I think they are outdated and not permitted anymore. At one time the older boilers had safety valves which were held closed with measured weights, and when the (pressure in the vessel psi) times the (valve port area in2) equaled the dead weight , the valve disc would lift and open and relieve the pressure.
Dead Weights are still in use on COnservation Vents.
A Pallett which corresponds to a Spring Loaded PRV Disc is fitted with weights and opens in repsonse to overpressure. However, these are very low pressure. Typically less than 5 psi and are Non-Code Applications. ASME Sec VIII is considering lowering the minimum pressur efor a Code Stamped valve from 15 down to 3 psi. This would open the door for these types of valves , but only in low pressure.
I saw these installed on the discharge lines of blowers used for aeration of effluent in treatment plants.
Believe it or not : It is believed that Archimedis single handedly sailed the 4000 ton ship Syracusia from the dock when dock workers failed and quoted his famous saying.
