No Slide TitleIT IS A PROCESS BY WHICH WE
ANALYTICALLY DETERMINE WHETHER THE STRESSES IN THE
GIVEN PIPING SYSTEM ARE WITHIN THE ALLOWABLE LIMIT
UNDER ALL POSSIBLE LOAD COMBINATIONS
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TO PREVENT FAILURES DURING OPERATING CONDITIONS
TO PREVENT EXCESSIVE LOADING OF PIPING AND ASSOCIATED STRUCTURAL ELEMENTS AND TERMINAL EQUIPMENTS
TO DETERMINE THE SUITABLE SUPPORTING SYSTEMS FOR THE PIPING SYSTEM
TO FIND OUT REACTIONS AT ANCHOR AND RESTRAINTS
TO FIND OUT DEFLECTIONS AND MOVEMENTS DUE TO VARIOUS LOADS
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AT WHAT STAGE OF THE PROJECT THE ANALYSIS IS DONE ?
P&IDs PFDS EQUIP-MENT LAYOUT
PIPING LAYOUT
PREL. ISOMETRICS
STRESS ANALYSIS
FINAL ISO
SUPPORT DESIGN
FABRICATION DRAWINGS
PRIMARY OR SUSTAINED LOADS
Pipe and associated component weight
Weight of medium carried by the piping and weight of insulation
EXPANSION LOAD
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OCCASIONAL LOADS
Wind Pressure
Seismic Force
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The internal pressure of the fluid in the pipe
- produces longitudinal stress and
Longitudinal stress = p.d / 4t
Hoop stress = p.d / 2t
The pressure stress in combination with the stress caused by the self weight (and fluid, insulation weight) of the pipes , is called SUSTAINED STRESS.
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STRESSES IN THE PIPING SYSTEM
Thermal stress caused by the restriction of expansion / contraction caused by the temperature of the fluid inside the pipe
These stresses are called Secondary stresses
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STRESSES IN THE PIPING SYSTEM
The OCCASIONAL LOADS produce bending stresses on piping system and the same are normally added to the Sustained stresses to compute the effect of combination of loading
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CHARACTERISTICS OF PRIMARY / SUSTAINED STRESS ?
Caused by ‘non self-limiting’ loadings (sustained) – Stresses remain as long load is sustained.
2. The stresses are not diminished by the deformation of the pipe.
3. They have the potential to cause collapse of the piping system with a single application of the load.
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Caused by ‘ self-limiting’ loadings – Stresses limited by boundary restraints.
2. The stresses diminish as the piping system deflects under thermal loads
3. Stresses will not typically fail the system on a single application of the load; Failure occur due to accumulation of fatigue caused by cyclic loading applications.
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Weight + Pressure
AFTER CACULATION OF THE STRESSES WHAT IS DONE ?
THE SUSTAINED AND SECONDARY STRESSES MUST BE LESS THAN THE ALLOWABLE LIMIT SET BY THE ASME CODE
B31.1 - POWER PIPING
B31.3 - PROCESS PIPING
THE LIMIT SET BY
OTHER NATIONAL CODES LIKE FDBR, SWEDISH, BS 7159, STOOMWEZEN – AS APPLICABLE
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WHAT SHALL BE DONE?
2. Introduce an Expansion loop
3. Redesign / Relocate the supports for the piping.
4. Introduce Expansion Bellows
Preliminary isometric of the piping
Pipe material specification and size
Pipe design parameters like, pressure and temperature
The code of design applicable
Tentative support locations
Allowable forces and moments at the connecting points
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The thermal movements do not affect the adjacent piping system
The associated equipments and structures are not overloaded.
The support loads are finalised for the selection and design of support elements and auxiliary supporting structures
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The analysis can be done either manually or in computer.
Nowadays this is done only in computers, using any one of the following software:
a. ROHR2 (developed in Germany )
b. Caesar-II (developed in USA )
c. CAEPIPE
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FOR
SCOPE COVERED BY ASME B31.1
Minimum requirements for the design, materials, fabrication, erection, test and inspection of power and auxiliary service piping systems for electric generation stations.
6 Chapters; 14 Appendices, 2 tables
Chapter – 1 Scope
Chapter – 2 Design
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DESIGN
Allowable stress Sh or Sc – are listed in ASME for various materials – Calculated based on the following factors:
TYPE OF STRESS FACTOR OF SAFETY
Tensile strength 3.5
Average stress to cause creep rupture 1.0
In 100,000 hrs.
100,000 hrs
Of 0.01%/1000 hrs
L : DEVELOPED LENGTH
U : ANCHOR DISTANCE
iMc
z Margin Available
Where
f = Stress range reduction factor depending on no. of
thermal cycles.
PDO
4tm Z
PDO
4tm Z Z
(1.15 or 1.2)
PDO
External pressure
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STRESS INTENSIFICATION FACTOR
It is defines as the ratio of the max stress intensity to the nominal stress
Used as a safety factor to account for the effect of localized stresses on piping under a repetitive loading
Applied to all components where stress concentration is possible
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SYSTEM ANALYSED IN ROHR2 PACKAGE
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UNEXPLAINED ROTATING EQUIPMENT VIBRATIONS.
FLANGE ALIGNMENT PROBLEMS.
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THE PIPING SHOULD NOT BE OVER-FLEXIBLE ALSO, BECAUSE
1. It costs more since more pipe bends have to be used
2. The pressure drop across the pipeline will be more since the
length is more and therefore the operating cost is more.
3. May create change in flow pattern (from laminar to turbulent)
4. May create operational problems e.g. loops in suction line
5. A flexible pipe is usually weak against wind, seismic and
prone for vibration