Using insulation to protect and support small bore ines

[Winnower]

Hi,

I am currently struggling with pipe stress and pipe support of some 1" or less pipes and tubes. The lines are hot, some 500c (low allowable stresses), and the wind loads are high. The lines are insulated and traced and the overall diameter of the pipe, tracing, insulation and cladding is around 150mm (it varies from line to line). This causes a problem due to the wind load over stressing the line. We are using a three second wind gust speed of 63m/s. This causes the pipe supports to have to be very close together, around 1200mm centres, which I have been told is undesireable.
To add to the problem we do not have linestops qualified for use on these lines (though we do have guides). Because of the temperatures the pipe supports are outside of the insulation, we have load bearing insulation at the support/guide points)

So that's the problem;

I have an idea that we could utilise the stiffness of the insulation (the insulation is mineral wool 50-75mm thick and the cladding is 0.9 aluminium, 1m lengths, banded together, so quite stiff) to actually support the pipe (as opposed to the pipe supporting the insulation which is the usual way). The advantage of this is that the pipe is then shielded from the wind loads. We would design the pipe supports to fix firmly to the insulation to take the wind. The problem I have is how to model this with CAESAR and how to justify it within the bounds of the ASME B31.3 and EN 13480 codes (we are checking to both codes). I think within CAESAR we could model the constant support that the pipe would get from the mineral wool with frequent supports say every 300mm, the mineral wool would compress at the elbows to allow the pipe to expand. As for the codes, can we justify the lack of wind load by saying the pipe is shielded from the wind by the insulation. I think that the insulation would be stiffer than the pipe, so that allows it to support the pipe.

The question is,

Does anyone have an opinion on the above solution and/or does anyone have any experience of this type of solution/problem.

 

[LittleInch]

Interesting idea, but I think you're giving far too much strength to your tin can aluminium sheeting and you'll need a lot thicker outer shell than that. Have you considered PU foam with a PE 3 or 4mm thickness outer pipe as a pre-fabricated pipe? That would hold up a whole lot better and probably be thinner for the same level of insualtion.

You need to think how this will be constructed as well. Normally insualtion as you describe will be added after pipe installation - now you're doing it the other way round and need to think through how that would work.

My motto: Learn something new every day

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

 

[Winnower]

Thanks for your comments Littleinch,

Apologies for my delayed reply, I have been away for some factory acceptance testing of some of our components so I did not have a chance to respond.


I don't think that polyurethane and polyethylene would stand the temperatures I am dealing with, the lines are 250c and 510c steam lines.

I don't want to have to change the specs of the job too much, so my intention is to keep things simple and utilise the pre-formed mineral wool insulation with the aluminium cladding sections as I have described previously and as per the rest to of the project.

As for the construction method, it would also be standard. The pipe would be installed first (with some temporary "blue nylon" supports initially) and then the insulation and permanent pipe supports would be added afterwards, as normal. We can do this because the problem I have is mainly with the wind load and the temperature, obviously during installation the lines would be cold and we would not be installing if the wind speeds were too high, so the piping/insulation system would not be over stressed during installation.

I did a quick calculation and the stiffness of a tube of aluminium cladding (OD = 153mm, 0.9mm thick, E=70.5x10^9) is about 65 times as stiff as a 1/2" sch40s (2.77mm wall, E=195x10^9) stainless pipe.

I understand of course that our cladding is not a nice continuous extruded tube of aluminium but a rolled sheet and of course our cladding is also made up from 1m long jointed sections. But if we make the banded joints nice and strong, maybe use a few rivets then I wonder if we can approach the stiffness of a continuous tube?

Do you (or does anyone) know whether this solution would be code compliant. Shielding the pipe from the elements, snow, wind, ice with insulation which is not covered by the code (it is not pressure retaining piping) seems like a very useful tool to have at your disposal. The pressure retaining, code compliant, piping then nestles snugly within its exoskeleton of cladding, free to expand and contract at will with only the mineral wool on the elbows to stop it. The pipe would just compress the mineral wool and as long as the elbow to elbow lengths were not too long (say 3 or 4 metres) then this would not be a problem.

 

[LittleInch]

The joints between the sheets, even with rivits are your weak point and simply never going to accept any real load. If you're going to do this you can't use the standard stuff, but use actual pipe - steel or aluminimum tube, wrap your pipe and then insert it into your strong outer tube of what ever length you have between elbowws. Analysis should allow you to use a continuous support as a uniformly distributed load, the same as laying it on the ground, to avoid any issue with multiple point supports. If your elbows are relatively close together then you shouldn't have an expansion issue to deal with.

Ducted pipe should have no issues with any code I am aware of, but I just don't trust the strength and rigidity of the cladding - sorry.

My motto: Learn something new every day

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

 

[Winnower]

Thanks again Littleinch,

I understand your concerns with the strength of the rolled cladding. They are my concerns too.

I will perhaps conduct some tests to look at exactly how stiff the cladding is and what loads it can take.

Yes, continuous support would be the correct way to model this system in CAESAR and I agree that as long as there are no long runs of piping then the expansion would just be absorbed into the insulation at the elbows.

Thanks for your comments,
regards,
Winnower.

 

[Winnower]

Thanks again Littleinch,

I understand your concerns with the strength of the rolled cladding. They are my concerns too.

I will perhaps conduct some tests to look at exactly how stiff the cladding is and what loads it can take.

Yes, continuous support would be the correct way to model this system in CAESAR and I agree that as long as there are no long runs of piping then the expansion would just be absorbed into the insulation at the elbows.

Thanks for your comments,
regards,
Winnower.

 

[LittleInch]

No problem. What you will need to know is what sort of load your insulation will accept. A few tests are needed I think....

My motto: Learn something new every day

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

 

[LittleInch]

Ps look at foam glas by pittsbugh corning. No issues with temperature and had high strength.

I'm not a salesman for it but think it could be good for your use.

My motto: Learn something new every day

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