FRICTION COEFFICIENT

Hi ,

I had a question about pipe support.
when we modeling a rigid guide or limit stop in the line do we have to consider friction coefficient(Mu) for them or we can leave it blank?

Thanks
Mahmoud

I consider friction on every restraint (except anchors) type. If the pipe is in contact with a restraint and has forces that cause the pipe to move in a plane normal to the restraint then there will be friction forces.

that is a good question.

I'm usually tempted to ignore friction on guides due to non-constant contact., er....

if you are being really pedantic, you might do as above

This would also go with the wind loads, EQs, and supports near rotating equipments where there is vibration in the piping systems making pipe-steel contact is not constant? Hmmm....

Throw me some thoughts please...

any support where gravity does not ensure contact would be a start (although line stops could be an exception)

i think you have to look at the way the pipes want to move and make a call.

If i remember rightly, you could find an old post of mine here where i state one should build the model as accuratly as possible. but i give that up once my experience grew, and all those gaps and friction usually means the model refuses to converge

mostly i ignore it unless i think its important

but make your own informed choice.

there is no golden rule, just judgement

Another consideration is that friction at best is an educated guess of what conditions you are going to find. In Critical systems I feel that one should always consider friction as a hinderance. If putting in Friction Lowers your stress or Nozzle loadings I avoid putting it in. If Friction increases your Nozzle Loadings then I usually leave it in. As well talk to your structural engineer to make sure both you and them are on the same page about how you are communicating the frictional loads.

Of course healthy doses of engineering judgement are needed to apply this but as a general rule friction should never help you.

a good day to all,

in addition of Mr. Mahmoud question.

my senior engineer told me not to input friction of guide support for severve cases(a strict in jugement) also for sensitive equipment, i was confuse because if i neglect friction coefficient in my own judgement i am not relating anymore the piping configuration into a real world situation. can anyone help us to clarify this matter.

thank you

regards,

junmar

We always model friction on our supports (guides, linestops, v-stops) and then we often carry out a sensitivity analysis to that friction factor to see how the model is responding.

I am surprised at the number of people who so not consider friction - how do you compensate for the friction loads when you give your support loads to your structural engineers to design?

so its a good practice to always considered friction loads in our piping configuration?

our practice in piping group, we give load (loading data) to our structural engineers group throught by computing its weight (pipe + inside water weight + insulation & cover metal) and multiplied by designed allowance, also we use the caesar2 result to that anchor points. this loads will be one of the bases of the structural designed.

so if you have notice we only submit vertical load and in anchor points base caesar2 result and didnt considered friction load (guide, linestops, v-stops) to structural group.

regards,

junmar

oh... writen in all caps in Technical Discussions in caesar manual. But before going to this, did you know the stifness of your support or its base?

Sometimes we need to look and analize the whole situation and not be dependent on the end result of the software. The whole system should be analize with integral approach. IMO this is Engineering...

Regards,

B31.3 (2006) Paragraph 319.4.3 "The significance of all parts of the line and of all restraints introduced for the purpose of reducing moments and forces on equipment or small branch lines, and also the restraint introduced by support friction, shall be recognized."

B31.3 (2006) Paragraph 321.2.1 (b) "...the effects of friction in other supports of the system shall be considered in the design of such anchors and guides." [Discussing the design of supports and restraints whose function is to control the loads imposed on terminal equipment.]

It is clear that the intent of B31.3 is to consider friction for terminal point forces and moments. This seems obvious from the philosophy of the Code, which is that displacement-induced loads on piping can be treated less stringently than sustained loads because they are self-limiting.

So, it seems that a beginning stress analyst should model support friction on all supports and then, if the model converges, begin to remove support friction from supports far from terminal points until changes in the loads imposed on the terminal points begin to vary. Once that happens, it seems that he/she will have discovered which supports produce friction loads that have significance to the terminal point loads.

As Superpiper noted above, after you have done this for a while, you will acquire some sort of "feel" for which supports need to have friction modeled in the first place. When you start doing this for the first time, I recommend that after you get a satisfactory run, you should ADD friction to the frictionless supports nearest the terminal points to see if your judgment is indeed validated.

It's going to require some effort from you to do this, but if this were easy we would all be making minimum wage or, more likely, doing something else for a living.

I would talk with your structural guys to see if they are considering any lateral loading due to friction. On large lines sitting on T-post type supports or on Pipe racks we have found in can make a big difference in the amount of bracing structural requires.

Again like all things engineering judgement and good communication to explain how the load will be applied and its duration are important so that structural doesn't over design as well.

Good day my fellow stress engineers,please i need some clarifications about the effects of including/NOT-including support frictions during analysis for getting the proper (well designed) axial loads on STOPS, especially when analysing very long lines with several loops and at times cryogenic lines.
please if you have some suggestions/links/attachments that will help me understand these things i will be very grateful.
thank you in anticipation.

on very long lines, the cumlative friction effects sum up so that a 2virtual anchor" is created. at tis point, lines start to buckle and can "walk" along the rack etc. therefore its vital anchors are placed in the line to keep the pipework centralised. anchor loads will end up being Huge, in the region of E for the material, some people just make the anchor as stout as possible and let nature take care of the rest.

friction on these systems is the friend of nozzles and the enemy of supports, so its best to do runs with and without.

on larger lines, where support forces can really build up (think 36"+) i have used friction reducing pads (graphite) so as to reduce stresses on the pipe wall from the saddle.

ambient effects are critical in long lines, make sure you account properly for solar and winter variations

regards

to Bom, GraigB and Greg F thank you for the knowledge you have shared it strive me to learned more and study.

to answer bom and Greg F question. About support stiffness, we have company standard to what kind of support to be use relating to the pipe to be supported, and stiffness of support usually we considered it as rigid as defualt in caesar2. also about lateral force due to friction, in the cases of T-post we submit horizontal force aside from vertical to structural group, but usually in pipe rack we neglect horizontal force not unless it is apparent to be considered.

again thnak you guys

regards,

junmar

Thank you so much Mr Superpiper for your prompt response, You are right its a 36" line,T1=210C,T2=80C...2 loops with AXIAL STOPS placed b4 and after each loop,(3AxialStops),centrally placed. Am confused bcos the axial loads on the stops are very unbalanced,the first 1 upto 10T +ve X direction,the second 7.5T and the third 1T both in -ve X direction..all with friction (MU=0.3) !!!!...without friction,the loads just got reduced !!
so pls. how do I deal wit this to send the proper axial loads to the structural dept.?
thanx again,

Are those loops identical? It should not vary that much.. It is required to submit the result of anchor load with/with out friction which ever is higher. While +Y support horizontal load can be disregard(but first consult the loading data team).

Regards!