FRP Flanges Explained: Fixed vs Loose (Stub End) Flanges
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Every FRP (GRP) piping system ends in flanges: at pumps, valves, tank nozzles and wherever fiberglass meets steel. Choosing between a fixed flange and a loose flange with stub end affects how easy the line is to install, how it behaves under load and how much it costs. This article explains the options and how to specify them. For the full picture of pipe and fittings, start from our FRP pipe and fittings guide.
Fixed FRP flanges
A fixed flange is laminated in one piece with its hub, so the flange face and the bolt holes are fixed in position. We supply them from DN15 to DN350 in three versions:
- with pipe: the flange comes with a short spigot (standard 200 mm), ready to be joined to the line by lamination or bonding;
- without pipe: to be laminated onto your own pipe or fitting;
- backing flange (blind): to close a line or a nozzle.
When to use them: on prefabricated spools, tank nozzles and equipment connections where the bolt orientation is defined in advance. They are compact and need fewer parts.
Loose FRP flanges and stub ends
A loose flange is made of two parts: a stub end (in Italian "codolo"), laminated onto the pipe, and a separate flange ring that slides over it and can rotate freely. The gasket seals on the stub end face; the ring only carries the bolts.
We supply the stub end with or without pipe, the loose ring, the backing flange or the complete assembly, from DN15 to DN350.
When to use them: when bolt holes must be aligned on site, for example connecting to existing equipment or steel flanges, on long lines where small rotations accumulate, and on connections that are dismantled often. Being able to rotate the ring saves time and avoids forcing the pipe into position, which is a frequent cause of stress in FRP joints.
Fixed vs loose at a glance
| Fixed flange | Loose flange + stub end | |
|---|---|---|
| Bolt alignment | defined at fabrication | adjustable on site (ring rotates) |
| Parts | one | two (stub end + ring) |
| Best for | spools, tank nozzles, equipment | connections to existing plant, frequent dismantling |
| Sizes in our range | DN15 to DN350 | DN15 to DN350 |
Drilling standards
An FRP flange must match the flange it bolts to. Our standard drilling options are:
- EN 1092 PN6, PN10, PN16 (the European standard used for most process and water piping);
- ANSI / ASME 150 lb, for connections to American standard equipment;
- undrilled, when you want to drill to a special pattern.
Up to DN150 the PN16 and PN10 drilling patterns coincide; from DN200 they differ, so always state the class. Other patterns are available on request.
Large flanges and tank manways (DN400 to DN600)
For tanks, scrubbers and vessels we produce manway flanges in DN400, DN450, DN500 and DN600, with or without nozzle or as blind covers, undrilled or drilled (ANSI 150 lb on DN600), with thickness from 20 to 45 mm according to the load. Sizes above DN600 are made on request.
Installation tips for FRP flanges
- Use gaskets designed for FRP flanges and compatible with the fluid.
- Use washers under nuts and bolt heads to spread the load on the laminate.
- Tighten in a cross pattern in several passes, up to the torque specified by the manufacturer or the design: FRP flanges must not be tightened like steel ones.
- Do not pull misaligned pipes together with the bolts: correct the alignment or use a loose flange.
- Re-check bolt torque after the first thermal cycle, as recommended by the installation procedure.
Materials
Our flanges are made by filament winding, with a vinyl ester liner at least 1.3 mm thick and an isophthalic polyester structural layer. Different resins, colours and thicknesses can be produced for specific services. Why vinyl ester in contact with the fluid? See Vinyl ester vs polyester resin.
Request a quote
Send us DN, type (fixed or loose), drilling standard, quantity, fluid and temperature through the contact form: we ship across Europe and worldwide, with freight quoted case by case.