This document specifies the product characteristics and test/assessment methods for workshop fabricated cylindrical, horizontal steel tanks, single (type S) and double skin (type D) intended to be used for the underground storage of water polluting liquids (both flammable and non-flammable), specifically used for storage and/or supply of fuel for building heating/cooling systems, and of hot or cold water not intended for human consumption at normal ambient temperature conditions (−20 °C to +50 °C) within the following limits: - from 800 mm up to 3000 mm nominal diameter and; - up to a maximum overall length of 6 times the nominal diameter; - for liquids with a maximum density of up to 1,1 kg/l and; - with an operating pressure (Po) of maximum 50 kPa (0,5 bar(g)) and minimum – 5 kPa (–50 mbar(g)) and; - for double skin tanks with a vacuum leak detection system where the kinematic viscosity does not exceed 5 × 10−3 m2/s.