The PTFE-lined ISO tank carries cargoes that destroy LDPE within hours and that even concentrated stainless cannot tolerate. PTFE (polytetrafluoroethylene), FEP (fluorinated ethylene propylene), and PFA (perfluoroalkoxy alkane) sheet liners 2 to 5 mm thick mount inside a carbon-steel or 316L stainless shell via fixpoint anchoring. The combination resists essentially every common industrial acid, base, oxidiser, and halogenated solvent at temperatures up to 120 deg C continuous. AGRU is the dominant European liner specialist; Marflex and the Chinese rotomoulding houses serve the Asian fleet.
What PTFE-lined is built for
The cargoes that justify the PTFE premium: hydrofluoric acid 70% (UN 1790, the canonical PTFE-lined cargo because nothing else resists anhydrous or near-anhydrous HF at concentration), 98% sulphuric acid (UN 1830) and oleum (UN 1831) where the lining provides corrosion margin over bare 316L for long voyages, sodium hypochlorite (UN 1791) at elevated temperatures or extended residence times, chlorine in aqueous solution (UN 1017), halogenated solvents that swell or dissolve rubber and PE (chloroform, methylene chloride at concentration, certain fluorinated solvents).
Construction and materials
Carbon-steel Q345R shell or 316L stainless cylinder, 6 mm reference-steel thickness, with a 2 to 5 mm PTFE / FEP / PFA sheet liner welded along seams and mechanically anchored via fixpoint pins to prevent movement under vacuum or thermal cycling. The liner thickness depends on the chemistry: 2 mm minimum for chemistries where mechanical stress is low, 4 to 5 mm for high-cycle fleets and aggressive cargo combinations.
PTFE has near-universal chemical resistance but is mechanically weaker than the steel substrate it sits inside. The fixpoint anchoring system distributes mechanical stress so the liner doesn’t tear under vacuum or vibration, and the welded seams maintain liner integrity at the high-stress regions near the manlid and outlet. Liner inspection regimes are stricter than for PE (more frequent visual checks, occasional spark testing). Premium price runs about 30% above an unlined T14 stainless build.
When PTFE-lined is the right choice
PTFE-lined is the right tank for HF service at any concentration, for sodium hypochlorite where elevated temperature or extended residence time matters, and for any cargo where the corrosion-rate margin of bare stainless is unacceptable. For concentrated 98% sulphuric specifically, the trade is currently split between bare 316L (which passivates above 70% concentration and gives multi-decade tank life) and PTFE-lined (which provides corrosion margin against off-spec dilution events that would attack stainless rapidly). Operators with tighter quality requirements increasingly prefer PTFE for sulphuric.
When PTFE-lined is the wrong choice
PTFE-lined is the wrong tank when PE-lined is sufficient. The 30% price premium is wasted on cargoes (HCl 35%, NaOH 50%, NaClO 10 to 15%, phosphoric acid) where LDPE serves for 5 to 8 years at half the lease rate. PTFE is also the wrong choice for cargoes that mechanically abrade the liner (slurries, abrasives) where the softer fluoropolymer wears faster than expected.
How a PTFE-lined booking is verified
Pre-loading inspection covers the standard plate stack plus a lining-condition check that includes a fixpoint-anchor inspection (visible from inside through the manlid), a seam check on the welded liner edges, and the AGRU or Marflex liner certificate listing installation date, fluoropolymer grade (PTFE / FEP / PFA), liner thickness, and last spark-test record. Spark testing on PTFE liners is more sensitive than on PE: a 5,000 to 10,000 volt spark scan finds pinholes that a visual inspection misses.