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April 04, 2018
The Evolution of Composites: Pre-Formed vs. Form Fitting

Posted by: Natalie Swift

Rigid coil composite repair systems typically use a fixed-layer count of unidirectional fiberglass reinforcement with a methyl methacralyate resin matrix. They have the distinction of being the first composite pipe repair system to gain widespread use in the pipeline industry, but the technology used in this system has not been evolved alongside advancements in the composite repair industry.

This two-part blog series will address the potential limitations of rigid coil composite repair technology in comparison to Milliken Infrastructure Solutions’ (MIS) composite repair products. It will mainly focus on A+ Wrap, our most popular repair solution that is composed of bidirectional fiberglass reinforcement pre-impregnated with a polyurethane resin matrix.

A primary difference between the two repair systems is the pre-formed and form fitting fabrics. This characteristic has multiple effects on the repair’s installation, which we will discuss in this post.

Pre-formed vs. Form Fitting

Rigid coil systems are pre-formed into predetermined lengths by the manufacturer. A+ Wrap, however, is form-fitting—the fabric remains uncured and flexible when applied. This fundamental difference between the A+ Wrap and rigid coil systems illustrates how rigid coil systems have not evolved to solve installation difficulties. The rigid coil system is made by winding the fabric around cylindrical mandrels and curing the composite into a pre-formed rigid coil. Using a pre-formed system creates several issues that inhibit the effectiveness of the composite repair because the system will not fully conform to the pipe’s shape.

Intimate Contact

Achieving intimate contact between the pipe and the composite is difficult with pre-formed systems. Neither the pipe nor the mandrels used to create the coils are perfectly round—they have some degree of ovality. It is unlikely that the major and minor diameters of the pipe and the coil will align properly, so there will probably be gaps between the coil and the pipe, which prevents intimate contact. Intimate contact is important because it increases the composite’s ability to transfer load from the pipe.

The importance of intimate contact can be explained by visualizing the composite repair as a rope being wrapped around the pipe. If the rope is wrapped around the pipe loosely, the gaps present will create slack in the rope. To pull tension through the entire rope, all of the slack will need to be removed. The tension in the rope represents the load transfer from the pipe to the composite, and better intimate contact allows for better load transfer. A lack of intimate contact means that there are bigger gaps and thus more slack that needs to be removed. There is also a greater likelihood of failure in leak repairs if intimate contact is not achieved. The A+ system is form fitting, so it will conform to the shape of the pipe it is applied to which will create intimate contact when applied correctly.

Complex Geometry

When the defect is on a part with complex geometry, such as a bend, tee, flange or reducer, the precured rigid coil will not be able to conform to the pipe’s shape. It is impossible to wrap a bend with only rigid coils and cover the entire surface; some gaps will be present. The A+ Wrap system, however, will conform to any pipe geometry to form a solid monolithic repair. This makes it much easier for the installers to adjust to in field conditions.

In the second part of this series, we will look at other situational repair setbacks you may find using a rigid coil system, and how these difficulties can be alleviated with a composite wrap system.

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Category: Feature Articles, Pipe Wrap

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