Air-gaps in pipe joints insulated with PUR half-shells
Paper i proceeding, 2004
Prefabricated half-shells as thermal insulation in pipeline joints come with obvious advantages.
Most importantly, the PUR insulation can be optimised in the factory, there is no need for handling
chemicals in the field and the jointing procedure is simplified. However, the technique has never
become very popular in Sweden. One reason for this is the risk of air-gaps between the half-shells
and the pipe ends. Air-gaps may cause an excessive radial heat flow leading to overheating and
premature ageing of the joint casing and the shrink seals. It has also been shown that voids in the
joint insulation may promote the accumulation of water in the joint when leaks are present in the
shrink seals. Furthermore, air-gaps due to half-shells will expose the medium pipe to permeating
water and the risk for corrosion.
The increase in temperature on the casing pipe caused by heat flow through an air-gap is treated
analytically based on previous laboratory experiments.
The presence of air-gaps was confirmed at excavation of a pipeline after nine years of service,
where air-gaps with an approximate width of 4 mm were observed between the half-shells and the
pipe ends on both sides of the joints. Previous tests undertaken by joint manufacturers have shown
that air-gaps may open up as the half-shells contract along with the joint casing when this cools
down after shrinkage. When the pipeline is heated to service temperature, the gaps may close. However,
measurements on various types of district heating pipe PUR foams indicate a tendency of the
foam to shrink in the axial direction when aged in high temperatures.