Stress analysis is a science and an art performed behind the scenes of a project and invisible to the average observer; invisible unless something fails. Sure, there are pipe supports but they can be passed off as keeping the pipe off the ground in the battle with gravity. Anyone who ever put up a shelf has some idea how to counteract gravity. And for many applications, it is that simple.
But then we enter the realm of pipes containing chemicals or operating at high temperatures; sometimes both combined, and the simple anti-gravity approach isn’t enough. Nor is it an obvious need in the perception of most people; in part because thermal growth is slow. But the benefits are readily apparent, especially if something bends or breaks.
In the United States, piping design, fabrication, installation, testing, and certification is governed by the ASME B31 Code for Pressure Piping series of Piping Codes. The Process Piping Code is ASME B31.3; the Power Piping Code is ASME B31.1. In Canada, piping design, fabrication, installation, testing, and certification is governed by CSA Z662.
There are additional B31 code sections applying to underground fuel transmission lines, refrigeration piping, low temperature, low pressure building services piping, and Hydrogen piping. Pressure Piping codes provide strict formulas for calculating the minimum wall thickness of various pipe materials based on design pressures, design temperature and allowable stresses in the selected pipe material. The codes also provide general guidance on the type of acceptable pipe and fitting materials, as well as, on the need for specific considerations in the system design for support spacing, and corrosion resistance. The details of the piping design are left to the system engineer.
In spite of advanced finite element analysis tools, determining the optimal solution for a given problem is still very much dependent upon engineering judgement. Even when or if, stress analysis of a system is required depends on experienced engineer judgement. When is stress analysis necessary, perhaps required, and not required?
When To Do Pipe Stress Analysis
There are numerous “rules of thumb” for when to apply pipe stress analysis, and we have summarized a few of them. Note that these are NOT provided to allow an inexperienced person to make a decision that an experienced engineer should make. These “rules of thumb” are provided so an inexperienced person can appreciate some of the ways to look at a situation and converse with an experienced person.
Different industries may use stricter or simpler guidelines for performing stress analysis. Apart from the legal or contractual obligations that may exist, some general guidelines for when stress analysis should be done include:
When system operating temperature exceeds 150F and the pipe diameter is 4 inch or above. If the temperature exceeds 300F, analyze lines smaller than 4 inch
Any pipe above 12 inches diameter (some say 8 inches diameter).
Any pipe 2 ½ inch and larger connected to rotating equipment or heat exchangers.
Any pipe 6 inch and larger connected to pressure vessels.
Double wall pipe with a differential temperature between the inner and outer pipe of 40F or greater.
Short hot runs of pipe anchored at both ends
When professional review of the system shows the system to lack flexibility
If the system is complex (branching)
When seismic analysis is required
A general, less stringent guideline is sometimes referred to as “The 1500 rule”:
If the line size (nominal pipe size) times (x) the temperature (degrees F) is below 1500 then the line "may" not need formal stress analysis. Example 3' (x) 400 degrees (F) = 1200
If the line size (nominal pipe size) times (x) the temperature (degrees F) is above 1500 then the line "may" need formal stress analysis. Example 4" (x) 400 degrees (F) = 1600
Remember that thermal stress analysis works in both directions i.e. for hot or cold piping. The farther away the operating temperature of the pipe is from the ambient temperature during pipe installation, the more likely stress analysis is required.
When Pipe Stress Analysis May Not Be Required
The system duplicates a successfully operating arrangement or can be judged adequate through comparison to a previously analyzed system. Comparison must be by a competent professional.
No thermal growth (ambient temperature fluids), small diameter HVAC chilled water, and plumbing.
Why Perform Pipe Stress Analysis
In order to keep stresses in the pipe and fittings within code allowables.
In order to keep nozzle loadings on attached equipment within allowables set by manufactures or recognized standards (API 610, API 617, the Hydraulic Institute, ect.)
In order to keep pressure vessel stresses at piping connections within the ASME Section VIII allowable levels.
In order to calculate design loads for sizing supports and restraints.
In order to determine piping displacements for interference checks.
In order to help optimize piping design.
Our structural Engineers / piping stress engineers have a bachelor's and Master's degree in mechanical / structural engineering and province licence (P.Eng.) in Alberta, Saskatchewan, British Columbia and Ontario. We review, validate, certify and stamp piping and structural packages. Also, check industries we serve.