For Self-Acting Valves with Internal Sensing, there is no clear distinction between actuator and valve, as the pressure-sensitive element (diaphragm or piston) is located in the flow path between the two.
Advantages
Ease of installation as it is not necessary to make any disconnections on the system pipes to connect the process fluid pressure to the valve/actuator.
The lack of packing makes the valve more sensitive to pressure variations.
Emissions to the outside are reduced to a minimum, making it an ideal solution for toxic/lethal or simply very expensive fluids.
It normally has small overall dimensions.
Disadvantages
The design becomes complex and uneconomical for valves with very large sizes and high-quality materials.
The valve becomes a "black-box" whose exact degree of opening is unknown.
Pressure reducing valves have the trim mounted from below (valve normally open) so the installation must also provide suitable clearance spaces in the area below the valve.
