Skip to main content

Introduction to Piping Stress Analysis

Pipelines Operating at High Temperature must be flexible enough to prevent thermal expansion from causing Excessive Stresses either in Pipe or pipe components.  These Loads due to expansion cause excessive undesirable forces and bending moments at joints, anchor, and guides etc. 

“Flexibility can be defined as a measure of the ability of a Piping system to absorb its own expansion or that of the connecting equipment while satisfying the Design conditions”.

Nature of expansion of the pipes is mainly depended upon the temperature it is exposed and type of material of construction. Every material got a "Coefficient of Expansion" which is indicative of a particular material’s nature of expansion and can be defined as a ratio between free expansion in Linear direction to the original length.

 When the Pipe is heated, it would try to expand axially but as the Piping network does not allow a “Free expansion” and there will be many restraints which resist expansion such as friction, Supports of Piping, Connected Line, Connected Equipment etc.

       Pipe with Fixed Ends: 

Imagine a pipe secured between two rigid plates. As the pipe attains high temperature, it will start to expand linearly. Here the ends are closed, the pipe will exert reaction forces at the ends. The magnitude of this force will be proportional to the Temperature applied and expansion in length due to the temperature change.(We Know that Change in Length divided by Total length equals to Strain).

Assume no Buckling Takes Place and Starin =α
                                                                           Stress=α x E  ( Where E=Modulus of Elasticity) 
From the Value of stress, We Get Force.
                                                               Force=Stress x Area.
Modulus of Elasticity of Particular material is known from the code and Area can be calculated from the Diameter of the pipe.

So it is evident that the Reaction force at the end is directly proportional to the Stress Developed and the Size of the pipe.

In order to reduce this Forces, traditionally Piping Designer give a change in direction with the help of a bend. This will increase flexibility and reduce thermal loads.Detail discussion about the same will do in some other post.

Change in the direction between fix points Reduces Loads on the nozzle as the flexibility is increased here.
But when we introduce flexibility, attention should be given about Over flexibility of the system. Giving more flexibility lead to other issues.Too much flexibility cause pressure loss in the system, which is an undesirable situation where Maintiang pressure is very important such as Pump Piping.

Generally, Piping Systems are Stress analyzed for,

1) To Prevent over stressing of the material of construction
2) Avoid Joint leakage caused due to excessive forces and moments.
3) To prevent Failure or malfunction of the connected Equipment due to excessive Piping Reactions.
4) To Calculate Magnitude of Force and Displacement on Support locations to Design Appropriate     Supports.

Flexibility between two fixed points in a piping can be checked by the following Simple rule. Consider the following arrangement.


Here,
                                       D=Diametr of Pipe (m,m)
                                       ∆ =Resultant Displacement (m,m)
                                       L=Devoloped Length of the Pipe between Anchor Points (mm)
                                       U=Distance Between Anchors(m.m)
Then the System Should be Sufficiently flexible if satisfy the following condition

Where K=208.3

But This method is applicable only for systems with only two fixed points and no intermediate restraints won't be there. Which is not the practical case in a Big Plant Piping network.

Unlike Earlier times, Complexity of Piping Stress Analysis reduced to a great extent by the introduction of Computer-based Stress Analysis. Bentley AutoPIPE and CAESAR II by COADE are two of the most popular pipe stress analysis applications in the Market








Labels:

Comments

Post a Comment

Most Popular

Wall Thickness Calculation of Pipes

Nominal Wall Thickness Calculation Of metallic pipes needs the following information as a Pre-requisite. a) Design Pressure, b) Design Temperature, c) Material Specification, d) Pipe Size e) Applicable Code As Per B 31.3, the required thickness of a Straight Section of Pipe is given by the relation t m =t+c Where , t m =Minimum thickness required including, mechanical corrosion and erosion allowance t=Thickness Required to withstand internal pressure c=Sum of Mechanical Allowance (thread, groove depth) added with corrosion and erosion allowance. Let's Go through each term in the above equation i) Thickness Required to withstand internal pressure [ t ] Nominal wall thickness should be Greater than or equal to the sum of the minimum required thickness and Manufacturer’s Negative tolerance. As per the Code Minimum Thickness t =PD/2(SE+PY) Here, P=Design Pressure (Gauge) D=Outside Diameter of Pipe S=Allowable Stress Value of
Post a Comment
Read more

Piping Specification-details and the preparation process

Piping Specifications are usually in the form of a manual containing the below-listed points, but not limited to             1 )       Pipe Class and Specifications.             2)       Pipe and fittings thickness table for the different class of piping.             3)       P & ID Abbreviations and Symbols.             4)       List of fluids with Design conditions and Pipe Class.             5)       List of Valves and Gaskets.             6)       Table of Branch connections for different Pipe Classes.             7)       Table of Bolt length.             8)       Valve Dimensions.             9)         Piping Scope Limit for instrumentation called as “Hook Up Diagram”.            10)      Special Items list. Licensor transfers the basic piping specification as part of their basic engineering package, it generally includes a)        List of mediums that have to handle with design conditions. b)       Dimensional and material standard o
Post a Comment
Read more

Details of Sustained and Displacement Stresses-Core of Pipe Stress Analysis

Piping flexibility analysis as per B31.3code requirements is dealing with two types of stress named as Sustained Stress and Displacement stress .  Both types of stress must be considered separately because sustained stresses are associated with sustained forces while displacement stresses are associated with fixed displacements. SUSTAINED STRESSES: Sustained Stresses are stresses caused by loads that are not relieved as the piping system deflects. See the diagrammatic representation below to get an idea about this.  The weight of the valve placed at the end of the cantilevered pipe induce Stress at the T-Joint. Regardless of the magnitude of the displacement (∆), the magnitude of the load (the weight of the valve) which causes the stress is unchanged.  Therefore, to avoid catastrophic failure, the magnitude of any sustained stress must not exceed the yield strength of the material.  Another example of a sustained stress is the hoop and longitudinal stresses induced by the
Post a Comment
Read more