pipe stress analysis information for frp

8/13/2019 Pipe Stress Analysis Information for Frp

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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. JLK 26 June 2013Rev Description Issued By Date

RPS A-150 and P-150

Pipe Stress Analysis Information

Prep. J. Kendall Date: 26 June 2013 Document No. E-433Chk. B. Hebb Date: 27 June 2013 Page No. Page 1 of 10


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Pipe Stress Analysis Information 26 June 2013

Table of Contents

1.0 INTRODUCTION 3

2.0 PIPE PROPERTIES 3

3.0 FITTINGS 43.1 Elbows 43.2 Reducers 53.3 Tees & Laterals 53.4 Reducing Branches 63.5 Flanges 6

4.0 CODE STRESS RECOMMENDATIONS 7

4.1 General 7

4.2 Load Cases 74.3 Corrosion Allowance 7

4.4 Occasional Loads 7

4.5 Hydrotest Allowable Stress 7

5.0 RIGIDLY RESTRAINED PIPE 8

5.1 Column-type Buckling 8

5.2 Allowable Stress 9

5.3 Pressure Loads on Anchors 10


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1.0 INTRODUCTION

The purpose of this document is to provide piping data and recommendations to carry outstress analysis of RPS A-150 and P-150 piping in accordance with ASME B31.1 orB31.3.

2.0 PIPE PROPERTIES

Notes:1. T includes 0.11 erosion/corrosion liner.2. Allowable Stresses apply when the axial stress is tensile. If the axial stress is

compressive, refer to Section 5.0.

3. Allowable stresses are axial stresses. They are based on maintaining a Design Factor of 6for combined loads, calculated as follows:

where: p = Axial pressure stress

up = Pressure axial strength (18500 psi)

b = Non-pressure axial stressub = Non-pressure axial strength (9000 psi)allow = p + b

Note: 1 pipe is hand lay-up construction which has an axial strength (pressure and non-

pressure) of 13500 psi.


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3.0 FITTINGS

3.1 Elbows:

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):

1 + 2.53 x (P / Eh) x (D / (2 x T))2x (R / T)

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where: P = Design pressure

Eh = Hoop modulusR = Bend radius

6. Allowable stresses are based on a maintaining a Design Factor of 6 undercombined loading. Elbows have been analyzed using FEA to verify the requireddesign factor.

7. Flexibility factors (k) based on BS7159:1989 Eqn. 7.5 (with minimum value of1.0).


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3.2 Reducers:

The thicknesses of reducers will be no less than those of the pipe. To obtain the thicknessfor a specific reducer, average the pipe thicknesses from Section 2.0 for the two sizes ofinterest. 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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3.4 Reducing Branches

Reducing branches should be modelled using the appropriate tee thicknesses. TheFlexibility Factor is 1.0 for all sizes and the allowable stresses are the same as for pipe.SIFs 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-idealinstallation conditions.


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4.0 CODE STRESS RECOMMENDATIONS

4.1 General

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 shouldbe 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 thesame 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 thestructural 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 Sp

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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(1.4E6 psi)

5.0 RIGIDLY RESTRAINED PIPE

A rigidly restrained pipe system is one that utilizes anchors along straight runs of pipingto prevent thermal expansion. The restrained thermal expansion manifests itself ascompressive 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 ofthe 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 addressthose 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 T

where:

E = Axial modulus of the pipe

A = Cross-sectional area of pipe

= Coefficient of thermal expansion

T = Change in temperature from installation temp to max operating temp.

The critical buckling load is calculated as follows:

Fcr

2

Es Is

L2

where:

Es= Axial modulus of structural layer

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 Fcris at least 15% higher than Fth.


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5.2 Allowable Stress

The allowable stresses listed in Section 3 apply only if the axial stress in the pipe istensile. For compressive axial stress, the allowable code stress should be determined asfollows (this will limit the strain to 0.0024):

where:

allow = allowable code stress due to combined loads of pressure, thermal,

weight, etc (kPa)

P = PressureTL= 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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5.3 Pressure Loads on Anchors

Depending on the pipe stress analysis software being used, the pressure elongation (orBourdon effect) for FRP piping may result in an understatement of the axial pressureloads 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 thicknessTS= structural thickness

At= cross-sectional area of total pipe wall

ha = Poisson ratio (axial strain due to hoop load)

Ea= Axial ModulusEh= Hoop Modulus

pipe stress analysis information for frp

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