Can I distribute force over pipe length?

[StructureMan44]

We have a 96ft long 18”Ø pipe with pressurized water at 400°F. We’d like to cut the pipe and weld in a valve. Before we can do this the pipe will be drained and brought down to an ambient temperature of 50°F. With a cross sectional area of 90.7in² and a coefficient of thermal expansion of 0.0000065 for A36 steel, I calculate the pipe length change as 2.6in or if constrained it will develop a force of 5987kips. We want to constrain the pipe so that when the cuts are made to insert the valve the pipe doesn’t move. Obviously a force of 5987kips is huge and cannot easily be constrained. Is my analysis correct? Is it correct that the 5987kips force will be present at the cut or will it be distributed along the pipe in some way?

 

[racookpe1978]

Well, if you have a pipe at 400 deg C (now during operation, after it heated up from shutdown (20-30 deg C) = the original temperature it was fabricated at), how much movement do you expect between 25 deg C fabrication temperature and the 50 deg C repair temperature?

(Why so high a repair temperature? 50 deg is very, very hard to work with safely and economically.)

 

[BigInch]

The pipe is above ground?

Don't try to constrain the pipe axially. If it is not buried, it will return to its previous no or low thermal stress state as it cools back down to near the temperature at which it was originally installed. If there are any residual stresses, they should be small and any further movement will also be small. Just worry about some possible residual non-thermal stresses, which may cause some small, but unpredictable movements. Provide restraints against those (potential lateral movements) to keep the cutting instruments from binding and throwing some guy's arm out of joint.

 

[IRstuff]

The OP wrote F twice, so I'd think that they didn't mean C

Nevertheless, it seems like you're talking about the pipe cooling DOWN to 50F, so the pipe should shrink, shouldn't it?


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[BigInch]

Exactly, it should be in axial compression at 400F and revert to no or little axial force as it nears its original installation temperature. If it is buried, it could be locked into the soil in the expanded state, which if so would result in it going into some amount of tension as it cools.

 

[StructureMan44]

Thank you for the help guys. The pipe is in the air suspended by 5 hangers and should reduce in length due to temperature cooling. Both ends of the pipe have 90 degree bends in different directions and I have no idea if the pipe is in tension or compression at the operating or ambient temperature. My goal is to find a conservative axial design force with which to design supports/restraints such that when the pipe is cut in two places to insert the valve it doesn’t move at all possibly hurting someone or making the addition of the valve difficult (opening to large or small).

What sort of recommendations or rules of thumb exist for determining a reasonable amount of force that exists after the pipe has cooled?

 

[BigInch]

You must know the original installation temperature and the temperature on the day that you make the cut. The thermal stresses present at the time can then be determined and the movements resulting from unleashing those forces can be estimated. The more flexibility in the line (lots of bends, it's good that you have two 90s) the less residual stresses will remain in the pipe. The more flexibility you have will also allow much movement with little stress, which is helpful as it also doesn't require a lot of force to restrain that movement.

 

[bimr]

Not really. Presumably the pipe was installed at ambient temperature and allowed to grow at the ends as it heats up. That is the normal design approach.

Why do you think it is constrained? If the pipe was installed up against a wall or other structure at the pipe end, wouldn't it have pushed away obstruction? Why don't you inspect the pipe at the ends and determine if it is constrained?

 

[LittleInch]

Before you can take this any further you need to work out this bit - "I have no idea if the pipe is in tension or compression at the operating or ambient temperature"

where do you get an cross sectional area of 90.7 sq inches?? I make that a wall thickness of 1 3/4" ?

If you really don't know and your issue is springing of the pipe, which can happen in many pipes, the thing to do is cut the pipe in sections around the joint to say 50% of the circumference. Then weld on axial clips over the cut sections which are bolted together. Then cut the rest of the pipe and ease off the bolts in turn.

However I'm with most people here - first figure out what the pipe looked like when it was built and how it was anchored, or not once it heated up.

In answer to your first question - If the pipe has expanded and was then anchored at your 90 elbows then yes, the force will apply equally over the length if it is suspended on hangers. However this means that the anchors at each end will then see nearly 3000kips each so will nee to be pretty meaty....

I think this is not a real issue as the piping sounds quite flexible and so will simply expand and contract with very little residual force, but unless you post a sketch showing support and anchor positions, it is very difficult to say any more.

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[Bohlool]

just cut the line for the length of valve+flanges in cold unstressed condition

It is almost impossible to keep the line totally fixed during cutting and welding the flanges under that stress, besides the amount of strain in this section is proportional to the length of cutting(valve+flanges)/total length (96ft) which is negligible.

there are 2 erection scenarios:
1- assemble line in stress free condition and let the supports and end connections absorb the expansion force during operation, which is commonly done.
2- Cold sprang the line as per ASME B31.3 319.2.4, in this case the required short/long cut shall be calculated and the line the be cut shorter/longer than the valve+flanges length by that amount. in this case the line will be assembled under stress, but in service due to thermal expansion its reactions on supports and end connections will reduce.

as the design of the line is part of expansion design of that system, one of above mentioned scenarios had already been selected during the design. so it doesn't seem necessary to make recalculation, just cut the line for the length of valve+flanges in cold condition. if there is some thing wrong with the initial mechanical/expansion design, the whole system shall be redesigned. but keep in mind, with cold spring the operating reaction (hot case) reduces, but in shutdown / cold cases there would be much more reactions on end equipment, as the material is stiffer in cold condition.

 

[rconner]

A watching of the video at

https://www.youtube.com/watch?feature=player_detailpage&v=0z3FmgzTeK8

is probably in order (for those of you old enough to remember, maybe kind of reminiscient of Slim Pickens riding the bomb down in Dr Strangelove?) All have a good weekend.

 

[pennpiper]

StructureMan44,
I have some fears about your project and I would like to ask some questions. You may think some of the questions are irrelevant (or non of my business) but please trust me, you and your project are the prime concern here.;

1. Your background relative to piping? What is your background, training and actual experience re; engineering, design of piping systems?

2. What kind of plant is this pipe in? How long has it been in operation? What is the maximum operating pressure of the piping system? Pressurized water for what?

3. You said the primary pipe segment is 96' long with pipe legs at the ends turning in opposite directions. This is a classic "Z" configuration. What is the length of the two legs?

4. What kind of Valve (Type and Rating) do you intend to install? Where in the 96' run of pipe do you intend to place the valve? Will this Valve be operated via a Handwheel or will it have an Electric, pneumatic or other type of automatic operator on it? What is the Operational and maintenance access to this new valve location?

5. With out knowing the type and rating of the Valve or the location of the valve there are critical issues for support. An 18" Class 300 Gate Valve (just a guess) in Carbon Steel (without an operator) will weigh about 2,745 lbs. Higher Class Valves will weigh much more. How close to one of the existing supports will this new Valve be located? Have you checked this support for this new concentrated load?

As you develop the answers to these questions, I think you will see the importance of them.
Regards,


Sometimes its possible to do all the right things and still get bad results

 

[StructureMan44]

BigInch: This piping was installed in the 1970s, I don’t know the time of year the installation occurred but the ambient temperature at the time of install is much closer to the ambient temperature of the valve cutting then the operating temperature.

bimr: Although it has 90 degree bends at both ends, one of those ends leads directly into a water heater making the end rigid (please see the attached pdf)

LittleInch: You are correct, the pipe has a wall thickness of 1.781in. Unfortunately I can’t tell the Contractor how to makes the pipe cuts (means and methods). Thank you for the confirmation, I was worried the force would remain uniform over the entire length.

Bohlool: I think this situation is scenario 1, with an operating temperature of 400°F I’m assuming cold spring was not incorporated in the design.


Thanks for the feedback everyone, I agree that some stress is relived with movement of the bounding 90 degree elbows at each end of the pipe. However I wonder if not all the stress will be relived. Once the pipe is heated the pipe will grow and the elbows will move or bend to relieve the stress. When it cools back down possibly not all the growth will be eliminated and this could also induce stress. It is these two stresses I am concerned with.

Is there any code guidance that recommends an upper bound for pipe stress that will eliminate the possible movement at the pipe cuts? I have found struts of many sizes that can restrain the pipe but I don’t know how to determine what force to design for.

 

[StructureMan44]

Pennpiper: sure thing, thank you for your questions:

1. My background is structural. I have little background in piping but I want to learn as it’s very interesting to me. My work on this will be reviewed by someone with many decades in pipe design.
2. This is in a coal plant in operation since 1977. The design maximum operating pressure is 3200 psig at 400°F for use in a high pressure feed water heater.
3. Both legs are short (3ft and 9ft), the attached pdf gives a better description than my words can
4. It’s an 18” class 2500 valve with a manual operator (an elector operator will be added later) weighing 10.6kips. An existing platform provides access.
5. The new valve is close to on support. The supports have been sized by a piping engineer. The installation process is to install the these restraints, makes the cuts and install the hanger and valve. We don’t want the pipes to move when cut. This will allow the cuts to be made at the correct length for the valve to be inserting in.

 

[LittleInch]

I think you're over complicating this. If the responsibility is with the contractor then all you need to tell him is that there is a possibility that there could be some residual stress and potentially movement of the pipe. If the hangers are spring constant force then you will need to lock them during the work and change them to handle the extra load.

The stress analysis must be interesting with pipe that thick and pressures and temp that high, but I don't think you've got any real permanent movement from the ambient position. Your stress engineer should be able to show you where the expansion goes.

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Also: If you get a response it's polite to respond to it.

 

[BigInch]

There will be little to no thermal stress to worry about. Worry more about supporting the unsupported length after as it is cut. I'm sure your contractor, if he's got any experience at all, knows much more about this than you give him credit for.

 

[StructureMan44]

Thanks big and little inch. The responsibility for installation of the valves is the Contractors. They have asked (and paid) for us to design the restraint. Currently I am designing the restraints to support 2x the vertical load (all unsupported pipe, valves, and fittings) in the vertical direction and 1x the vertical load in the axial and lateral direction. I don’t know if this is conservative enough.

 

[pennpiper]

StructureMan44,
Thanks for the information and the sketch. I now have a better understanding of the whole situation.
The information and advice given by BigInch and LittleInch is all good and should give you all you need.
Please come back after the Valve is installed and tell us how it went.

p.s.: Good luck and welcome to the world of Piping.

PennPiper/Jop

Sometimes its possible to do all the right things and still get bad results

 

[dhengr]

StructureMan44:
It seems to me that you are missing an important part of the installation process with your questions and concerns. The pipe growth you mention does happen at the operating temps. and it’s taken up by bending and movement at the end elbows and their extended runs of pipe, without over stressing anything, it is hoped. You should be working on this at ambient temp. and no pressure, and it is fairly reasonable to assume that give or take a few degrees these were to conditions during original installation. If the original design (pipe stress analysis) was on the money, in the first place, you should not have any appreciable pipe yielding, which would be what would cause locked in stresses (loads) which could kick back at you. Some loads could be locked in at restraint points and you have to check (determine) this. Maybe by loosening these restraints a bit and seeing what moves and how much. With regard to thermal expansion, it should be back in the range of the original installation. You will make one circumferential cut and the pipe may move a bit because of some locked in stresses. Then you will make your final length measurement to make the second circumferential cut 5-6' away from the first, and do the end preps., and make the new welds. Big & Little Inches, et.al., know this topic much better than I do, and this seems so simple that maybe I’m missing something, or they just didn’t think to spell it out this way. They’ll correct me if I’m wrong, or maybe add to my thoughts. It would be good to know as much of the pipe’s history as possible to judge these things. You do want to restrain (gag down) the pipe just in case, both laterally and vertically, but you are not really restraining it longitudinally, other that some strong backs across the first cut, as LittleInch suggested, his bolted “axial clips,” 17JUL15,09:45. These clips might even yield a bit, but hold things in place and together. Of course, you have to support the ends of the pipe and the valve during this work.

 

[LittleInch]

That's a good post. Working with pipe this thick is not trivial. I've never seen pipe that thick outside a military museum and retrofitting a valve which I assume is weld end? will impart a lot of heat and could lead to distortion as I would imagine you're going to need PWHT.

Dhengr is correct, once cool loosen off any flanged bolts or supports as you encounter them moving in both directions away from your first cut line then make the cut and then fix them back in place.

Let us know how it goes and a picture of this pipe would be really interesting.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.