Pipe Expansion Loops: Benefits, Features & Usage

[Frank Peters]

Pipe loops are used to accommodate thermal expansion. I have seen many types and lengths but here is one example:

https://www.engineeringtoolbox.com/steel-pipe-expansion-loop-d_1069.html

I am a little unclear on why they can work. With any thermal expansion the stresses on the elbow joints should become very severe and could even lead to fatigue with repeated expansions.

Does increasing the length of the bend (I've some very long bends) lessen the stress on the elbow joints?

Some loops are circular rather than rectangular. What is the advantage, if any, of a circular loop?

Why aren't simpler in-line "sliding" joints or bellows joints used?

 

[jrmichler]

This is a case where you need to do the calculations to understand the solution. Calculate the forces and stresses in a straight length of pipe with fixed ends, then calculate the forces and bending stresses with a pipe loop added.

There is no such thing as a simple sliding joint or bellows that is as reliable and durable as a properly designed pipe loop.

 

[anorlunda]

You can experience it yourself. Take a long thin rod like a length of coat hanger wire. You can flex it in the bending direction easily, but you can not easily compress or stretch it in a straight line.

 

[JBA]

One advantage of thermal expansion loops when it comes to the maximum stress in the components is that the loop is designed with a known maximum pi[ping temperature change and therefore the maximum loop deflection and stresses in the loop.

As to the effect of longer legs; yes, the longer the legs the less angular deflection at the elbows for a given movement of the end(s) of the main run pipe and results in a lower stress on all four of the loop elbows. The increasing the length of the bottom section of the loop effectively gives a more flexible pipe length between its two end elbows and reduction in the stresses on the those items but has no effect on the main pipe connecting elbow stresses.

As stated above, there are no reliable sliding type joints because they require seals that can wear and leak. There are, bellows type compensators that can be used where there is no space available for pipe loops but they are much more expensive than fabricated piping loops and more easily damaged.

 

[Frank Peters]

Thanks for all the responses. My understanding is improved.

However, an ancillary question comes to mind. I have seen expansion loops that are oriented vertically and others that are oriented horizontally. A vertical expansion loop would seem to present a pumping pressure problem because the pipeline flow repetitively must work against gravity. This would be especially severe for liquid contents.

Are vertical expansion loops limited to gas phase flows?

 

[256bits]

A vertical expansion loop would seem to present a pumping pressure problem because the pipeline flow repetitively must work against gravity.

There is an up and a down to the loop. They cancel each other.

 

[Baluncore]

The expansion change is distributed not only at the tight elbows, but over the entire length of the unrestrained loop.

A sliding joint would only work well with low pressure.
Pressure in the pipe must be countered by tension in the pipe wall, or the ends must be firmly anchored.

A vertical loop upwards may fill with gas and so reduce flow rate by causing turbulence. It may store energy in an unsafe way which empties the entire liquid contents of the pipe if a failure occurs elsewhere.
A vertical loop downwards, like a 'U'-bend, may fill with sludge and so reduce the flow rate. If it fills with water it may block the flow of a less dense liquid or gas.