Direktförlagda böjar i fjärrvärmeledningar - Svensk Fjärrvärme

Summary
The objective of this projectisto clarify the loadin g condi tion s around directly buricd
district heaiing pipe bends, i.e,, bends laid directly in ihe ground withoui expansion
elements such as cushiofis, etc. In addilion, ihe purpose is to suggest critcria for Ihe
design or directly buried bends.
Inboth Sweden and Finland, directly buried bends tias reportedly been installed inmany
instances wilhoul any notable pipeline damagc. This in spile of current design
principlcs, which ptedict the load& on a direutly buried L bend to become so greal that
Ihe Lechnique is practicalty usclcss.
A reason fbrthc lack of correspondencebetween experience and thcory is tne
difficuHies in formulating relevant critcrra for lolerabEe stresses and defoiroations in the
pipe. Conservatfve design limits have traditionally been used, parlicuJaTly regurding
alJowable stresses in the PUR foam.
Based on the concepi Ehai the deformation of the PUR foam, and thus also Ihe
displacement of the stcclpipe relatrve to the Ciising pipe. must be lunircd only to avoid
overhealing of the polyethylene, an alteraativc method for design with respect to the
PURfoammaybedeveloped. The Fällors influencing the deformation of ihe PUR foam
ismaänlythe magnitude of the tempcrature indueed steel pipe displaecment and the
stiffnessandstrengthofthe backiill materia]. When ihe properties wf the PUR foam and
the backflll material are known and Ihe relationship between the steeJ pipe drsplacemenl
and Ihe displacement foree is clarificd, the maximum temperature on the casing pipe
maybc deiermined. Tfihe thennal motion of the steel pipe is speoilied, Ihe temperature
on the casing pipe can be calculaied and compared to a design valuc.
The conchtsiort is that the la-ying of bends directly in the ground haTdly poses any
problems with ordinary backflll materials and uompaction conditions. A ccrtain care
should be laken, ihough, when the bends are to be laid m very stiff and strong backfill.
where vcry high reactioti pressures may develop.
In general, however, the risk for pipeline damagc isminimised if directly buned bends
are laid m modcrately compacled backiill.
4