During periods of high demand liquefied gas was pumped from storage tanks and heated in vapourisers to a gaseous state for delivery into the NTS.[2] There are four tanks each of 50,000 m3 (total storage capacity of 200,000 m3), constructed of 9% nickel-steel alloy insulated with perlite, each with a containment bund to half the tank height. At times of low demand gas could be withdrawn from the NTS and liquefied by cryogenic cooling to minus 162 °C to replenish the storage tanks. An adjacent gas holder received gaseous boil-off from the tanks.