Satish Lele
lelepiping@gmail.com

Design Code for Thermal Stresses
The Code B31.3 provides:
  1. A list of acceptable piping materials with their allowable stress at various temperatures and numerous notes providing additional information on the use of each material.

  2. A tabulation of standards which include acceptable components for use in B31.3 piping systems such as:

    • ASME B16.5, which covers the dimensions, materials of construction, and the pressure temperature limitations of the common types of flanges found in refinery piping.

    • ASME B16.9, another dimensional standard for butt-welded fittings such as tees, crosses, elbows, reducers, weld caps, and lap joint stub ends. B16.9 fittings must also be capable of retaining a minimum calculable pressure.

    • ASME B16.11, another dimensional standard for socket-weld and threaded tees, couplings, and half-couplings. This standard also has a minimum pressure requirement. These are only a few of the more than 80 listed standards.

  3. Guidance in determining safe piping stress levels and design life.

  4. Weld examination requirements for gauging the structural integrity of welds.

  5. Pressure test requirements for piping systems before plant start-up.

With the above in mind, it might be assumed that the B31.3 Code is a designer's handbook. This belief could not be further from the truth. The Code is not a design handbook and does not eliminate the need for the designer or for competent engineering judgment. The Code provides only a means to guide the designer to analyze the design of a piping system, by providing simplified equations to determine the stress levels, wall thickness, or the design adequacy of components, and acceptance criteria for examination. The Code does not provide any instruction on how to design anything.
The Code's approach to calculate stress levels and assure safety in piping is a simplified one. Codes would be of little use if the equations specified were very complicated and difficult to use. Codes would find little acceptance if their techniques and procedures were beyond the understanding of the piping engineer. This is not to say, however, that designers who are capable of applying a more rigorous analysis should be restricted to this simplified approach. In fact, such designers who are capable of applying a more rigorous analysis have the latitude to do so provided they can demonstrate the validity of their approach.
Plants designed to B31.3 generally have a life of about 20 to 30 years. Plants designed to B31.1, on the other hand, may be expected to have a plant life of about 40 years. The difference between these two codes is the factor of safety in the lower to moderate design temperature range. B31.3 uses a 3 to 1 factor of safety, where B31.1 has a 4 to 1 factor. This factor can reflect differences in plant cost. For example, the same design conditions for a B31.1 piping system may require schedule 80 pipe wall thickness, while a B31.3 system on the other hand, may require only schedule 40 pipe wall thickness.
Plant reliability issues center on the effect of an unplanned shutdown. Loss of power to homes on a cold winter night is an example of a reason to have very high plant reliability in B31.1 piping systems. Here, the safety of the general public is affected. If a chemical plant is forced off stream for one reason or another, very few people are affected. A lesser reliability can be tolerated in B31.3 piping systems.
The types of plants for which B31.3 is usually selected are: installations handling fluids including fluidized solids; raw, intermediate, or finished chemicals; oil; petroleum products; gas; steam; air; and refrigerants (not already covered by B31.5). These installations are similar to refining or processing plants in their property requirements and include:

  • Chemical plants

  • Petroleum refineries

  • Loading terminals

  • Natural gas processing plants

  • Bulk plants

  • Compounding plants

  • Tank farms

  • Steel mills

  • Food processing

  • Beer breweries

  • Pulp & paper mills

  • Nuclear fuel reprocessing plants

  • Off-shore platforms

Principal Axis and Stress: The analysis of piping loaded by pressure, weight, and thermal expansion can appear to be rather complicated and difficult to accomplish. This complexity will be greatly simplified when the analyst has an understanding of the Principal Axis System.

PRESSURE DESIGN OF PIPING & PIPING COMPONENTS

Design Conditions: An essential part of every piping system design effort is the establishment of the design conditions for each process. Once they are established, these conditions become the basis of that systemís design. The key components of the design conditions are the design pressure and the design temperature.
Design Pressure and Temperature: Design pressure is defined as the most severe sustained pressure which results in the greatest component thickness and the highest component pressure rating. It shall not be less than the pressure at the most severe condition of coincident internal or external pressure and maximum or minimum temperature expected during service.
Design temperature is defined as the sustained pipe metal temperature representing the most severe conditions of coincident pressure and temperature. B31.3 provides guidance on how to determine the pipe metal temperature for hot or cold pipe.
Designers must be aware that more than one design condition may exist in any single piping system. One design condition may establish the pipe wall thickness and another may establish the component rating, such as for flanges. Once the design pressure and temperature have been established for a system, the question could be asked: Can these conditions ever be exceeded? The answer is yes, they can be exceeded. In the normal operation of a refinery or chemical plant, there is a need, on occasion, for catalyst regeneration, steam-out or other short term conditions that may cause temperature-pressure variations above design. Rather than base the design pressure and temperature on these short term operations, the Code provides conditions to permit these variations to occur without becoming the basis of design.
A review of Allowances for Pressure and Temperature Variations, Metallic Piping, reveals these conditions for variations is given in Code. Therein, the Code sets the first two allowable stresses for design:

  • The nominal pressure stress (hoop stress), shall not exceed the yield strength of the material at temperature.
  • The sum of the longitudinal stresses due to pressure, weight, and other sustained loadings plus stresses produced by occasional loads, such as wind or earthquake, may be as high as 1.33 times the hot allowable stress, Sh, for a hot operating system.
Pressure stress in the circumferential (principal) stress or hoop stress. The stress limit of the yield strength at temperature is simply a restatement of the maximum principal stress failure theory. If indeed, the hoop stress exceeded the yield strength of the material at temperature, a primary stress failure would occur.
The second stress condition, the longitudinal stress caused by pressure and weight, is a principal stress and, pressure, weight and other sustained loadings are (wind or earthquake stresses) primary stress loadings. The allowable stress, Sh, is defined as a stress limit value that will not exceed a series of conditions, one of which was 2/3 yield at temperature. Applying this 2/3 yield stress condition with the 1.33 Sh stress limit, we find again, a direct application of the maximum principal stress failure theory. That is, longitudinal principal stress must be less than 1.33 x 2/3 yield strength at temperature, the product of which results in a limit of about 90% yield. Again, the primary stress is less than yield at temperature. (Some factor of safety is included in this equation to account for the simplified technique of combining these stresses.)
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