Backing Ring Glossary

What is a backing ring?

A backing ring - also called a loose flange, stub-flange backing ring, or in German Losflansch - is a separate ring that fits behind the stub flange (collar) of a plastic pipe. When the bolts of a flange connection are tightened, the load is transferred through the backing ring rather than through the soft plastic of the pipe. This lets you bolt PE, PP, PVC and GRP piping to steel flanges, valves, pumps, and other flanged equipment without crushing or deforming the pipe end.

An FRP (fiber reinforced plastic) backing ring uses the same composite philosophy as the pipe itself: corrosion-free, lightweight, and chemically resistant. It pairs naturally with plastic piping where a steel ring would rust, leach, or require coatings.

How a flanged connection works (anatomy)

1. Pipe + stub flange - the pipe end has a short flared collar (the stub flange or stub end). The face of the stub flange is what seats against the gasket.
2. Backing ring - slipped over the pipe before the stub flange is welded or fused on, the backing ring lives behind the stub. Its bolt holes align with the counter-flange's holes.
3. Gasket - between the stub flange face and the matching counter-flange.
4. Counter-flange - the steel/cast/composite flange on the equipment side.
5. Bolts + nuts - pass through both backing ring and counter-flange, clamping the gasket between the two faces.

DN and PN explained

DN ("Diameter Nominal") is the EN-system pipe size in millimeters. It's a labeling convention, not the actual outer diameter - DN50 is "two-inch-nominal" pipe, DN300 is "twelve-inch-nominal." Use DN to specify across pipes, fittings, valves and flanges so everything matches.

PN ("Pressure Nominal") is the pressure rating class in Bar - PN6, PN10, PN16, PN25, PN40 are the common steps. The PN class drives the bolt circle, bolt count, and bolt diameter on EN1092 flanges. Always match the backing ring's PN to the counter-flange's PN.

Drilling standards: which one do I need?

The drilling standard tells you the bolt-hole pattern (number of holes, bolt circle diameter, hole size). Different standards are not interchangeable - you must order the backing ring drilling that matches the counter-flange.

• EN1092 - European standard. Sized by PN (PN6/10/16/25/40). Standard for most projects in Europe, the Middle East, Africa.
• ANSI B16.5 - American standard for 1/2"–24" flanges, sized by Class (150, 300, 600, 900, 1500, 2500). Common in North America and the international oil & gas sector.
• ANSI B16.47 - Continuation of B16.5 for 26"–60" diameters. Two series: Series A (heavier) and Series B (lighter).
• AWWA C207 - American Water Works Association. Used for waterworks (water/wastewater) flanges. Classes B, D, E, F.
• JIS B2220 - Japanese Industrial Standard. Common in Japan, Korea, parts of Southeast Asia.
• BS 4504 - British. Largely superseded by EN1092 but still cited in legacy specifications.
• AS - Australian Standard, common for projects in Australia and New Zealand.

Custom drilling patterns are available on request. If your counter-flange is non-standard or proprietary, send us the dimensions and we'll match it.

FRP versus steel backing rings

For plastic piping (PE, PP, PVC, GRP), an FRP backing ring is almost always the right call:

• Fully corrosion-free - no rust, no coating to flake off, no cathodic protection needed.
• ~40% the weight of an equivalent steel backing ring - lower install effort, lower freight cost.
• Electrically non-conductive - eliminates galvanic concerns where the pipe joins steel equipment.
• Chemical resistance spans most acids, bases, and salts - appropriate for chemical processing, water treatment, mining, marine, and offshore service.
• Lifecycle cost is lower than coated steel anywhere corrosion is the failure mode.

The trade-off: FRP has lower tensile strength than steel of the same dimensions, so the backing ring section is typically thicker than a steel equivalent. The flange face, gasket selection, and torque values may need to account for this - talk to our engineering team if you're substituting for a steel ring on a pre-engineered skid.

How to specify a backing ring

To order a stocked backing ring you need five things:

1. Pipe material - PE, PP, PVC, or GRP
2. Nominal size - DN15 through DN1200 (larger on request)
3. Pressure rating - Bar (or ANSI Class)
4. Drilling standard - EN1092 PN..., ANSI B16.5 Class..., AWWA C207 Class..., JIS B2220, etc.
5. Quantity

Browse our catalog by pipe material:

• Backing rings for PE & PP pipes
• Backing rings for PVC pipes
• Backing rings for GRP pipes

Manufacturing: Hot Pressing vs RTM

We use two manufacturing methods, chosen by part size:

• Hot Pressing (DN15–DN300) - high-pressure compression of Sheet Molding Compound (SMC) in heated steel tooling. Fast cycle, high volume, tight dimensional tolerances. This is the right method for high-running stock sizes.
• Resin Transfer Moulding / RTM (DN350–DN1200) - a closed-mold process where dry glass reinforcement is laid into the mold, then liquid resin is injected under pressure. RTM gives precise reinforcement positioning, low void content, and consistent quality at large diameters where hot pressing isn't practical.

Both methods produce ISO 9001 quality output with full material traceability.