Coefficient of static friction i.e 0.3 for Steel to Steel. 1

While carrying out Stress analysis we generally take coefficient of static friction as 0.3 to determine forces at sliding surfaces for Steel to Steel.

If :
1. Plant is located near Coastal Area.(Atmospheric corrosion will increase its roughness factor.
2. Lines routed on Structural Piperack, during rainy season
the bottom of pipe might get eroded due to rain drops flashing on structural surface near bottom of pipe affecting again in roughness factor.

My question is, shouldn't this factor of 0.3 be increased.
If yes, then what should be that figure.

 

[DSB123]

Amit,
Using 0.3 for friction steel/steel is being optimistic. We generally use 0.4 but in some cases 0.5. Where the steel is subject to corrosion and pitting the surfaces can actually become partially welded to each other (if un-protected) and no relative movement for some time with the friction coefficient being as high as 0.8 .

Hope this helps to some extent. It's up to the Stress analyst to consider the circumstances in which the piping system is situated.

Regards

 

Hello

Another thing to recognize is that most of the time (unlesss the steel-on-steel has "some" grease on it) there will not be a nice linear friction force. More often the force will increase and then "break-loose" in steps. Telescoping "slip-joint" type expansion joints do this too. There is no continuous and unchanging friction force - it builds up and then "pops".

Regards, John.

 

[gearjammer]

I take it that you are concerned with longetivity or service life of the instalation.
the effects of exposure to coastal environments can be
lessened.

you might sudgest that the replacement cost would outweigh the cost of a simple undercoating applied to the installation along with scheduled inspections by plant maintenance.

if the stress of friction is a concern then perhaps there is room for a little rolling friction steel/steel .003
vs the 0.3 sliding friction.

in this changing environment there is much expansion and contraction that might benifit from rolling friction.

 

[flamby]

I have never heard water used as lubricant but I fear this might be true in case of humid and rainy environments. I would prefer to stick to 0.25 to 0.3 in such cases instead of relying on corrosion. The loose rust, in fact, may aid in slipping.

 

[Amit74]

In advance thanks to all of you for answering my query.

Now I am looking for some design basis (on technical ground) as which figure should I consider for Stress Analysis from the range 0.3 to 0.8.

Please help me to come out with a relation between friction factor and Roughness factor( as it will get increase subject to corrosion, erosion/pitting the surface of support steel). Any literature that explains this topic please forward it to me.

Few more questions :
What is the roughness factor when we take coefficient of static friction as 0.3? Is it zero.
& What would be the roughness factor if I have to consider coefficient of static friction as 0.4, 0.5 or 0.8?

 

[KernOily]

This is a very good question, one I struggle with all the time.

In talking with a structural engineer, the value he uses is 0.3 for new steel on steel.

More fundamentally, I think, the question you should ask yourself is: what happens IF the friction coeff IS in fact 0.8? What if it's only 0.3? If it IS 0.8, does this large mu create huge reactions someplace in the piping system? If so, is it too expensive to design and build the supports or equipment to handle the reactions? Are expansion joints an option to handle the movements/friction reaction loads?

Has the plant owner given you any guidance on this?

If not, then calc your friction loads using mu=0.8, design your supports, do your cost estimates, and use that as a worst-case measure of conservatism. You could certainly stand up in court and defend that decision. The careful application of some judgment in this case might save your bacon and a lot of time (gee, they told me that when I took the PE . . . ;-) )

There are other more-or-less weatherproof solutions, also, such as the use of fluorocarbon slide bearing plates under the pipe shoes to reduce the mu down to about 0.06, I think it is. Thanks!
Pete