Would Newton Approve of How You Use His Law?

 Improve Safety & Accuracy in Pipe Force Predictions

ASME B31 piping code requires engineers to consider loads on pipes from waterhammer, steam hammer and other fluid transients. In principle, this means using Newton's Second Law which he published over three centuries ago. In 1687 to be specific. And in Latin if you want to read it in its original form!

When it comes to fluid transients, this is more tricky than one would think. Previous generations of engineers using slide rules had no choice but to simplify Newton. But doing this today is risky and possibly inaccurate.

Why is it risky? Because the simplifications to Newton (when estimating fluid transient forces, to be specific) typically only consider pressure forces. While it is true that pressure forces are often the dominant forces, that is not always true and is sometimes highly inaccurate. A complete expression of Newton's Second Law applied to fluid transients also includes friction forces and momentum changes.

AFT Impulse for liquid transients has included this complete force balance since Version 4 in 2007. AFT xStream for steam and gas transients has included this complete force balance since Version 1 in 2021.

Want to learn more? Below are links that give the short version and the long version of Newton applied to fluid transient forces:

• The short version is given is this brief overview hosted on ASME's website: Applying Newton's Laws to pipes forces caused by fluid transients is tricky
• The long version is given in these more detailed technical papers I co-authored for the 2022 ASME Pressure Vessel and Piping Conference: Accurately Predicting Transient Fluid Forces In Piping Systems

The method we use in AFT xStream for gas transients was developed by my colleague and co-author Scott Lang. And it is quite clever. It uses changes in fluid acceleration to determine the forces. In the ASME papers above we call it the Acceleration Reaction Method.

The bottom line is if you are still using only pressure forces to estimate fluid transient loads, you are using a method suitable for engineers in the bygone slide rule era. Today's tools, like those from AFT, allow engineers to do a proper and accurate force estimation. There is no excuse for using an incomplete form on Newton's Second Law any longer. I believe Isaac Newton would agree.