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 4 Comments

11 most important questions & answers from

ASME B 31.3 which a Piping stress engineer

must know

44 1

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.

3. for austenitic stainless steels and nickel alloy s hav ing similar stress–strain 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

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 25°C (50°F) 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.

Know ANSI/ASME Standardsduralabel.com

Free Guide shows Pipe Marking Standards for easy identification.

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 21°C (7 0°F),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?

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:

Related Posts:

1 . Major Stress related differences in Between 2012 edition and 2010 edition of ASME B 31.3

2. Centrifugal Pum ps: Interview questions for a Piping stress engineer

3. Piping Stress Job Interview Questions: Part 7

4. Piping Stress Job Interview Questions: Part 5

5. ST ORAGE T ANK PIPING ST RESS ANALYSIS AS PER API 650 USING CAESAR II

6. Piping Stress Job Interview questions: Part 3

7 . Piping Stress Job Interview questions

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This article has 14 comments

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

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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KK

Saturday 7 June 2014, 11:57 am

My Engineer advice me cut the dummy support 150mm which is suppose to be sit in the platform

frame after cutting the grating as per design and welded in a pad for a fire water 250mm line .due to

elevation difference of 25~ 40 mm they want don.t want to cut the grating rather than cut the horizontal

dummy leg and weld it 25~40 mm below without shifting the pad or not weld addition pad. I can’t agree

.Please suggest whether pad can be eccentric i.e on the top maximum 70 mm and on bottom 5mm

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