ASME B31.4 design code for transporting pipeline
In general, there are 4 types of stress that occur in the fluid transporting pipeline. The first is the material stress that occurs due to internal pressure. The second stress is stress that occurs in the pipe due to the weight of the pipe, the weight of fluid in the pipe as well as the weight of other accessories. The third stress is stress caused by expansion when the pipe is used at high temperatures. The fourth stress come from external load outside the pipe e.g. road and rail traffic loads. The most important stress is circumferential stress in the longitudinal joint of pipe due to internal pressure. This stress will govern the pipeline thickness.
minimum required thickness
The nominal wall thickness of straight sections of steel pipe shall be equal to or greater than tn determined in accordance with the following equation:
D = outside diameter of pipe, in. (mm)
Pi = internal design gage pressure, psi (bar)
S = applicable allowable stress value, psi (MPa), as determined by the following equation:
In setting design factor, due consideration has been given to and allowance has been made for the under thickness tolerance and maximum allowable depth of imperfections provided for in the specifications approved by the Code. The value of F used in this Code shall be not greater than 0.72. Where indicated by service or location, users of this Code may elect to use a design factor, F, less than 0.72 e.g. you can use F = 0.65 for some pipeline location.
Please note, the value of E used in this ASME B31.4 is not 100% same with the value of E in ASME B31.3.
Allowable stress values, S, shown in this Table are equal to 0.72E (weld joint factor) x specified minimum yield strength of the pipe. For slurry pipeline systems, allowable stress values are equal to 0.8E (weld joint factor) x specified minimum yield strength of the pipe. Please note, the S value is not related to service fluid temperature or design temperature like mentioned in ASME B31.3.
It is very interesting that the ASME B31.4 formula is similar to Barlow formula (that used in API 570) with some “other factors” addition. The “other factors” are F (design factor welding) and E ( weld joint factor). There is no temperature factor like mentioned in ASME B31.3. Please note, we use ASME B31.4 design code for new transporting piping, and we use Barlow formula to evaluate in-service piping as mentioned in API 570. In general, formula used in ASME B31.4 is more simple than used in ASME B31.3 or we can say the requirement in ASME B31.3 is stringent than ASME B31.4
Here some tips and trick to choose NPS schedule for ASME B31.4:
- read design condition as mentioned in Process Flow Diagram
- choose pipeline material specification or you can see in Pipeline Class document in your company or Client Engineering Standard
- calculate minimum required thickness
- consider mill allowance
- choose pipeline schedule that satisfy all requirements
- specially for elbow, tee and other pipeline component, please refer to Maintenance and Inspection strategy. For some critical pipelines, the client will use ILI for pipeline inspection. Make sure your pipeline system design is pig-able.
disclaimer: Please always refer to the new and latest ASME B31.4 edition for your work.