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9/4/2014 11 most important questions & answers from ASME B 31.3 which a Piping stress engineer must know | What is Piping
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ASME B 31 .3 is the bible of process piping
engineering and every piping engineer should
frequently use this code for his knowledge
enhancement. But to study a code similar to B 31 .3
is time consuming and also difficult because the
contents are not at all interesting. Also every now
and then it will say to refer to some other point of
the code which will irritate y ou. But still every
piping engineer should learn few basic points from
it. The following literature will try to point out 11
basic and useful points from the code about which
every piping engineer must be aware.
1 . What is the scope of ASME B 31 .3? What does it covers and what does not?
Ans: Refer to the ASME B 31 .3-Process Piping section from my earlier post.
Link: http://www.whatispiping.com/?p=44
Alternatively refer the below attached figure ( Figure 300.1 .1 from code ASME B 31 .3)
What is PipingAn attempt to explain process piping engineering basics in detail to help all the process piping engineering
professionals around the world.
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1 4th August 201 3 Anup ANSI B 31 .3 1 3 Comments
11 most important questions & answers from
ASME B 31.3 which a Piping stress engineer
must know
1
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2. What are the disturbing parameters against which the piping sy stem must be designed?
Ans: The piping sy stem must stand strong (should not fail) against the following major effects:
Design Pressure and Temperature: Each component thickness must be sufficient to withstand most
severe combination of temperature and pressure.
Ambient effects like pressure reduction due to cooling, fluid expansion effect, possibility of moisture
condensation and build up of ice due to atmospheric icing, low ambient temperature etc.
Dy namic effects like impact force due to external or internal unexpected conditions, Wind force,
Earthquake force, Vibration and discharge (Relief valve) reaction forces, cy clic effects etc.
Component self weight including insulation, rigid body weights along with the medium it transport.
Thermal expansion and contraction effects due to resistance from free displacement or due to thermal
gradients (thermal bowing effect) etc.
Movement of pipe supports or connected equipments etc.
3. How to calculate the allowable stress for a carbon steel pipe?
Ans: The material allowable stress for any material other than bolting material, cast iron and malleable iron
are the minimum of the following:
1 . one-third of tensile strength at maximum temperature.
2. two-thirds of y ield strength at maximum temperature.
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3. for austenitic stainless steels and nickel alloy s hav ing similar stressstrain behavior, the lower of two
thirds of y ield strength and 90% of y ield strength at temperature.
4. 100% of the average stress for a creep rate of 0.01% per 1 000 h
5. 67 % of the average stress for rupture at the end of 100 000 h
6. 80% of the minimum stress for rupture at the end of 100 000 h
7 . for structural grade materials, the basic allowable stress shall be 0.92 times the lowest value determined
(1) through (6) above.
4. What is the allowable for Sustained, Occasional and Expansion Stress as per ASME B 31 .3?
Ans: Calculated sustained stress (SL)< Sh (Basic allowable stress at maximum temperature)
Calculated occasional stress including sustained stress< 1 .33 Sh
Calculated expansion stress< SA = f [ 1 .25( Sc + Sh) SL]
Here f =stress range factor, Sc =basic allowable stress at minimum metal temperature and SL=calculated
sustained stress. The sustained stress (SL) is calculated using the following code formulas:
Here,
Ii = sustained in-plane moment index. In the absence of more applicable data, Ii is taken asthe greater of
0.7 5ii or 1 .00.
Io = sustained out-plane moment index. In the absence of more applicable data, Io is taken as the greater of
0.7 5io or 1 .00.
Mi = in-plane moment due to sustained loads, e.g.,pressure and weight
Mo = out-plane moment due to sustained loads, e.g.,pressure and weight
Z = sustained section modulus
It = sustained torsional moment index. In the absence of more applicable data, It is taken
as 1 .00.
Mt = torsional moment due to sustained loads, e.g.,pressure and weight
Ap = cross-sectional area of the pipe, considering nominal pipe dimensions less allowances;
Fa = longitudinal force due to sustained loads, e.g.,pressure and weight
Ia = sustained longitudinal force index. In the absence of more applicable data, Ia is taken as 1 .00.
5. What are steps for calculating the pipe thickness for a 10 inch carbon steel (A 106-Grade B) pipe carry ing
a fluid with design pressure 15 bar and design temperatre of 250 degree centigrade?
Ans: The pipe thickness (t) for internal design pressure (P) is calculated from the following equation.
Here, D=Outside diameter of pipe, obtain the diameter from pipe manufacturer standard.
S=stress value at design temperature from code Table A-1
E=quality factor from code Table A-1A or A-1B
W=weld joint strength reduction factor from code
Y =coefficient from code Table 304.1 .1
Using the above formula calculate the pressure design thickness, t.
Now add the sum of the mechanical allowances (thread or groove depth) plus corrosion and erosion
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allowances if any with t to get minimum required thickness, tm.
Next add the mill tolerance with this value to get calculated pipe thickness. For seamless pipe the mill
tolerance is 12.5% under tolerance. So calculated pipe thickness will be tm/(1-0.125)=tm/0.87 5.
Now accept the available pipe thickness (based on next nearest higher pipe schedule) just higher than the
calculated value from manufacturer standard thickness tables.
6. How many ty pes of fluid serv ices are available for process piping?
Ans: In process piping industry following fluid serv ices are available..
Category D Fluid Serv ice: nonflammable, nontoxic, and not damaging to human tissues, the design
pressure does not exceed 150 psig, the design temperature is from -20 degree F to 366 degree F.
Category M Fluid Serv ice: a fluid serv ice in which the potential for personnel exposure is judged to be
significant and in which a single exposure to a very small quantity of a toxic fluid, caused by leakage, can
produce serious irreversible harm to persons on breathing or bodily contact, even when prompt
restorative measures are taken.
Elavated Temperature Fluid serv ice: a fluid serv ice in which the piping metal temperature is sustained
equal to or greater than Tcr (Tcr=temperature 25C (50F) below the temperature identify ing the start of
time-dependent properties).
Normal Fluid Serv ice: a fluid serv ice pertaining to most piping covered by this Code, i.e., not subject to
the rules for Category D, Category M, Elevated Temperature, High Pressure, or High Purity Fluid Serv ice.
High Pressure Fluid Serv ice: a fluid serv ice for which the owner specifies the use of Chapter IX for piping
design and construction. High pressure is considered herein to be pressure in excess of that allowed by
the ASME B16.5 Class 2500 rating for the specified design temperature and material group.
High Purity Fluid Serv ice: a fluid serv ice that requires alternative methods of fabrication, inspection,
examination, and testing not covered elsewhere in the Code, with the intent to produce a controlled level
of cleanness. The term thus applies to piping sy stems defined for other purposes as high purity , ultra
high purity , hy gienic, or aseptic.
7 . What do y ou mean by the term SIF?
Ans: The stress intensification factor or SIF is an intensifier of bending or torsional stress local to a piping
component such as tees, elbows and has a value great than or equal to 1 .0. Its value depends on component
geometry . Code B 31 .3 Appendix D (shown in below figure) prov ides formulas to calculate the SIF values.
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8. When do y ou feel that a piping sy stem is not required formal stress analy sis?
Ans: Formal pipe stress analy sis will not be required if any of the following 3 mentioned criteria are
satisfied:
1 . if the sy stem duplicates, or replaces without significant change, a sy stem operating with a successful
serv ice record (operating successfully for more than 10 y ears without major failure).
2. if the sy stem can readily be judged adequate by comparison with prev iously analy zed sy stems.
3. if the sy stem is of uniform size, has no more than two points of fixation, no intermediate restraints, and
falls within the limitations of empirical equation mentioned below:
Here,
D = outside diameter of pipe, mm (in.)
Ea = reference modulus of elasticity at 21C (7 0F),MPa (ksi)
K1 = 208 000 SA/Ea, (mm/m)2 = 30 SA/Ea, (in./ft)2
L = developed length of piping between anchors,m (ft)
SA = allowable displacement stress range
U = anchor distance, straight line between anchors,m (ft)
y = resultant of total displacement strains, mm (in.), to be absorbed by the piping sy stem
9. How will y ou calculate the displacement (Expansion) stress range for a piping sy stem?
9/4/2014 11 most important questions & answers from ASME B 31.3 which a Piping stress engineer must know | What is Piping
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Ans: Expansion stress range (SE) for a complex piping sy stem is normally calculated using softwares like
Caesar II or AutoPipe. However, the same can be calculated using the following code equations:
here
Ap = cross-sectional area of pipe
Fa = range of axial forces due to displacement strains between any two conditions being evaluated
ia = axial stress intensification factor. In the absence of more applicable data, ia p 1 .0 for elbows, pipe
bends, and miter bends (single, closely spaced, and widely spaced), and ia =io (or i when listed) in Appendix
D for other components;
it = torsional stress intensification factor. In the absence of more applicable data, it=1.0;
Mt = torsional moment
Sa = axial stress range due to displacement strains= iaXFa/Ap
Sb = resultant bending stress
St = torsional stress= itXMt/2Z
Z = section modulus of pipe
ii = in-plane stress intensification factor from Appendix D
io = out-plane stress intensification factor from Appendix D
Mi = in-plane bending moment
Mo = out-plane bending moment
Sb = resultant bending stress
10. What do y ou mean by the term Cold Spring?
Ans: Cold spring is the intentional initial deformation applied to a piping sy stem during assembly to produce
a desired initial displacement and stress. Cold spring is beneficial in that it serves to balance the magnitude
of stress under initial and extreme displacement conditions.
When cold spring is properly applied there is less likelihood of overstrain during initial operation; hence, it
is recommended especially for piping materials of limited ductility . There is also less dev iation from as
installed dimensions during initial operation, so that hangers will not be displaced as far from their original
settings.
However now a day s most of the EPC organizations does not prefer the use of Cold Spring while analy sis any
sy stem.
11 . How to decide whether Reinforcement is required for a piping branch connection or not?
Ans: When a branch connection is made in any parent pipe the pipe connection is weakened by the opening
that is made in it. So it is required that the wall thickness after the opening must be sufficiently in excess of
the required thickness to sustain the pressure. This requirement is checked by calculating the required
reinforcement area (A1) and available reinforcement area (A2+A3+A4) and if available area is more than
the required area then no reinforcement is required. Otherwise additional reinforcement need to be added.
The equations for calculating the required and available area are listed below for y our information from the
code. Please refer the code for notations used:
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Related posts:
1 . Major Stress related differences in Between 2012 edition and 2010 edition of ASME B 31 .3
2. Centrifugal Pumps: Interv iew questions for a Piping stress engineer
3. Piping Stress Job Interv iew Questions: Part 7
4. Piping Stress Job Interv iew Questions: Part 5
5. STORAGE TANK PIPING STRESS ANALY SIS AS PER API 650 USING CAESAR II
6. Piping Stress Job Interv iew questions: Part 3
7 . Piping Stress Job Interv iew questions: Part 2
This article has 13 comments
9/4/2014 11 most important questions & answers from ASME B 31.3 which a Piping stress engineer must know | What is Piping
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arun
Wednesday 11 December 2013, 4:05 pm
Please clarify my following doubts
1) the equation provided for the sustain is bit different what i learned ( PD/4t+M/Z+F/A) ..but in your
equation u havent consiederd longitunal stress but considered torsioanl stress.please clarify me?
2) in the equation for expansion stress tosional stress is to be corrected
please correct me if iam wrong
Regards
arun
Reply
Anup
Wednesday 11 December 2013, 6:57 pm
Regarding your confusion:
I suggest you to read the latest version of the ASME B 31.3 code. Caesar used to calculate
the stress following your equation as no code equation was available in earlier versions of
the code. But now B 31.3 provides equations for calculating sustained stress.
The torsional term is also included in expansion stress calculation in latest version of the
code.
Thanks for reading my blog. Request you to subscribe with your email to get instant
updation about any of my posts.
Reply
arun
Wednesday 11 December 2013, 9:10 pm
Thanks for your quick reply .and clarify my doubts
iam satisfied with your reply ..
1) still iam confused that why did they ddint use Longitudinal stress Pd/4t in new
equation?
2) In previous version was also considered torsional stress in expansion stress
as
9/4/2014 11 most important questions & answers from ASME B 31.3 which a Piping stress engineer must know | What is Piping
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Sqrt of Sb2 +4St2..in your equation 4st2 have changed to 2st2
.,..this also new changes in new version?
thankx in advance
arun
Reply
Abba
Wednesday 11 December 2013, 8:59 pm
I find this site very informative. I have just attended an Intergraph C2 training for both statics and
dynamic.
thank you for sharing
Reply
rahul sapra
Tuesday 18 March 2014, 3:20 pm
Dear Admin,
Please send basic material for learning CAESAR II software which you got during your
training since i am new to this.
Reply
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