The boiler is one of two. The user initially develops controls for the one that is base-loaded. The boiler fires both fuel oil and gas. The gas is variable in flow, pressure, temperature and composition. Total duty control is first implemented. Three-element level control, with compensation for 'swell', can be installed on the steam drum. Cross-limiting control and flue gas O2 and CO controls are also commissioned. The boiler is then switched off base-load and steam header pressure control configured.