Dear all,

I have a question regarding friction forces. Do the friction forces disappear after a period of time of the pipe operating at a constant temperature or do they continue once the expansion of the pipe have finished?
I have read in this forum that friction forces are transient, not permanent, but I don't understand it very well..
Thank you for any help

Friction varies with time....

Imagine a 100KG cube at rest stationary on an incline....

You give it a nudge and once it starts to slide it slides all the way to the bottom... yet before you nudged it it was stationary

At time 0 it might take 35Kg of force to get it to move, however once it starts to move its sliding coefficient of friction is much lower and may be only 10Kg of force

So at rest the Static coefficients of friction is always greater than sliding coefficients....

Take a look at...

http://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htm

Hello Carletes,

The piping system is like a spring.

When the pipe supports are moved ("pushed") by the expanding piping, they slide on some surface that resists the sliding movement by friction. When the piping has expanded to its hot position, the force needed to overcome friction is no longer applied to the support - but some of the force has compressed the "spring" (piping system) and that portion of the force of expansion remains. If there were no friction, the support would have moved further and there would be no force stored in the "spring" (piping system).

When the piping system returns to ambient temperature it first releases the stored force of (cold-to-hot) friction, and then again moves ("pulls" on) the supports as it contracts, sliding the support to a new location, somewhere between the installed position and the hot position. If there were no friction, it would return to the point from where it started.

But really it doesn’t slide smoothly, rather the expansion/contraction builds up the force (stores energy in the "spring") against the support until it overcomes the resisting force of friction and then the support "hops" to a new position (exhausting the force to a level less than the opposing friction force). This continues until the temperature of the pipe reaches a steady state (hot or cold). But, in either case there will be some "stored" force in the "spring" that is something less than the force needed to overcome the sliding friction and make the supports move.

The (coefficient of) friction that opposes the movement of the supports changes due to time, temperature and a variety of other factors. No two engineers seem to be able to agree upon what is a reasonable friction coefficient to use in the piping model. That being the case, I am sure you will have several responses to your question.

http://www.engineeringtoolbox.com/friction-coefficients-25_778.html

Regards, John.

A better question might be what is the meaning of life?

While the real world may be cloudy on the issue of friction, the ideal world of CAESAR II (CAESAR II) is quite clear. I don't want to go through the algorithm now but just add on John Breen's approach...

In CAESAR II, we apply the friction force against expected motion when the pipe slides. But, instead of somehow recalculating the new, remaining friction force, we keep the original (mu*N)load. I don't know if it causes any trouble. That's just how we (and everyone else?) do it.

One thing we give you in CAESAR II though, is the ability to run the model with and without friction in the same analysis (using the "Friction Multiplier" in the Analysis processor. I figure the actual system response is somewhere between these two states.

Remember, because of this uncertainty, never take credit for reduced loads or stresses due to friction - always use the worst case. Again, that's my opinion.

Dave

............and an associated topic for discussion might be the forces (of gland seal friction) that "build up" in "slip joints" before they "pop" and compress to accommodate the pipe expansion (and do it again on "cool-down"). Are these impact loadings (should they be multiplied by a "dynamic load factor" (DLF) as is used int B31.1, Appendix II, Paragraph II-3.5.1.3)? Do the anchors get "hammered" in these systems?

And axial forces on piping systems due to excessive hanger angularity (hanger rods too short) might also be fun to discuss. Lots of things give us axial loadings in our piping.

So, should there be a calculated stress (F / Am) in the Code? And, when that stress is there should it be added to the (primary) "additive stresses" (and should it be added absolutely or algebraically)? Or, should the analyst calculate the possibility of "column buckling" in the pipe due to axial compressive forces?

Just thought I would ask.

Regards, John (the trouble maker)

PS, A better question might be what is the meaning of life?

A first hand experience I’ve had with friction was at a film plant where the massive gear pump ~ 10kips was placed on carefully placed, aligned, TFE slide plates.

The systems displacement stresses and the bolted joints (specially designed high pressure flanges with custom coiled O-rings “Helicoflex”) all depended upon the free movement of the pump, base, reducer gear and motor.

On the systems first start up I carefully watched the pumps position as we armed tings up the CAESAR II calculated break away temperature with the mu supplied by the TFE manufacturer was approximately 400F. Well 400 F came and went and the pump base did not move finally when we got to 550F it still had not budged.

So I got a come along hooked up to the base put some minor tension on the line and kicked the line… that nudge was great enough to break mu at that time and pump slid nicely to where it was supposed to be at 550F.. When we finally got to 800F it had displaced properly.

On subsequent cool downs and startups it did not stick anymore and continues to this ady to move…

So what mu was the correct mu?…. Beats me…. A person’s time is better spent on almost anything rather than obsessing about what one number mu is or is not. Design for the two possible extremes of mu, Low and High and hopefully the real world will land between those values.

A long time ago systems were designed without computer programs and there was no ability to add mu to a calculations numbers, yet designs were executed successfully. How did we ever do this??? By realizing that friction was always present and designing to minimize its effects…

I recently learned of a phenomenom that occours with teflon/teflon slide plates.
One of the slide plates can 'sink' into the other
effectively causing a low level line stop.
until the plates break free, the 'stiction' can be very high. Sounds just like mr Luf's pump.

Superpiper,

For this reason one should not use PTFE to PTFE pads as one tends to bed into the other.The standard practice in the industry these days is to use SS plate at the top and PTFE pad at the bottom, this also prevents dust accumulation as the PTFE pad should be designed smaller than the SS bearing plate on top.

Incidentally I have also seen ATCO pads (used as acoustice isolators) deeply embed themselves onto the steel beams supporting them,effectively they end up as line-stops and if they are chemically bonded to the u/s of the shoes they in time shear off altogether.

Regards
A