## Pipe wall thickness calculation as per ASME B31.3

Enter data as indicated in the form below and click on Submit Button to calculate pipe wall thickness for internal pressure. The theory behind the calculations is provided below.

The minimum pipe wall thickness including mill tolerance for pipe sizes from 1/2" to 24" will be listed below the Submit Button.

## Pipe Wall Thickness for Internal Pressure

As per section 304.1.2 of ASME B31.3, for a straight pipe under internal pressure, the design thickness shall be calculated as per the following equation:

t = \(\frac{PD}{2(SEW + PY)}\)

The minimum thickness of pipe, T shall not be less than t_{m}

where t_{m} = t + c

c = sum of mechanical allowances (thread or groove depth) plus corrosion and erosion allowances

D = Outside diameter of pipe

P = Internal design pressure (gauge)

S = Allowable stress of material from Table A-1 of ASME B31.3

t = Pressure design thickness calculated as per above formula

W = Weld joint strength reduction factor in accordance with para 302.3.5(e)

E = Quality factor from Table A-1A or Table A-1B of ASME B31.3

Y = coefficient from Table 304.1.1, valid for t < D/6

## Example of Wall Thickness Calculation as per ASME B31.3

**Problem**: Find the wall thickness of 8" (DN 200) ASTM A106 Gr. B pipe at design pressure of 800 psig at design temperature of 650°F considering a corrosion allowance of 1.6mm.

**Solution**

P = Internal design pressure = 800 psig

D = Outside diameter of pipe = 8.625" (219.1 mm)

S = Allowable stress of ASTM A106 Gr. B pipe = 17 ksi = 17,000 psi

E = Quality factor from Table A-1B = 1 for seamless pipe

W = Weld joint strength reduction factor = 1

Y = Coefficient from Table 304.1.1 = 0.4

### Allowable stress from Table A-1

The allowable stress should be selected from Table A-1 corresponding to the applicable material and design temperature. As per figure below, the allowable stress for ASTM A106 Gr.B is 17,000 psi at 650°F.

### Basic Quality Factor from Table A-1B

Table A-1B provides basic quality factors for longitudinal weld joints in pipes, tubes and fittings The quality factor should be selected from Table A-1B for the applicable material. As per figure below, the quality factor for ASTM A106 seamless pipe is 1. Note that the basic quality factor is lower (0.85) for electric resistance welded pipe.

### Weld Joint Strength Reduction Factor

As per para 302.3.5(e) of ASME B31.3, the weld joint strength reduction factor is the ratio of the nominal stress to cause failure of the weld joint to that of the base material for the same duration. In the absence of more applicable data (e.g., creep testing), the factor shall be taken as 1.0 at temperatures of 510°C (950°F) and below, and 0.5 at 815°C (1500°F) for all materials.

Hence for temperature of 650°F, W = 1.

### Coefficient Y

The value of coefficient Y from Table 304.1.1 of ASME B31.3 is valid for t < D/6. For carbon steel Y = 0.4 for temperature up to 900°F.

### Calculation

t = \(\frac{PD}{2(SEW + PY)}\)

Substitue the values in above equation gives

t = \(\frac{800 * 219.1}{2(17000*1*1 + 800*0.4) }\)

t = 5.06 mm

t_{m} = 5.06 + 1.6 = 6.66 mm

### Selection of commercially available pipe

After the minimum required thickness is calculated, the next step is to select the next higher wall thickness from commercially available pipe. For example from ASME B36.10, the next commercially available pipe wall thickness is Sch 30 (7.04 mm).

To this wall thickness, a negative (or under) mill tolerance of 12.5% (in this case) should be applied. Please note that if you are using API 5L grade pipes, the mill tolerances specified therein should be applied. After applying the mill tolerance, the available wall thickness is 7.04 mm x 0.875 (100%-12.5%) = 6.16 mm which is less than t_{m} = 6.6 mm. Hence selection of Sch 30 pipe is not acceptable. The next commercially available (preferred) thickness is Sch 40 (8.18 mm). After applying negative mill tolerance, the available wall thickness is 8.18 x 0.875 = 7.15 mm which is greater than t_{m} = 6.6 mm.

Hence the required wall thickness is **Sch 40 (8.18 mm)**