anchor forces on long straight pipe
anchor forces on long straight pipe
(OP)
I have a 6000 foot above grade crossing for a new water line on a steel bridge. Due to tight constraints and aesthetics we are using bellows type joints. The pipe is 8 inches in diameter and is at 150 psi. I am wondering what the anchor forces will be if I have 2,000 feet between anchors with the expansion joint in the center. I have the force of the joint which is 12,000 pounds. To guide the pipe we are looking at pipe slides every 45 feet to act as a guide and a support in one. Roller supports will be used every 15 feet otherwise. The guides are PTFE. The water is at maybe 60 degrees. Do I need to account for the friction force of the pipe and the slides in the anchor load calculation, or is the temperature and pressure low enough that it is negligible? I don't have a modeling program to help.
Thanks for the help!
Thanks for the help!





RE: anchor forces on long straight pipe
This could be a very expensive mistake not to get this analysed properly using stress analysis software.
Either that or many more bellows and anchors, say an anchor every 3 or 4 hundred feet with a bellows in the middle. As my strapline says, theta a good reason everyone else does it this way....
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: anchor forces on long straight pipe
RE: anchor forces on long straight pipe
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: anchor forces on long straight pipe
I'm not clear what material the pipe is. You've only said the guides or slides are PTFE.
Thermal stresses are not dependent on length, only on temperature changes (positive, or negative) and Young's modulus.
Force = stress x cross sectional area of the pipe wall.
If everything is always 60F, there would be no stresses. Is there day, night, summer and winter temperature variations where you live. Bridge temperatures may vary considerably more than the water, thus expect to generate the maximum friction forces possible at slides. Water may remain above freezing, while bridge temperatures could be much lower.
Independent events are seldomly independent.
RE: anchor forces on long straight pipe
It still just seems a long way between anchors and bellows to me...
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: anchor forces on long straight pipe
RE: anchor forces on long straight pipe
Have you tried suspending the pipe from hangers? Sounds like somebody has an "architectural" wet dream going if they demand you use bellows rather than expansion loops.
RE: anchor forces on long straight pipe
this is bellows, [<---12Kips BBBB 12 Kips--->]
That would require an anchor at each side of a bellows.
Anchor ----> [<---12Kips BBBB 12 Kips--->] <----12K Anchor
Normally you would need only one bellows. Why two? Unless the min/max expansion/contraction is greater than what one bellows can accept, use one.
if you had a figure like that above, with a bridge expansion joint at the center of the bridge, I'd try it like this,
===G====G===G====A== <-12K-> [BBBB] <-12K-> =A====G===G===G====
Guide Guide Guide Anchor Anchor Guide Guide Guide
each guide would only have a friction force = weight of pipe + contents * 0.3 friction factor, if steel on steel. PTFE slide plates have a friction factor of 0.1
Aside from the friction forces, the only real load in that arrangement above should be the 12 Kips compression (resisting the 12K tension force from internal pressure in the bellows) in the pipes connecting each end of the bellows to the anchor. That's what you get when you use only one bellows, with an anchor on each side. That way, thermal force is released by pushing all movement away from anchors and allowing it within the bellows. No restraint against movement = no thermal stress, or thermal force. Only friction forces remain that will try to buckle the pipe and they should be small enough to allow long distances between guides, especially if using TPFE slides.
If you have 1 bellows that will accept the full thermal expansion, then there will be no thermal load in the pipe from contraction during cold periods, or expansion when hot, except for what is caused by the friction forces of sliding at the guides. Very much less than pipe thermal loads in general.
Google the Euler buckling load formula, works for any section. Maximum compressive stresses are calculated by using that Euler buckling load divided by a safety factor, safety factor depending on what type of load caused the compressive stress. If thermal, I'd use sf=1.4 All you need to do is calculate radius of gyration of the cross section and know the length of the longest pipe between guides. Support adequately by brackets, or hangers, otherwise to avoid overstress from bending.
There is a difference in stress and force. Steel, bridge or pipe, if the same grade of steel, will both yield at the same tension stress. The bridge beam, having a larger x-sectional area, will probably be carrying many times the force in the pipe.
Independent events are seldomly independent.
RE: anchor forces on long straight pipe
RE: anchor forces on long straight pipe
if you have 134 supports in 2000 ft of pipe
the load on each slide (with water in the pipe) is 50.2 lbs * 14.92 ft = 750 lbs
x 0.1 friction factor = 75 lbs per slide plate
The 75 lbs friction is generated in each slide
The pipe pushes out ,away from my first anchor above, to the left.
Each slide won't resist more than 75 lbs, so the load in each pipe segment between slides cannot possibly be more than 75 lbs.
Pipe load = 75 lbs <-, friction on slide-> 75 lbs, carried on to next pipe segment is +75-75=0
Next segment, 75 lbs<- add 75lbs ->, net load to next segment = 0
etc.
You can see that there is no load greater than 75 lbs using that method.
Force between two anchors, not having an included bellows is HIGH!
a = 0.0000065 in/in-F
dT = temp change 20 deg
E = 30,000,000 psi
stress = a * dT * E = 3900 psi
thermal force = 3900 * Asteel
Asteel = 8.4 in2 = 32,760 lbs
A.) Use one anchor, allow growth/contraction in each direction, do not restrict expansion/contraction, except on each immediate side of the bellows and your maximum load is 75 lbs in any segment. Bellows anchors have 12 kips.
B.) Use two anchors, the compression load in ALL pipe segments in between is 12,000 lbs compression, the bellows resisting load, if there is a bellows in between the anchors.
C. No bellows, or if bellows max expansion is reached, pipe anchored both ends could be a high load of 32,760 lbs thermal + 12,000 lbs bellows resisting compression load (44,760 lbs compression total) in all pipe segments with only a 20 deg temp rise and the expansion of 0.26" was totally restrained. or 32,760 lbs tension -12,000 bellows load = 20,760 tension in all pipe segments with a 20 deg drop in temperature. If the bellows accepted some movement, the load would be proportionally reduced.
Independent events are seldomly independent.
RE: anchor forces on long straight pipe
=A=======G====G===G====[BBBB] =====G===G===G=====A=====G====G===G====[BBBB] =====G===G===G=====A====
The bellows joint is 1,000 feet away from the anchors, which are 2,000 feet apart. I have 3 joints over the length of the 6,000 foot bridge.
change in temperature is assumed to be 100 degrees between summer and winter. Pipe will be dry in the winter and drained to prevent freezing.
Thanks for the help! Hopefully this is my last question:)
RE: anchor forces on long straight pipe
For pipe design purposes, it looks like 12,000 lbs in all pipes between anchors, provided the bellows accept all movement and don't bind up. Theoretically the center anchor will have no net force, since the +12Kip load balances the -12K load, but I'd design it for 12,000 lbs anyway.
I'd assume no friction at the guides for pipeline design purposes, but actually design them for at least 1000 lbs friction load (probably more than necessary, but a nice round number) and of course the vertical load of weight of pipe + contents.
Temperature will not make any difference, since supposedly the 2 bellows will accept all the thermal movement.
Independent events are seldomly independent.
RE: anchor forces on long straight pipe
RE: anchor forces on long straight pipe
The friction loads are ACTUALLY resisted by and at each guide.
12000 lbs - 750 lbs/guide = 9750 lbs remaining load at the outside anchors.
The center anchor has no net force.
see diagram
Independent events are seldomly independent.
RE: anchor forces on long straight pipe
How are you not getting the pressure thrust and friction loads on your anchor to not be additive? I think you're signs are flipped on your friction.
Also, the joints themselves do have spring rates. If we're talking about a 100F temperature change over 2000' of carbon steel pipe, that's over 15" of compression into each joint....
...actually, I think you're going to have a tough time getting a single joint design to absorb that much compression without squirming at the design pressure the OP is looking for.
Edward L. Klein
Pipe Stress Engineer
Houston, Texas
"All the world is a Spring"
All opinions expressed here are my own and not my company's.
RE: anchor forces on long straight pipe
Same could happen on a long cold water line. Empty and hot from the sun then cold water is flowed in from one end.
I have seen linear expansion bellows used on long lines but these have internal and external guides and bellows restraints.
Kevin
RE: anchor forces on long straight pipe
It is physically impossible for a steam line to have no expansion on one side of an anchor point and full expansion on the other side unless you have a closed valve at the anchor. Even then there would be a temperature gradient. I agree that during warm-up there is a temperature gradient along the line but to consider full expansion on one side of an anchor and no expansion on the other is very conservative to say the least.
RE: anchor forces on long straight pipe
As you feed steam into a line the hot 100C steam moves up the pipe. Behind this front the line is hot and ahead is cold.
I have seen this many time times.
Kevin
RE: anchor forces on long straight pipe
Independent events are seldomly independent.
RE: anchor forces on long straight pipe
====R====R====R====R====EEEE====R====R====R====R====
R-Rigid Mechanical Joint
EEEE- Expansion Coupling
Steer clear of frictional DIP restraints. Stick with rigid factory welded restrained joints. Just a thought.