Scope of using electrothermal composite pipe fittings for TCP
A pipe coupler for high-pressure (100bar service, 350bar burst) composite hydrogen pipelines, without the use of elastomeric sealing mechanisms (e.g. o-rings), thereby avoiding seal maintenance costs and effort associated with digging up subterranean lines over a pipe’s lifetime of up to 50 years. The project aimed to reduce the technical barrier of high as-installed cost of pipelines, by utilizing fiber reinforced plastic (FRP) and thermoplastic composite pipe. Further DOE targets include those from the Fuel Cell Technologies Office Multi-Year Research, Development, and Demonstration Plan.
The resulting coupler design employs electrofusion due to its proven use on low pressure unreinforced lines and for its simplicity in the field. This electrofusion coupler design utilizes established concepts for pipe couplers and combines them in a new and unique method. Existing pipe couplers are employed on thermoset pipes in high pressure, with elastomeric seals as the sealing mechanism, and on thermoplastic low-pressure pipes by way of electrofusion.
After many design and test iterations, the final design achieved two of the three main design criteria (leak rate and burst pressure) and came close to achieving the third (fatigue performance).Further work is needed to fully achieve the remaining goal of fatigue performance.
The initial proposal of a continuous fiber reinforced plastic (CFRP) composite electrofusion coupler aimed to increase the burst rating and ensure robustness for field installation in harsh environments. The target service pressure rating was set at 100 bar to support hydrogen pipeline applications. Later design iterations in the project took into account both thermoset-based pipes and bonded thermoplastic layered pipes, emphasizing the versatility of thermoplastic composite pipe solutions.
The principle of electrofusion involves the use of resistive wire heating to melt-bond the thermoplastic resin of the pipe liner to the thermoplastic resin of the inner layer of the coupler, resulting in a fluid-tight seal. The coupler incorporates a structural continuous fiber reinforced composite layer, bonded to the sealing layer, to withstand burst pressure and axial stress. Furthermore, the coupler-to-pipe contact area is designed to distribute the axial load evenly across the bonding region. The layering of this coupler, from the inner to outer diameter, comprises an HDPE thermoplastic liner with embedded wires (serving as heating elements) to provide bonding and sealing, a CFRP composite layer positioned at an optimal wind angle for enhanced mechanical loading, and a neat thermoplastic resin layer for environmental protection and damage prevention.
The initial design employs automated fiber placement techniques to manufacture electrofusion couplers using repeatable and reliable continuous fiber reinforced plastics (CFRPs). Automation equipment is utilized to place and consolidate both the thermoplastic coated wire and preimpregnated continuous fiber composite tape. The material is heated, melted, and solidified in-situ, eliminating the need for post-processing. This innovative technology enables the use of high-pressure thermoplastic composite pipes for hydrogen service while maintaining the ease of installation and dependable performance that have established electrofusion couplers as the industry standard for low-pressure applications.
The fundamental concept behind this approach involves adapting the electrofusion process to simultaneously bond the inner thermoplastic liner of a FRP pipe and the outer structural layers to the coupler. This allows for a fluid-tight seal on the inner diameter of the pipe and a structurally secure connection on the outer diameter.
The next article will introduce the detailed design for this new TCP pipe fittings & connection.