ASME B31.3 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 B31.3 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 a 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 of.
1. What is the scope of ASME B31.3? What does it cover and what does not?
Ans: The Process Piping Code, ASME B31.3 is usually applicable for the piping systems in petroleum refineries; chemical, petrochemical, pharmaceutical, paper, textile, ore processing, onshore and offshore petroleum and natural gas production facilities; semiconductor, cryogenic plants; food and beverage processing facilities; and related processing plants and terminals. ASME B31.3 provides design, fabrication, erection, test, inspection, assembly, and material requirements for piping systems that carries following fluids:
- Petroleum products
- Raw, intermediate, and finished chemicals.
- Gas, steam, air, water
- Fluidized soilds
- Cryogenic fluids
Packaged equipment piping design is also covered by the B31.3 code.
The process piping code ASME B31.3 typically does not cover the following:
- The piping systems designed for internal gage pressures at or above zero but less than 105 kPa (15 psi), provided the fluid handled is non-flammable, nontoxic, and not damaging to human tissues as defined in 300.2, and its design temperature is from −29°C (−20°F) through 186°C (366°F).
- Power boiler and power system piping following ASME B31.1
- Fired heater internal piping.
- Pressure vessels, heat exchangers, pumps, compressors, and other fluid handling or processing equipment, including internal piping and connections for external piping.
Alternatively, refer the below-attached figure ( Figure 300.1.1 from code ASME B31.3)
2. What are the disturbing parameters against which the piping system must be designed?
Ans: 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 the most severe combination of temperature and pressure.
- Ambient effects like pressure reduction due to cooling, fluid expansion effect, the possibility of moisture condensation, and build-up 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 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 is 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 1000 h
- 67% of the average stress for rupture at the end of 100000 h for temperatures up to and including 815°C.
- For temperatures higher than 815°C (1,500°F), (100 × Favg)% times the average stress for rupture at the end of 100000 h. Favg is determined from the slope, n, of the log time-to-rupture versus log stress plot at 100 000 h such that log Favg = 1/n. Favg shall not exceed 0.67.
- 80% of the minimum stress for rupture at the end of 100000 h
- for structural grade materials, the basic allowable stress shall be 0.92 times the lowest value determined (1) through (7) 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:
- 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 the 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?
Ans: The pipe thickness (t) for internal design pressure (P) is calculated from the following equation.
- D=Outside diameter of the 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 calculates 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?
Ans: 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 (1035kPa), the design temperature is from -20 degree F to 366 degrees F.
- Category M Fluid Service: a fluid service in which both of the following conditions apply:
- highly toxic fluid such that a single exposure to a very small quantity of the fluid, caused by leakage, can produce serious irreversible harm to persons on breathing or bodily contact, even when prompt restorative measures are taken.
- after consideration of piping design, experience, service conditions, and location, the owner determines that the requirements for Normal Fluid Service do not sufficiently provide the leak tightness required to protect personnel from exposure.
- Elevated 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 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 pipe systems defined for other purposes as high purity, ultra-high purity, hygienic, or aseptic.
7. What do you 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. ASME B31.3 Appendix D, up to edition 2018 used to provide provides formulas to calculate the SIF values which is reproduced in the following figure. However, From ASME B31.3 Edition 2020 onwards the Appendix D is deleted from the code and the code now suggests to use ASME B31J or FEA calculations to find the values of Stress intensification factors.
8. When do you feel that a piping system is not required formal stress analysis?
Ans: Formal pipe stress analysis will not be required if any of the following 3 mentioned criteria are satisfied:
- 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).
- if the system can readily be judged adequate by comparison with previously analyzed systems.
- if the system is of uniform size, has no more than two points of fixation, no intermediate restraints, and falls within the limitations of the empirical equation mentioned below:
Here, D = outside diameter of the 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?
Ans: Expansion stress range (SE) for a complex piping system is normally calculated using software like Caesar II, Start-Prof, Rohr-II, or AutoPIPE. However, the same can be calculated using the following code equations:
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=1.0 for elbows, pipe bends, and miter bends (single, closely spaced, and widely spaced), and ia =io
it = torsional stress intensification factor.
Mt = torsional moment range between any two conditions being evaluated.
Sa = axial stress range due to displacement strains= (ia X Fa )/Ap
Sb = resultant bending stress range due to displacement strains.
St = torsional stress range= (it X Mt )/2Z
Z = section modulus of pipe
ii = in-plane stress intensification factor
io = out-plane stress intensification factor
Mi = in-plane bending moment
Mo = out-plane bending moment
10. What do you mean by the term “Cold Spring”?
Ans: 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 overstraining 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, nowadays most of the EPC organizations do not prefer the use of the Cold Spring while analyzing any system.
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 the available area is more than the required area then no reinforcement is required. Otherwise, additional reinforcement needs to be added. The equations for calculating the required and available areas are listed below for your information from the code. Please refer the code for notations used:
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