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Piping Stress Analysis Training

Introduction and How to Analyze using CAE Software

By: Ferdian Harjono

Timeline (2014/06/17) :

▪ 08:00 – 10:00 -> Basic Piping Introduction & TrainingOverview

▪ 10:00 – 10:30 -> Coffee Break▪ 10:30 – 12:00 -> Introduction to Pipe Stress Analysis

Code Compliance▪ 12:00 – 13:30 -> Lunch Break▪ 13:30 – 15:30 -> Basic Piping Stress Analysis Theory

Timeline (2014/06/18) :

▪ 08:00 – 10:00 -> Piping Stress Analysis Workflow▪ 10:00 – 10:30 -> Coffee Break▪ 10:30 – 12:00 -> Piping Stress Analysis Workflow

(Cont’d) ▪ 12:00 – 13:30 -> Lunch Break▪ 13:30 – 15:30 -> Equipment Analysis

Overview :

Piping Flexibility▪ Piping Support: Purpose▪ Pipe Support Span Calculation▪ Pipe Support Type▪ Piping Stress Engineer Workflow

Scope of Work of Piping SystemSubfield1. Piping Material

- Material Selection, Specification for welding, insulation,painting, fabrication, etc.

2. Piping Design

- Designing Piping System from one equipment to another

equipment / one area to another area based on severaldesign basis

3. Piping Stress Analysis

- Analyzing the stresses in piping system to assure the safetyof the plant (enough flexibility = No overstress occur inthe piping system)

Basic Pipe Stress Analysis: WhyHow?Pipe Stress Analysis

Why? Safety

“Piping Stress Analysis adalah suatu metode terpenting untukmeyakinkan dan menetapkan secara numerik bahwa sistem pemipaandan engineering adalah aman ” – Achmad Chamsudi

Actual Loads not exceed Allowable Loads – Codes Standards

Analyze the piping system using piping stress analysis software (CAESAR

/ AutoPIPE)

Limit the Stresses in piping▪ Limit the deflection in piping▪ Limit the loads on connected equipment▪ Limit the loads on supports▪ Check leakage at flange joints▪ Avoid unintentional disengagement of pipes from support▪ Avoid excessive displacement

Objectives of Pipe Stress Analysis

Limit the Stress in Piping

Limit the Deflection in Piping

Limit the Loads on Connected Equipment

Limit the Loads on Support

Check Leakage at Flange Joints

Check Leakage at Flange Joints Cont’d)

Avoid Unintentional Disengagement ofPipes from Support

Avoid Excessive Displacement

Codes Standards

Codes : General rules for design, materials, fabrication,installation, and inspection prepared in such manner thatit can be adopted and made into law

▪ Standards : Documents prepared with the basis of properengineering practice and written with mandatoryrequirement

+ Beneficial : + Consistency and safety in design+ Avoiding chances of failure+ Reduction in plant shutdown

Mandatory Requirement from Codes

Piping component Thickness calculation for internal pressure▪ Piping component Thickness calculation for external pressure▪ Reinforcement requirement for branch▪ Minimum flexibility requirement▪ Piping support and their design▪ Allowable stresses for various materials▪ Piping Fabrication and layout requirement▪

Piping inspection and testing method

Terminology of StressTerminology of Stress Strain

Stress ( σ) = Force / Cross Sectional Area

Strain ( ε) = ΔL / L (Longitudinal Strain)

= ΔD / D (Lateral Strain)

Modulus of Elasticity (E) = σ / ε ----- Pipe Span Calculation

Terminology of Stress Cont’d)

Yield Strength : Max Stress a material can withstandwiththout permanent deformation

▪ Ultimate Tensile Strength : Max stress a material canwithstand before uncontrolledyielding and breaking

▪ Allowable Stress : To account for uncertainty in loading anbehavior of materials, a factor of safetywas introduced to find allowable stress

a material

Allowable Stress = Yield Strength (UTS) / Factor of safety

Terminology of Stress Cont’d)

Longitudinal Stress▪ Circumferential / Hoop Stress▪ Radial Stress▪ Axial Stress▪ Bending Stress▪ Shear Stress

Loads Stresses in Piping:Stresses in Piping Cont’d)

Longitudinal Stress– Stress Developed normal to the cross sectional of pipe, caused byinternal pressure▪ Sl = PD/4t

Loads Stresses in Piping:Stresses in Piping

Circumferential / Hoop Stress– Stress developed tangential to the cross section of pipe and is due tointernal pressure loading (parallel to pipe circumference)▪ Sc= PD/4t

Radial Stress– Stress developed in radial direction across the wall thickness of pipe.It’s value is equal to internal pressure at the inside of pipe wall andequal to atmospheric pressure at the pipe’s external surface

– Note – At the outer radius of pipe, radial stress is zero whereas the bendingstress is maximum. Hence this stress component has been traditionallybeen ignored during stress calculation.

Axial Stress– Stress developed due to external loads in axial direction in pipe▪ Sc= PD/4t

Bending Stress– Stress developed in pipe due to loads acting in a plane normal to theaxis of pipe. These are due to temperature, weight of pipe, weight ofcontents, snow, wind, or earthquake.▪ Sb= M/Z

▪ M = Bending Moment▪ Z = Section Modulus of Pipe

▪ Shear Stress– Sum of 2 components i.e. Torsional Stress and Direct Shear Stress.

Direct shear stress is negligible (max at neutral axis of pipe andzero at max distance from neutral axis. Neglected for calculationbecause very small)

Torsional Stresses are due to twisting moment on pipe▪ St= T/2J

▪ T = torque▪ J = Polar Moment of Inertia

▪ Distribution of Stresses in Piping

▪ A Piping System need to accommodate their own expansionas well as the expansion of connected equipment so as toavoid:– Failure due to overstress– Leakage at Flange Joints– Distortion of connected equipment

Piping Flexibility

Temperature

Piping Flexibility Cont’d) ▪ A Piping System need to accommodate their own expansion

as well as the expansion of connected equipment so as toavoid:– Failure due to overstress– Leakage at Flange Joints– Distortion of connected equipment

Methods of Providing PipingFlexibility▪ The method to provide flexibility for 2 different types are as

under:

– Axial Flexibility▪ Can be provided by using certain types of bellows

– Bending Flexibility▪ Can be provided using loops, offsets, bends, bellows, etc.

Methods of Providing PipingFlexibility (Axial Flexibility)▪ Consider member of cross sectional Area A, length L, and

Modulus of Elasticity E subjected to an axial Load P▪ Deflection, δ = PL/AE

▪ Stiffness, k = P/ δ = AE/L

▪ Axial Flexibility = 1/k = L/AE

Methods of Providing PipingFlexibility (Bending Flexibility)Consider the same member is subjected to Bending Load P

– Deflection, δ b = PL3/CEI– C = Constant depending upon Type of Supports, Loading and

Location of applied Bending Load. For example – – Deflection, δ b = PL3/48EI, for Simply Supported Beam– Stiffness k = P/ δ b = 48EI/L3

– Bending Flexibility = 1/k = L3/48EI

Piping Support: Purpose

• Carry weight of Pipe, Fittings, Valves, with / without Insulation,with Operating / Test Fluid

• Provide adequate stiffness against external loads like Wind, Ice,Snow, Seismic Loads etc.

• Avoid overstressing of Piping Material• Avoid sagging which creates draining problem• Control Thermal expansion / contraction in the desired manner• Withstand /dampen vibration produced by connected equipment

or Flow induced vibration, Acoustic induced vibration etc.

Pipe Support Span CalculationCont’d)

• Based on Stress– Total Bending Stress (Sustained Load) shall not exceed hot

allowable Stress,– PD/4t + M/Z ≤ Sh

– For Pipe connection having one end simply supported andother end fixed,

– Bending Moment, M = 5wl 2/48– Hence Span can be calculated by eq.

PD/4t + 5wl 2/48Z ≤ Sh

Based on Deflection- For Pipe connection having one end simply supported and

other end fixed,

– Maximum Deflection

δ = wl4/128E I

–Where,–δ = Maximum Deflection–E = Modulus of Elasticity–l = Pipe Span–I = Moment of Inertia,–w = Unit weight of Pipe

Pipe Support Type Cont’d)

▪ Shoes– Used for Carrying weight of insulated Pipe– Used where less space is available (w.r.t hanger)– Used where frictional resistance is not a criteria (else slide plates

are used)– Local stress check required where CA>1/8” or wall thickness <

STD (for single web shoe)

– Trunnion / Dummy Support▪ Used generally for vertical pipes▪ Used where clearance between BOD and grade is higher (w.r.t

Shoe)▪ Used where elbow required to be supported▪ Local stress check is essential for dummy support

• Flexible Hanger – Variable Spring:• Used for piping with vertical movement but less than 35-40 m• Used to distribute load when pipe is thermally moving up or d• Used where load variation is less than 25% (from different loa

conditions)

• % Variability = (Hot Load – Cold Load) / Hot Load x 100 = Ky / Ho

Flexible Hanger – Variable Spring (Selection):

1) Calculate Piping Load for Spring (Hot Load)2) Calculate thermal Movement (y) and its direction at that location3) Locate hot load in load table (preferred at center of table) and

spring suitable for that thermal movement4) Calculate Installed Load = Hot Load ± (Thermal Movement [y]

Spring Rate [k] )5) Calculate Variability if Hot and Cold or Installed Load in same

column6) If Variability is within Project Specification (or < 25%) then OK7) Else move to adjacent column where Hot and Cold load can be

located in working Range8) Repeat step 3 to 69) From Dimension table find length of Spring Assembly10)Compare with Available clearance

11) If not OK repeat step 3 to 10

i. Anchor:• It is a mechanical connection (Welded or Bolted) between

Pipe and Structure• They do not allow movement and rotation of Pipe and

completely fix the line in place• Equipment Nozzles are also anchors but experience thermal

movement

ii. Directional Anchor (Line Stop):• Stops movement of Pipe in axial direction but allow

sideways movement

Pipe Restraint

iii.Guides:• Stops movement of Pipe in sideways direction but allow

axial movement

iv. U - Clamps:• They act as guide or anchor depending on construction and

installation•

Used for small bore (less than 2”) pipes

Pipe Restraint Cont’d)

v. Sway Struts:• It retains movement in one direction while allowing for

movement in another direction• Used where normal Guide or DA is not feasible due to non

availability of Steel Structure• Used where resistance due to friction is not acceptable• Effective under Tensile or Compressive Force• Allows angular motion or misalignment of ± 5°

vi. Vibration Absorbers – Snubbers (Hydraulic):• Used where Pipe experiences vibration due to earthquake,

wind, sudden valve opening/closing, rotating equipmentetc.

• It allows slow Thermal growth but resists sudden vibration• It goes to restraint mode based on particular velocity or

acceleration of motion depending on its design called

Activation Level• It consists of Double acting Cylinder and Piston assembly

(A), Flow control Device (B), and Reservoir (C)

vii.Vibration Absorbers – Snubbers (Mechanical):• It utilizes an inertia Mass which generates different

magnitude of resistance force against input Load inproportion to its acceleration level (M.a = F)

• Low input acceleration generates negligible level ofresistance force allowing free movement of Pipe

• High input acceleration generates sufficient resistance force

to completely restrain the vibration of Pipe• Mechanical Snubbers are preferred as no Oil Leakage or

change of Oil required

viii.Sway Brace:

• It has a pre – loaded spring which can be extended orcompressed in order to give increasing resistance in bothdirection

• The increase in resistance allows it to dampen vibrations onpipe from equipment, opposes pipe sway and absorbsShock Forces

Stress Symbol

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