Piping Stress

STRESS TYPE & CLASSIFICATION 

 Stress  Types

of stress

 Classes

of stress

 Basis

for establishing allowable stress

 Code

allowable stress

STRESS TYPE & CLASSIFICATION 

 Stress  Types

of stress

 Classes

of stress

 Basis

for establishing allowable stress

 Code

allowable stress

Type of stress  1. 2. 3. 4. 5. 6. 7. 8. 9.

Tensile Compressive Shear Bending Bearing  Axial Discontinuity Membrane Principal

Type of stress  10. Thermal 10.Thermal 11.Tangential 11. Tangential 12.Torsional 12. Torsional 13.Load 13. Load Induced 14.Strain 14. Strain Induced 15.Circumferential 15. Circumferential 16.Longitudinal 16. Longitudinal 17.Radial 17. Radial 18.Normal 18. Normal

Membrane Stress   Outside

of Pressure Vessel Design there is no membrane stress.  An example of membrane stress is the uniform stress across the thickness of a pressurised shell.

Discontinuity stress   Additional

stress produced where abrupt changes in geometry, materials and/or loading occur in an FRP laminate

Principal Stress   Stresses

in principal plane is called principal stress

Thermal Stress 

Tangential Stress  A

stress which acts along a plane in the interior of a body



CLASSES OF STRESS Stress

1) 2) 3)

Primary Unrelenting load General loading (Pm + Pb ) Local loads (PL + Pb ) Not self limiting Internal Pressure External Sustained External forces & moments

1)

Secondary Relenting loads (Self limiting) - Local yielding and minor  distortion can satisfy the conditions which caused the stress to occur. - Can not cause structural failure due to restraints offered by the body to which the part is attached. - Thermal stress

Peak  - the additional stresses due to to stress intensification in highly localised areas. - both sustained and self limiting loads. - Significant in fatigue condition. - additive to part section. 1) Stress at the corner of a discontinuity.

- Gross structural discontinuity. 2) Thermal stresses in a wall in the surface temperature. 3) Stress due to notch effect (Stress concentration)

Pm --> Primary Membrane:       

Circumferential and Longitudinal stress due to pressure. Axial stress. Bending of horizontal vessels over the saddles due to Longitudinal Stress. Membrane stress in the centre of the flat head. Axial compression due to weight. Membrane stress in the nozzle wall within the area of reinforcement due to pressure external loads.

Pb-->

Primary Bending: 



Bending stress in the centre of a flat head or  crown of a dished head.



Bending stress in a shallow conical head.



Bending stress in the ligament of closely spaced openings.

PL-->

PM + Membrane stress at local discontinuities.





Head - shell juncture.



Cone - cylinder juncture.



Nozzle - shell juncture.



Shell - flange juncture.



Head - skirt juncture.



Shell - stiffening ring juncture.

Secondary Stresses: 

Self limiting.



Local yielding and minor distortions can satisfy the conditions which caused the stress to occur.



Can not cause structural failure.



Radial loads on nozzles produce secondary means stresses in the shell at the junction of the nozzle.



Discontinuity stresses.



Thermal expansion (start up - shut down) loads.



Loads caused by vibration.



The non-uniform portion of the stress distribution in a thick walled vessel due to internal pressure.



Bending stress at a gross structural discontinuity.



Peak Stresses  

Both sustained loads and self limiting loads.



Significant in fatigue calculations.



Stress due to notch effect.



Stress at the corner of a discontinuity.



Thermal stresses in a wall caused by a sudden change in the surface temperature.

CATEGORIES OF STRESS Stress Classification/Category

 Allowable Stress

General primary membrane

Pm

<

Sm

General primary bending

Pb

<

1.5 Sm < 0.9 S y

< <

1.5 Sm 3 Sm

<

2 Sa

Pm + P b Pm + Pb + Q

(Secondary)

Pm + Pb + Q + F

(Fatigue)

FLEXIBILITY ANALYSIS

Expansion and Flexibility: In addition to the design requirments for pressure, weight and other loadings. Piping systems subjected to thermal expansion or construction or to similar movements imposed by other sources shall be designed in accordance with requirements for the evaluation and analysis of flexibility and stresses specified herein:-

Flexibility: -

-

to prevent pipe movements from causing failure from over stress of pipe material or anchors, leakage at joints or detrimental distortion of connected equipment resulting from excessive thrusts and moments. shall be provided by changes of direction in the piping through the use of bends, loops or offsets or provision shall be made to absorb thermal movements by utilising expansion, swivel or ball  joints.

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.

8.

Metallurgical considerations. For any system, these criteria must be considered and satisfied.

Emprical formula for finding flexibility of the system having only two terminal points and pipe of uniform size.

D -------(L  – U)2

≤

208.3

l U

D =

Outside dia of pipe

= Resultant expansion in mm L

= Developed length of line axis between anchors (m)

U

= Anchor distance (m).

PIPING DESIGN CRITERIA 1.

2.

3.

P (D  – t) Allowable internal Pressure stress = ----------2t Allowable sustained local stress  AF Q SL = P x ----- + ---- Am Am

< Sa -- (1)

-- (2)

Allowable occasional load stress: The sum of longitudinal stresses due to pressure, weight and those produced by occasional loads (such as wind, earthquake) may exceed the basic material allowable stress.

4.

Allowable test load stress: The maximum stress during pressure tests shall not exceed 90% of  the yield at test temperature.

5.

Allowable stress  – range for expansion stresses: S A = f (1.25 Sc + 0.25 Sh) Sc = Cold allowable stress. Sh = Hot allowable stress. f

= Stress range reduction factor for cyclic condition.

Cycles 7000 and less 7000 to 14000 14000 to 22000 22000 to 45000 45000 to 100000 100000 and over

f  1.0 0.9 0.8 0.7 0.6 0.5

Theory of failure   Failure

theory is the science of predicting the conditions under which solid materials fail under the action of external loads.  The failure of a material is usually classified into brittle failure (fracture) or ductile failure (yield)  Depending on the conditions (such as temperature, state of stress, loading rate) most materials can fail in a brittle or ductile manner or both