Pipe Support Design for Friction

[rcatalina85]

Hello,

I am currently working on a design that involves a pipe bridge and pipe supports, and there is some disagreement on how the friction loads are determined for the supports. This is not a typical senario, as the pipe loop that goes over the bridge serves as a verticle expansion loop. The pipe analysis shows large vertical loads on the two pipe supports adjectent to the bridge (25 kips) whichs is about 3 times the actual pipe weight. So the thermal loading is pushing down on these supports as the loop expands.

My approach was to use a four column braced frame to resist the friction loads, but I am told it looks rediculous and is not typical. I used a friction factor of 0.3 which results in a lateral load of 7.5 kips per pipe (total of 2 pipes). The mechanical engineer did not include any friction in their pipe analysis but I took it upon myself to add it into the support design since it will be steel on steel. I mentioned using different materials for the slide supports, and they are concerned that they will not last and do not want to rely on them.

I have not dealt with vertical pipe loads of this magnitude and maybe I am missing something here....I would appreciate some input.

Thanks

 

[Bagman2524]

Can you use a spring hanger or snubber to take some of the movement and reduce the force on the pipe bridge?

 

[bridgebuster]

I'm a little confused - you're citing veretical, thermal, and expansion loads. In elevation does your bridge look like this?

++++ <-----Vertical Expansion Loop?
+ +
+++++++++++ ++++++++ <----Pipe

If you're looking for a reliable, long lasting, durable bearing for the pipe try a PTFE/stainless steel sliding bearing. Fabreeka sells them; the CoF is typically less than 10%.

 

[bridgebuster]

my sketch got messed up

 

[LittleInch]

I've read this three times and still can't imagine your design. Can you add a sketch or drawing please to stop us all guessing? I can't get my head around the thermal loads transferring to vertical ones without understanding your piping layout.

My motto: Learn something new every day

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

 

[rcatalina85]

Sorry if I wasnt clear on the original post. I have attached a sketch of the system.

The support on the left is an Anchor loacation. The two supports on the sides of the bridges are slides. After putting some thought into it, the expansion loads are the final loads during operation, and the initial slip and friction force will occur when just the weight of the pipe and water are present. I am sure there will be incremental loading between the intial slip of the pipe and final operational loading, but I think taking 30% of the operation load is too conservative.

 

[delagina]

0.3 of pipe operation weight is standard pipe friction.

what i'm confused is are you putting pipe friction on the location where there is anchor load?
because you only put friction load on supports where there are no anchors.

 

[rcatalina85]

No friction loads at the anchor support. Just the slide locations. Delagina, the thing I am trying to sort out is the difference in the net weight of the pipe Vs. the vertical load from the operational case is about 9 kips Vs. 25 kips on the slide supports on either side of the bridge. So you are saying 0.3 x 25 kips for the friction load? Those loads are per pipe (2 total). That is what I designed around originally and the engineer reviewing my calcs thought I was crazy.

 

[delagina]

you are not crazy, that is a standard calculation for pipe friction, 0.3 of pipe operation weight.

you may want to talk to pipe stress maybe he can do a horizontal loop instead before the pipe bridge because you are getting huge pipe friction.

 

[IFRs]

If the movement from thermal expansion is very small, you can make the argument that the structure will deflect under the friction load and therefore it does not have to resist the friction load. Otherwise, design for it.

 

[JimKSF]

Is the 25 kip load due to thermal expansion of the vertical sections of pipe unloading the pipe bridge and transferring the weight of the pipes back to the supports on either side? If so, then whatever the supports end up supporting is the normal force you have to consider for friction.

One thing to consider is that while a 0.3 coefficient of friction is standard for (clean) steel to steel and is often used for pipe supports, most pipes are painted. And most steel supports are galvanized and/or gets dirty/rusty. That can affect the COF. We recently did some tests for COF with a variety of UHMW samples against a 12x12 steel plate painted to the same piping painting specs used at a refinery. There was a wide variety in the rsults (+/- about 80%) but most of the samples came back with a static COF less than .17. It might be worth looking into for your case.

We modified the ASTM for testing tile materials for friction, using the painted steel plate as the tile and replacing the neoprene "heel" material from Goodyear with the UHMW samples. ASTM C1028 Standard Test Method for Determining the Static Coefficient of Friction of Ceramic Tile and Other Like Surfaces by the Horizontal Dynamometer Pull-Meter Method.

Out loads were lateral and seismically induced. There were anchor stations that took the bulk of longitudinal exapnaion loads out.

 

[IFRs]

I often reduce the friction by using readily available shoes with Teflon pads. Then the friction factor becomes 0.07 and is easier to handle.

 

[LittleInch]

i think JimKSf has it right. when you say the force on the supports increases during thermal expansion, what section of pipe and contents doe this refer to? The whole expansion loop or just the bits either side of the bridge support?

What problem is this extra load casuing you exactly? - Is it vertical load or is it friction on that support making the right hand side of your pipe less able to move thermally into the expansion loop? A constant force spring hanger or support can help reduce your load quite considerably.

My motto: Learn something new every day

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

 

[paddingtongreen]

It would be good to know where the anchors are. The supports nearest the bridge should be spring hangers.

When the pipe expands, it tries to lift off the bridge, the lower levels push towards the bridge and force upward bending of the portion on the bridge, the verticals expand upward, both of these actions transferring load to the lower adjecent supports. With only two pipes, the loads are real. Spring supports allow the pipe to deflect at the support, and leave load at the top, the horizontal expansion loads at each end of the bridge would cancel out.

Another option, if the movements are large, is "Cold Spring", search for it.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[rcatalina85]

Little Inch,

The extra load which is vertical load (hence frictional load) is resulting in a non "Standard" structural support frame as shown in the image of my earlier post.

I appreciate all of the input and keep it coming.

I guess it comes down to a couple of options:

1. Either design for friction under the operational loads using 0.3 factor for steel on steel, or use teflon or other low friction material to reduce the friction loads under the current pipe configuration.
2. Change the pipe layout/support configuration (Mechanical engineer will love this option).

Paddingtongreen,

How is the cold spring method implemented when operational tempertures are 300+ degrees F? Seems like it would be difficult to implement. Also, this system is not in constant use, so I would still have to deal with the system shrinking.

 

[JimKSF]

I'm confused. Any vertical load on the support isn't frictional, its normal to the support.

The load due to thermal expansion of the horizontal run of pipe would be frictional. In the horizontal direction.

 

[LittleInch]

You'll need to forgive me here but what exactly is wrong with a "four column braced frame support". I've seen many simple supports fall over if not properly fixed in position and a support which has fallen over is not a support anymore.

If you want to avoid some of the issues why don't you just use a simple hanging support. Actual movement is very small so nearly all forces are vertical. If the extra cost is less than your major looking support then you've done your job.

300F and intermittent use is new info –it's much better if you tell us everything at the start and not drip feed information....

My motto: Learn something new every day

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

 

[dhengr]

Rcatalina85:
I think you should get with the pipe stress guys and determine what they had in mind, and what they had planned for in their design. It seems to me that you almost have a situation where at one extreme, you could have the main bridge lifting the two side support frames off the ground; and in the other extreme you could have the two side support frames trying to carry the entire main bridge, all through unintended pipe bending and axial forces. ‘The pipe loop that goes over the bridge serves as a vertical expansion loop,’ but that is primarily to absorb/tolerate horiz. expansion and contraction through a gradual/tolerable pipe bending action around that loop. It seems you are tying the pipe down to the main bridge or side support frames too tightly. I think you should take full advantage of min. friction saddles. Also, maybe the inner side support frame, on the left side of your sketch isn’t even needed, and the one on the right side should be moved further away from the main bridge. What would happen if those two side support towers, were just simple A-frames, perpendicular to the pipe line and hinged at their bases and under the pipe saddle?

 

[paddingtongreen]

300 degrees isn't much, but what is the horizontal distance between anchor points? is the growth measurable? Could you cut out half of that length as a cold spring?

Normally in a case like this, the nearest supports at the lower level should be as far away from the bridge as they legitimately can be to maximize flexibility for the vertical expansion. On the oil refineries, I have done that and still used a spring support at that first support to allow even more deflection.

You need to get the piper to show you the behavior of the pipe through the sequence of events, you will understand the loads better if you understand that.



Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin