This refers to the safety of life, the danger of boilers, pressure vessels (including cylinders), pressure piping, elevators, lifting machinery, passenger ropeways, large-scale amusement facilities.
By this definition, air compressors, hydrogen compressors, other forms of compressors, are general-purpose machinery,
But its auxiliary equipment, including gas storage tanks (pressure vessels), pressure piping, etc., are special equipment.
And safety valves belong to the safety accessories of special equipment.