Stress Analysis Piping Systems
Piping Stress Analysis
Piping Stress analysis is a term applied to calculations, which address the static and dynamic loading resulting from the effects of gravity, temperature changes, internal and external pressures, changes in fluid flow rate and seismic activity. Codes and standards establish the minimum requirements of stress analysis.
Purpose of piping stress analysis Purpose of piping stress analysis is to ensure: Safety of piping and piping components. Safety of connected equipment and supporting structure. Piping deflections are within the limits
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Piping Stress Analysis
Interrelated with Piping layout and support design Layout should take care of sufficient flexibility for thermal expansion, and simplified supports Pipe section properties to be suitable for intended service, temperatures, pressures and anticipated loadings.
Support locations and types to satisfy nozzle loads, valves accelerations and piping movements.
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ASME B31.3 Process Piping Course from web BECHT Engineering Company, Inc.
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The Stress-Strain curve (1/3)
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The Stress-strain curve (2/3)
The stress-strain curve characterizes the behavior of the material tested. It is most often plotted using engineering stress and strain measures, because the reference length and cross-sectional area are easily measured. Typical regions that can be observed in a stress-strain curve are:
Elastic region Yielding Strain Hardening Necking and Failure
It should be emphasized that the extent of each region in stress-strain space is material dependent, and that not all materials exhibit all of the above regions. In the elastic region, the slope of the stress-strain curve is the Young's Modulus. Thus s = e E
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Stress-strain curves for structural steel (ASTM A36) at elevated temperatures (3/3)
The yield and ultimate strength decrease with 12/10/2012
temperature as does the modulus of elasticity.
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Examples
What's the Young's Modulus of a steel bar that has a crosssectional area of 0.73 in2, is 4 inches long, and supports a load of 3.9x107 lbs, deforming 0.2 percent?
E = 9.36 x 10-6 psi E = 2.67 x 1010 psi E = 3.74 x 10-11 psi E = 2.67 x 10+10 lbs.
During plastic deformation, the volume of the specimen, as well as its cross-sectional area, decrease. Yes/No?
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Failure Theories
The two theories commonly used are
Maximum principal stress theory Maximum shear stress theory
Maximum principal stress theory forms basis of B31 series codes.
Yielding in a pipe component occurs when the magnitude of any of the three mutually perpendicular principal stresses exceeds the yield strength of materials.
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Stress Categories The major stress categories are
Primary Stresses – These are developed by the imposed loading. Limits are intended to prevent plastic deformation and bursting. These satisfy equilibrium between internal and external forces and moments of the piping system. Primary stresses are not self-limiting. Secondary Stresses – Primary + Secondary stress limits are intended to prevent excessive plastic deformation leading to incremental collapse. These are developed by the constraint of displacements of a structure – e.g. Thermal expansion or movement of an anchor. Secondary stresses are self-limiting. Peak Stress – limit is intended to prevent fatigue failure from cyclic loading. Examples are stress concentrations at discontinuity and thermal gradient through pipe walls
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Secondary Stresses
Piping system must satisfy an imposed strain pattern rather than be in equilibrium with imposed forces. Local yielding and minor distortions tend to relieve these stresses.
Therefore, secondary stresses are self limiting. Only ductile materials with a well defined minimum yield points are used in piping wherever thermal stresses are encountered
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Classifications of Loads
Sustained loads: present throughout normal plant operations (pressure, weight etc.) Occasional loads: infrequent intervals during plant operations e.g. earthquake, wind, transients e.g. water hammer, relief valve discharge Expansion loads – thermal expansions, seismic anchor movements, thermal anchor movements, building settlements.
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B31.3-2008 Process Piping Code The loadings required to be considered are Pressure Weight (live and dead loads) Impact Wind Earthquake induced horizontal forces Vibrations Discharge reactions Thermal expansions and contractions Temperature gradients Anchor movements.
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Stresses due to sustained loads
The sum of longitudinal stress SL due to pressure, weight, and other sustained loads must not exceed Sh (basic allowable stress at maximum temperature).
SL ≤ Sh SL = P D / 4 t + Sb
The thickness of pipe used in calculating SL shall be the nominal thickness minus mechanical, corrosion, and corrosion allowances.
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Stresses due to occasional loads
The sum of the longitudinal stresses due to pressure, weight and other sustained loads and of the stresses produced by occasional loads such as earthquake or wind shall not exceed 1.33Sh Earthquake and wind loading need not be considered as acting simultaneously.
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Stress range due to expansion loads
The displacement stress range SE shall not exceed SA SE ≤ SA Where SE = (Sb2 + 4 St2)1/2 Sb resultant bending stress psi = [(ii Mi)2 + (io Mo)2]1/2 / Z Mi in-plane bending moment in-lbs Mo out of plane bending moment in-lbs ii = In plane stress intensification factor io = out of plane stress intensification factor St = torsional stress, psi = Mt / (2Z) Mt = torsional moment in-lbs For definition of SA, see next slide
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Allowable Displacement Stress Range SA [Refer ASME B 31.3-2008, 302.3.5(d)]
(1/2)
The computed displacement stress range SE shall not exceed allowable displacement stress range. SA = allowable displacement stress range = f (1.25 Sc + 0.25 Sh) when Sh > SL SA = f [1.25 (Sc + Sh) – SL], Sc = basic allowable stress at minimum metal temperature, psi Sh = basic allowable stress at maximum metal temperature, psi f = Stress range factor = 6.0 (N)-0.2 ≤ fm fm = maximum value 1.2 for ferrous materials with SMTS < 517 MPa (75 ksi), metal temp ≤ 371 °C (700 °F), otherwise fm = 1.0
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Allowable Displacement Stress Range SA [Refer ASME B 31.3-2008, 302.3.5(d)]
(2/2)
N equivalent number of full displacement cycles during the expected service life of the piping system
When computed stress range varies, whether from thermal expansion or other conditions, SE is defined as greatest computed displacement stress range. The value of N can be calculated as N = NE + Σ(ri5 Ni) for i = 1, 2, …., n
ri = Si / SE, Si is any computed displacement stress range smaller than SE
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Stress Range Factor f
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Stress Intensification Factor (SIF)
A Stress Intensification Factor (SIF) is defined as the ratio between the peak stress and average stress in a given component:
SIF = Actual Peak Stress / Nominal Stress in Part
A. R. C. Markl and his team (1950’s) developed the original SIFs still used in ASME piping Codes today.
In his study, Markl determined that girth butt-welds typically resulted in stresses approximately 1.7 to 2.0 times the stress in non-welded piping. As a result, all of the piping codes have been “base lined” to include the factor of 2.0 for girth welds:
B31.3 SIF
Actual (Peak Stress)due to Moment M Stress in Girth Butt Weld due to Moment M
or Actual (Peak Stress)due to Moment M B31.3 SIF 2 * (Moment M) / (Section Modulus Z) 12/10/2012
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ASME B31.3, sample. Refer code for complete table
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Analysis of Integral welded attachments (IWA)
Used to support piping systems Local stresses are evaluated using Welding Research Council (WRC) Bulletin #107. WRC #107 approach has limitations on attachment parameter (0.01 ≤ b ≤0.5) and shell parameter (5 ≤ g ≤ 300) Shell parameter g = Dm / (2T), where Dm = Do-T Attachment parameter b = 0.875 (do/Dm) for circular attachments or b1 = C1/Dm an b2 = C2/Dm for rectangular attachment
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Add figure
WRC 107 Nomenclature applicable to Cylindrical shells
Vc Concentrated shear load in the circumferential direction, lb VL Concentrated shear load in the longitudinal direction, lb Mc external overturning moment in the circumferential direction with respect to the shell lb ML external overturning moment in the longitudinal direction with respect to the shell lb
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WRC Bulletin 107 LOCAL STRESSES IN SPHERICAL AND CYLINDRICAL SHELLS DUE TO EXTERNAL LOADINGS
K. R. Wichman, A. G. Hopper, and J. L. Mershon
WRC Bulletin 107 presents the results of an analytical and experimental research program aimed at providing methods for determining the stresses in pressure vessel nozzle connections subjected to various forms of external loadings. Based on the work of P.P. Bijlaard, the Bulletin covers the sign conventions, parameters, calculation of stresses, nondimensional curves, and limitations on application for spherical and cylindrical shells and an abridged calculation for maximum stress in spherical shells.
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http://www.forengineers.org/wrc/
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WRC Bulletin 198 SECONDARY STRESS INDICES FOR INTEGRAL STRUCTURAL ATTACHMENTS TO STRIAGHT PIPE; STRESS INDICES AT LUG SUPPORTS ON PIPING SYSTEMS
W.G. Dodge; E.C. Rodabaugh, W.G. Dodge and S.E. Moore
This report presents a simplified method for calculating the stresses induced in straight pipe by thrust and moment loadings applied to lugs and other integral attachments. Following the philosophy of the nuclear power piping portion of Section III of the ASME Boiler and Pressure Vessel Code, appropriate secondary stress indices are defined. A simple and conservative formula for computing the stress indices is developed using analytical results as a guide. A comparison is made between experimental stress indices and those obtained using the simplified analysis procedure developed here as well as the more complex analysis procedures of Welding Research Council Bulletin 107 (WRC-107 method). The method is extended to attachments having a variety of cross sections.
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WRC Bulletin 297 LOCAL STRESSES IN CYLINDRICAL SHELLS DUE TO EXTERNAL LOADINGS ON NOZZLES
J. L. Mershon, K. Mokhtarian, G. V. Ranjan, and E. C. Rodabaugh
WRC Bulletin 297 presents methods and data for treating two normally intersecting cylindrical shells, i.e., cylindrical nozzles radially attached to cylindrical vessels (shells). Stresses in both the nozzle and vessel can be determined, and the range of vessel diameter-to-thickness ratio covered is increased over that in Bulletin 107. The analytical method used was derived and developed by C.R. Steele on the basis of the thin shell theory.
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Shear Stresses Formula for Shear Stress calculations (WRC 107)
Square/ rectangular IWA
Circular IWA
VL
t = VL / (2C2T)
t = 2 VL / (πdoT)
VC
t = Vc / (2C1T)
t = 2 VC / (πdoT)
MT
t = MT / F
t = 4MT / [2π(do)2T]
F = larger of
T [Cmax + Cmin] (Cmin/2) or [1.57 + 0.093 (Cmax/Cmin)] (Cmin)2 (Cmax/8)
Where Cmax = maximum of C1 and C2, Cmin = minimum of C1 and C2
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Expansion Joints
When piping elbows, bends, and expansion loops may not provide adequate flexibility in piping, expansion joints may be used to absorb the expansion and contractions of piping. Following may be its applications
Thermal movements inducing excessive stress Space restrictions Large reactions to pipe anchors Large reactions to equipment nozzles
In expansion joints, pressure forces must be resisted by pipe supports and anchors. (Pressure force = internal pressure x max sectional area where applied.) Types can be rubber hoses or metal bellows, etc.
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Expansion Joints
Expansion joint is defined as an assembly containing one or more bellows used to absorb dimensional changes caused by thermal or mechanical movement in a pipeline or duct. Expansion joints assembly consist of Bellow (Flexible Element) and end connections (Pipe, Flange or any special as per requirements). As per design requirements Limit Rods (Tie Rods), Internal Sleeve (Liner), Shroud (External Cover) are also provided Three basic types of movements absorbed by Expansion Joints.
Axial Lateral Angular
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Flexible Pipe loops
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Types of Pipe Loading Conditions(1/9)
Sustained Load: Pressure
Internal pressure in piping induces stresses in pipe wall – not result in pipe support loading. The longitudinal stress in pipe due to internal pressure SLP = P D / 4 t It can also be represented as SLP = P d2 / (D2 – d2) = P (Af / Am)
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Where SLP = longitudinal stress psi P = Internal design pressure psig D = Outside diameter in d = Inside diameter in Af = Flow area in2 Am = metal area in2 t = pipe wall thickness in
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Types of Pipe Loading Conditions(2/9)
Sustained Load: Weight
Design load on piping supports will include
Live loads: weight of medium transported or for test Dead loads: weight of piping, fittings, insulation, valves, flanges etc.
When a gas or steam piping is to be hydrotested, its effects also need to be considered. If pipe supports are not designed for this load, temporary supports may be needed,
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Types of Pipe Loading Conditions(3/9)
Thermal Expansion Loads
Piping systems are to be analyzed for maximum operating temperature. Free thermal analysis may be performed considering terminal points and anchors and equipment nozzles. A thermal stress < 10,000 psi means adequate flexibility in piping system. Equipment nozzle displacements due to thermal expansions need to be considered The thermal stress developed in the pipe are in fact “stress range” i.e. difference between thermal expansion and highest and lowest temperatures. Loads due to differences in expansion characteristics as in bimetallic, lined, jacketed or metallic-non metallic piping also need to be considered.
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Types of Pipe Loading Conditions(4/9)
Occasional Loads - Seismic
Piping Seismic analysis, if required, may be performed by one of the three methods eg
Time history analysis Modal response spectrum analysis Static analysis
A basic equation of motion for any piping system subjected to seismic excitation is M d2x/dt2 + C dx/dt + kx = f Where M mass matrix of system C damping matrix K stiffness matrix d2x/dt2 acceleration vector dx/dt velocity vector x displacement vector f external loading vector, function of time.
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Types of Pipe Loading Conditions(5/9)
Seismic anchor movements (SAM)/ Support settlements
A piping system supported on two seismically independent structures that move out of phase will experience stresses due to differential displacements. The analysis is done by applying corresponding displacements at pipe supports or anchor locations. Usually evaluated as secondary stress If SAM is less than thermal stress range, effective secondary stress range is sum of two. If SAM is more than thermal stress range, effective secondary stress range is twice SAM stress.
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Types of Pipe Loading Conditions(6/9)
Dynamic Loads (1/4) Dynamic Effects may consist of the following Impact – caused by external/ internal conditions Wind – exposed piping Earthquake – horizontal forces to be considered Vibration- due to any source as impact, pressure pulsations, flow vortices, resonance in compressors and wind. Discharge reaction – let down or discharge of fluids.
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Types of Pipe Loading Conditions(7/9)
Dynamic Loads (2/4) Safety Relief Valve Discharge Analysis Safety Relief Valve are of two types
Open discharge – discharging to atmosphere Closed discharge – to a closed system
Static analysis method for a open discharge system is as given in ASME B 31.1 The reaction force due to steady state flow following opening of valve F = W V / g + (P-Pa) A F = Force reaction at exit W = mass flow rate lbm/s G = Acceleration due to gravity 32.2 ft/sec2 P = Static pressure at exit, psia Pa = Atmospheric pressure, psia A = Exit area in2
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Types of Pipe Loading Conditions(8/9)
Dynamic Loads (Continued 3/4)
The Dynamic Load Factor (DLF) used to account for increased laod caused bu sudden application of discharge load, range from 1.1 to 2.0. Calculate the safety valve installation period T = 0.1846 (W h3 / EI)0.5 Where T = safety valve installation period sec W = Weight of safety valve, installation piping, flanges, attachments etc. lb h = distance from run pipe to centreline of outlet pipe in E = Young’s Modulus psi I Moment of inertia of inlet pipe in4 Calculate the ratio t0/T determine the DLF. Moment = force x distance x DLF. The stress is than calculated accordingly.
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Types of Pipe Loading Conditions(9/9)
Dynamic Loads (Continued 4/4)
The water hammer loading to be discussed
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Computer Programs
CAESAR II Autopipe Triflex Windows Ansys CAEPIPE (prounounced "K-pipe") SIMFLEX
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ASME B 31.3-2008 §302.3 Allowable Stress and Other Stress limits
Basic allowable stress in tension for metals and design stresses for bolting materials as per Table A-1 and A-2 Bolting materials design stress 1/4 x minimum tensile strength or 2/3 minimum yield strength For materials except Bolting, CI, malleable iron etc. basic allowable stress is Lower of 1/3 ST or tensile strength at temperature. Lower of 2/3 of SY or 2/3 of yield strength at temperature For Aus SS and Ni alloys, lower of 2/3 of SY and 90% of Yield strength at temperature. Other criteria for creep rate and stress for rupture as per code
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Equipment & piping
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Allowance for thermal expansion
Using Figure, find the approximate expansion from 15°C, of 100 metres of carbon steel pipework used to distribute steam at 265°C Temperature difference is 265 - 15°C = 250°C. Where the diagonal temperature difference line of 250°C cuts the horizontal pipe length line at 100 m, drop a vertical line down. For this example an approximate expansion of 330 mm is indicated.
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ASME B31.3:2006 §319.4 Flexibility Analysis (Sht 1 of 3) No formal analysis of adequate flexibility is required for piping system which Duplicates or replaces a system operating successfully. Judged adequate by comparison with previously analyzed systems Is of uniform size, no more than two points of fixation, no intermediate restraints, and falls within D y / (L-U)2 ≤ K1 D Outside diameter of pipe mm (in) Ea reference modulus of elasticity at 21 °C (70 °F) MPa ksi K1 = 208,000 SA/ Ea (mm/m)2
= 30 SA / Ea (in/ft)2
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30 x (25.4 mm/ 1 in)2 (3.2808 ft/ 1m)2 = 208,328 (mm/m)2
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ASME B31.3:2008 §319.4 Flexibility Analysis (Sht 2 of 3)
L developed length of piping between anchors m (ft) SA allowable displacement stress range MPa (ksi) B31.3 §302.3.5(d) SA = f (1.25 Sc + 0.25 Sh) U anchor distance straight line between anchors m (ft) y = resultant of total displacement strains, mm (in) to be absorbed by the piping system
All the parameters may be taken from ASME B31.3-2006 code various appendices as below. Cold and hot allowable stresses Sc and Sh from Table A-1, Basic Allowable Stresses in Tension for Metals.
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ASME B31.3:2008 §319.4 Flexibility Analysis (Sht 3/3)
Thermal Coefficients a from Table C-1 (Total thermal expansion) or Table C-3 (Coefficients for Metals), at the maximum temperature Ea value from Table C-6, Modulus of Elasticity, US units for Metals. Stress range Factor f from Fig 302.3.5 of B31.3 or by given formula in par. 302.3.5(d). Number of cycles may need to be assumed based your knowledge of the operation. y resultant of total displacement strains to be absorbed by piping system may be calculated using the above thermal expansion value with no restraints. Therefore, y = a x U x Dt, Dt is change in temperature, U anchor distance
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Example (Sht 1 of 2) h
Dy K1 , (L U )2
Given that: Formal Analysis not required 5 SA K 2 . 08 10 (mm / m) 2 Pipe material: Carbon Steel, C ≤ 0.3% 1 Ea Pipe Size: 8" sch 40 SA = f (1.25 Sc + 0.25 Sh), Assume f = 1 Temperature range: 30°C to 400°C (Range F: 86 °F to 752 °F) Ea at 21 °C (70 °F) 2.95 x 107 psi (Refer B31.3 Table C6)
From Table A-1; ASME B31.3-2006 Sc = 20,000, psi Sh = 12,900 psi Length m 100 Find: h where 2h = L - U 12/10/2012
Determine no. of sizes of expansion loops needed in pipe sizes
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Example (Sht 2 of 2)
Thermal Expansion Coeff. a = 7.54 x 10-6 in/in °F Refer ASME B31.3 table C3.
e = a DT; y = e U DT
y = 7.54E-6 x (752-86) x 100 = 0.502 m (19.8 inch)
SA = 1 x (1.25 x 20,000 + 0.25 x 12,900) = 28,225 psi K1 = 2.08x105 x 28,225 / 2.95 x 107 = 199 (mm/m)2 (L – U) = (D y / K1)0.5 = (219.1 x 502/ 199)0.5 = 23.5 m h = 23.5 / 2 = 11.75m The number of sizes of expansion loops will be decided by piping engineer to suit his overall layout. Both 2D and 3D loops may be considered. Three loops of 4m each may be considered.
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Expansion Loops Load due to axial expansion causes bending stresses to be developed, increasing upwards in the vertical pipes and becoming a maximum at the loop elbows. That bending moment stays at that maximum bending moment level for the entire length of the top horizontal pipe until it gets to the next elbow and starts reducing until it reaches the bottom pipe on the other side of the loop.
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Expansion loops
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Min. Leg length “Z”
Z-Shaped Expansion Compensator Minimum leg length of Zshaped pressfit piping temperature expansion loops are indicated in the diagram below.
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http://www.engineeringtoolbox.com/pressfit-pipes-expansion-loops-d_1169.html
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Min. Leg length, “U”
U-Shaped Expansion Compensator Minimum leg length of Ushaped pressfit piping temperature expansion loops are indicated in the diagram below.
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Pipe Stress Analysis
CAESAR II (1/6) Why carry out stress analysis?
The reasons one does a pipe stress analysis on a piping system are as follows to comply with legislation to ensure the piping is well supported and does not sag or deflect in an unsightly way under its own weight to ensure that the deflections are well controlled when thermal and other loads are applied to ensure that the loads and moments imposed on machinery and vessels by the thermal growth of the attached piping are not excessive to ensure that the stresses in the pipework in both the cold and hot conditions are below the allowables
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Pipe Stress Analysis
CAESAR II (2/6) How to carry out piping stress analysis?
The piping system is modelled using analysis software such as CAESAR II, available from COADE Software. The model is constructed from piping general arrangement drawings, piping isometric drawings and piping and valve specifications. Once the system is accurately modelled, taking care to set the boundary conditions, comprehensive stress analysis calculations are done, modifications to the model are made to ensure compliance with the above requirements. The modifications may include one or more of the following tools
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Pipe Stress Analysis
CAESAR II (3/6) Restraints A device which prevents, resists or limits the free thermal movement of the pipe. Restraints can be either directional, rotational or a combination of both. Anchors A rigid restraint which provides substantially full fixity, i.e., ideally allowing neither movements nor bending moments to pass through them. True anchors are usually difficult to achieve. A seemingly solid gusseted bracket welded to a house column does not qualify as an anchor if the column does not have the strength to resist the loads applied to it.
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Pipe Stress Analysis
CAESAR II (4/6) Expansion Loops A purpose designed device which absorbs thermal growth; usually used in combination with restraints and cold pulls. Neutral Planes of Movement This refers to the planes on the 3 axes of a turbo machine or pump from where expansion of the machine starts e.g. the fixed end of a turbine casing. This information is normally provided by the equipment manufacturer. If not available from this source, the fixed points of the machine must be determined by inspection and an estimation of the turbine growths calculated. A pipe restraint positioned in line with a neutral plane prevents differential expansion forces between the pipe and the machine.
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Pipe Stress Analysis
CAESAR II (5/6) Cold Pull or Cold Spring This is used to pre-load the piping system in the cold condition in the opposite direction to the expansion, so that the effects of expansion are reduced. Cold pull is usually 50% of the expansion of the pipe run under consideration. Cold pull has no effect on the code stress, but can be used to reduce the nozzle loads on machinery or vessels. Spring Hangers Used to support a piping system that is subjected to vertical thermal movements. Commercially available single coil spring units are suitable for most applications. Supplier's catalogues adequately cover the selection of these springs. According to Hooke's law, the spring's supporting capacity will vary in direct proportion to the amount of displacement the spring undergoes due to thermal movement. This variation between cold and hot should be between 25 and 50% of the hot loaded condition.
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Pipe Stress Analysis
CAESAR II (6/6) Solid Vertical Support In places where vertical thermal movement does not create undesirable effects, or where vertical movement is intentionally prevented or directed, solid supports in the form of rollers, rods or slippers are used. It is important that free horizontal movement of the pipe is not impeded unless horizontal restraint is desired. Slippers and rollers must be well designed and lubricated.
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Example
Example 1 Comparison between 4” Sch 40 (I = 7.23 in4) and 10” Sch 40 pipe (I = 160.8 in4), cantilever, length 4’. The force required for an end displacement of ¼” for each pipe will be F = 3 E I / l3 D 3 x 29 x 106 I / (4 x12)3 x ¼ = 196.7 x I For 4” pipe, force is 1,422 lbs (6325 N) while for 10” pipe 31,629 lbs (140,690 N) or 22.4 times for same deflection. If the force 1,422 lbs is applied on 10” pipe, the deflection will be F 0.0112” or 0.283 mm. Example 2 The force developed in a restrained 10” Sch 40 pipe subjected to a temperature 200 °F from an installation temperature of 70 °F shall be F = E a A (metal area) = (29 x 106 psi) x (0.99 in/100ft / 12) x (11.91 in2) = 284,946 lbs Force F for 4” pipe (metal area 3.17 in2) = 109,213 lbs
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Nozzle loads - example
Example 3 Displacement of a 4’ pipe from 70 °F to 200 °F is 0.99 x 4 / 100 = 0.0396”or 1 mm. This 1 mm deflection of middle portion will give bending forces 3EI / l3 D (assume cantilever) as below For 4” pipe, = 224 lbs For 10” pipe = 4981 lbs Corresponding moment on nozzle will be F l / 2 = 9,962 lb-ft (13,507 N-m) for 10” case. This load in larger pipe size is well above allowed by equipment codes. Thus, more flexibility is required in piping.
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Nozzle
Nozzle
4’
4” Sch 40
4’ Nozzle
Nozzle
4’
10” Sch 40 4’
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AVEVA Pipe Stress Interface 11.6
Integrates pipe stress analysis with AVEVA PDMS design
Analysing piping designs to check stresses and to design the supports is a specialist job. AVEVA Pipe Stress Interface provides an interactive and intuitive method of specifying a stress network and delivers major savings by automating the two-way flow of information between PDMS designers and stress analysts - Coade's CAESAR II .
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Piping Flexibility Analysis
NPS 4 CARBON STEEL AMB to 315 °C (600 °F) SG Contents 1.0
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Piping Flexibility Analysis
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ASME B31.3-2008 Appendix S
This was added in 2004 edition and further elaborated in 2006 edition, by adding two more examples. The Index of the Appendix S is as under: Piping System Stress Analysis Examples S300 Introduction S300.1 Definitions and Nomenclature S301 Example 1 : Code Compliant Piping System S302 Example 2: Anticipated Sustained conditions considering pipe lift off S303 Example 3: Moment Reversal Each of these para. S 301, S302, S303 are subdivided as in enclosed table
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Ex. 1
Ex. 2
Ex. 3
Paragraph
S301
S302
S303
Example Description
S301.1
S302.1
S303.1
Design Conditions
S301.2
S302.2
S303.2
Computer Model Input
S301.3
S302.3
S303.3
Pressure Effects
S301.4
S302.4
S303.4
The Operating Load case
S301.5
S302.5
S303.5
The sustained load case
S301.6
S302.6
S303.6
Displacement stress Range Load Case
S301.7
S302.7
S303.7
-
S302.8
S303.8
Code Compliance – Satisfying the Intent of the code
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Appendix S: Piping System Stress Analysis Examples 1, 2, 3 figures
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ASME B 31.3-2008 APPENDIX P
Alternative rules for performing flexibility analysis were added, as Appendix P, in ASME B31.3, the Process Piping Code, 2004 edition. These rules are considered to be more comprehensive than before; they were designed around computer flexibility analysis. To determine stress range, the difference in stress states, considering all loads, is computed.
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DESIGN CODES REQUIRE SURGE ANALYSIS
B31.3 Ch. II Design, PAR 301.2 Design Pressure PAR 301.2.1 (a) The design pressure of each component in a piping system shall be not less than the pressure at the most severe condition of coincident internal or external pressure and temperature (minimum or maximum) expected during service, except as provided in Para. 302.2.4 PAR 301.2.2 (b) Sources of pressure to be considered include ambient influences, pressure oscillations and surges, improper operation, decomposition of unstable fluids, static head, and failure of control devices. PAR 301.5 Dynamic Effects PAR 301.5.1 Impact fores caused by external or internal conditions (including changes in flow rate, hydraulic shock, liquid or solid slugging, flashing, and gysering) shall be taken into account in the design of piping.
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BSI BS EN 13480-3 Metallic industrial piping - Part 3: Design and calculation - AMD 16050: December 17, 2005; CORR 16362: June 29, 2007 (1/2)
12.3 Flexibility Analysis 12.3.2 Stress due to sustained loads
The sum of primary stresses σ1, due to calculation pressure, pc, and the resultant moment, MA, from weight and other sustained mechanical loads shall satisfy the following equation:
s1
pc d o 0.75 i M A fh 4 en Z
(12.3.2-1)
where MA is the resultant moment from the sustained mechanical loads which shall be determined by using the most unfavourable combination of the following loads:
piping dead weight including insulation, internals and attachments;
weight of fluid;
internal pressure forces due to unrelieved axial expansion joints etc.
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BSI BS EN 13480-3 Metallic industrial piping - Part 3: Design and calculation AMD 16050: December 17, 2005; CORR 16362: June 29, 2007 (2/2) 12.3.3 Stress due to sustained and occasional or exceptional loads
The sum of primary stresses, σ2, due to internal pressure, pc, resultant moment, MA, from weight and other sustained mechanical loads and resultant moment, MB, from occasional or exceptional loads shall satisfy the following equation:
s2
where MB is the resultant moment from the occasional or exceptional loads which shall be determined by using the most unfavourable combination of the following loads:
pc d o 0.75 i M A 0.75 i M B (12.3.3-1) k fh 4 en Z Z
wind loads (T≤ TB/10); snow loads; dynamic loads from switching operations (T ≤ TB/100); seismic loads (T ≤ TB/100);
k=1
if the occasional load is acting for more than 10 % in any 24 h operating period, e.g. normal snow, normal wind; k = 1,15 if the occasional load is acting for less than 10 % in any 24 h operating period; k = 1,2 if the occasional load is acting for less than 1 % in any 24 h operating period, e.g. dynamic loadings due to valve closing/opening, design basis earthquake;
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The code may need to be studied in detail, once a copy is available.
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Piping Stress Analysis Where do I start?
Five factors influencing piping Temperature -200 °F to 1000 °F
Weight – Live load/ Dead load
Vibration (mechanical/ acoustics)
“While analysis cannot create a good design, it can confirm a good design” 12/10/2012
Pressure Vacuum to High pr. Force Internal / External 72 of 82
Piping Stress Analysis
Where Do I start? (1/10) Prior to beginning to model a piping system for a stress analysis, following may need to be done: 1. Identify what is to be achieved in the analysis. Possible reasoning for conducting a piping stress analysis: a) Stresses in a specific piping system and to determine if these stresses are within the range allowed by the Piping Code? b) Loads on a piece of rotating equipment? c) Loads on a heat exchanger, pressure vessel or tank nozzle? d) Loads on one or more structural anchors? e) Loads on one or more pipe supports?
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Piping Stress Analysis
Where Do I start? (2/10) f) Movements of portions of the piping system due to thermal growth or contraction? g) Effects of wind loads on the piping system and/or attached equipment? h) Effects of earthquake loads on the piping system and/or attached equipment? i) Effects of wave loading on the piping system and/or attached equipment? j) Effects of soil resistance to movement for underground or buried piping system and/or any attached equipment? k) Effects of changes in temperature, pressure and weight on flanged couplings and to determine if there is a tendency for the connections to leak?
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Piping Stress Analysis
Where Do I start? (3/10) Once the objective has been established, check each of the following steps. 2. Determine which piping code will govern the design of the piping system. For Process Piping, use ASME B 31.3-2006 3. Collect all the plan and elevation drawings necessary to fully document the piping routing. 4. Obtain or construct an isometric drawing “Stress ISO” of the entire piping system. 5. Collect all the necessary physical properties for all of the piping components in the piping system as follows: a) Nominal Pipe Diameter
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Piping Stress Analysis
Where Do I start? (4/10) b) Pipe Schedule or Pipe Wall Thickness. c) Corrosion Allowance. d) The Weight per unit length of the contents. e) The Insulation weight per linear unit length. f) Piping Material density, modulus of elasticity and coefficient of expansion. g) Operating Temp (Min and max, if applicable), Design Temp, Upset Condition Temp and Base or Ambient Temp. h) Operating Pressure (Internal or External), Design Pressure and Upset Condition Pressure. i) Flange Rating and Flange Type
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Piping Stress Analysis
Where Do I start? (5/10) j) Valve Type (Gate, Globe, Butterfly, etc.) Rating or Valve Weight and Length. k) Elbows and/or Bends Radius or Bend Radius Ratio, Fitting Thickness and the number of miter points, if applicable. l) Reducer length, inlet and outlet diameters, schedule or wall thickness, concentric or eccentric and, if eccentric, the flat side orientation. m) Branch Connections - welding tee, weld-in contour insert, weld-on fitting, fabricated tee with the reinforcing pad thickness, extruded tee with the crotch radius or lateral fitting data.
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Piping Stress Analysis
Where Do I start? (6/10) n) Expansion Joint Properties – Translational Spring Constants in force/unit length of travel – Axial and Lateral or Shear and Rotational Spring Constants in moment/degree of rotation – About the axis of the expansion joint (normally considered to be totally rigid) and about the radial axes. o) Structural Members – Details of any structural member that is welded or bolted to the piping system and is expected to act as part of the piping system. 6. For all Anchors, the location of the anchor point in the piping system. A complete definition of the equipment or structure to which the piping system is connected.
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Piping Stress Analysis
Where Do I start? (7/10) 7. The location of each restraint acting on the piping system as well as the specifics as to how each restraint affects the piping system. The following covers restraints acting along one of the X, Y, Z axes.
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a) Translational Restraints b) Limit Stops c) Imposed Movements d) Imposed Forces e) Dampers f) Frictional Resistance to Movement g) Existing Spring Hangers h) New Spring Hangers to be Designed i) Rotational Restraints j) Imposed Rotations k) Imposed Moments 79 of 82
Piping Stress Analysis
Where Do I start? (8/10) 8. Special Effects such as cold spring must be defined. The location of the cold spring in the piping system must be specified. 9. Special Loading Conditions a) Wind Loading – The piping components on which the wind loads are to be applied must be identified. b) Wave Loading – The piping components on which the wave loads are to be applied must be identified. c) Seismic Loads - The magnitude of the loading must be quantified and a decision as to the analysis method to be employed must be made. d) Soil Interaction – The piping components on which the soil interaction is to be modeled must be identified.
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Piping Stress Analysis
Where Do I start? (9/10) 10. Once all the physical data has been collected, the Global (overall) Axis System (X, Y, Z) must be oriented on the isometric drawing for easy reference. (The standard right-hand rule axis system is used with Y being the vertical axis. Consider gravity exerting a negative Y force on the piping system.) 11. Now we are ready to begin assigning data point numbers to all pertinent piping components in the piping system. All such data point numbers should be placed on the isometric drawing. A data point must be assigned to any location in the system for which output data is desired.
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Piping Stress Analysis
Where Do I start? (10/10)
The logic of the data collection effort that should occur prior to beginning to model a piping system for a stress analysis. It is to be clearly understood the scope of stress analysis, and the results expected. It is always better to draw one Stress ISO instead of referring many Iometrics. Each blind or a nozzle shall be assigned a node number. Additional data may be needed for each of the above categories, than listed in this summary.
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