There is NO ASME Requirement for an Open Bonnet PRV on Air, just a lifting Lever. The Bonnet (open or enclosed) houses the spring.
Per ASME Section VIII, Division 1, Paragraph UG-136(a)(3), "Each pressure relief valve on air, water at the valve inlet that exceeds 140°F (60°C), excluding overpressure or relief events, or steam service shall have a substantial lifting device which when activated will release the seating force on the disk when the pressure relief valve is subjected to a pressure of at least 75% of the
set pressure of the valve."
Notice ASME uses the generic term Pressure Relief Valve. In ASME, a Safety Valve is used for compressible fluid, typically steam or air. Typically the Safety Valve has an Open Bonnet with a visible spring. Conversely, a Safety-Relief Valve has a Closed Bonnet so that can be used for compressible or incompressible fluids. Typically, incompressible fluids do NOT have a Lift Lever, unless it is hot water as defined by UG-136(a)(3) as mentioned above. The Relief Valve is primarily for Liquid (incompressible) fluid. I hope this is helpful.
JAC