District heating and cooling
Sealing joints on a district heating network already welded in
Every joint is a gap in the casing, and the foam behind it has no defence of its own.
Water in the foam, and a carrier pipe corroding where nobody looks
A wet joint does not announce itself. Output falls slowly, pumping cost creeps up, and the surface shows nothing. When the trench is opened again the polyurethane foam has taken up water. Its insulation value is gone and the steel has been sitting wet for years.
A bonded pre-insulated pipe is a carrier pipe, foam around it, and a casing around the foam. The casing is continuous except at a joint, where it stops so the carrier pipes can be welded. The manufacturer's stated requirement is narrow: keep water off the insulation that the weld joint leaves exposed.
Two published figures decide whether it holds. Peel strength is 14 N/cm minimum. The method is ISO 21809-3. Adhesive softening point is 130 °C minimum. The method is ASTM E28. On a heating line, read that second one twice.
This order of work is our reconstruction
The manufacturer publishes the parts, their charts and their test data, but not this sequence as a numbered procedure. It is assembled from those sheets and from what each part is described as sealing. The instructions supplied with the parts override it.
The order the parts actually go on
Two companies fit these parts, weeks apart, and only one of them ever sees the trench.
The sealing cap goes on at the pipe works
A heat-shrink cap with a hot-melt or mastic lining, moulded from cross-linked polyolefin, closing the casing end down onto the carrier pipe. The manufacturer states it can be factory installed and then protects the pipe through transport and storage. The pipe maker buys it.
The carrier pipes are welded in the trench
Nothing on this page belongs on the girth weld. It is made, checked, and the joint insulated by the method the pipe system calls for. The caps sit clear of that, on the casing either side.
The wrap closes the joint over the casing
GTWS is described for field joints on buried or exposed steel line, and on insulated pipe. Cross-linked polyethylene over hot-melt adhesive, 500 mm wide at every size, Ø159 mm to Ø1016 mm, each with its closure patch.
Mastic fills the steps a wrap would bridge
The cap arrives carrying its own: 30 mm minimum along the body, 15 mm along the finger, on every code in both charts. Cold-applied butyl goes into what is left.
The four parts, and the code you order each against
Every figure below is from the manufacturer's selection chart for that code.
GPPC1, single outlet
One carrier pipe leaving one casing. The chart opens at GPPC1-1030, recovering 60 mm onto 30 mm, and closes at GPPC1-225300, 570 mm onto 300 mm. Some codes are marked tool-on-demand, so raise those before they reach a drawing.
GPPC2, double outlet
Two carrier pipes leaving one casing, which is how a twin run is built. GPPC2 1565S is 115 mm onto 65 mm and GPPC2 55175AA is 355 mm onto 180 mm. Recovered wall stays between 2.0 mm and 3.5 mm.
GTWS, the wrap over the joint
Flat length runs 620 mm at Ø159 mm to 3430 mm at Ø1016 mm, always 500 mm wide. The closure patch is 100 mm to 200 mm long. Cathodic disbondment is 3 rad. The method is ISO 21809-3.
Mastic, worked in by hand
Cold-applied butyl in strips and rolls, 2.5 mm thick in black and 1.7 to 2.0 mm in red, on a 35 mm minimum width. Black dielectric strength is 6 kV/mm minimum, red 10 kV/mm. The method is ASTM D149.
Two purchase decisions, not one
Different companies, different documents, conditions with nothing in common.
| Decision | At the pipe works | In the trench |
|---|---|---|
| Who is buying | The pre-insulated pipe manufacturer, into a bill of materials | The network contractor, against a joint schedule |
| What goes on | The sealing cap, single or double outlet, over the casing end | The wrap over the finished joint, mastic worked into the steps |
| Ordered against | Both diameters the cap bridges, casing at D and carrier at d | Pipe outer diameter, which sets flat length and patch length |
| Fitted onto | A clean dry casing, indoors, on a known heating setup | A casing handled, cut back, and often wet |
| How it goes wrong | A code picked against the wrong d, found at the joint face | An overlap lifted in service, found years later as wet foam |
Two of the manufacturer's sheets disagree on the cap's temperature limit
One sheet gives the continuous temperature limit for GPPC1 and GPPC2 as -40 °C to +100 °C. Another gives -40 °C to +125 °C. Both carry the same charts and the same 18 N/mm² tensile minimum, so this is not two products. The method named on both is IEC 216. On a heating network that 25 degree spread is the whole question. Get it confirmed in writing against the code you order.
Corrugated HDPE on the same site is a jointing problem, not a sealing one
Ducting, drainage and cable protection runs arrive on the same job in corrugated HDPE. Those are jointed with the GMW coupler, a tube of cross-linked polyolefin with a hot-melt lining sealed to IP68. It stands in where the alternative is a butt weld, a snap-fit or a split coupler. At 3:1 shrink, GMW 70/22 recovers 70 mm onto 22 mm and the chart runs to GMW 863/250.
Two limits. It is a tube, so one pipe end has to be free before it can be threaded on. And it is a jointing and sealing part: the sheet publishes tensile, elongation, hardness, dielectric and temperature data, and no pressure rating at all. The manufacturer's own page is Heat Shrink Coupler for HDPE Pipe Jointing, and the same family closes a post-tensioning duct joint.
Buried or overground, and what the adhesive actually sees
Underground the load is mechanical and chemical. Backfill drags across the seam as it settles, and the seal holds against ground water for the life of the line. The wrap's sheet claims resistance to bacteria, mildew, solvents and the cathodic-protection current the line itself carries.
Overground the load is ultraviolet and thermal cycling. The caps are stated resistant to UV rays and ozone, and the wrap to prevent the coat separation ultraviolet light causes. Those are claims about the material, not about your flow temperature. Read the 130 °C softening point against the cap's continuous limit before a riser goes up in direct sun. Impact guarding at boot height stays your own design decision.
Where this is the wrong part
Say these out loud on site before anyone lights a torch.
- Foam that is already wet. Nothing here dries insulation out. A cap closed over saturated foam seals the water in with the steel.
- The girth weld itself. The cap sits on the casing, the wrap sits over the joint, and neither replaces the joint insulation between them.
- Any pressure duty on the carrier pipe. No part on this page carries a published pressure rating.
- Specifying the mastic from its published data alone. The manufacturer lists it among what it supplies to pre-insulated pipe makers, then describes it as a sealant for cable joint and termination kits.
Where each of these is written up in full
This page is the sequence. The dimensions live below, once each.
- Pre-insulated pipe sealing cap, single and double outletD, d, L, l, recovered wall and mastic lengths, code by code.
- Three-layer sleeve for the joint itselfBacking, adhesive and closure patch, peel quoted hot as well as cold.
- Pipeline sleeve size chart, 159 to 1016 mmFlat length and patch length against outer diameter.
- Cathodic disbondment and ISO 21809-3What a 3 rad result is, and what it does not tell you.
- How to install a wraparound sleevePreparation, torch work and the overlap, written once.
- The manufacturer's district heating and cooling pageGala Thermo Shrink Pvt. Ltd., Palghar and Surat SEZ.
Send the casing OD, the carrier OD and the flow temperature
Those three numbers pick the cap code and settle the temperature question in one message. Add whether the run is single or twin, and buried or above ground.