Ultra Compact Double Wall Pipe in Pipe

Double wall pipe , or pipe in pipe, can result in some large space allowance for piping installations. As long as there is no limit on the amount of space available then this is probably acceptable, but in many cases, such as offshore or marine applications, space equals money. Therefore it makes sense to design the double wall pipe to be as compact as possible. This doesn’t only apply to the space taken up by the piping, but the increase in material mass and welding times associated with unnecessarily large double wall pipe.
The inner pipe is defined by the process requirements, and there is little room for making changes here. This is typically the flow area defined by the flow bulk velocity, and the wall thickness defined by the pressure requirements. Material selection is also defined by material compatibility and temperature requirements. The outer pipe is also defined by process requirements to some extent, flow velocity is not used so much except with ventilated type double wall systems, but in the case of cryogenic pipe in pipe design factors such as insulation will need to be considered. Further for many applications the outer pipe will also need to meet the same pressure design criteria as the inner pipe, which leads to an increase in wall thickness as the outer diameter increases. This is yet another reason to keep the outer pipe as compact as possible. The guiding principle of the AF Flex type double wall piping was in fact to provide a compact flex support for this very reason. Therefore it should be considered that some parameters of outer pipe sizing are at the discretion of the company or contractor supplying the double wall pipe, and therefore the sizing of the outer pipe can be considered a competitive parameter. In addition, in the first article entitled “Double wall pipe design”, the simplest form of double wall pipe was considered where there was sufficient heat transfer between the outer and inner pipe so that the inner & outer pipe could be considered as a single pipe, not moving independently of one another. So two key factors for double wall pipe are:
1) Outer pipe sizing
2) Thermal displacements between inner and outer pipe
Considering the two main factors above, AF Pipes has developed the AF Slim type double wall pipe for an extremely compact pipe in pipe installation, with improved thermal conductivity through the cross section. This is particularly interesting for heat exchangers, and applications where there is a large temperature gradient between the ambient and the process, as the inner and outer pipe expands and contract together. Additional insulation layers can be added to the outer surface of the outer pipe, however should vacuum insulation be required, then it is recommended to apply the traditional AF Flex type solution. The AF Slim double wall pipe arrangement greatly increases the simplicity of the double wall system, as the thermal movements are taken care of only on the outer pipe, using traditional methods of pipe installation. In order to provide such a compact solution, the inner pipe is fixed within the outer pipe using proprietary tooling and methods that have been developed specifically for production of the AF Slim double wall pipe modules. The pipe spools can be delivered completely ready to install, or installed on site depending on the requirements of the end product. In some cases it is easier to complete the production on site, as it may be necessary to access the outer pipe ends, for example if welding into a tubesheet.
Once the inner pipes have been fixed in place, a series of helical passages between the inner and outer pipe run longitudinally down the pipe spool, which are used for leakage detection in this annular space. The passages will be connected to a pressure or temperature type leakage detection system, which monitors the annular space, and will trigger the control system in the case of pressure or temperature rise or fall, depending on exact customer requriements. Here it should be noted that due to the small annular space, the AF Slim double pipe is not suitable with ventilation leakage detection systems. In many cases this is translated as an additional benefit, as a pressure leakage detection is generally cheaper, and easier to install. Therefore it is important to also consider whether a ventilated double wall pipe system is really the optimum solution, particularly if cheaper and easier alternatives are available.
Where a full double wall pipe arrangement is required, the specially design AF Slim couplings & bends can be used to complement the straight pipe spools, and provide a full double pipe arrangement that can be designed to suit any installation. In order to locate leaks in the double wall piping, the AF Slim couplings have built in leak detection plugs, that can be opened when performing leak detection. The couplings themselves are available in welded or threaded versions, depending on customer specifications.