Double-Wall Piping Fabrication
Double-wall piping exists because a single barrier is not acceptable — for cryogenic duty, for toxic media, or because class requires containment of a gas fuel line. That makes the annular space, and the ability to prove it is intact, the whole point of the system.
Meritus coordinates double-wall and vacuum-insulated pipe fabrication from Chinese shops, with testing focused on the parts that cannot be inspected after installation.
Scope we cover
Vacuum-insulated LNG and cryogenic transfer lines; pipe-in-pipe assemblies for gas fuel service; double-walled sections for toxic and hazardous media; bayonet and vacuum-jacketed connections; annular space leak detection and ventilation provisions; supports, guides and expansion arrangements for cryogenic movement.
Fabrication control
The inner line has to be welded, tested and proven before the outer jacket closes over it. We define hold points around that sequence, witness the inner-line pressure and leak testing before jacketing, and verify insulation and vacuum integrity afterwards — because once the jacket is on, a defect in the inner line is a cut-and-rebuild.
Thermal and movement design
Cryogenic pipework moves. We check that expansion loops, guides, anchors and cold-shoe arrangements match the thermal analysis, and that supports are installed where the drawing says rather than where the shop found it convenient. Every order runs through the same quality chain: shop pre-qualification, material traceability against mill certificates, in-process and final inspection, and a documentation pack released before shipment.
Failure modes we inspect for
The outer jacket closed before the inner line was fully tested — irreversible without cutting. Vacuum integrity confirmed at the shop and lost in transit because the transport arrangement ignored it. Cold shoes and guides installed where convenient rather than where the thermal analysis requires. And annular leak detection provisions fitted but never functionally proven.
How an order runs
Enquiry and specification review; shop shortlisting against the actual scope rather than a general vendor list; quotation with the inspection plan priced in, not added later; kick-off with agreed hold and witness points; in-process inspection at those points; final inspection, document pack release and shipment; arrival support if anything is queried. For double-wall work the sequence is the control: inner line welded, tested and released, then jacketed, then vacuum and insulation verified.
Frequently asked questions
Why use double-wall piping?
Because a single barrier is not acceptable — for cryogenic duty, for toxic or hazardous media, or because class requires containment and monitoring of a gas fuel line.
What is the difference between pipe-in-pipe and vacuum-insulated?
Pipe-in-pipe provides secondary containment with a ventilated annulus; vacuum-insulated piping uses an evacuated annular space for thermal performance. Some systems do both, and the testing regime differs accordingly.
How is the inner line proven?
Pressure and leak testing witnessed before the jacket closes, with the hold point formally recorded — after jacketing, a defect means cut and rebuild.
Is annular leak detection tested?
Yes, functionally, not just installed. Detection and ventilation provisions are proven as part of the release.