The valve is connected into a compressed air main with the air flowing through ports 1 and 2 of the valve body. When the pressure in the main increases, valve member 3 is pushed upward, compressing helical spring 6, until the valve stem runs up against bonnet 5. Upon a further increase in pressure, sleeve 4 slides upward, continuing to compress spring 6 and releasing air through clearance a, the clearance between sleeve 4 and the valve stem and holes b to the atmosphere. The force exerted by spring 6 (and the pressure at which the air is released) can be regulated by turning threaded bonnet 5.
$3598$SHP,Va$