2. What is Stress Analysis ?
The objective of pipe stress analysis is to ensure safety against failure
of the Piping System by verifying the structural integrity against the
loading conditions, both external and internal, expected to occur
during the lifetime of the system in the plant. This is to be
undertaken with the most economic considerations.
3. Objectives of the stress analysis
• Ensure that stresses in piping components in the systems are within the
allowable limits.
• Ensure the nozzle loadings are within the allowable limits.
• Ensure that sustain vertical displacement is within 10mm.
• Ensure the safety against the occasional loadings such as Seismic and
wind.
• Solve dynamic problems developed due to mechanical vibrations,
acoustic vibration, fluid hammer, pulsation, relief valves etc.
4. Requirement of stress analysis as per ASME CODE
“The Piping Engineer has the following choices to
establish that the required flexibility has been provided in
the piping layout.”
The Piping Engineer has the following choices to establish that the
required flexibility has been provided in the piping layout.
As per clause 119.7.1/319.4.1 of the code ASME B 31.1/B 31.3, no
formal analysis is required in systems which
Duplicates of successfully operating installations or replacements.
Can readily be judged adequate by comparison with previously
analyzed systems.
Satisfy equation specified in clause 119.7.1(A3)/ 319.4.1 (c)
5. Analyzing the layout by an approximate method. Approximate method
shall be applied only if they are used for the range of configuration for
which adequate accuracy has been demonstrated.
Carrying out a comprehensive analysis.
i) Analytical
ii) Model test
iii) Chart method
6. Information Needed for stress analysis.
1. Outside diameter of piping, wall thickness (or nominal diameter,
schedule number)
2. Temperature, internal pressure
3. Material of piping. (Expansion coefficient, Young's modulus, and
material density will be selected for this material.)
4. Insulation thickness and insulation material. (If not given, standard
thickness for calcium silicate will be selected.)
5. Specific gravity of contents
6. Any wind load to be considered? If yes, the direction of application
is important.
7. Any anchor initial translation. (For towers, exchangers, and so on,
nozzle initial
translation is important.)
7. 8. Corrosion allowance for piping
9. Flange rating, (ANSI B16.5) 10. Standard valve weight and fange
weight will be used. (For special valves mark the weight on pipe stress
isometric.)
11. Long radius elbows will be used. (If short radius or any other bend
radius, mark on the isometric.) For short-radius elbow, radius= diameter
12. Any allowable loading from manufacturers on pumps, turbines,
compressors? (From the vendor drawing for equipment.) 13. Any
preference to use expansion loops, expansion joints, and so on,
if needed?
14. Mark type of intersection (reinforced fabricated tee, etc.)
15. Mark support locations (available steel crossing, and so on) on the
isometric
16. Is hydraulic testing load condition to be considered to get structural
support loads?
17. Pipe stress isometrics (x-, y-, z-axis) piping plans, and sections are
necessary.
9. Causes of pipe stress:-
The two common causes of pipe stress are weight and thermal loads which causes
loads on equipment nozzles.
Weight
Weight causes the pipe to sag, which puts stress into the piping material and forces
onto equipment nozzle. It includes the weight of pie, weight of the insulation, weight
of valves, instruments etc.
Thermal
When temperature of the pipe is higher the size of the pipe increases which causes the
nozzle loads to increase and the nozzle loads are further increased when the supports
restrain the pipe from moving. Thus improperly stress analyzed system will cause very
high loads on connecting equipment nozzles.
The other causes of the pipe stress are the occasional loads caused due to Wind,
earthquakes, dynamic loads due to equipment operation like Reciprocating Compressor,
Pilot safety valve reaction force, Slug flow etc.
10. Steps involved in the stress analysis can be listed as
Identify the potential loads that the piping system would encounter during the life
of the plant.
Relate each of these loads to the stresses and strains developed.
Get the cumulative effect of the potential loads in the system.
Decide the allowable limits.The system can withstand without failure.
After the system is designed, to ensure that the stresses are within the safe limits.
11. Types of loads
All the American code for Pressure Piping classify the loads mainly into three types.
Sustained Loads: Those due to forces present during normal operation
Occasional Loads: Those present ring rare intervals of operations
Displacement Loads: Those due to displacement of pipe
12. Impact of excessive pipe stress
Overstressing Piping Components.
Overstressing Piping Nozzle.
Impacting on mechanical components
15. Thermal Expansion
Thermal Expansion. Expressed as the coefficient of linear
expansion, thermal expansion is a ratio of the change in
length per degree of temperature, to a length at a given
standard temperature (such as room temperature, or the
freezing point
of water). The units of the coefficient are length of growth
per unit length per degree of temperature. The value of the
coefficient varies with temperature.
16. Thermal Expansion of Pipe:
All pipes will be installed at ambient temperature. Pipes carrying hot fluids such
as water or steam operate at higher temperatures. It follows that they expand,
especially in length, with an increase from ambient to working temperatures.
This will create stress upon certain areas within the distribution system, such as
pipe joints, which, in the extreme, could fracture. The amount of the expansion
is readily calculated using Equation 1 or read from an appropriate chart such as
Figure.
Expansion (mm)=LΔTα ----------------------1
Where:
L = Length of pipe between anchors (m)
DT = Temperature difference between ambient temperature and operating
temperatures (°C)
a = Expansion coefficient (mm/m °C) x 10-3
17. 1) Case Study On Thermal Expansion:
A 30 Mtr Carbon steel pipe is to be used to transport a steam at 4 bar (g) & 152 deg
Cel. If the pipe installed at 10 Deg Cel, Determine the expansion using
Equation 1
Expansion (mm)=LΔTα………………………(a)
L=30 Mtr
ΔT= T1-T2……………………………………………(b)
T1 =152 Deg Cel
T2 =10 Deg Cel
ΔT= (152-10)
ΔT=142 Deg Cel.
α- 0-200 =14.9 X10-3 MM / M deg Cel for carbon steel pipe.
Expansion : 30 x 142 Deg Cel X14.9 X10-3 MM / M deg Cel
Expansion: 63.5 mm
18. Alternate Method for Thermal Expansion Calculation.
2) Case Study On Thermal Expansion:
Using below chart find the approximate expansion from 15°C, of 100 metres of
carbon steel pipework used to distribute steam at 265°C.
Temperature difference is 265 - 15°C = 250°C.
