RPS Composites Inc.
RPS A-150 and P150
Pipe Stress Analysis Information
V
Changes to: W, p, Eax, Eh, G, Vh/ax, Sigma allow and CTE for Pipe Propts.
Rev
Description
JLK Issued By
26 June 2013 Date
RPS A-150 and P-150
Pipe Stress Analysis Information Prep. Chk. Chk.
J. Kendall B. Hebb
Date: 26 June 2013 Date: 27 June 2013 2013
Document No. Page No.
E-433 Page 1 of 10
E-433 Rev. V 26 June 2013
Pipe Stress Analysis Information
Table of Contents
1.0
INTRODUCTION
3
2.0
PIPE PROPERTIES
3
3.0
FITTINGS 3.1 Elbows 3.2 Reducers 3.3 Tees & Laterals 3.4 Reducing Branches 3.5 Flanges
4 4 5 5 6 6
4.0
CODE STRESS RECOMMENDATIONS 4.1 General 4.2 Load Cases 4.3 Corrosion Allowance 4.4 Occasional Loads 4.5 Hydrotest Allowable Stress
7 7 7 7 7 7
5.0
RIGIDLY RESTRAINED PIPE 5.1 Column-type Buckling 5.2 Allowable Stress 5.3 Pressure Loads on Anchors
8 8 9 10
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E-433 Rev. V 26 June 2013
Pipe Stress Analysis Information
1.0
INTRODUCTION
The purpose of this document is to provide piping data and recommendations to carry out stress analysis of RPS A-150 and P-150 piping in accordance with ASME B31.1 or B31.3. 2.0
PIPE PROPERTIES
Notes: 1. 2. 3.
T includes 0.11” erosion/corrosion liner. Allowable Stresses apply when the axial stress is tensile. If the axial stress is compressive, refer to Section 5.0. Allowable stresses are axial stresses. They are based on maintaining a Design Factor of 6 for combined loads, calculated as follows:
where:
σp
= Axial pressure stress σup = Pressure axial strength (18500 psi) σb = Non-pressure axial stress σub = Non-pressure axial strength (9000 psi) σallow = σp + σb
Note: 1” pipe is hand lay-up construction which has an axial strength (pressure and nonpressure) of 13500 psi.
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Pipe Stress Analysis Information
3.0
FITTINGS
3.1
Elbows:
E-433 Rev. V 26 June 2013
Notes: 1. T includes 0.11” erosion/corrosion liner. 2. The thicknesses listed above apply to the elbow extrados and are the correct values to be used in the stress analysis. Intrados thicknesses range from 140% to 200% of the listed values. 3. For 45° bends, k should be reduced by 30%. 4. For flanged elbows, reduce k as recommended in ASME B31. 5. To account for pressure stiffening, divide SIF and k by (Ref. BS7159:1989, Eqn 7.7): 2 1/3 1 + 2.53 x (P / Eh) x (D / (2 x T)) x (R / T) where: P = Design pressure Eh = Hoop modulus R = Bend radius 6. Allowable stresses are based on a maintaining a Design Factor of 6 under combined loading. Elbows have been analyzed using FEA to verify the required design factor. 7. Flexibility factors (k) based on BS7159:1989 Eqn. 7.5 (with minimum value of 1.0).
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Pipe Stress Analysis Information
3.2
E-433 Rev. V 26 June 2013
Reducers:
The thicknesses of reducers will be no less than those of the pipe. To obtain the thickness for a specific reducer, average the pipe thicknesses from Section 2.0 for the two sizes of interest. SIF = 1.3. k = 1.0. 3.3
Tees and Laterals:
Tees and laterals contain significant additional reinforcement to achieve the required pressure rating, and hence can be several times thicker than pipe in local areas. However, for the purposes of the stress analysis, it is recommended that the appropriate pipe wall thicknesses be used. The Flexibility Factor is 1.0 for all sizes and the allowable stresses are the same as for pipe. The SIFs are as follows:
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Pipe Stress Analysis Information
3.4
E-433 Rev. V 26 June 2013
Reducing Branches
Reducing branches should be modelled using the appropriate tee thicknesses. The Flexibility Factor is 1.0 for all sizes and the allowable stresses are the same as for pipe. SIF’s are as follows:
3.5
Flanges
Flanges can be analyzed using SIF = 1.0 and k = 1.0. The allowable stresses are the same as for pipe. It is recommended that loads on flanges be minimized as much as possible as the actual stresses in the flanges may be higher than calculated due to less-than-ideal installation conditions.
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Pipe Stress Analysis Information
4.0
CODE STRESS RECOMMENDATIONS
4.1
General
E-433 Rev. V 26 June 2013
FRP does not yield in the same manner as a ductile material such as steel. It is therefore not recommended that higher allowable stresses be used for FRP piping when analyzing displacement-type load cases (eg. thermal load cases). Displacement load cases should be treated in the same manner as sustained loads such as pressure and weight. 4.2
Load Cases
It is recommended that all operating loads be analyzed as either Operating or Sustained stress cases. It is not appropriate to use Expansion stress cases for FRP piping, as the same allowable stress should be used for primary and secondary load cases. Occasional load cases should be analyzed using Occasional stress cases. It is recommended that torsional stresses be included in the longitudinal stress calculation.
4.3
Corrosion Allowance
The piping properties for the total wall, i.e. liner plus structural layers, should be used for calculation of piping loads, but the piping stresses should be calculated based only on the structural wall. The liner (or “corrosion allowance”) should be deducted from the total wall thickness after the loads have been calculated and prior to calculating piping stresses. 4.4
Occasional Loads
The allowable stresses listed in Sections 2 and 3 are appropriate for long term loadings, i.e. sustained loads. When occasional loads such as wind or seismic are combined with the sustained (operating) loads, it has been RPS practice to increase allowable stresses by 20%. 4.5
Hydrotest Allowable Stress
Allowable axial stress for the hydrotest load case can be calculated as follows: Shydro = Sallow + 0.5 x S p where: Shydro = Allowable stress for hydrotest load case. Sallow = Allowable stress at design pressure (see Sections 2 & 3). Sp = Axial stress due to design pressure.
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Pipe Stress Analysis Information
5.0
E-433 Rev. V 26 June 2013
RIGIDLY RESTRAINED PIPE
A rigidly restrained pipe system is one that utilizes anchors along straight runs of piping to prevent thermal expansion. The restrained thermal expansion manifests itself as compressive stress in the pipe and axial thermal loads on the anchors. This type of support system is most often used for small diameter piping on pipe racks or other long straight runs. Its use is generally limited to smaller diameters of pipe as the magnitude of the thermal loads on the support structures can be excessive with larger diameter pipes. Pipe stress analysis software can be utilized to analyze this type of pipe system, but there are several additional requirements that must be borne in mind. This section will address those requirements. 5.1
Column-type Buckling
The restraint of thermal expansion of the pipe will result in compressive loads in the pipe. It is therefore necessary to ensure that the spacing between guides is adequate to prevent column-type buckling. The thermal load is calculated as follows: Fth
E⋅ A ⋅ α ⋅ ∆T
where: E = Axial modulus of the pipe A = Cross-sectional area of pipe α = Coefficient of thermal expansion ∆ T = Change in temperature from installation temp to max operating temp.
The critical buckling load is calculated as follows: 2
Fcr
π ⋅ Es⋅ Is
2
L
where: Es = Axial modulus of structural layer (1.4E6 psi) Is = Moment of inertia of structural layer L = Spacing between guides
If the critical buckling load is less than the thermal load, the spacing between guides should be reduced. A good rule of thumb is to ensure F cr is at least 15% higher than F th.
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E-433 Rev. V 26 June 2013
Pipe Stress Analysis Information
5.2
Allowable Stress
The allowable stresses listed in Section 3 apply only if the axial stress in the pipe is tensile. For compressive axial stress, the allowable code stress should be determined as follows (this will limit the strain to ≤ 0.0024):
where: σallow =
allowable code stress due to combined loads of pressure, thermal, weight, etc (kPa)
P = Pressure TL = liner thickness TS = structure thickness Note: The absolute value function in the above formula for the allowable stress is required if the pipe stress analysis software being used reports the code stress as positive regardless of whether the axial stress is tensile or compressive.
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E-433 Rev. V 26 June 2013
Pipe Stress Analysis Information
5.3
Pressure Loads on Anchors
Depending on the pipe stress analysis software being used, the pressure elongation (or “Bourdon effect”) for FRP piping may result in an understatement of the axial pressure loads on the anchors. The understatement is typically not large, particularly in comparison to the typical thermal loads on the anchors, and it can usually be ignored. If required, the actual pressure load on the anchor can be calculated from:
where:
P = pressure ID = inside diameter of pipe TL = liner thickness TS = structural thickness At = cross-sectional area of total pipe wall νha = Poisson ratio (axial strain due to hoop load) Ea = Axial Modulus Eh = Hoop Modulus
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