Pipe thickness – What Is Piping

Pipe wall thickness calculation is one of the important basic activities for every piping engineer. Process plants deal with the fluids which flow inside the pipe at high-pressure and temperature. So, the pipe deals with high circumferential pressure which can cause the bursting of the pipe if the pipe thickness is not enough. Hence, The designers need to find out the required piping thickness as per section “304.1.2 of ASME B31.3” to resist the internal line pressure. The operation must be leak-free. In this article, I have simplified the pipe thickness calculation procedure. A sample pipe wall thickness calculation problem is discussed mentioning the calculation steps. Process Piping Code ASME B 31.3 is used as the basis for the Pipe thickness calculation. Few Important Points required for Pipe thickness Calculation Before starting the piping thickness calculation, the engineer should be aware of the following points: Process plants are designed for 20 years or 7200 cycles. (Considering 1 cycle per day; Total no of cycles in 20 years=20*360=7200 cycles)Pressure and temperature can vary from line to line and from time to time.Fluid could be corrosive and toxic in the system.Corrosion allowance for ferritic mostly taken 3 mm and zero mm for austenitic steel.Mill tolerance for the seamless pipe is 12.50% and 0.3 for the welded pipe. How do I Calculate Pipe Thickness? Let's consider the following details for the pipe thickness calculation of a seamless Carbon Steel pipe. MOC (Material of Construction) of the pipe - A106 Gr. BNPS (Nominal Pipe Size) - 4”Manufacturing type of the pipe (SMLS, EFW, ERW) – Seamless (SMLS)Design Pressure (PSI) – 1200 PSIGDesign Temperature – 500° FMechanical, corrosion and erosion allowances – 3 mmMill Tolerance – 12.50% of the thickness Pipe Wall Thickness Calculation Formula As per clause 304.1.2 (a) of ASME B 31.3, the internal pressure design thickness for straight pipes with t<D/6 can be calculated using the following formula (Equation (3a): Fig. 1: Internal Pressure Design Thickness Equation per ASME B 31.3 Here, P: Internal Design Gage Pressure=1200 PSIG as per problem definition D: Outside Diameter of the pipe The equation for the pipe wall thickness is based on the outside diameter of pipe, rather than the inside diameter. This is because the outside diameter of pipe is constant, it is independent of the wall thickness. Hence, the pipe wall thickness can directly be calculated easily using the pipe outer diameter. Outside diameter has to be taken from the below standards- ASME B36.10M: for ferritic steel (seamless & welded wrought steel pipes).ASME B36.19M: for austenitic steel (stainless steel pipes) Fig. 2: Outside Pipe Diameter from ASME B 36.10M So from Fig. 2, D= 114.3 mm S: Allowable Stress value of the Pipe Material (A 106-B) at Design Temperature (500° F) Refer to Table A-1 (or Table A-1M) of the ASME B31.3 (Fig. 3) for getting the value of the allowable stress. Travel in the horizontal (x) direction for allowable stress value and vertical (y) direction for pipe material, and the match point to get the value (refer to Fig. 3). If required, use interpolation to calculate the middle value.Note: the value of the allowable stress in Table A-1 is given in KSI, So we need to convert the value in PSI. Fig. 2: Allowable Stress Value from Table A-1 of ASME B31.3 As per Fig. 3, the allowable stress for ASTM A106 Gr.B is 19,000 psi at 500°F. E: Quality factor Quality Factors are used in Pressure Design and applied at Longitudinal and Spiral Weld Joints and for Castings. The maximum Value of quality factors is 1.0. The value of E, Longitudinal Weld Joint Quality Factor, or Casting Quality Factor can be found in Table A-1A or Table A-1B of the ASME B31.3. The weld joint factor (E) is 1 for our problem case (Refer to Fig. 4). Fig. 4: Quality Factor for Longitudinal Weld W: Weld Joint Strength Reduction Factor As per section 302.3.5(e) of ASME B31.3, The weld joint strength reduction factor, W, is the ratio of the nominal stress to cause the failure of a weld joint to that of the corresponding base material for an elevated temperature condition of the same duration. It only applies at weld locations in longitudinal or spiral (helical seam) welded piping components. Weld Joint Strength Reduction Factors are used because at elevated temperature the weld joint creep rupture strength can be lower than the base metal. The value of W can be found from Table 302.3.5 of ASME B 31.3 (Refer to Fig. 5) and for our problem the value of W=1 Fig. 5: Weld Joint Strength Reduction Factor Y: Values of Coefficient from Table 304.1.1, The factor "Y" depends on temperature. At elevated temperatures, the factor Y increases leading to a decrease in the calculated required pipe wall thickness. Refer to Table 304.1.1 of ASME B31.3 for finding the value of Y, It is Valid for t < D/6 and materials shown below The value of Y may be interpolated for intermediate temperatures. For material A106 Gr. B, Y is given 0.4 (refer to Fig. 6) Fig. 6: Values of Coefficient Y Pipe Thickness Calculation Steps Step 1. Put the above values in the equation shown in Fig. 1 t=(1200*114.3)/{2(19000*1*1+1200*0.4)}=3.52 mm; Hence calculated thickness (t)= 3.52 mm Step 2. Add the corrosion to the calculated thickness. tc = t + c = 3.52 + 3tc = 6.52 mm Step 3. Add the mill tolerance to the thickness after adding corrosion value. tm = tc + 12.50 % of the pipe thicknesstm =tc/0.875 =6.52/0.875 = 7.45 mm (This is required thickness) Step 4. Check the next Ordering thickness available in ASME B36.10M considering the required thickness. Fig. 7: Dimensions and weights of Steel Pipes So from Fig. 7, The Ordering thickness is 8.56 mm or Schedule 80. Note:1. Ordering thickness for seamless pipe will always be the next greater value available from Schedule to schedule.2. Whereas for welded pipe any next greater value will be the ordering thickness.3. Extra thickness can be calculated by ordering thickness minus the required thickness = (8.56 – 7.45) = 1.11 mm. Use of the Extra thickness available in the pipes for calculating the life of a pipe after 20 years.For calculating the maximum pressure holding capacity of the pipe.For checking extra thickness is sufficient to take care of thinning, if the same pipe is used for manufacturing the bend.The extra thickness also minimizes deflection and reduces the number of support.To compare with flange pressure holding capacity, to declare pipe is stronger than the flange. Still in doubt!! Simply click here and attend this course to clarify all your doubts from industry experts. Few more useful Resources for you.Pipeline wall thickness calculation with exampleMeaning of Pipe Schedule / Schedule Numbers?Piping Layout and Design Basics