thickness calculation as per ASME B31.3

In general, there are 3 types of stress that occur in the process piping in a plant. The first is the material stress that occurs due to internal pressure. There are 2 kinds of this stress i.e. circumferential stress and longitudinal stress. Simply put, circumferential stress has a value of 2 times of longitudinal stress so that the designers often only calculate circumferential stress. Circumferential stress occurs in longitudinal joint of pipe. There are several types of longitudinal joint in the pipe depend to the pipe type (e.g. API 5L grade B) and there is also a seamless pipe (e.g. ASTM A106 grade B). Each longitudinal joint has a different safety factor. We call it as coefficient value, the largest is 1.

The second stress is bending 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. In general, piping designer or piping engineers will choose NPS (Nominal Pipe Size) pipe based on required thickness to hold material stress caused by internal pressure. Then pipe designer and pipe engineer will calculate the second and third stress using pipe stress analysis software. For big and giant project, usually there are a lot of pipe stress engineers involved. These engineers will review pipe support type and its location to make sure the simulated stress is align with ASME B31.3 code.

pipe thickness calculation for process piping based on ASME B31.3 design code.

required thickness to hold internal pressure:

Where

• P = Internal design pressure (psig)
• D = Outside Diameter of pipe (in)
• S = Allowable stress value from material table A-1 ASME B31.3 (psi)
• E = Joint quality factor from Table A-1A (for casting) or A-1B (for tube/pipe)
• W = Weld joint strength reduction factor from Para 302.3.5.e
• Y = Temp dependent coefficient from Table 304.1.1

the Process engineer should define the internal design pressure (P) and design temperature of the process piping. Usually we can find it in a document called as ” Line list or line index”. This document consist of all piping that will be built in a plant. The pipe outside diameter (D) can be found in this document: NPS nominal pipe size

 

 

The weld strength reduction factor (W) in ASME B31.3 has a value between 0.0 and 1.0 that is determined by the material and the design temperature. The weld strength reduction factor will be equal to 1.0 at low temperatures and will decrease at high temperatures. For the design temperature the weld strength reduction factor will be determined by interpolation. this W variable is relatively new in ASME B31.3 that not mentioned in old edition.

ASME B31.3 Table 302 weld joint strength reduction factor (W)

 

It is very interesting that the ASME B31.3 formula is similar to Barlow formula (that used in API 570)  with some “other factors” addition. The “other factors” are W (welding), E ( joint quality factor) and Y ( Temperature dependent coefficient ). Please note, we use ASME B31.3 design code for new process piping construction, and we use Barlow formula to evaluate in-service process piping as mentioned in API 570.

After we calculate required thickness to hold internal pressure ( some time we call as t req ), then we add some thickness as corrosion allowance ( some time called as ca ). So the minimum required thickness is

t min = t req + ca

the corrosion engineer should define the corrosion allowance of the process piping for each service fluid. The value will be vary for each plant. Normally if you work for Oil and Gas Company or other World-class company, you can find it in the corrosion monitoring procedure. If you choose stainless steel pipe ( e.g. ASTM A312 TP304) then you will not need to add a corrosion allowance. After you calculate minimum required thickness, then you can choose or select pipe schedule with mill tolerance consideration. The pipe schedule can be found in this document: NPS nominal pipe size

related article:

1. pipeline ASME B31.3 design code
2. Example calculation of minimum required thickness
3. pipeline inspection