Respected all forum members,

Can we have to considerd friction co-efficient at guide and limit support?

If yes or no then whats the logic.
My senior has suggested me one restrain configuration as attached snap
In which co-efficient of friction is considered in +Y where as it is not considered in "-Y" and "X".will you please justify that what is the reason??

Please share your views

Thanks in advance

Aftab Alam

From my point of view and my usual approach, the following are my suggestions,

1. We must provide friction values for all supports, unless you may know that the pipe will have no contact with the restraints during all conditons of operation (this is something not ease to find out, if so then no need of Caesar-II) because when the pipe displace and hit the guide, it will have friction on both side of the pipe (i.e.) at side (due to guide)and bottom (due to rest).
2. If the pipe contact with the restraints the load varies depend upon the friction value and it is more conservative because for structural support the axial and lateral loads play a vital role in designing.

Please clarify, if i am wrong.....

I agree with sillyman. You should model friction on all supports. If the pipe is not in contact with the support then the friction force will be zero and CAESAR II will show this. If the pipe is in contact with the support then there will be a friction force proportional to the restraint load and friction coefficient.

Remember that friction acts in a plane normal to the line of action of the restraint. So on a +Y restraint, friction will be acting in the X-Z plane. On guide that restrains the pipe in the X-direction then the friction force will be in the Y-Z plane. This means that the guide friction will add to the total Z-force on both restraints (the +Y and X restraints in this example). The guide friction may also change the value in the Y direction in the Restraint Summary, showing possibly a too high or too low restraint load there. To clarify the force on each restraint separately, look at the Restraint Report, NOT the Restraint Summary. The Restraint Report shows the loads on each individual restraint instead of the summation of loads on all restraints in each direction.

You are 100% right, but..... It should be also inserted the gap, but nobody can be sure that in site will be the same, it should also be inserted the real stiffness and so on. It will also create some trouble with the convergence. So normaly if it is not a critical issue, you can ignore.

One more thing, you can not take advantage from friction effect, so if you discover that in calculation seems to help you, ignore it.

Regards,

Hi Mr. Dave,

As per ASME B31.3-2010 under clause 321.2.1(b), it is stated that "In addition to the other thermal forces and moments, the effects of
friction in other supports of the system shall be considered
in the design of such anchors and guides.".

Since Mr.Dave, you are a Coade cum ASME B31.3 process piping section committee member, i am requesting reply from you. Since it involve both Code and software query.

From the code statement, my understanding related to caesar-II input spread sheet for defining the restraints such as guides, limit/axial stops is that when guide is included in the piping stress analysis, friction coefficient shall be included for guides also and it should not be ignored. Is this what code means? Please clarify.

If ignoring the friction will aid in reducing the stress/support loads/equipment nozzle loads etc. are such that we are not complying with the code. Which is not acceptable? Am i correct!!!!

Since for some people the code statement defines as follows, that is "the friction should be provided for a rest supports near/adjacent to the guides or anchors, but shall not provided for guides or anchors." I dont know what their understanding is...... But i totally disagree with them...

But i may be or may not be correct.... So please provide an eloquent solution, so in future if the same query arised we can give a solution by referring this thread..... Please clarify...

Paldex,

You do not need Dave's statement on that. Isn't that obvious?

In some cases the friction can cause accumulation of forces on equipment nozzles under thermal expansion or contraction. Some cases the opposite, for example a line stopper can get larger forces if the friction is less on other supports.

If you know the exact friction values use it. But friction coefficient can change over the years, under the working conditions and environment as well. So our job is to make sure the pipe is working within reasonable assumptions and mainly under contract requirement.

Coming to the adding friction to all supports (I am going to use an assumption that the following supports are having vertical downwards loading at all time):

1. If There are a zero gap line stopper and zero gap guide at the same location, I would not put friction coefficient to these supports because there will not be any movement mathematically and friction load will be zero in all time either by hand calculation or computer analysis.

2. If there is a zero gap line stopper without a guide I would put friction coefficient to that line stopper because of the expectation of lateral movement under thermal loads.

3. If there is a zero gap guide without stopper I would put friction coefficient to that guide due to the expectation of thermal movements.

4. I would do the same thing for all above with gaps at supports as well. When the gap closed the acting support will start taking load, if there is still a motion afterwords it will take the friction loads as well.

If there is uplift at the -Y supports with gap, and there is a guide at the same support location, if the pipe is under thermal movement -Y support will start taking friction loads if the gap closes earlier than the motion finishes similar to the guide at the same location.

There are many possibilities that you normally consider in the modeling and analyzing piping systems and their supports. We, as a stress engineer, need to see the possibilities and find a way to simulate them in making some assumptions by complying the code rules and engineering requirements.

In technical side there is no straight answer, there are possibilities. Code Committee is trying to warn us about these possibilities to consider. In case there is no need to put a support to a location, no one will question that. Therefore our judgment is important, and it is questionable.

It became very long, and may not be the answer you are looking for. But we all need to put our hats on the table, and think more when we question someone.

Kind regards,

Ibrahim Demir

Hi Demir,

Thanks for your reply, i agree with u. But my worry is there are some people who mis-read the code and try to ignore many critical issues such as support frictions, max. displacement stress range (instead of min metal to max. metal temperature, they check either min to installation or max. to installation neglecting the overall displacement stress range). The one to whom i argue regarding the friction support is my main contractor who argue the same thing about the max. displacement stress range...

My actual requirement is, what is the meaning (understand by all) of the statement from the code "In addition to the other thermal forces and moments, the effects of friction in other supports of the system shall be considered in the design of such anchors and guides.". Thats it...

Thankyou...

This is an example of the meaning of:

"In addition to the other thermal forces and moments, the effects of friction in other supports of the system shall be considered in the design of such anchors and guides.".

L1 is operating with friction, L2 is operating without friction.

Observe in the "friction2.CAESAR II" file the differences in nodes no 20 (anchor) and 70 (guide).


It is obvious that friction can not be ignored in the design of the supports.

However, in "friction3.CAESAR II" I deleted the mu only in the guides 30, 120, 150 and 180.

Observe the differences between the two files. So, for 180 it is not critical if the friction is ommited in the guide.

For 70 it may be critical if you remove the friction in the guide because is a large load in the guide.

So judgement need to be made on a case by case basis.

Regards,

I think it's all here now.

Remember that your quote comes from the section on Piping Supports (para. 321). We are looking for the worst case in sizing these supports. That greatest magnitude does not necessarily come from the "friction analysis" or from the "no friction analysis". Test both when you are unsure but remember that, when it comes to friction, your CAESAR II evaluation is only an approximation.

Hi Dave,

Thanks for your reply. So i conclude from your reply, that friction shall be provided in guides also. It can be igonored, if it is assumed to be negligible else friction shall be specified at guides.

Piping - Guide friction may or may not be ignored.
Pipeline - Guide friction shall not be ignored. It is requirement to include the friction at guides...

There is no clear statement in the Codes regarding the use of friction. It's up to you to "consider" it.

I'm skeptical on using friction on restraints that do not carry deadweigt as the normal load may be changing.