stress presentation.pptx

8/14/2019 Stress presentation.pptx

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What is Stress Analysis? Stress-Strain Relationship

Purpose of piping stress Analysis

How piping component fails Stress Categories

Classification of Loads

Requirement of ASME B31.3


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What is Stress?Stress is the internal resistance per unit area to the

deformation caused by the applied load.

Strain:Strain is the unit deformation under applied load.

Stress-Strain Relationship:

Stress is directly proportional to strain- Hooks Law


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Purpose of piping Stress Analysis:

To comply with legislation.

To check the stresses are within allowable limits.

To check the force and Moment on the equipment nozzles are withinallowable limits.


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How Piping and Components Fails?

Failure by general yielding:Failure is due to excessive plastic deformation.

Yielding at Sub-Elevated Temperature:Body undergoes plastic deformation under slip action

of grains.Yielding at Elevated Temperature:

After slippage, Material recrystalizes and hence

yielding continues without increasing load. This phenomenon is called asCreep.

Failure by Fracture:Body fails without undergoing Yielding.

Brittle Fracture:This type of fracture is occurred in brittle materials.

Fatigue:Due to cyclic loading ,Small crack is developed which grows after

each cycle and result in sudden failure.


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Theories of Failure

Maximum Principal Stress TheoryThis theory states that yielding occurs when the magnitude of

any of the three mutual perpendicular Principal Stresses exceeds the yieldpoint of the material.

Maximum shear stress TheoryThis theory states that yielding occurs when the maximum

shear stress exceeds the shear stress at the yield point in a tensile test.

Stress categories

Primary StressesThese are developed by imposed loading and are

necessary to satisfy the equilibrium between the external and internal forceand moment of the piping system.


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Secondary stressesThese are developed by the constraint of the

displacement of structure.These displacements can be caused either by

thermal displacement or by outwardly imposed restraint and anchor pointmovements.

Peak StressesUnlike loading condition of secondary stress which cause

distortion, Peak stress cause no significant distortion. Peak stresses arethe highest stress in the region under consideration and are responsiblefor causing fatigue failure.


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LoadsVarious loads on the piping can be grouped into the followingthree categories

Primary loads (sustained loads)

Secondary loads (expansion loads)

Occasional loads (wind, earthquake, etc.)

Load Combinations Considered Primary loads

Secondary loads

Primary + occasional loads


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The piping is subjected to several loads. Some of them are:

internal pressureexternal pressureself-weightloads from connected componentsthermal loadsequipment movementssteam hammerwater hammerwind loadsseismic loads and foundation relative settlement


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The following constitute the primary loads:internal pressureexternal pressure

self-weightweight loads from connected

components.The following constitute the secondary loads:

thermal loadsequipment thermal movements

foundation relative settlement

The following constitute the occasional loads:surge loadssteam hammerwater hammer

wind loadsseismic loads


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Requirement of ASME B31.3

Stresses due to Sustained loadsSL< Sh

SL = (PD/4t) + SbSh = Basic Allowable Stress at Hot Condition.

Stresses due to Occasional loadsThe sum of longitudinal loads due to

Pressure ,weight and Stresses produced by Occasional loads such as

earthquake ,wind shall not exceed 1.33Sh.

Stress range due to Expansion loadsThe displacement stress range SE

Shall not exceed SASE< SA


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Allowable stress range for expansion stresses:SA = f (1.25 Sc + 0.25 Sh)

Sc = Cold allowable stress.

Sh = Hot allowable stress.

f = Stress range reduction factor for cyclic condition.

Cycles f

7000 and less 1.0

7000 to 14000 0.9

14000 to 22000 0.8

22000 to 45000 0.745000 to 100000 0.6

100000 and over 0.5


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Parameters to be considered for flexibility analysis:-

1. The appropriate code that applies to the system.

2. The design pressure and temperature.

3. The type of material.

4. The pipe size and wall thickness of each pipe component.

5. The piping geometry including movements of anchors and restraints.

6. The allowable stresses for the design conditions set by appropriate

code.

7. Limitations of forces and moments on equipment nozzles set by API,

NEMA or the equipment manufacturers.


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Types of Supports:RestraintsGuide

AnchorLimit stopsHangers

Restraints:Restraints are provided in the piping primarily to transfer

the occasional loads to the supporting structure. Inclined restraints arealso used. Usually the restraints are double acting. Struts and ties,which are single acting, are also used. A single acting restraint is adevice, which carries only tension or compression.


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Limit Stops:Limit stops are used to limit the stresses in the piping and to

reduce the anchor reaction. The behavior of the limit stops is non-linear.

The limit stop has zero rigidity up to certain movement. After thispredetermined movement, the limit stop comes into action. The activerigidity of the limit stop can be finite or infinite. This depends on theconstruction of the limit stop.

Anchors:Anchors is used to arrest all movements. It is a rigid support.

Guide:It will arrest the lateral movement of the piping.


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Hangers:Variable Load Hanger VLH):Variable load hanger is a special type of hanger, which accommodate thevertical thermal movements, while carrying the vertical load. Usually

variable load hangers are made of helical springs. The load varies fromcold condition to hot condition.

Constant Load Hanger CLH):Constant load hanger is a special type of hanger, similar to the variableload hanger. There are several types of constant load hangers. The load

variation in the constant load hanger from cold to hot is usually limited to0%.

Semi-Constant Load Hanger SCLH):Semi-constant load hangers are similar to the constant load hangers. Theload variation permitted in the semi-constant load hanger is usually 10%.

stress presentation.pptx

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