Navigating ASME B31
Once design conditions are established, piping can be specified. The first item to determine is what material to use. As stated previously, different materials have different temperature limitations. Paragraph 105 provides additional limitations for various piping materials. Material selection is also dependent on the system fluid, such as the use of nickel alloy for a corrosive chemical piping application, the use of stainless steel to convey clean instrument air, or the use of carbon steel with high chromium content (greater than 0.1%) to prevent flow accelerated corrosion. Flow accelerated corrosion (FAC) is an erosion/corrosion phenomenon that has been demonstrated to cause severe wall thinning and pipe failure in some of the most critical piping systems. Failure to properly account for thinning of piping components can, and has, led to drastic consequences, such as in 2007 when a desuperheating water line ruptured at KCP&L’s IATAN power station, killing two workers and seriously injuring a third.
Equations 7 and 9 in Paragraph 104.1.1 define minimum required wall thickness and maximum internal design pressure, respectively, in a straight pipe under internal pressure. Variables in these equations include maximum allowable stress (from Mandatory Appendix A), pipe outside diameter, material coefficient (as given in Table 104.1.2 (A)), and any additional thickness allowance (as described below). With so many variables involved, specifying a proper piping material, nominal diameter, and wall thickness can be an iterative process that may also incorporate fluid velocity, pressure drop, and pipe and pumping cost. Whatever the application, it is necessary that the minimum required wall thickness be verified.
Additional thickness allowances may be added to compensate for several reasons, including FAC. An allowance may be required to account for material removal due to threading, grooving, etc., required to make a mechanical joint. Per Paragraph 102.4.2, the minimum allowance should be equal to the thread depth plus the machining tolerance. An allowance may also be required to provide for additional strength to prevent damage, collapse, excessive sag, or buckling of pipe due to superimposed loads or other causes as discussed in Paragraph 102.4.4. Allowances may also be added to account for welded joints (Paragraph 102.4.3) and pipe bends (Paragraph 102.4.5). Finally, an allowance may be added to compensate for corrosion and/or erosion. Per Paragraph 102.4.1, the thickness of this allowance is in the judgment of the designer and should be consistent with the expected life of the piping.
Nonmandatory Appendix IV provides guidance for controlling corrosion. Protective coatings, cathodic protection, and electric isolation (e.g., dielectric flange) are all methods to prevent external corrosion in buried or submerged pipe. Corrosion inhibitors or internal linings may be used to prevent internal corrosion. Care should also be taken to use a hydrostatic testing water of appropriate purity, and to completely drain piping after hydrostatic testing, if necessary.
The minimum pipe wall thickness, or schedule, required per the previous calculations may not be constant over a range of pipe diameters and may require the specification of different schedules for different diameters. Corresponding values for schedule and wall thickness are defined in ASME B36.10 Welded and Seamless Wrought Steel Pipe.
When specifying piping material and performing the calculations discussed previously, it is important to ensure the maximum allowable stress values used in calculations match the material being specified. For example, if A312 304L stainless steel pipe were mistakenly specified instead of A312 304 stainless steel pipe, the wall thickness provided could be insufficient due to the significant difference in maximum allowable stress values between the two materials. Similarly, the method of pipe manufacture should also be specified properly. For example, if calculations are performed using maximum allowable stress values for seamless pipe, then seamless pipe should be specified. If not, seam welded pipe may be provided by the fabricator/erector, which could result in insufficient wall thickness due to a lower maximum allowable stress value.
As an example, assume piping is sized for water with a design temperature of 300 F and a design pressure of 1,200 psig. A 2- and 3-in. carbon steel (A53 Grade B Seamless) line will be used. Determine the proper pipe schedule to specify to meet the requirements of ASME B31.1, Equation 9. First, indicate the design conditions:
P = 1200 psig
T = 300 F
Next, determine the maximum allowable stress value at the above indicated design temperature for A53 Grade B from Table A-1. Note that the value for seamless pipe is used because seamless pipe will be specified:
SE = 17,100 psi
A thickness allowance must also be added. For this application a 1/16-in. corrosion allowance is assumed. A separate milling tolerance will be added later.
A = 0.0625 in.
The value of y is determined from Table 104.1.2(A):
y = 0.4
The 3-in. pipe will be specified first. Assuming a Schedule 40 pipe and a 12.5% milling tolerance, the maximum pressure is calculated:
Do = 3.5 in.
tm = 0.216 in.* 0.875 = 0.189 in.
Schedule 40 pipe is satisfactory for 3-in. pipe at the design conditions specified above. Next, check 2-in. pipe using the same assumptions:
Do = 2.375 in.
tm = 0.154 in.* 0.875 = 0.135 in.
The 2-in. pipe will require a heavier wall thickness than Schedule 40 at the design conditions specified above. Try a 2-in. Schedule 80 pipe:
Do = 2.375 in.
tm = 0.218 in.* 0.875 = 0.191 in.
Schedule 80 pipe is satisfactory for 2-in. pipe at the design conditions specified above.