Include Friction in EXP case: Yes or No?

[KernOily]

Hi guys. Looking for opinions here. We have a big pi**ing match going on here in the office as to whether friction should be included in the expansion case.

I say yes, because friction represents a load on the system, and the B31 codes direct us to account for all loads.

One of the other guys says NO, because excluding friction provides a measure of conservatism in the deaign of restraints. He is very experienced and seasoned and I have a lot of respect for his counsel but in this case I disagree.

My point is this conservatism is unnecessary and can lead to overdesigned supports and anchors, nigh inconstructible and at the least embarrassing/expensive. He counters that the seismic load usually governs the lateral loads on the restraints anyway (I am in UBC Zone 4) and so those are used to design the supports. But there are plenty of systems where this is not the case.

In the olde days, i.e. pre-computers, it was more difficult to account for friction using hand calc methods for pipe stress. I think part of this indiviudal's reluctance may be a carryover from those days, not sure though.

As best I can tell, 31.3 is silent on this particular issue. The CBC (Calif. Bldg Code) does require the exclusion of friction in the seismic cases, so that one is settled. If I'm wrong on this, somebody please correct me.

This is bugging me. What say ye?

 

[doberdorks]

Well, the weasel answer (mine) is run it with both, if the system is good both ways, basis are covered (until you start talking aobut what coefficient of friction to use ;-) ).

However, I think if what you are saying is "design only with friction", then you are depending on getting the friction right to be sure your supports are right -- to me friction is a black art, I prefer not to depend on it (must be that summer job working in aviation where they drilled into my brain "never depend on friction")

My thoughts only
SLH

 

[richay]

This is rather a "moot point" if you do things right. By default the Expansion case is a "stress RANGE" (note the word "range"). In CAESAR II this is achieved by subracting two load cases - typically Operating minus Sustained. If you consider friction in both of these cases, then your Expansion case will contain the "difference in friction effects" between these two cases.

Note also that in the "Load Case Editor" that you can't alter the "friction multiplier" for Expansion cases - it is not active here.

Now, if your setting up your expansion case as something like "T1 + D1", and then you want to play with friction - I disagree with the whole load case.

Richard Ay
COADE, Inc.

 

[KernOily]

Hi Rich. Thanks for the reply. I'm a little confused though (what else is new). There is no movement associated with the SUS case. So how then is friction a part of that load case? L2-L1 (EXP - SUS) should still have friction assigned only to EXP. Am I missing something? Thanks!

 

[4Pipes]

Just to stir things up, I'm going to fully support Doberdorks and KernOily. I will completely disagree with Richay.

Richay's reply relies too much on his first sentence. IMHO friction has more affect on badly designed systems but cannot be ignored on even on the best.

Friction changes deflection and will lock up supports if there isn't enought force to break away. It changes deflection as seen in calcs and on site. The deflected shape and hence stress range will then be completeley different. Try a simple calc, say an L with several supports on each leg. Run it with and without friction. Not an example of good design but it illustrates the point.

 

[C2it]

Ther may well be small movements associatd with a SUS case, due to pressure expansion and Bourdon effects in the piping system.

You are missing that Caesar derives the EXPansion case from OPErating minus SUStained by default. You can build whatever combination you like.

I agree with doberdorks, never depend on friction. However, never ignore it either.

 

[richay]

Assuming this set of load cases :

1) W + T1 + P1 (OPE)
2) W + P1 (SUS)
3) L1 - L2 (EXP)

Then case 3 (L3) is the range between case 1 and case 2. If friction is modeled at the supports, then case 3 will also include the "change" in friction effects between cases 1 and 2. If there is no movement in case 2 (the Sustained case) then friction effects in this case will be minimal, hence the range (case 3 - EXP) will include most of the friction effects from case 1 (OPE).

Richard Ay
COADE, Inc.

 

[Sid7]

KernOily

For more discussion on the friction you may find Coade website forum useful too. Friction is discussed in more detail.

I hope you use the EXP case for stresses only and not for loads!

Friction is a double edged sword. It may help/hurt your system. That is why, the use of Mu and the magnitude of Mu (thanks doberdorks) depends purely on the situation it affects.

The practice of using Mu in seismic cases for system loading is somewhere questioned, cant recall where I read it.

But the practice of using Mu for getting conservative loadings on the supports is ok. Again how much Mu is a different issue.

But to consider a system will experience no friction at all is unrealisticaly conservative in most situations, IMHO.

Well again this can not be generalised and this is why you wont be having any agreement with your senior in this issue.

My 2 cents.
regards,

Siddharth
These are my personal views/opinions and not of my employer's.

 

[NozzleTwister]

For my two cents worth on Friction, below is an excerpt from the Stress Criteria that I wrote on the philosopy for friction to be used on a recent project.

Loads due to friction are transient and only occur when the pipe slides on its support, usually during thermal expansion or contraction. Reactions on equipment nozzles, especially rotating equipment, resulting from friction shall be minimized or eliminated where possible by the careful placement of directional anchors and guides.

Note that computer analysis that includes friction is not considered conservative for all restraints and nozzles. While computer analysis with friction will increase the loads on some restraints, it may serve to unrealistically dampen the thermal forces on other restraints and reduce moments at nozzles.

All piping system calculations performed by computer will include analysis without friction effects. Where friction effects will have an impact on equipment nozzle loadings, additional analysis will be performed including friction. At the discretion of the analyst, friction forces on anchors and guides may be manually calculated and added to the operating loads or a computer analysis that includes friction effects at supports may be performed.

For restraints, the largest loads resulting from all analyses of a calculation will be transmitted to Structural Engineering and used for support design.

Equipment nozzle qualification will be reviewed using loadings without friction effects and where relevant, with friction effects.


NozzleTwister
Houston, Texas

 

[doberdorks]

Well, just checked a random output file

Calculated full range expansion stress(based on 2 OPE cases w/ friction)
51% of allowable
Calculated full range expansion stress(based on 2 OPE cases w/o friction)
44% of allowable

<1% difference in operating stresses.

The job I'm currently working on dictates that nozzle loads with friction may exceed allowable nozzle loads by 50%.

SLH

 

[NozzleTwister]

I find that with friction, manytimes forces on nozzles go up and moments reduce.

NozzleTwister
Houston, Texas

 

[DSB123]

All,
Some good points here on whether to include friction or not. My stance on this is to consider both situations since in reality the actual situation will be somewhere between since there are variations in friction factors from support to support, variations in load distribution to what the analysis states due to construction tolerances or piping component weights so to try and predict what a system will do is a near impossibility in reality. Stress analysis results from a computer give the Engineer a "thumbs up" as to whether the system will be acceptable but the Engineer needs to consider practicalities and the limitations of the software. My view is to run both situations and ensure the system meets both bounding conditions.
NozzleTwister is correct.
The problem also is that friction factors change throughout the life of a piping system. Corrosion between the underside of the pipe shoe and the steelwork increases the friction factor and poor maintenance means that who knows what the friction factors are. The value used of 0.3 between steel to steel is IMHO a very dubious figure - fine for first installation but not for a system which has been subject to environmental conditions.Then we have the low friction PTFE/SS values that are used. Unless these are maintained (and the PTFE surface is not exposed) then friction factors can be higher than 0.1 normally used. My point is that if you have to rely on the level of friction to achieve an acceptable system then good luck.