Re: Friction Effects in Piping Systems

"Logic" is overrated. I saw someone said the ground holds him up because of friction.
The concept of friction is simple, yet few still escape the idea. By definition, friction is the resistance that is encountered when two surfaces "slide" or "tend to slide" over one another. When the force between the surfaces of two bodies pressing upon each other is normal to the surface of contact, friction force is absent. While standing on your feet, a flat surface exerts a reaction to counter your body weight. This reaction is a force, not friction. That force is equal to and in the opposite direction of your body mass times the pull of gravity (1st year high school physics; F=Ma). An incline surface also exerts a force to counter your body weight. Additionally, since you "tend to slide", there is this resistance that keeps you from sliding down the incline. This resistance is called friction force. The coefficient of friction determines if you'd get wiped off the incline or stay put.

Let's now look at pipes.
Pipes grow or shrink when heated or cooled. During thermal transient
(that period of heating or cooling) the pipe would slide over the
stationary supporting steel thus creating sliding resistance we called friction force. When the process of cooling or heating ends
(steady state), sliding or the tendency to slide ceases to exist,
thus friction force is absent (Please don't ask me about residual forces; Another territory, and mostly insignificant).

Do you need to consider friction in piping analysis? You betcha. Do you need to include it in the analysis? Not always. It all depends on the system and its configuration. In most cases friction may be safely ignored. Notice the word "safely" You the engineer must make that decision. Like flow induced vibration, friction is a reality you cannot casually overlook.

What coefficient of friction to use?
Depend on the materials in contact. If project specification fails to specify this value, use data from engineering handbook(s). Most projects I work with specify a value of 0.3 between carbon steel. Some use 0.5.
If project allows, use tested material such as Lubrite or Teflon. Theirs are tested, so the values do not wander all over the chart like data in many engineering handbooks. Friction is fickle. You consider friction only when it can "significantly" hurt your system. Received on Thu Apr 24 09:38:00 2003