Underground storage of flammable liquid is no longer acceptable as a single steel shell in most jurisdictions. Secondary containment means a second barrier must catch the leak if the primary fails. There are three ways to build that barrier, and they are not interchangeable.
SF double-walled tank
Steel inner shell (Q235B/Q345R, 6–8 mm) for strength and product contact; FRP outer shell (4–6 mm glass-reinforced vinyl ester) for soil-side corrosion resistance. Annular gap 3–5 mm forms the interstitial space. Standard for gasoline, diesel, kerosene, biodiesel blends up to B20. Advantages: familiar steel welding inside, no metal corrosion outside, 30-year design life. Limits: FRP outer cannot take high temperature (>80 °C) or steam cleaning.
FF double-walled tank
Both inner and outer shells are FRP. Used where chloride soil is extreme, or where the end user forbids metal entirely (some food-grade solvent stores). Better chemical resistance than SF for certain acids, but lower impact tolerance-needs careful backfill and traffic-load calculation if under a truck lane.
SS double-walled tank
Steel inner + steel outer with annular gap, sometimes filled with dry sand or connected to a vacuum. Used in high fire-rating scenarios, LNG-adjacent zones or where local code distrusts FRP. Heavier, pricier, but fully weld-inspectable on both shells.
Leak detection-non-negotiable
A double wall without monitoring is a delayed accident. Three common systems:
Vacuum decay: annulus held at −25 kPa, alarm if pressure rises
Pressure sensor: annulus pressurized to +10 kPa, drop triggers alarm
Liquid sensor probe: standing sensor at tank bottom annulus, detects first droplet
Procurement verification list (paste into your PO)
Inner shell RT/UT coverage percentage and acceptance standard
FRP outer spark test (holiday detector) voltage and result
Factory interstitial pressure test certificate, 24 h hold
Leak detector model, alarm setpoint, cable length
UL 58 / UL 1746 / GB 50156 clause mapping
FRP gel coat UV stabilizer percentage
Cathodic protection stub if soil resistivity < 50 Ω·m
Manhole reinforcement drawing signed by PE
Backfill specification (clean sand, 150 mm lifts, no rock)
30-year structural warranty wording, not just "quality guarantee"
Zhongneng Huajian ships SF tanks with steel-shell NDT report, FRP spark test video on USB, interstitial air-pressure curve, and a leak-probe pre-wire to the manhole junction box.
