Calculating the Required Structural Thickness of Pipe Using Beam Stress Theory

Introduction

Whether tasked with designing a new piping system or performing analyses on an existing piping system that suffers from some type of damage, the analyst must account for all variables that could have an effect on the system.  One of the items that has been an area of uncertainty to the piping system designer or Fitness-For-Service analyst is the structural strength component of the minimum thickness calculation.  Design and analysis standards, such as ASME B31.3, or API 579, refer to the use of the structural strength requirements for design or analysis of piping systems.  Although this is a requirement in the analysis process, none of the standards prescribe how to perform this calculation. There are many approaches that could be used in determining the structural component for the required pipe wall thickness.  The processes include Beam Flexural Stress calculations, Beam Deflection calculations, use of a Percentage calculation, or simply a fixed minimum thickness value.  This article will go through the methodology of each process, but will focus on the use of Beam Flexural Stress calculations and Beam Deflection calculations to determine the structural component of the required pipe wall thickness calculation.

The minimum wall thickness calculation should be based on the effects of internal and/or external pressure, the stresses induced from thermal influences, the effects of fluid dynamics, the weight of the fluid, the weight of insulation system, the weight of the pipe itself, and even the effects of short term items like snow, ice and wind and when applicable seismic activity.  

If the analyst has access to the stress analysis model, the values for weight, wind and thermally induced stresses are usually indicated and may be extracted from the analysis. In the absence of a stress analysis model, the analyst must determine which stresses have the most influence on the piping system, and calculate the required wall thickness as a result.

Different Calculation Methods

This article will provide information on four (4) potential methods to select from when attempting to calculate the minimum required structural component of pipe wall thickness.  The analyst should review and select the calculation process that provides the most accurate and credible results.  

The first method I’ll cover in determining the minimum required wall thickness of new pipe, is to calculate the thickness required to contain the product pressure, add an amount for Future Corrosion Allowance, then specify the next higher/thicker schedule pipe, which should not be a value less than a predetermined amount.  This is frequently seen in small bore pipe, less than NPD (nominal pipe diameter) 2.  The facilities or firm who choose to use this method have historical information that supports the use of this philosophy.  It is uncommon to have a petrochemical facility specify less than Schedule 80 pipe for less than NPD 3, and less than standard wall for NPD 3 to NPD 12.  This practice is usually based on the typical structural steel support spans, the fact that some small bore may be threaded rather than welded, and the experience that the Schedule 80 provides a significantly longer life with a minor incremental cost.  Although this process works well for new pipe specification, it does not provide the analyst the technical basis for the thickness specified.

The second method that can be used for determining structural wall thickness is to use a percentage of the nominal wall thickness.  Some analysts use 25% to 35% of Standard Wall nominal thickness as the structural component of piping, and then add the pressure calculated thickness and Future Corrosion Allowance to determine the minimum required thickness.  Many of the petrochemical facilities and commercial Asset Integrity Software systems use a default percentage (typically 50%) of nominal thickness to determine the structural thickness.  Although these methods provide a fast and simple process to yield a numerical value, the analyst should be cautious about the technical validity of the value.  As recommended by some of the corrosion monitoring software providers, the 50% default for structural thickness is intended to serve as an Alert or Action Level thickness.  If a piping system corrodes to this level, it should either be replaced or further detailed analysis performed.