General Introduction
The field welded joints on coated pipe
are exposed, and are protected by methods such as:
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Heat-Shrinkable Sleeves |
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Cold applied tapes |
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Fusion bonded or liquid epoxy |
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Coal tar enamel or asphalt enamel |
The use of heat-shrinkable products or "shrink sleeves" is the most common method used to protect joints worldwide due to:
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design flexibility and compatibility with pipeline conditions and pipe coatings |
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high reliability and superior service performance |
Heat-Shrinkable Sleeve Function
The function of a heat-shrinkable sleeve
is to protect the pipeline field joint from corrosion. The
sleeve adhesive achieves this by preventing moisture
and air ingress to the pipe surface. The exterior polyolefin backing mechanically protects the adhesive
and the joint. The adhesive further acts to hold
and anchor the sleeve around the joint through
adhesion to the substrate and the backing.
Sleeve Description
The heat-shrinkable sleeve consists of a cross-linked
and stretched sheet which upon heating, will
shrink to its original length. This sheet is coated
with a protective heat-sensitive adhesive. When
heat is applied to the sleeve wrapped around a
joint, the adhesive melts and becomes a liquid
while the sleeve backing begins to shrink. The
radial shrinking forces of the sleeve squeezes
the fluid adhesive into all the pipe surface irregularities,
while the sleeve conforms tightly to the joint
profile. On cooling, the adhesive solidifies,
creating a tough bond to the pipe and the coating.
Types of Shrink Sleeves
Product Design
There are three basic configurations of heat-shrinkable sleeves used to protect joints:
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 |
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| Wrapid Sleeve™ |
CanusaWrap™ |
Canusa Tube™ |
As mentioned above, heat-shrinkable sleeves consist of a backing and adhesive.
Sleeve Backing Types
Cross-linked polyolefin backings are designed based on type of service. Canusa-CPS manufactures sleeves from high density polyethylene (HDPE), Linear low density polyethylene (LLDPE) and polypropylene (PP). The choice of backing is dependant on project construction conditions, pipeline in-service conditions and type of pipeline.
Adhesive Types
Adhesives are broadly categorized as either mastic or hot melt although within those types, a number of grades exist. As a general description:
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Mastics
Mastics are tacky, pressure-sensitive, and relatively soft adhesives. Therefore they bond easily to substrates upon contact and pressure.
The heat generated during sleeve installation helps the adhesive flow and thus enhances
bonding. Mastics are more forgiving than
hotmelt adhesives to installation conditions,
such as pipe cleanliness and heat input. Mastics
have good peel strength and
good cathodic disbondment resistance as they are self healing, i.e. a puncture in the backing is
commonly sealed by the mastic adhesive. |
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Hotmelts
Hotmelts are non-tacky, harder adhesives based
on semicrystalline resins. They have very high
shear strengths, and often high peel strengths and are formulated to operate at high temperatures. |
Sleeve Thickness
Thickness provides the mechanical protection,
for example, impact and penetration resistance.
Sleeves typically are provided to give 2.8 mm
(111 mils) applied thickness for normal applications,
and 3.3 mm (131 mils) where extra mechanical protection
is needed.
Cross-linking Technology
Materials of a thermoplastic nature such as polyolefins
are composed of long molecules arranged
in a random order. The strength of such materials
depends upon the distance between the molecules,
and the crystalline nature of the molecular structure.
It is these crystals which provide most of the
strength of the material. When a material is
heated above the crystalline melting point, these crystals disappear and the molecules
can then slip past each other and flow.
Exposure of some plastic materials to high-energy irradiation can cause the permanent cross-linking, or intermolecular joining, of molecules. This linking results in the chemical bonding of the plastic structure into a new three-dimensional structure.
When the material has been cross-linked, it will resist melting or flowing. When the material is heated, the crystals still disappear as before, but it will no longer flow or change shape because the cross-links act as ties between the molecules. The cross-linked structure, however, is elastic. This when it is heated to a temperature where the crystals have melted, the material behaves like rubber.
By using irradiation cross-linking, products can be designed to have superior benefits over standard polyolefins. These cross-linked products are supplied in an expanded condition and have a perfect elastic memory. When heated, they will shrink and tightly encapsulate the object over which they have been placed. They are ideal for protecting pipe joints and a variety of unusual configurations and pipe fittings.
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