There are basically two grades of propane storage containers, DOT cylinders and ASME tanks. DOT cylinders which are used by most of the propane companies require the cylinder be removed from service every 10 years to have the relief valve replaced, they are basically over sized BBQ cylinders. ASME tanks are what we use exclusively for HVAC and their service life is 25 years at which point the tank must be removed from service and the relief valve replaced.