11 Most Important Questions & Answers From B 31 PIPING...
What is Piping An attempt to ex plain process piping engineering basics in detail to help all the process piping engineering professionals around the world.
11 most important questions & answers from ASME B 31.3 which a Piping stress engineer must know 1 4 th August 2 01 3
ANSI B 3 1 .3
1 4 Com m ents
ASME B 31 .3 is the bible of process piping engineering and ev ery 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 ev ery now and then it will say to refer to some other point of the code which will irritate y ou. But still ev ery piping engineer should learn few basic points from it. The following literature will try to point out 1 1 basic and useful points from the code about which ev ery piping engineer must be aware.
1 . What is the scope of ASME B 31 .3? What does it cov ers 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 Alternativ ely refer the below attached figure ( Figure 300.1 .1 from code ASME B 31 .3)
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 sev ere combination of temperature and pressure. Ambient effects like pressure reduction due to cooling, fluid ex pansion 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 ex ternal or internal unex pected conditions, Wind force, Earthquake force, V ibration and discharge (Relief v alv e) reaction forces, cy clic effects etc. Component self weight including insulation, rigid body weights along with the medium it transport. Thermal ex pansion and contraction effects due to resistance from free displacement or due to thermal gradients (thermal bowing effect) etc. Mov ement 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 max imum temperature. 2. two-thirds of y ield strength at max imum temperature.
3. for austenitic stainless steels and nickel alloy s hav ing similar stress–strain behav ior, the lower of two thirds of y ield strength and 90% of y ield strength at temperature. 4. 1 00% of the av erage stress for a creep rate of 0.01 % per 1 000 h 5. 67 % of the av erage stress for rupture at the end of 1 00 000 h 6. 80% of the minimum stress for rupture at the end of 1 00 000 h 7 . for structural grade materials, the basic allowable stress shall be 0.92 times the lowest v alue determined (1 ) through (6) abov e.
4. What is the allowable for Sustained, Occasional and Ex pansion Stress as per ASME B 31 .3? Ans: Calculated sustained stress (SL)< Sh (Basic allowable stress at max imum temperature) Calculated occasional stress including sustained stress< 1 .33 Sh Calculated ex pansion 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 1 0 inch carbon steel (A 1 06-Grade B) pipe carry ing a fluid with design pressure 1 5 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 v alue at design temperature from code Table A-1 E=quality factor from code Table A-1 A or A-1 B W=weld joint strength reduction factor from code Y =coefficient from code Table 304.1 .1 Using the abov e formula calculate the pressure design thickness, t. Now add the sum of the mechanical allowances (thread or groov e depth) plus corrosion and erosion
allowances if any with t to get minimum required thickness, tm. Nex t add the mill tolerance with this v alue to get calculated pipe thickness. For seamless pipe the mill tolerance is 1 2.5% under tolerance. So calculated pipe thickness will be tm/(1 -0.1 25)=tm/0.87 5. Now accept the av ailable pipe thickness (based on nex t nearest higher pipe schedule) just higher than the calculated v alue from manufacturer standard thickness tables. 6. How many ty pes of fluid serv ices are av ailable for process piping? Ans: In process piping industry following fluid serv ices are av ailable..
Category D Fluid Serv ice: nonflammable, nontox ic, and not damaging to human tissues, the design pressure does not ex ceed 1 50 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 ex posure is judged to be significant and in which a single ex posure to a v ery small quantity of a tox ic fluid, caused by leakage, can produce serious irrev ersible harm to persons on breathing or bodily contact, ev en when prompt restorativ e measures are taken. Elav ated 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 cov ered by this Code, i.e., not subject to the rules for Category D, Category M, Elev ated 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 ex cess of that allowed by the ASME B1 6.5 Class 2500 rating for the specified design temperature and material group. High Purity Fluid Serv ice: a fluid serv ice that requires alternativ e methods of fabrication, inspection, ex amination, and testing not cov ered elsewhere in the Code, with the intent to produce a controlled lev el 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 v alue great than or equal to 1 .0. Its v alue depends on component geometry . Code B 31 .3 Appendix D (shown in below figure) prov ides formulas to calculate the SIF v alues.
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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 1 0 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 fix ation, 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 = dev eloped 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 (Ex pansion) stress range for a piping sy stem? Ans: Ex pansion stress range (SE) for a complex piping sy stem is normally calculated using softwares like Caesar II or AutoPipe. Howev er, the same can be calculated using the following code equations:
here Ap = cross-sectional area of pipe Fa = range of ax ial forces due to displacement strains between any two conditions being ev aluated ia = ax ial 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 = ax ial 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 1 0. 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 serv es to balance the magnitude of stress under initial and ex treme displacement conditions. When cold spring is properly applied there is less likelihood of ov erstrain 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. Howev er now a day s most of the EPC organizations does not prefer the use of Cold Spring while analy sis any sy stem. 1 1 . 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 ex cess of the required thickness to sustain the pressure. This requirement is checked by calculating the required reinforcement area (A1 ) and av ailable reinforcement area (A2+A3+A4) and if av ailable 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 av ailable area are listed below for y our information from the
code. Please refer the code for notations used:
1 . Major Stress related differences in Between 2012 edition and 2010 edition of ASME B 31.3 2. Centrifugal Pum ps: 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. ST ORAGE T ANK PIPING ST RESS 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
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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
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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
from the OD of dummy leg. Reply
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