Volume XLV: Q&A from ASME B31.3

ASME PRESSURE VESSELS

The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America.

11 Most Important Questions & Answers from ASME B 31.3 Which a Piping Stress Engineer Must Know

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 you. 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?

ANSWER: Refer to the attached figure below (Figure 300.1.1 from code ASME B 31.3)

2. What are the disturbing parameters against which the piping system must be designed?

ANSWER: The piping system 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 buildup of ice due to atmospheric icing, low ambient temperature etc.
  • Dynamic effects like impact force due to external or internal unexpected conditions, Wind force, Earthquake force, Vibration and discharge (Relief valve) reaction forces, cyclic 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 equipment, etc.

3. How to calculate the allowable stress for a carbon steel pipe?

ANSWER: The material allowable stress for any material other than bolting material, cast iron and malleable iron are the minimum of the following:

  • One-third of tensile strength at maximum temperature.
  • Two-thirds of yield strength at maximum temperature.
  • For austenitic stainless steels and nickel alloys having similar stress–strain behavior, the lower of two thirds of yield strength and 90% of yield strength at temperature.
  • 100% of the average stress for a creep rate of 0.01% per 1 000 h
  • 67% of the average stress for rupture at the end of 100 000 
  • 80% of the minimum stress for rupture at the end of 100 000 h
  • 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?

ANSWER: 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:

Where,

  • Ii = sustained in-plane moment index. In the absence of more applicable data, Ii is taken as the greater of 0.75ii or 1.00.
  • Io = sustained out-plane moment index. In the absence of more applicable data, Io is taken as the greater of 0.75io 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 carrying a fluid with design pressure 15 bar and design temperature of 250 degree centigrade?

ANSWER: The pipe thickness (t) for internal design pressure (P) is calculated from the following equation.

Where:

  • 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/0.875.

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 types of fluid services are available for process piping?

ANSWER: In process piping industry following fluid services are available.

  • Category D Fluid Service: 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 Service: a fluid service 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 service: a fluid service in which the piping metal temperature is sustained equal to or greater than Tcr (Tcr=temperature 25°C (50°F) below the temperature identifying the start of time-dependent properties).
  • Normal Fluid Service: a fluid service 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 Service.
  • High Pressure Fluid Service: a fluid service 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 Service: a fluid service 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 systems defined for other purposes as high purity, ultra high purity, hygienic, or aseptic.

7. What do you mean by the term SIF?

ANSWER: 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) provides formulas to calculate the SIF values.

8. When do you feel that a piping system is not required formal stress analysis?

ANSWER: Formal pipe stress analysis will not be required if any of the following 3 mentioned criteria are satisfied:

  1. If the system duplicates, or replaces without significant change, a system operating with a successful service record (operating successfully for more than 10 years without major failure).
  2. If the system can readily be judged adequate by comparison with previously analyzed systems.
  3. If the system 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:

Where,

  • D = outside diameter of pipe, mm (in.)
  • Ea = reference modulus of elasticity at 21°C (70°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 system

9. How will you calculate the displacement (Expansion) stress range for a piping system?

ANSWER: Expansion stress range (SE) for a complex piping system is normally calculated using software like Caesar II or AutoPipe. However, the same can be calculated using the following code equations:

Where

  • 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 you mean by the term “Cold Spring”?

ANSWER: Cold spring is the intentional initial deformation applied to a piping system 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 deviation from as installed dimensions during initial operation, so that hangers will not be displaced as far from their original settings.However now a days most of the EPC organizations does not prefer the use of Cold Spring while analysis any system.

11. How to decide whether Reinforcement is required for a piping branch connection or not?

ANSWER: 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 your information from the code. Please refer the code for notations used:

 

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