What do we do when a pipeline must pass beneath a road, a riverbed, a national highway, railway embankments, bridges or culverts, flood-protection dykes, motorways, or protected heritage areas in old city centres?
What happens when project specifications require the protection of active pipelines by inserting them into casing pipes? Such requirements typically appear when pipes run alongside major roads, when they must cross railway embankments, watercourses, flood-protection dykes, motorways, or when transmission mains pass through protected urban zones.
When we asked various contractors how they usually solve these issues, the answer was staggering: “We hammer in a few wooden wedges and seal the ends with foam.” We have even encountered detailed design documents specifying the size and shape of the wooden slats - though not necessarily the type of wood—mounted on the pipe with galvanised wire...
Ing. Viktor Szokoly
General Manager Proex Top SRL
Despite such improvised practices, there are in fact fully engineered, professional solutions consisting of polypropylene pipe spacers and insulator rings, combined with specialized end-sealing bellows.
The wide range of models and sizes ensures proper installation for pipes from 25 mm up to 2500 mm.
| Type | Max. static load per ring |
|---|---|
| PA/PE 0.75 – PA/PE 1.5 | 85 kg |
| PA/PE 2.0 – PA/PE 3.0 | 100 kg |
| PA/PE 4.0 | 200 kg |
| PA/PE 6.0 – PA/PE 12.0 | 250 kg |
| AZ/AC 1 / AZ/AC 2 | 200 kg |
| GKO-mK | 250 kg |
| MA | 650 kg |
| RGV | 1000 kg |
| GKO-gl | 4000 kg |
| GKO-gs | 14,200 kg |
Each model has been tested for static load-bearing capacity on every ring.
The height of the support blocks, which ensures concentric alignment, ranges from 25 mm to 125 mm, offering extensive flexibility for matching to the dimensions of the casing pipe.
What information is required to select the correct technical solution?
- The outside diameter of the active/transport pipe; and whether it is additionally protected with an anticorrosive coating (e.g., steel pipes with PE outer coating).
- The inside diameter of the casing pipe.
- The material of the casing pipe and the type of joint used between sections.
- The full weight of the active pipe (pipe weight per metre + water weight per metre).
- The length of the under- or over-crossing.
- The required level of concentricity.
Typical spacing of spacer rings:
- For pipe diameters up to DN 300 mm: every 2.5 m
- For DN 301 – 600 mm: every 2.0 m
- For DN above 600 mm: every 1.5 m
For special cases, the distance between spacer rings may be adjusted according to actual site conditions.
Additional benefits include:
- High resistance to stress and cracking
- Polypropylene provides elasticity while maintaining structural stability
- Low weight
- Being a plastic material, it offers excellent electrical insulation and reduces eddy-current effects, lowering cathodic-protection costs
- Polypropylene has self-lubricating properties, minimising friction between the two pipes during insertion
Example: On a gas pipeline project in Turkey, after pulling 500 m of DN 1000 pipe in one continuous operation, measurements showed that the last support blocks had lost only 2 mm from their original thickness.
Complex crossings with multiple pipes, including pipes belonging to other utility operators, can also be constructed, as shown in the attached image.
How is end-sealing achieved?
End sealing for these protective casings is performed using rubber end-sealing bellows.
These are clamped and tightened onto both pipes with V4A stainless-steel bands, creating a watertight closure that prevents soil or water from entering the annular space between the concentric pipes. Multiple models exist, including multidimensional designs and variants fitted with inspection windows for video monitoring.
Rubber sealing bellows
Installation of collars
One may wonder why modular LinkSeal® systems, presented in the previous issue, are not recommended for end-sealing. The answer is straightforward: those systems cannot function as structural support points for pipelines, and no support-and-sliding system for pipe-in-pipe installations can guarantee perfect geometric concentricity.
Returning to the previous issue, one might also ask whether our colleagues in Iași could rehabilitate more efficiently the three adduction lines that under-cross the Moldova River by applying these solutions?!