Thermal Expansion, Anchoring, Guides and Supports

[inertia1]

Hello everyone,

I am currently designing and detailing the support system for a stack of Stainless steel and GRE pipes. I have already calculated the loads and minimum support spans using the manufacturers engineering guide.

The site is in Qatar experiencing an average maximum change in ambient temperature of 14 deg C. I can easily calculate the change in length due to thermal expansion and the force put on anchor points. However, in this environment the pipes will be exposed to sunlight therefore will experience some solar heating. From your experience is it necessary to calculate this effect for thermal expansion? Or would a better approach to increase the change in temperature to a conservative level? Also is it necessary to include any losses from convection (wind at night)?

It is also my understanding that U-bolts should not be used as a support but only as guides that do not touch the GRE pipe, and be used with a pipe support cradle to provide a minimum of 120deg contact with the GRE pipe. The cradle being attached to the pipe, so any wear due to movement will take place on the support steelwork and the cradle.

This is where I get confused.....surely the weight of the pipe (and contents) will be supported at the guide location. Does this mean the guide spacing should conform to the minimum support length? Or are guides not designed to take any loads and only prevent buckling? If buckling may occur from thermal expansion I would like to use a U-bolt configuration to reduce cost, instead of the manufacturers expensive support shoe, which will be used for anchors to spread the load over the surface of the pipe?

Furthermore, along this stretch of pipework I have branches for each service at 40m centres. I assume that best practice would be to begin by anchoring the T-branch then progress with the design on that basis?

The manufacturer advises that thermal expansion joints or loops are not required as there is enough flexibilty in the pipe itself. However, to take up any misalignment in the civil works, my colleauges have advised me to include them. Should I have expansion joints for only this reason, or include them in the design for thermal expansion.

As you may be able to tell I am getting confused and have been searching for a definitave answer or method.

Thanks in advance for all your help!

 

[BigInch]

Daytime temps on pipe in the Gulf can be 175ºF if exposed to sun. If that means a lot of growth to your pipe that you are trying to restrain, high axial forces can result if the configuration is not flexible (read on).

"U" bolts are a relatively concentrated wearing surface, thus should be avoided where cyclical movement occurs frequently.

Guides are not designed to prevent buckling. They should only prohibit lateral movement, not buckling forces. If your pipe wants to buckle (ie reaches buckling stress), there's far too much axial stress that should be relieved by other means, such as allowing some axial growth via "snaking", or in other words, allowing axial growth, but without significant axial loads. Your pipe is not a structural column that is being built to take floor loads. Pipe should expand and in the process, relieve stresses. Only anchor and guide pipe where you MUST. Never because you want to just "keep it straight". If you want to keep it straight, let it expand without restraint and most of the time, it expands pretty close to straight. If you want to keep it from moving laterally from wind loads, etc. OK, a loose guide might do it then. You don't want a pipe reaching buckling stress with just a guide holding it. The guide flys off and then ... the pipe-column collapses.

A guide will support gravity and other loads, unless there is an adjustable spring set for constant force or something. If you were to remove the guide (on your calculation paper) and then calculate the deflection due to dead and live loads, say using a continuous beam formula, then reverse the calculation using the deflection equation for a point load and find the equivalent point load for the same deflection, that is the apporximate load that the guide is taking if you replace it.

Does this mean the guide spacing should conform to the minimum support length? Yes (if you do not have any other support nearby)

Don't anchor any pipe unless you must prevent it from moving (into equipment areas where misalignment or other upsets may occur).

If you have to anchor a header, anchor at the center length-wise and let it grow outwards in both directions. Why do need to anchor it? You may be better served by providing an expansion loop or some other type of flexibility based on offset configuration and a few 90's in the run to the header area.

Pipe is not flexible, for axial loads, only bending. If your configuration does not have a bend, it can reach very high axial loads very easily if there are axial growth restraints. In bending pipe is relatively flexible, if the length over which the bending moments occur is long.

The manufacturer advises that thermal expansion joints or loops are not required as there is enough flexibilty in the pipe itself. I presume the manufacturer does not know your configuration, so there's a lot of other stuff that he doesn't know. Stress is only one of them.

However, to take up any misalignment in the civil works, my colleauges have advised me to include them. You don't use loops for civil misalignment, you use pads, pipe shoes, bearing plates, grout, wear plates, shims, rubber seals, silicone cement or whatever. Never expansion loops!

Hope you're not confused now.

http://virtualpipeline.spaces.msn.com

"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein

 

[jistre]

Expansion joints are tricky beasts. Because Murphy, his law, and the universe abhor order, they will find some way to bite you. The nature of expansion joints means that somewhere down the line, they WILL fail. You're going to have something like a metallic joint that fatigues or corrodes, or possibly elastomers that degrade over time. The manufacturer usually even will give you a number of cycles to replacement. However, if you get a slight ding in a metal joint during construction, an abrasion in a rubber joint, some chemical spilled on the joint, or any number of other "unknowables", you can see significantly reduced lifetimes. This kind of liability in terms of spills, unplanned downtime, and maintenance and replacement costs over the lifetime of the plant is something I try to figure out some way to avoid even in the situations where they're clearly warranted. I prefer to add expansion loops or reroute piping over expansion joints if possible. However, there are some situations in existing installations or set designs in which there will just be no other option than to use an expansion joint.

However, I would never specify one just to accomodate imprecise installation work. You're preloading the joint to begin with just to make up the connection. Then, as the piping cycles, it's not that hard to go outside the physical limits of the joint and you've got problems.

Just my two cents, though. Each company/engineer is different.

 

[inertia1]

Thanks for advice so far guys.

So BigInch would you advise me to actually calculate the temperature change due to solar heating for that location (which I did do once at university for solar heating systems) or would just using a conservative temperature change from day to night would be enough? Say 70 deg C? This will give me a change in length between anchor points of 25.2mm.

I have calculated the force at the anchor points. Then I have used this to check that the critical buckling force is not reached under the thermal load. From this I have optimised the support distance to suit.

I assume this will allow the pipe to 'snake' between the guides (will use a support shoe/bracket), to give the 25.2mm expansion/contraction. In this situation I cannot use loops, thats why they have been discounted. Therefore should I include an expansion joint (rubber below or similar)?

Thanks for your advise regarding the confusion with expansion joint for misalignment.....I understand this better now

 

[BigInch]

I'd use the 175 ºF temp.

I don't like too many "contacting" guides if you want the pipe to snake. Some loose fitting guides might be ok. As long as friction loads are less than an "equivalent lateral buckling force" (don't ask me how to calculate that. 5% of buckling load???), the pipe should snake.

If you work it so the buckling stress is small (very long length with little lateral friction forces), stress x pipe area is all the anchors will ever see, so they won't need to be too large.

If you can take up that 25mm of axial growth with the length along the bends in the snake, I wouldn't think you need any flexible connections.

http://virtualpipeline.spaces.msn.com

"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein

 

[BillBirch]

The selection of an upper temperature for stress analysis of piping is a difficult one. I have experienced these problems in designing piping in the Australian desert using all sorts of materials including GRE, HDPE, carbon steel and stainless steel.

We had witnessed on-site black bulb temperatures of around 75 degrees C, and using that data on stress runs, perfectly functioning installed systems should have failed. This led us to discontinue to use this extreme case, based on the following rationales -

a) Only the sector of pipe in direct sunlight will be at the highest temperature, the shaded sector, at a lower temperature will tend to hold back the pipes tendency to grow to the largest extent.

b) Fluid flow or bulk fluid temperature will how a larger impact on the inner wall temperature of the pipe. TYhis is evident in the B31.3 determination of design temperature, based on fluid temperature. Note that I did not use the term design temperature above when addressing the environmental effect.

c) Considering the two points above, we came to understand that there will be a temperature gradient throught the pipe wall, and the "fibres" at the various locations within the wall thickness will want to expand at different rates.

There is no easy answer to this one, because your may have more or less shade. Offshore, for example we have run GRE lines under decks or alongside beams where maximum shade is available.

As for snaking, that is the only reasonable way the treat composites or plastic pipes. They do not behave like steel where an expansion loop or direction change will elastically absorb the thermal growth. Composites and plastics when addressed in this manner will tend to locally deform. In my experience,I have used say, teflon coated U-bolts loosely fitted with an upper lock nut every two or three supports. It is extremely important that there is sufficient clearance to allow rotation from the pipes longitudinal axis at the guides.

Anchoring at every offtake is not advised as you may find that you have not accumulated sufficient length between branches to promote buckling. My approach to this is to keep a tally of the expected growth of each branch and make sure that lateral movement of the branches can be accommodated along the way - if not, take control and anchor again.

Inertia1, congratulations on a well-written analysis of the same sort of challenges that I had over twenty years ago!
Regards,
Bill Birch