Becht Engineering has performed many design, analysis, and design review projects on double wall piping systems constructed of metallic, thermoplastic (e.g. polypropylene), and thermosetting (e.g. FRP) material. We have found, particularly in our design review activities, that a commonly misunderstood and/or neglected aspect of the design is differential thermal expansion between the inner and outer pipe. This can be from normal operating temperatures of the contained fluid, or during occasional events such as steam out.
The common situation is when the inner pipe may contain a hot fluid and expand lengthwise relative to the outer pipe. It also can contract relative to the outer pipe when containing cold fluids. An additional contributor, which can be significant for plastic piping systems due to their lower stiffness, is longitudinal expansion due to internal pressure. The relative strength of the inner and outer pipe is also an issue when nonmetallic pipe is used to enclose metallic pipe. The differential thermal expansion can be accommodated using a floating system where the inner pipe is free to expand within the outer pipe (this is normally the required option when nonmetallic pipe encloses metallic pipe) and a fully restrained system in which the inner and outer pipe are connected, and differential thermal expansion is restrained by axial forces in the piping. Design of the connecting elements and consideration of buckling are key elements to the latter design approach.
Becht Engineering has designed many double containment systems using both of the above approaches.