objective of stress analysis, steps involved in stress analysis
What is the objective of stress analysis?
A . To ensure that the stresses in piping components in the system are within allowable limits.
B. To solve dynamic problems developed due to mechanical vibration, fluid hammer, pulsation, relief valves, etc.
C. To solve problems associated due to higher or lower operating temperature as: -
I. Displacement stress range.
II. Nozzle loading on connected equipment.
III. Pipe displacements.
IV. Loads & moments on supporting structure.
What are the steps involved in stress analysis (or any stress package carries out)?
A. Identify the potential loads that the piping system would encounter during the life of the plant.
B. Relate each of these loads to the stresses and strains developed.
C. Get the cumulative effect of the potential loads in the system.
D. Decide the allowable limits the system can withstand without failure as per code.
E. After the system is designed to ensure that the stresses are within safe limits.
Sources of sustained load generated in piping system, Inputs required for stress analysis of a piping system
How the loads are classified in stress analysis package?
A. Sustained Loads : Those due to forces present during normal operation.
B. Occasional Loads : Those present during rare intervals of operation.
C. Displacement Loads : Those due to displacement of pipe.
(Self - limiting stresses due to thermal effects).
What are the sources of sustained load generated in piping system?
A. Internal fluid pressure.
B. Dead weight of pipe with fluid and its attachments. Sustained load is calculated as:- weight of pipe with fluid + internal fluid pressure load + load due to springs (W+P1).
What are the inputs required for stress analysis of a piping system?
A. Pipe size.
B. Fluid temperature.
C. Pipe material.
D. Model.
E. Design pressure.
F. Insulation thickness.
G. Specific gravity.
H. Friction coefficient.
calculate the operating load, occasional loads
How do you calculate the operating load?
W+P1+T1
T1 – Load due to thermal expansion.
Give some examples for occasional loads.
A. Wind load.
B. Seismic load.
C. Forces due to relief or blow down.
D. Pressure wave generated due to water hammer effects.
What is the failure theory subscribed under ASME B31.3?
A. Maximum principal stress theory (rankines theory).
B. Maximum shear theory.
C. Octahedral shear theory.
Answer: - A. maximum principal stress theory or rankines theory.
Select the failure stress range for fatigue failure due to thermal expansion as per B31.3?
A. SA = (Sc+ Sh) 1.6f
B. SA= 1.25 (SC+ Sh)
C. SA = (1.25 Sc+0.25Sh)f
Where, SA= allowable expansion stress range.
Sc and Sh = basic allowable material stress in cold & hot conditions respectively.
f = stress range reduction factor (1 for 7000 cycles.)
Answer: - C. SA = (1.25 Sc+0.25Sh)f
What is the desired life cycle for piping in operation?
Desired life cycle for piping in operation is 20 years (7000 cycles). The normal no. of cycles for which the displacement or thermal stresses are designed is 7000 cycles.
calculate the stress developed due to thermal expansion and calculate the thermal expansion in a pipe
How do you calculate the stress developed due to thermal expansion?
Stress developed, Σ = ∈ x α , (α= Δ L/L)
……. Where, E = young’s modulus.
Δ L = increase in length due to thermal expansion.
L = original length of the pipe.
How do you calculate the thermal expansion in a pipe?
Δ L = α x Δ T x L
In the codes and many reported calculations, α is used as inclusive of Δ T. Thus the above formula is
written as:- Δ L = α x L
Where, α = coefficient of thermal expansion from ambient to operating temperature .
L = length of the pipe.
What do you mean by Stress Intensity Factor (SIF)? Give some examples.
Stress Intensity Factor (SIF) is the ratio of maximum stress intensity to normal stress. It is used as safe factor to account for the effect of localised stress on piping under respective loading. In piping it is applied to welds, fittings, branch connections etc where stress concentration and possible fatigue failure may occur.
Example: - SIF for reducer and weldneck flange is 1.0
SIF for socket weld flange is 1.3
pressure for hydro - test
How much should be the pressure for hydro - test?
Hydrotest pressure should be ca lculated as follow except as provided against point No D.
A. 1.5 times of design pressure.
B. For a design temperature above the test temperature, minimum test pressure can be calculated as:
Pt = (1.5 X P X St) / S
………..Where, Pt : minimum test pressure.
P : internal design pressure.
St: allowable stress at test temperature.
S : allowable stress as design temperature.
C. If a test pressure as per above would produce a stress in excess of the yield strength at test temp. The test pressure may be reduced to maximum pressure that will not exceed the yield strength at test temp.
D. If the test pressure of piping exceeds the vessel pressure and it is not considered practicable to isolate piping from vessel, the piping and vessel may be tested together at test pressure of the vessel when approved by owner and provided the test pressure for vessel is not less than 115% of piping design pressure adjusted for temperature as per point No.B.
Calculate the pipe spacing and fluid is used in heat exchanger
How do you calculate the pipe spacing?
Pipe Spacing (mm) = ( Do+ Dt) / 2 + 25mm + Thickness of Insulation (mm).
Where: D0 : OD of small size pipe (mm).
Dt: OD of flange of large size pipe (mm).
Which fluid is used in heat exchanger in shell side and tube side?
Generally corrosive fluid is used from the tube side (as tube can be easily replaced) and cleaner fluid is used from shell side. Sometimes hot fluid is also used from the shell side.
What is reynolds number and what is the value of reynolds number upto which the flow is laminar?
It’s a dimension less number to classify the nature of flow.
Re= a v d / f
………Where, Re: raynold’s no.
a : density of fluid.
d : diameter of Pipe.
v : average velocity of fluid.
f : viscosity of fluid.
flow is laminar upto Re=2100
drip leg in steam line and thickness of pipe during stress analysis as per ASME B31
Why do we provide drip leg in steam line?
To remove condensate when there is a rise of same in the pipe along the flow direction. If drip leg is not provided in steam line, the condensate which forms inside the pipe will result in water hammer effect causing damage to piping system.
What is the design standard followed for the calculation of allowable forces / moments in nozzles of centrifugal compressor & Steam turbines nozzle?
The strain sensitive equipment piping to be routed and supported to limit nozzle loading and moment in equipment within allowable limits furnished by respective vendors or in absence of vendor data API 560/610/615/621/661 & NEMA SM23. (referred by API 617) is used for compressor & steam turbine nozzle.
What is the mill tolerance to be considered for the thickness of pipe during stress analysis as per ASME B31?
(i) 1%
ii) 2.5%
(iii) 7.5%
iv) 12.5%
Answer : iv
static load and dynamic load
Differentiate between static load and dynamic load?
Static loads are those which are applied slowly enough so that the system has time to react and internally distribute the loads, thus remaining in equilibrium. In equilibrium, all forces and moments are resolved (i.e., the sum of the forces and moments are zero), and the pipe does not move. Dynamic loads are those which changes quickly with time. The piping system may not have time to internally distribute the loads, so forces and moments are not always resolved & resulting in unbalanced loads, and therefore pipe movement. Since the sum of forces and moments are not necessarily equal to zero, the internally induced loads can be different either higher or lower than the applied loads.
different types of dynamic loads and What is dynamic analysis
Give different types of dynamic loads with example?
A. Random – wind, earthquake.
B. Harmonic – equipment vibration, pulsation, acoustic vibration.
C. Impulse – fluid hammer, relief valve opening, slug flow.
What is dynamic analysis and why it is used?
Dynamic analysis is performed for all two phase lines in order to ensure that the line supported is safe from vibrations loads which may occur during normal operation as well as in start up or any upset condition. (diesel mixed with hydrogen in DHDT process).
loop piping system, absorbed in loops in normal design and allowable stress range for CS pipes
What are WRC 107 / WRC 297?
Localised stresses at nozzle to shell is calculated by WRC 107 / 297 and these computed stress values shall be limited in accordance with ASME sec VIII for pressure vessels.
Why loop is provided in piping system?
To adjust thermal expansion.
What is the maximum expansion absorbed in loops in normal design?
10 Inches.
What is the allowable stress range for CS pipes?
2070 kg/cm2