Pipe Support Friction Factors 3

[DSB123]

Just wanted to bring this old one up again.
Friction Factors for pipe supports where a pipe shoe sits on steelwork. Nearly everyone uses a friction factor of 0.3 for steel to steel contact. How can this be correct for pipe systems running along pipe racks or outside where we all know that maintenance is not what it used to be. The general friction factors for steel to steel are for static friction are around 0.72 to 0.74 and for sliding friction the values are quoted as 0.57. So I ask what is the justification of using 0.3 for pipe stress analysis ? Does the pipesupport "know" it is a pipe support therefore the maximum frictional load cannot be greater than 0.3 times the perpendicular load ? No is the answer so come on the Pipe Stress guys out there make a justification for using 0.3 in your pipe stress analysis when before movement occurs a load of 0.72 times the perpendicular load needs to be achieved and to keep the pipe moving a loading of 0.57 times the perpendicular load needs to be maintained?

 

[rconner]

Mine is perhaps kind of a stupid question, but isn't design support "friction" really what the specific application designer wants it to be? It seems that if I wanted to have a very low and most dependable friction coefficient between a pipe and support, I would perhaps put a roller or bracketed (above and below) roller on top of same, or instead provide a slick e.g PTFE (teflon) pad or other slide bearing etc. On the other hand, if I wanted quite high and dependable axial friction, I might put e.g. a rubber pad between the pipe and saddle, and also strap the pipe firmly down to same.

That being said and on the other hand, I would think however that (while there indeed would be exceptions) with a great many process piping applications (unless great thermal variations, pressures/velocitie or long lengths etc. are involved), due to the high modulus and low thermal expansion coefficient of steel and other common parameters it might make very little practical difference to the pipe or support what "coefficient of friction" were assumed, and indeed just "steel-on-steel" might work reasonably well, and for quite a while.

 

[DSB123]

rconner,
you have switched to a different track here. The discussion is about steel to steel friction not the selection of an interface to provide you with a particular value of friction. Try not to muddy the water on a particular discussion and open another thread if you want to discuss "selection of interfaces to provide particular friction factors".

LittleInch,
I have worked on plants, on pipe trenches in particular, where "T" post supports have failed laterally where there have been no lateral restraints on the pipes supported. Granted the failure has been a combination of some corrosion but the loading causing failure was lateral friction loading. That is why the Company owning the assets stipulated a minimum friction factor of 0.5 for the design of supports.

 

[BigInch]

If T supports had a lateral friction coefficient of 0.5 the support would never sway to the side more than the pipe. As long as the pipe was designed properly, expansion to the side minimized to acceptable values, the pipe would hold the T support in place now wouldn't they.

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[DSB123]

BigInch,
The lines I mention did not have the lateral expansion minimised!!!! The original premise was to allow the lines to "snake" to accommodate the longitudinal expansion. Not a good idea but that was the designer at the time's idea. So no the pipe would not hold the T post in position in that case.

 

[LittleInch]

DSB, You clearly have reasons to believe that "the loading causing failure was lateral friction loading", but I have difficulties with that. Any support which was designed so slender deserved to fail, but if the pipes actually moved laterally, I wonder if the structural designer actually allowed the weight to be further away from the centre?? Or it was designed with two pipes and only one was there or had fluid in it. Too many potential failure modes to pin it on fiction force. Going from 0.3 to 0.5 would only add 1.66 to your force - surely that was within the safety factor on the structural design of the T piece?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[DSB123]

LittleInch,
Who said the friction factor went from 0.3 to 0.5? Who knows what the friction factor was as friction factors are pure guesswork anyway. My point is that the basis of design must be robust enough to accommodate variations in parameters. Using a friction factor of 0.3 for the design of pipe supports subject to pipe movement is questionable in view of the possible friction factor variation.

 

[LittleInch]

DSB,

You implied it went from 0.3 to 0.5, but design for a T support is much more affected by the weight and position of the pipe on the support than the friction force. I accept that the design should be robust, but equally taking a higher FF than would exist can limit the movement being calculated in other locations and then could lead to pipes falling off or limit stops being incorrecty sized.

0.3 would seem to fall between the two for me and just because one support failed, the impact of increasing the FF on the pipe also needs to be considered, not just the support.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[DSB123]

LittleInch,
Nowhere in my post regarging the failure of the T-post did I mention or imply any friction factor values. I was just providing an example of where friction was a contibutor to the failure. Nowhere in the postings has anyone provided a sound engineering basis for using 0.3 universally as a friction factor. Responses such as "well it's been used for years" or "It's universally accepted" does not provide a robust "sound" basis IMHO. You might as well stick your hand in a container with various friction factor values and pull one out and use that one as the friction factor (but how do you defend that?).

 

[Duwe6]

"I was just providing an example of where friction was a contibutor to the failure"

And you know this how? Friction would give you torsion; did the T-post rotate prior to falling over? Most likely the mechanism that allows pipe shoes to work well is a layer of mill scale and/or rust acting as a friction reducing agent. Who cares? Hundreds of thousands of years of pipe history has proven that steel shoe-on-steel pipe rack works well. So something not described in your calculations is happening. So what? If it works well, it works well. Period.

The aerodynamic calculations available up to the 1970's [1990's?] proved that a bumblebee was aerodynamically incapeable of flight, say nothing of achieving a hover. It doesn't matter that we can't adequately explain it if there is proven history that it works.

And yes, it bothers me that I'm not smart enough to be able to calculate it; but I can accept "It Works".

 

[TGS4]

DSB123 - I have reviewed again what you wrote. Solely for the design of the support (whether it is the slender T-post in a latter example or a pipe rack), at a location where there is not a restraint, such as a guide or directional anchor, indeed a higher coefficient of friction will generate a higher lateral load in the direction of movement (or impending movement). And, in a pipe rack situation, where there are hundreds of pipes, you could argue that the difference between a μ of 0.3 and 0.5 could add up to be a big number. You could indeed argue that IF you assume that the direction of movement (or impending movement) is all simultaneously in the same direction and that there is no feedback between any movement of the structural steel and the pipe.

I have long argued (much to the chagrin of my structural engineering colleagues) that, rather than the structural steel holding up the pipes in a pipe rack, it is actually that the pipes hold up the steel.

Try this out for a thought experiment:

All of your pipe supports are perfectly frictionless. You have a long hot pipe in a pipe rack. The total thermal expansion exceeds a specified amount (typically 4-6 inches), and therefore a thermal expansion loop is required. The load on the directional anchors on either side of the expansion loop it determined solely by the loop loads. Every other pipe support on that horizontal steel member is frictionless and therefore does not add (or subtract) from the total load. OK - that's situation #1.

Situation #2 - same basic layout, but in this case there is friction - you can choose the coefficient. Now the total load at the direction anchor is the sum of the loop loads PLUS the friction loads on the pipe supports from the DA up to the start of the loop. You would be correct to say that the higher the coefficient of friction, the higher the total DA force. HOWEVER, all of the other pipes that are on that horizontal structural member ALSO have the same coefficient of friction that you assumed for supports on the first line. Given that the structural steel is going to translate (small, but the stiffness of structural steel is never infinite), the weight*coefficient for ALL of the other pipes is going to resist the movement of the steel. Without doing the proper interaction evaluation, you cannot know a priori that a higher coefficient of friction is better or worse.​

Second example of where the selection of a particular coefficient of friction can help or hinder you:

Imagine a very hot pressure vessel, on a short support skirt. The base ring of the vessel has anchor bolts anchoring it to the concrete foundation. When I run the calculations using a high coefficient of friction (2.0 for example, indicating that it is "sticky"), then the radial expansion of the base ring and hence the bottom of the skirt is restricted. This, in turn, results in higher stresses at the skirt-to-shell junction. However, if I assume frictionless (or at least a low coefficient of friction, corresponding to low-friction slide plates), then the resulting stresses in the skirt-to-shell junction are less. However, now with the base ring free to expand thermally, now it is bearing on the anchor bolts, putting them into a shear load - something that they generally are not designed for.

I have seen problems in the skirt-to-shell junction of such short skirt vessels. And, I have seen identical vessels completely shear off their anchor bolts due to the thermal expansion of the base ring.

Unless I design for a range of values or I purposely select materials that have predictable values, I won't know which failure mode might govern.​

Make sense? In my second example, friction hurts the skirt-to-shell junction, but helps the anchor bolt shear. Because of multiple failure modes in multiple failure locations, it is completely inadequate to simply choose a value. Rather a range of values needs to be evaluated to capture ALL of the potential failure modes.

 

[BigInch]

The "big" engineering companies use 0.1 x 0.3 W for refinery type pipe racks loaded with pipe. Reasoning is that all pipe friction forces do not act in the same direction and most wind up canceling themselves out. They use the 10%f factor for safety in case they don't act that way. Actual value used for f makes little difference in that situation. Vertical loads are 20 psf across the rack. Isolated pipes, or extra heavy pipes and anchors at expansion loops are treated separately. What's the justification for that, other than 100+ years of experience.

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[DSB123]

Duwe6,
How do you know friction would produce torsion? Not if the pipe was directly above the post it would not!!! Did I say it was offsett - no - you should not read what you want to read but read exactly what is there. What the hell has a bumblebee's flight got to do with the topic? Nothing!!!
You say "Hundreds of thousands of years of pipe history has proven that steel shoe-on-steel pipe rack works well". I suppose you have a "photo" to prove this statement - oh forgot no cameras hundreds of thousands of years ago but then come to think of it I doubt there were any pipes at that time let alone pipe shoes!!! Facts are important and your statements are not fact.

 

[BigInch]

surely he means man-years, ie. not 1 guy working since the days of the pyraminds

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[DSB123]

BigInch,
That's not what the statement says. Accuracy in statements is paramount if misunderstanding is to be avoided!!!

 

[BigInch]

I can see past that. Not too big a stretch there.

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[LittleInch]

Don't know - some of the pipe stress engineers I've worked with look like they've been working since the days of the pharaohs.... ;-)

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Duwe6]

"you have a "photo" to prove this statement. . ."

Obviously no photo's taken off-site. How 'bout we get a leetle more rigiorus:

At the close of 2013 the US Energy Information Administration "EIA" found that there were 143 operating refineries in the United States. Of those, I have worked in 10, and have been close enough to see the detailed layout of the piping and equipment of 19 more. Of these, 100% used both tee-shoes and steel pipe directly onto the pipe rack crossbeams. Thus it is more than an educated guess that over 90% of the remaining refineries follow suite giving us over 100 locations using these supports. The oldest unit I have been in was built before the 1930's [Baway NJ, and the newest one I built; GOHDS unit Commerce City refinery CO [which had sections that were pre-WWII]. The weighted average seems to come out at 50-years, but lets drop that guesstimate by 20%, giving 40-years of 'Continued Good Service'. Multiply that by the reduced total of 100 refineries, and you get 4500-unit years of proper performence of steel-on-steel sliding supports. So if there are ONLY 230 steel-on-steel supports in each ENTIRE refinery, counting all of each refieery's half-dozen to dozen-&-a-half units, that gives a grand total of 920,000 years of pipe support experience.

after these calc's, I am revising my "tens of thousands" to well over 1-million support-years of experience

 

[kacarrol]

I'm a little surprised there isn't a reference to Peng on here...that's okay, I'll provide one!

http://www.pipestress.com/papers/Friction.pdf

 

[DSB123]

Duwe6,
Let's end this as I bow down to your undoubtedly greater expertise as I have neve built a refinery on my own before "and the newest one I built; GOHDS unit Commerce City refinery". It has always taken a few hundred more people. Well done!!!!

 

[Duwe6]

Good reference kacarrol, excellent paper - I had only heard of Peng periferally, not his accomplishments. Thank you.