Restrained and Unrestrained pipe

What is restrained and unrestrained pipe section in ASME B31.4 and ASME B31.8?
Some people says that "restrained" is buried piping, and unrestrained is above ground. Is it right?

 

[pennpiper]

"Is it right?"
Well yes! and no! Depending on the situation and the interpretation of the term "Restrained".

In most cases, in Piping, Restrained means there are Anchors and there are Guides involved.
Example: A long Steam line on an above ground pipe rack. There will be Expansion Loops required to adsorb the thermal expansion. For these Loops to function properly there needs to be Anchors, (say 50meters [+/-] away) on each side of the Loop and Pipe Guides at every other pipe support.
These Anchors and Guides are called "Restraints".
Anchors can be Fixed Anchors, preventing (or restraining) movement in all directions. There are also Anchors that allow movement in lateral directions but prevent movement in a lineal directions.
Guides are most often to "Restrain" or control the growth movement in pipes caused by thermal expansion.
I hope this is helpful.


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

 

[bimr]

833.1 Restraint

(a) The restraint condition is a factor in the structural behavior of the pipeline. The degree of restraint may be affected by aspects of pipeline construction, support design, soil properties, and terrain. Part 833 is applicable to all steel piping within the scope of B31.8. For purposes of design, this Code recognizes two axial restraint conditions, "restrained" and "unrestrained". Guidance in categorizing the restraint condition is given below.

(b) Piping in which soil or supports prevent axial displacement of flexure at bends is "restrained". Restrained piping may include the following;

(1) straight sections of buried piping

(2) bends and adjacent piping buried in stiff or consolidated soil

(3) sections of above-ground piping on rigid supports

(c) Piping that is freed to displace axially or flex at bends is "unrestrained". Unrestrained piping may include the following:

(1) above-ground piping that is configured to accommodate thermal expansion or anchor movements through flexibility

(2) bends and adjacent piping buried in soft or unconsolidated soil

(3) an unbackfilled section of otherwise buried pipeline that is sufficiently flexible to displace laterally or which contains a bend

(4) pipe subject to an end cap pressure force

 

[LittleInch]

Like pennpiper says it is a bit variable.

You need to read the entire section 833, 834 & 835 t see what difference it makes.

Basically Restrained pipe is buried pipe, where there is no thermal expansion and hence any thermal effects results in additional axial stress. However in poor or unconsolidated soil at bends, this is regarded as unrestrained.

Unrestrained pipe has no calculation in axial stress that includes for thermal stress, therefore all thermal expansion is assumed to be free or it falls into the "sufficient flexibility" category -833.7 (3)

fully restrained buried pipe is only assumed for a straight level pipe when you are 150-250m away from a bend or end point.

Everything in between falls into the grey zone where you need to look at it carefully and see if a more complex stress analysis including some sort of computerized analysis.

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