Piping Stress

(1)

(2)





Stress



T

ypes of

stress



Classes of stress



Basis for establishing allowable stress



Code allowable stress

STRESS TYPE &

CLASSIFICATION

(3)





Stress



T

ypes of

stress



Classes of stress



Basis for establishing allowable stress



Code allowable stress

STRESS TYPE &

CLASSIFICATION

(4)



1.

1. Tensile

Tensile

2.

2. Compressive

Compressive

3.

3. Shear

Shear

4.

4. Bending

Bending

5.

5. Bearing

Bearing

6.

6. Axial

Axial

7.

7. Discontinuity

Discontinuity

8.

8. Membrane

Membrane

9.

9. Principal

Principal

Type of stress

(5)



10.

10. Thermal

Thermal

11.

11. Tangential

Tangential

12.

12. Torsional

Torsional

13.

13. Load Induced

Load Induced

14.

14. Strain Induced

Strain Induced

15.

15. Circumferential

Circumferential

16.

16. Longitudinal

Longitudinal

17.

17. Radial

Radial

18.

18. Normal

Normal

Type of stress

(6)

(7)

(8)

(9)





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.

(10)





Additional stress produced where abrupt changes in

geometry, materials and/or loading occur in an FRP

laminate

(11)





Stresses in principal plane is called principal stress

(12)

(13)

(14)

(15)

(16)

(17)

(18)



(19)





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

body

(20)

(21)

(22)

(23)

(24)

(25)

Primary

1) Unrelenting load

2) General loading (P_m + P_b ) 3) Local loads (P_L + P_b )

Not self limiting

Internal Pressure

External Sustained External forces & moments

CLASSES OF STRESS

Stress

Secondary

1) 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

- Gross structural discontinuity.

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.

2) Thermal stresses in a wall in the surface temperature. 3) Stress due to notch effect

(26)



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.

(27)



P

_b

--> 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.

(28)



P

_L

-->

P

_M

+ Membrane stress at

local discontinuities.



Head - shell juncture.



Cone - cylinder juncture.



Nozzle - shell juncture.



Shell - flange juncture.



Head - skirt juncture.

(29)



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.

(30)



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.

(31)

CATEGORIES OF STRESS

Stress Classification/Category

General primary membrane

P

_m

General primary bending

P

_b

P

_m

+ P

_b

P

_m

+ P

_b

+ Q

(Secondary)

P

_m

+ P

_b

+ Q + F

(Fatigue)

Allowable Stress

<

S

_m

<

1.5 S

_m

< 0.9 S

_y

<

1.5 S

_m

<

3 S

_m

<

2 S

_a

(32)

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.

(33)

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.

(34)

Emprical formula for finding flexibility of the system having

only two terminal points and pipe of uniform size.

U

l

D

---

≤

208.3 (L

–

U)

2

D =

Outside dia of pipe

= Resultant expansion in mm

L = Developed length of line

axis between anchors (m)

U = Anchor distance (m).

(35)

P (D

–

t)

1. Allowable internal Pressure stress = --- < S

_a

-- (1)

2t

2. Allowable sustained local stress

AF

Q

S

_L

= P x --- + ---

-- (2)

Am

3. 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.

(36)

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 S_c+ 0.25 S_h)

S_c = Cold allowable stress.

S_h = 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 0.7 45000 to 100000 0.6 100000 and over 0.5