Why Does PA11 Retain Pressure Sheath Integrity After Condensate Aging in Unbonded Flexible Riser Service?

The pressure sheath in an unbonded flexible riser is not a standard tube; it is a thick-wall extruded cylinder that must survive sour gas condensate, explosive decompression, annular fluid chemistry, and hydrostatic collapse at water depths beyond 3,000 m. Rilsan BESNO TL is charged as 100 wt% virgin PA11 with no PA12 dilution in the pressure sheath compound; a carbon black masterbatch is only used in outer protective layers at 2–3 wt%, never in the fluid-contact bore. The principal equipment is a single-screw extruder with 30:1 L/D and barrier screw geometry and a grooved feed section. Predrying at 80–90 °C to a moisture content below 0.08% is mandatory because residual moisture above 0.10% produces hydrolysis, viscosity loss, surface shark-skin, and a measurable decline in ASTM D638-14 elongation at break. Melt temperature is held at 230–250 °C; adapter and die zones are trimmed to 225–235 °C. The sheath is extruded over the interlocked steel carcass under controlled melt pressure; typical pressure before the screen pack is 15–25 MPa, and equipment selection must tolerate pressure fluctuations driven by carcass gap variation without altering wall thickness. Cooling water inlet temperature is kept below 40 °C to control crystallinity and shrinkage; quench water colder than 20 °C can freeze in residual stress and reduce environmental stress crack resistance against methanol-bearing condensate. Finished sheath wall thickness is typically 6–15 mm. Terminal product types are unbonded flexible riser pressure sheaths, flowline liners, and jumper spools qualified to API Spec 17J and ISO 13628-2:2020 with NORSOK M-710 rapid gas decompression testing in methane and sour gas. Operational boundaries include limiting continuous service temperature to 90 °C and avoiding free methanol above 70 °C without cutback testing because PA11 can initiate stress cracking in acidic condensate.

An air-brake tube line processing Rilsan BESNO TL for commercial vehicle coiled tubing operates with a 45 mm single-screw extruder at 25:1 L/D and a polyamide barrier screw. The resin is fed as 100 parts by weight of BESNO TL; where black tube is required, carbon black is incorporated as a 2.0 wt% coextruded cap layer or precompounded masterbatch, not as a dry pigment. Predrying at 80 °C to 0.08% moisture prevents steam-induced porosity. Melt temperatures are maintained at 225–240 °C at the adapter and 220–230 °C at the die. The tube is pulled through a vacuum calibration sleeve with slot vacuum at −0.08 MPa and ultrasonic wall thickness scanning; draw ratio is held at 1.05–1.15 to avoid inner bore deformation and axial orientation that would reduce radial burst strength. The finished air brake tubing is tested against SAE J844 Type A and ISO 7628-1; cold impact conditioning at −40 °C for 4 h is followed by a drop test, and boil burst testing at 100 °C verifies that the tube remains above minimum burst pressure. FMVSS 571.106 applies when the assembly is installed as part of a full vehicle braking system. Terminal product types include tractor-trailer air brake coil assemblies, suspension air lines, and push-in fitting rigid truck lines. The continuous service ceiling is 100 °C, with excursions to 125 °C accepted only for short intervals because PA11 loses burst strength as it approaches its crystalline melting point near 189 °C.

Fuel Vapour Barrier Structures Under SAE J2260 and CARB LEV III Emission Limits

The coextrusion of PA11 as the outer jacket over EVOH in a three-layer fuel line imposes a narrow processing window because the EVOH barrier must not be overheated above 225 °C while Rilsan BESNO TL requires 230–240 °C at the die to avoid cold slugs. A representative multilayer construction contains 38 wt% BESNO TL as the outer layer, 24 wt% BESNO TL as the inner bore layer, 10 wt% EVOH, and the balance as anhydride-grafted tie resin. The PA11 layers serve as impact and moisture protection for the EVOH; the inner BESNO TL wall determines connector retention and burst. The process uses five extruders feeding a spiral mandrel die, gravimetric layer control, and post-die vacuum calibration. Layer thickness is verified by ultrasonic cross-section scanning at 2 m intervals, with total wall thickness held between 1.0 mm and 1.5 mm. ASTM D638-14 tensile elongation of the extruded PA11 layer after fuel aging is monitored above 150% to ensure crush resistance at clip points. Compliance for fuel permeation is demonstrated by SAE J2260 and the applicable California ARB LEV III evaporative emission limits; converter-specific permeation data must be generated on the production die because published data for this exact layer construction is limited. Terminal products include fuel feed lines, fuel vapor return lines, onboard refueling vapor recovery lines, and quick-connector assemblies in flex-fuel applications. The operational limitation is the EVOH layer’s moisture sensitivity; the outer PA11 layer must remain free of pinholes because moisture ingress can collapse the EVOH barrier during outdoor storage without sealed end caps.

When submersible pump cable sheathing is specified to IEC 60092-351 and NEK TS 606, the line must balance melt strength against pressure-tool leakage at the crosshead. Rilsan BESNO TL is run as the primary jacket compound at 100 wt%, with carbon black masterbatch at 3.0 wt% and a processing aid at 0.2 phr to reduce die drool. The conductor is preheated to 80 °C before entering the crosshead to prevent abrupt crystallization at the insulation interface. Melt temperature at the crosshead is controlled at 230–245 °C, and the extruder is operated with a 24:1 L/D barrier screw. The jacket is pressure-extruded to maintain adhesion to the underlying insulation; tube extrusion, by contrast, is used only for bedding layers. Cooling is achieved in a 15 m water trough with segmented temperature zones from 50 °C to 25 °C, followed by air wiping and online spark testing at 3 kV DC. The jacket wall thickness is typically 1.5–2.5 mm. The PA11 jacket provides abrasion resistance and hydrocarbon resistance in engine rooms; low-temperature impact strength is evaluated by IEC 60811-506 after −40 °C conditioning. Terminal products include submersible pump cable jackets, offshore crane cable sheathing, and marine low-voltage control cables. When flame-retardant performance is required, BESNO TL is not used as an FR compound; halogen-free FR masterbatches must be qualified separately because most flame-retardant systems shift PA11 elongation at break and low-temperature impact.

Drinking-Water Contact Tubing: Migration Kinetics, NSF/ANSI/CAN 61, and 21 CFR 177.1500

In drinking-water transfer lines, extraction after 24 h at 60 °C in 10% ethanol and 3% acetic acid is used to screen the finished tube before certification. Rilsan BESNO TL is processed as 100 wt% virgin PA11; no regrind is used for potable-water contact because reprocessing shifts the low-molecular-weight fraction and can increase total organic carbon release. The production line uses a dedicated stainless-steel single-screw extruder at 225–240 °C with a polished chrome plate die to minimize surface roughness and biofilm adhesion. After extrusion, tubes are rinsed at 70 °C with deionized water for 30 min and then conditioned at 50% relative humidity for dimensional stabilization. Final certification is based on NSF/ANSI/CAN 61 for drinking-water system components and 21 CFR 177.1500 for food-contact polyamide; EU applications require migration testing under EU 10/2011 and EN 1186-1. The finished tube is controlled for total organic carbon, turbidity, and threshold odor; because PA11 absorbs water and swells by approximately 1.8% at saturation, dimensional change across the wall is compensated by specifying the inner diameter after moisture conditioning. Terminal products include beverage dispense tubing, drinking water filtration manifolds, and food-processing plant water lines. The operational boundary is continuous exposure to chlorinated water at 60 °C; above 65 °C or free chlorine above 2 ppm, surface oxidation may proceed and fitting retention should be revalidated because the tube loses hoop stiffness as it plasticizes.

When Subsea Chemical Injection Lines Encounter Methanol Carryover at 70 °C

In subsea chemical injection service, the tube bore is exposed to methanol, xylene, scale inhibitor, and monoethylene glycol at temperatures that can reach 70 °C. The construction uses Rilsan BESNO TL in the bore layer at 100 wt% of the fluid-contact wall, with a carbon-black-filled PA11 outer cap layer accounting for 10–15 wt% of total wall thickness for UV and abrasion protection. The dual-layer tube is coextruded on a 30:1 L/D line with two melt streams joining in a spiral mandrel die; bore-layer melt temperature is 235–245 °C, and the cap layer is held at 220–235 °C to minimize pigment dispersion defects. Online inspection includes laser micrometer diameter measurement, wall thickness eccentricity below 5%, and spark testing at 2 kV to detect pinholes in the cap layer. The assembled tube is pressure-tested at 1.5× the rated working pressure with hydraulic oil before spooling. Compliance is anchored to API Spec 17E for subsea control systems and ISO 13628-6 for subsea production control systems, with material qualification under NORSOK M-710. Terminal products are methanol injection lines, subsea hydraulic control line bundles, and chemical dosing conduits installed in umbilicals. The known incompatibility is continuous exposure to free methanol above 70 °C at elevated pressure, which requires cutback testing because methanol plasticizes the PA11 bore and can reduce collapse resistance; published data for this specific configuration is limited, so qualification must be performed on the production tube.