The 'flareable' inventory at a refinery is not enough to cause problems for overflying aircraft. The sites marked on the chart are there because typically they are on major gas pipelines - in the event of a major malfunction the gas inventory is massive - pipelines like the Interconnector at Bacton have thousands of tons of gas which whilst very unlikely to 'get out' could really spoil your day. In contrast Grangemouth has possibly 10 tonnes and much of that at much lower pressures that the transmission lines.
That is of course why it does not have a warning on the chart!