Fugitive emissions, at times called `leaks' are unintended, unknown releases of unburned gas, venting is the release of unburned gas for known and planned reasons, such as maintenance activities or to drive valve controllers or pumps. Combustion or combustion slip refers to the fraction of fuel gas that remains uncombusted during any combustion process, such as engine or turbine exhaust, heaters, or other process equipment. All combustion processes have some slip.