas of industrial plants. Piping loads can vary greatly fromproject to project as can the loads from wind and earthquake. Clearly, it isdifficult to define specific criteria for the design of such structures. Thisguideline, however, sets forth general requirements, which the Engineer shouldincorporate into piperack designs if possible.
3.2 This guideline applies to the following three basic types of steel piperacks:
• Strutted main piperacks
• Unstrutted secondary or miscellaneous piperacks
• "T" supports
3.3 Structural steel design shall be in accordance with the referenced AISCspecifications and codes. The plastic design method in the AISC manual shallnot be used in steel design. Steel for piperack design will normally be A-36 orASTM A992/A992M.
3.4 Piperacks and their foundations shall be designed to support loads associatedwith full utilization of the available rack space, and any specified futureexpansion.
3.5 Foundation concrete shall be designed in accordance with ACI 318. Theminimum 28 day compressive strength of concrete shall be 4000 psi, and shallbe noted on the drawings.
3.6 Piperack superstructures and foundations shall be designed for the loads andload combinations specified in Sections 4.0 and 5.0 of this guideline.
3.7 The deflection requirements for piperack beams and transverse bents shall be asfollows:
The maximum allowable beam deflection Dmax due to total load shall be asfollows:
Dmax = L/240 L = the Span Length
The maximum allowable drift limits for piperack shall not exceed H/150(where H = piperack height).
The maximum allowable seismic drift limits for piperack shall be in accordancewith ASCE 7 - 95 Table 9.2.2.7 (Category IV Structure in accordance withASCE 7 Table 1-1 classification). Piperacks shall be considered as building.
The maximum allowable drift limits for piperack shall not exceed H/100(where H = piperack height).
3.8 Connections for steel piperacks shall conform to the following requirements:
a. Shop connections may be either bolted or welded. Field connections shallbe bolted where possible. Connections may be field welded whenconditions are such that a bolted connection is not suitable.
b. Bolted connections for primary members shall utilize high-strength boltsconforming to ASTM A-325-N, bearing-type connections with threadsincluded in the shear plane. However, slip-critical-type connections shallbe used in connection subject to vibration or repeated stress reversal.
c. Standard connections shall be designed by the fabricator in accordancewith the project construction specifications and loads shown on thedrawings. Moment connections and special connections, however, shall bedesigned by the engineer and shall be shown on the engineering drawings.
d. Moment connections shall preferably be of the bolted end plate type.
4 Primary Loads
The following loads shall be considered in the design of piperack superstructures andfoundations:
D - Dead Load
PL - Product Load
Pt - Test Load
TL - Thermal Load
W - Wind Load
E - Earthquake
O - Other Loads
The above loads are defined as follows:
4.1 Dead Load (D)
4.1.1 Dead load shall include the weight of all process equipment, pipes,valves and accessories, electrical and lighting conduits, trays, switchgear,instrumentation, fireproofing, insulation, structural steel plates andshapes, etc. Foundation concrete weight along with any soil overburden
shall also be considered as dead load. All piping shall be consideredempty of product load (PL) when calculating dead load.
4.1.2 Piperacks shall be designed for present and future dead loads. Unlessstipulated otherwise by Saudi Aramco, piping and electrical loads shallnot be less than the following:
a. A minimum pipe deck load of 23 psf (1.10 kPa) shall be used forthe design of major piperacks. This is equivalent to 8-inch (203mm) diameter, Schedule 40 pipes spaced at 15-inch (381 mm)centers.
b. Along with the minimum pipe deck loads specified above, aconcentrated load shall be added at pipes that are larger than 12inches (300 mm) nominal diameter on the support. Theconcentrated load in pounds, PDL, shall be calculated using thefollowing equation:
PDL = S (WDL - pDL D)
Where:
S = Pipe support spacing (ft)
WDL = Large pipe weight per foot (plf)
pDL = Average pipe deck loading (psf)
D = Large pipe diameter (ft)
c. Single level and double level electrical cable trays shall have aminimum uniformly distributed weight of 20 psf (0.96 kPa) and40 psf (1.92 kPa), respectively. The cable tray load shall beconsidered as dead load. Tray locations shall be as shown onelectrical drawings.
4.2 Product Load (PL)
4.2.1 Product load shall be defined as the gravity load imposed by liquid orviscous material in piping during operation.
4.2.2 Piperacks shall be designed for present and future product loads. Unlessstipulated otherwise by Saudi Aramco, product loads shall not be lessthan the following:
a. A minimum product load of 17 psf (0.81 kPa) shall be used at eachlevel for the design of major piperacks. This is equivalent to 8-inch(203 mm) pipes full of water spaced at 15-inch (381 mm) centers.
b. Along with the minimum piping product loads specified above, aconcentrated load shall be added at pipes that are at least largerthan 12 inches (300 mm) nominal diameter on the support. Theconcentrated load in pounds, PPL, shall be calculated using thefollowing equation:
PPL = S (WPL - pPL D)
Where:
S = Pipe support spacing (ft)
WPL = Large pipe product load per foot (plf)
pPL = Average product loading (psf)
D = Large pipe diameter (ft)
4.3 Test Load (Pt)
The test load shall be defined as the gravity load imposed by the liquid(normally water) used to pressure test the piping. Large vapor lines may requirehydrotesting. If so, it may be possible to test them one at a time while the otherlines on the support are empty and thus avoid the heavy pipe support loading.When such procedures are used, special notes should be placed on the structuraland piping drawings to specify test procedures. Small vapor lines are normallyconsidered filled with water.
4.4 Thermal Loads
Thermal loads shall be defined as forces caused by changes in the temperatureof piping. For piperack design, both friction forces (FF) and anchor forces(AF) shall be considered. Pipe supports must be designed to resist longitudinalloads arising from pipe thermal expansion and contraction. On the averagepipeway, the lines expand and contract varying amounts at random times. Theseloads are applied to the transverse beams either through friction or through pipeanchors. Thermal loads shall be considered as dead load and included in theappropriate load combinations.
4.4.1 Friction Forces (FF)
Friction forces caused by hot lines sliding across a pipe support duringstart-up and shut-down are assumed to be partially resisted by adjacentcold lines. The resultant longitudinal friction force, however, shall betaken as the larger of the following:
a. 10% of the total operating weight of all lines tributary to thesupport
b. 30% of the total operating weight of those lines tributary to thesupport, which will expand or contract simultaneously.
The 10% of the total piping weight shall be taken as an estimatedlongitudinal friction forces (FF) applied only to local supporting beams.However, an estimated friction force equal to 5% of the total pipingweight shall be accumulated and carried into piperack struts, columns,braced anchor frames, and foundations.
Pipe friction loads shall not be combined with wind or seismic loads forthe design of piperack struts, columns, braced anchor frames, andfoundations, when there are multiple frames. During high wind orearthquake, the vibration and deflection of the supports under load willlikely relieve the friction forces.
4.4.2 Anchor Forces (AF)
Anchor forces may dictate the use of horizontal channels or horizontalbracing as well vertical bracing at anchor bents. This should not occurtoo frequently since Piping Engineering like to anchor large lines on onlya few bents in a pipeway. Anchor and guide forces and locations shallbe obtained from the piping stress analysis and piping isometricdrawings.
Pipe anchor and guide forces (AF) produced from thermal expansion,internal pressure, and surge shall be considered as dead loads. Piperacksbeams, struts, columns, braced anchor frames, and foundations shall bedesigned to resist actual pipe anchor and guide loads. For local beamdesign consider only the top flange as acting in horizontal bending unlessthe pipe anchor engages both flanges of the beam. Anchor and pipeforces shall be obtained from the checked pipe stress analysis computerrun.
Anchor and guide loads (excluding their friction component) shall becombined with wind or seismic loads.
4.4.3 Temperature Force (TF)
Thermal forces caused by structure expansion and contraction should beconsidered in the design with the structural steel checked for temperaturechange. Range of temperature change shall be in accordance withSAES-A-112. Refer to Section 7.1.6 for requirements. Designtemperature shall be defined as the difference between the highest andlowest one day mean temperature plus the metal temperature for the
sunheating effects on structural steel which can be estimated at about20°C.
4.5 Wind Load (W)
4.5.1 Wind loads on all pipe, equipment, structural members, cable trays,platforms, ladders, and other attachments to the piperack shall beconsidered in the design. Wind pressures, wind pressure distribution,and pressure coefficients shall be computed and applied in accordancewith ASCE 7 - 95 and the Saudi Aramco Best Practice SABP-006 "WindLoads on Piperacks and Open Frame Structures".
4.5.2 The total wind load per foot on pipes, F, can be determined using thefollowing equation:
4.5.3 For major piperacks, the design lateral wind load on pipes at each pipedeck shall not be less than the wind load computed for 12-inch (300 mm)pipes at 15-inch (381 mm) centers.
4.5.4 Longitudinal wind load on piperacks is negligible compared to otherlongitudinal forces and, therefore, can normally be disregarded.
4.5.5 For detailed wind load calculations on piperacks, refer to criteriaspecified in Saudi Aramco Best Practices SABP-006 "Wind Loads onPiperacks and Open Frame Structures".
Earthquake loads shall be computed and applied in accordance with ASCE 7 -95. The earthquake loads in ASCE 7 are limit state seismic loads and thisshould be taken into account when using allowable stress design methods andapplying load factors from other codes, etc.
ASCE's Guideline for Seismic Evaluation of Design of Petrochemical Facilitiesshall also be used for seismic design. The Rw factors in ASCE's SeismicGuidelines Tables 4.4 may be converted to R factors for use with ASCE 7 bydividing by 1.4. For steel piperack, with an Ordinary Moment Resisting Frame,the Rw value is 6. Therefore, the response modification factor to be used inASCE 7 is 6 divided by 1.4 equals to R = 4.29.
Seismic zones, effective peak acceleration, effective peak velocity and site soilcoefficient shall be determined in accordance with SAES-A-112"Meteorological And Seismic Design Data". All plant area structures shall beconsidered essential facilities.
The Importance Factor I shall be Category IV.
4.7 Other Loads (O)
Piperacks may be subjected to loads not covered by the six categories describedabove.
The following load combinations of loads are for use in conjunction with theallowable stress method of design. The load combinations shown below are themost common load combinations but may not cover all possible conditions.Any credible load combinations that could produce the maximum stress orgovern for stability should be considered in the calculations. Theses loadcombinations shall be considered in superstructure and foundation design ofpiperacks.
D + PL + FF + TF + AF (if any) Load Comb. 1(Max. Operating Gravity Loads)
0.75 (0.9 D + W) Load Comb. 2(Min. Dead Load + Wind)
0.75 (D + PL + AF + W or E) Load Comb. 3(Max. Oper. Gravity + W or E)
0.80 [D + Pt + (1/4 W or 1/4E)] Load Comb. 4(Test Load + W or E)
where:
D = Dead Load
PL = Product Load
AF = Anchor Force
TF = Temperature Force
Pt = Test Load
W = Wind Load
E = Earthquake Load
5.1.1 Wind forces and earthquake forces shall not be considered to actsimultaneously.
5.1.2 The engineer should use his judgment in selecting potential criticalcombinations. Load conditions that have primarily a localized effectgenerally do no need to be included in the main analysis as these loadsmay be considered during individual structural component design.
5.1.3 In combinations involving Test Load (Pt), and W or E load, only ¼ of theload need be considered. For wind load, this is justified becausehydrotests are not conducted during high winds and, for earthquake load;the probability of shocks occurring during hydrotest is low.
5.2 Loading Combinations and Load Factors – Strength Design
The following load combinations of loads are for use in conjunction with thestrength design method and may be used for foundation design. The loadcombinations shown below are the most common load combinations but maynot cover all possible conditions. Any credible load combinations that couldproduce the maximum stress or govern for stability should be considered in thecalculations.
1.4D + 1.4Pt + (0.57W or 0.63E) Load Comb. 4(Test Load + W or E)
6 Allowable Stresses and Strength Requirements
6.1 Structural Steel
The allowable stresses and stress increases specified in the AISC manual shallbe used for all piperack steel design with the following exception:
Exception:
Under test conditions, the allowable stress for all structural steel elements andtheir connections may be increased 20% when a partial wind or earthquake loadis included.
6.2 Anchor Bolts
The design of anchor bolts shall conform to requirements of Paragraph 4.7 ofSAES-Q-005 and SABP-001.
6.3 Cast-in-Place Concrete
Strength design methods of ACI shall be used in piperack footing design. Forfooting design requirements see SAES-Q-005 and Saudi Aramco Best PracticeSABP-002 "Spread Footings Design".
7 Piperack Superstructure Design
7.1 General
7.1.1 The principal structural components of a piperack are the transverse bentbeams, the bent columns, longitudinal struts, and vertical bracing.Design criteria applicable to each of these components are presentedbelow.
7.1.2 In general, the pipe support framing system is designed as rigid framebents with fixed or pinned bases in the transverse direction and as bracedframes in the longitudinal direction.
7.1.3 A determined effort should be made early on the project to establish thecorrect number of transverse beam levels required for piping andelectrical support, and the number of longitudinal beams required tosupport pipes entering or leaving the pipeway. Additional longitudinaland/or intermediate transverse beam may be required to support
electrical conduit, instrumentation lines, or other small lines. Electricalconduit and cable trays usually must be supported every 10 feet.
7.1.4 Structural components of the piperack must be capable of resisting theaxial loads, shears, moments, and torsion produced by the loadcombinations given in Section 5.0 of this guideline.
7.1.5 An elastic analysis shall be used to determine moments and forces inpiperack members.
7.1.6 Structural Steel Expansion
For piperack design, provisions shall be made for thermal expansion ofsteel, with the structural steel checked for temperature change. Slottedconnections (sliding connection) shall be provided in each segment ofthe piperack between vertical bracing to allow for structural steel thermalexpansion. The maximum segment for the piperack shall be limited to140 feet (42.5 meters) in length unless calculations show otherwise.Details and requirements for the slotted connection shall be provided onthe engineering drawings.
7.2 Transverse Bent Beams
7.2.1 In computing the allowable bending stress, Fb, the unbraced length shallbe taken as the span of the beam and the AISC factor Cb shall be used toaccount for end fixity. A Cb value of 1.0 is a very conservative and safeassumption. In no case shall the assumption of lateral support frompiping be used in computing Fb.
7.2.2 Generally, the depth of horizontal members should not be less than 1/24of the span.
7.2.3 If top flange lateral loads are significant, the transverse beam shall beinvestigated for bending about the y-y axis and for torsion. This can beestimated by using My x 2 / Sy.
7.2.4 In axial load design, the total span of the beam should be used, modifiedby the appropriate effective length factor for each direction. This factorshould be equal to 1.0 for the weak direction of the beam.
7.2.5 Special consideration shall be given to the design of transverse beamswhich support large vapor lines to be hydrotested or which support largeanchor or guide forces. Horizontal bracing may be required locally if thelocal bending stresses are too high.
7.3.1 In strutted piperacks, columns shall normally be designed with pinned orfixed bases depending on the lateral drift requirements.
7.3.2 In unstrutted piperacks, column bases shall be considered pinned in thetransverse direction and fixed in the longitudinal direction. The majoraxis of columns should normally be perpendicular to the longitudinaldirection of the piperack (i.e., plane formed by column web is parallel tolongitudinal direction).
7.3.3 "T" support column bases shall be considered fixed in both the transverseand longitudinal directions. The major axis of columns may be turned ineither direction.
7.3.4 Column base plates for major and miscellaneous piperacks and "T"supports that are to be attached to concrete foundations shall be four-boltbase plates.
7.4 Longitudinal Struts
7.4.1 In areas where gravity loading of struts is anticipated, struts shall bedesigned for axial loads produced by longitudinal pipe loads plus gravityload moments and shears. Such struts should be designed for the actualload but not less than 50% of the gravity loading of the loaded transversepipe support beam. This loading requirement will account for the usualpiping and electrical conduit that is "rolled-out" of the piperack.Concentrated loads for large pipes shall also be included in design.
7.4.2 Where gravity loading is not anticipated, struts shall be designed foraxial load only. The primary source of axial loads is longitudinal pipeloads.
7.5 Vertical Bracing
7.5.1 Vertical bracing may be used to transmit transverse and longitudinalforces to the foundations. K-bracing or X-bracing is usually used for thispurpose.
7.5.2 Braced bays in strutted piperack systems should be spaced at 140 feet(42.5 meters) maximum. Longitudinal bracing should be provided inabout every fourth bay.
7.5.3 Compression bracing for steel piperack systems shall normally bedesigned with wide flange and structural tee shapes. For tension bracing,single angle, double angle or structural tees may be used.
8.1 Foundations shall be designed in accordance with the project soil reportrecommendations and SAES-Q-005 "Concrete Foundations".
8.2 The type of foundation to be used for piperacks shall be established based on thesoil report recommendations.
8.3 In piperack foundation design, buoyant load shall be considered whenapplicable. The buoyant load included in the design shall be based on projectwater table elevations (permanent or temporary), which produce the mostunfavorable effect on the foundation.
Revision Summary31 August, 2002 New Saudi Aramco Best Practice (SABP-007).
Design typical piperack bent in Uthmaniyah Gas Plant. The piperack configuration shall be asshown in example 1 (Figures 1 through 6), and with a 3-sec. Gust wind speed of 96 mph perSAES-A-112. Earthquake zone is 0, therefore seismic loads need not be considered in theanalysis and design.
Assumptions:
The main beams shall be W10X33 for the cable tray support and W12X40 and W12X45 for thepipe supports. The beam levels are 20.00, 25.00 and 20.00. The beams are rigidly connectedto the columns (i.e., moment connection)
The columns are W14X53 and fixed at the base.
The longitudinal struts (W10X33) located at levels 17.50, 22.50 and 30.00 acts as struts totransfer thermal load to the vertical bracing of the rack. These levels will be considered asbraced in the longitudinal direction. Refer to Figure 2 for Bent Framing arrangement.
Primary loads to be considered are as follows:
D, PL, FF, TF & W (assume no anchor loads and no pipes will be tested at this bent)
Load Combinations to be considered are as follows:
Structural steel shall be designed based on SAES-A-112. The Design Temperature shall be thedifference the highest and lowest one-day mean temperature. For Uthmaniyah it will be 106-43= 63°F plus metal temperature for the sunhearting effects on structural steel which can beestimated at about 36°F or (20°C).
Design Temperature = (63 + 36) = 99°F (103°F is used in this example – say ok)
Wind Loads
Design wind forces are determined by the equation listed below, where F is the force per unitlength of the piping or cable tray (For Force Coefficients and details, refer to Structural DesignBest Practices Guidelines for "Wind Loads on Piperacks and Open Frame Structures"):
F = qz G Cf Ae ASCE 7 Table 6-1
Design wind pressure, for 30 ft elevation from Table 1
qz = 26.59 psf
Gust effect factor, G = 0.85 (ASCE 7, Section 6.6.1)
The guidelines require the consideration of the piping or cable trays separately from thestructural members. The following calculations are only for piping and cable trays without thestructural support members:
Unity Check:Ensure that unity check for all structural members are less than 1.0
Beam Deflection:Ensure that maximum beams vertical deflection is less than L/240where L = span length
Lateral Drift:Ensure that maximum lateral drift for the piperack is less than H/150 for load combinationswith wind load and H/100 for earthquake case.
Connections & Columns Base Plate:Design Beam/Column moment connection based on AISC Steel Manual procedure.Design vertical and horizontal bracings connections based on member loads and in accordancewith the AISC Steel Manual procedure.Design Columns Base Plates based on AISC Steel Manual procedure.
Foundations:Design columns Foundations in accordance with the requirements of SAES-Q005 and theSaudi Aramco Best Practices SABP-002 "Spread Footings Design".
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